169
Chapter 8
Leafhoppers (Homoptera: Cicadellidae):
a Major Family Adapted to Grassland Habitats
K. G. Andrew Hamilton
Biodiversity Program, Research Branch
Agriculture and Agri-Food Canada
K. W. Neatby Bldg., 960 Carling Ave.
Ottawa, Ontario, Canada K1A 0C6
Robert F. Whitcomb1
Plant Virology Laboratory
Crops Research and Entomology Research Division
USA Department of Agriculture,
Agricultural Research Service
Beltsville, Maryland 20705 USA
Abstract. Many Canadian grasslands are not, as commonly believed, merely extensions of those in the United
States. Analyses of their leafhopper faunas indicate that grasslands have unique Pleistocene histories that confer
equally unique features of ecology and biodiversity, with 470 species of grassland-endemic leafhoppers of which
at least 223 species in 66 genera occur in Canada. Of the latter, at least 132 species are strict monophages.
This rich endemic fauna appears to reflect a combination of glacial and postglacial adaptations to environmental
factors, such as short summers, which in many cases permit only a single generation per year. Also remarkable
is the apparent resilience of leafhopper populations to stressors, including floods and habitat fragmentation.
This resilience allows relict populations to persist in grasslands isolated in agricultural and forested landscapes.
However, these populations cannot readily disperse across altered landscapes and may be vulnerable to extinction
under global warming. Such isolated grasslands may be either postsettlement ecosystems or postglacial-age
ecosystems, a distinction that is strongly reflected in their leafhopper faunas. Postglacial-age grasslands include
glacial-age refugia, post-Altithermal relicts of maximum prairie extent, and grasslands that emerged on glacially
scoured landscapes such as alvars. Relict populations of native grasses and their leafhopper specialists indicate
that Altithermal prairie extended as far east as Windsor, Ontario, while an older periglacial grassland reached to
Lake Champlain near the Quebec–New York border. Grasses that were adapted to sandy conditions, and their
insect herbivores, probably followed glacial moraines all the way to the Atlantic Ocean after each glacial retreat.
Résumé. Contrairement à la croyance générale, les prairies canadiennes ne sont pas toujours de simples
prolongements des prairies américaines. L’étude de leurs populations de cicadelles montre qu’à partir du
Pléistocène, elles ont acquis des caractéristiques uniques aux plans de l’écologie et de la biodiversité, avec 470
espèces de cidadelles endémiques desquelles on retrouve au moins 223 espèces réparties en 66 genres dans les
prairies du Canada. De ces derniers 66 genres, il y a au moins 132 espèces qui sont des monophages stricts. Cette
faune endémique riche semble refléter une combinaison d’adaptations glaciaires et postglaciaires à des facteurs
environnementaux—par exemple, les étés courts qui ne permettent souvent de produire qu’une seule génération
par année. Il convient également de souligner l’apparente résistance des cicadelles aux agents stressants, y compris
les inondations et la fragmentation de l’habitat. Cette résistance permet à des populations reliques de persister
dans des prairies isolées au milieu de territoires agricoles et d’écosystèmes forestiers. Toutefois, ces populations
ne peuvent pas se disperser facilement au-delà des paysages modifiés et pourraient être exposées à l’extinction
sous l’effet du réchauffement climatique. Ces prairies isolées peuvent être de deux types—écosystèmes formés
après l’arrivée des Européens, ou écosystèmes remontant à l’époque postglaciaire—et cette distinction se reflète
clairement dans les populations de cicadelles. Les prairies d’âge postglaciaire comprennent des refuges d’âge
Deceased.
1
Hamilton, K. G. A. and R. F. Whitcomb. 2010. Leafhoppers (Homoptera: Cicadellidae): a Major Family
Adapted to Grassland Habitats. In Arthropods of Canadian Grasslands (Volume 1): Ecology and Interactions in
Grassland Habitats. Edited by J. D. Shorthouse and K. D. Floate. Biological Survey of Canada. pp. 169-197.
© 2010 Biological Survey of Canada. ISBN 978-0-9689321-4-8
doi:10.3752/9780968932148.ch8
170
K. G. A. Hamilton and R. F. Whitcomb
glaciaire, des reliques de l’expansion maximale des prairies remontant à l’époque postérieure à l’altithermal, et
des prairies qui se sont formées sur les paysages érodés par les glaces—par exemple, les alvars. Les populations
reliques d’herbacées indigènes et de leurs espèces spécialisées de cicadelles donnent à conclure que la prairie
altithermale s’étendait vers l’est jusqu’à Windsor (Ontario), tandis qu’une prairie périglaciaire plus ancienne
atteignait le lac Champlain, près de la frontière du Québec et de l’État de New York. Les graminées qui étaient
adaptées à des habitats sableux et les insectes qui s’en nourrissaient ont probablement suivi les moraines glaciaires
jusqu’à l’Atlantique après chaque période de retrait des glaces.
Introduction
All native Canadian grasslands bear eloquent testimony to glacial activity. Vast sand and
gravel moraines that stretched in huge arcs across the continent, together with erosional
features such as exposed rock outcrops and deep coulees carved by immense outpourings
of glacial meltwater, were all prime substrates for the development of unique grasslands.
Today, although many of these features have been obliterated by erosion or concealed by
encroaching vegetation, they nevertheless underlie the structure of many northern grassland
ecosystems. The biodiversity and distribution of their phytophagous insects developed on
such plants and substrates over the last 10,000 years of postglacial temperatures. This is
reflected most noticeably in their associated suites of endemic herbivores (Fig. 1). Thus, the
history of North American grasslands and their insect fauna is intimately tied to glaciation.
This is particularly true of interglacial grasslands, adapted to warm summers, which had to
find a refugium south of the ice cap for the preceding 40,000 years.
Regions farther south were hardly less affected by the Ice Age than were Canadian
flora and fauna. Vastly expanded boreal forests during this period displaced most of the
interglacial vegetation. As world climates ameliorated, the interglacial vegetation again
spread northward into Canada from refugia that have been thought to be near (or on) the
Gulf coast (Ross 1970). From this perspective, the faunas of Canadian grasslands have often
been considered to be simply extensions of those that exist today in the south-central United
States (e.g., Oman 1949). In this chapter, we examine this thesis from the perspective of
plant-feeding bugs that are adapted to living in native grasslands. Our separate experiences
Fig. 1. “Whitcomb’s beauty,” Stirellus bicolor (Van Duzee), a leafhopper whose polymorphism RFW was able to
demonstrate through controlled breeding experiments. This insect shows latitudinal differences in host specificity,
being polyphagous in the tropics, oligophagous in the southern United States, and monophagous on the Canadian
prairies, where it is restricted to little bluestem and is an indicator of tallgrass prairie. All individuals shown
are females. Photographs, clockwise from spring variety, lower left, courtesy of Lynette Schimming, Graham
Montgomery, Charles Schurch Lewallen, Tam Stewart, Tyler C. Christensen, and Samuel Houston.
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
Membracidae
Cercopidae
Delphacidae
adult
adult
male brachypter
nymph
nymph
female macropter
171
Cicadidae
Fig. 2. Leafhopper relatives on the prairies, right to left from upper right: Okanagana synodica (Say), Cicadidae;
Muirodelphax arvensis (Fitch), Delphacidae; Lepyronia gibbosa Ball, Cercopidae; Campylenchia rugosa
(Fowler), Membracidae. Photographs courtesy of Dan Johnson, Tyler C. Christensen, University of Minnesota,
Bill Johnson, Andy Daun, and Lynette Schimming.
in sampling the northern and southern faunas indicate that the grasslands that now occupy
glaciated terrain differ in biodiversity and ecology from those of unglaciated sites, both
on the Great Plains and on the adjacent desert plains of the Southwest United States.
Our target organisms are the phytophagous true bugs known as leafhoppers (Homoptera:
Cicadellidae). We examine these insects as tools for differentiating biotic communities and
distinguishing different types of grasslands. Somewhat surprisingly, leafhoppers can also
be used as indicators of the prehistoric origins and pre-settlement characteristics of native
grasslands in Canada (Hamilton 2005). We also discuss possible reasons for the life history
strategies of these insects, their present biogeography, and reactions to stressors.
Leafhoppers and Relatives
Leafhoppers (Fig. 1) are jumping insects belonging to Homoptera, suborder Auchenorrhyncha. They suck vascular fluids with slender beaks that arise at the back of the
head. Unlike aphids and their kin (Homoptera, suborder Sternorrhyncha), leafhoppers
and their relatives have tiny, bristle-like antennae and are thus sometimes called “shorthorned” bugs. These insects are both abundant and diverse in North American grasslands.
There are almost 1,500 species of all such bugs known from Canada (Maw et al. 2000),
of which at least 258 species are confined to grasslands (Hamilton 2004a). Of the total
Canadian fauna, the great majority (1,088 known species) are leafhoppers. The remainder
(Fig. 2) are mainly planthoppers (Fulgoroidea, chiefly the family Delphacidae), with
smaller numbers of cicadas (Cicadidae, 20 native species in Canada), spittle bugs or
froghoppers (Cercopidae and Clastopteridae, 36 species), and treehoppers (Membracidae,
more than 100 species).
The majority of cicadas, spittle bugs, and treehoppers reside in forested lowlands of
southern Canada and include only a few species characteristic of native grasslands. All four
species of cicadas in the grasslands of Canada are confined to southern Alberta. These are
not prairie endemics but are derived from intermontane western (Cordilleran) grasslands,
such as those of Utah. The only common prairie spittle bug, Philaenarcys bilineata (Say),
is abundant and widespread not only on the plains, but also in boreal marshes (Hamilton
1982: Map 32). Three spittle bugs characteristic of prairies in the eastern United States are
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K. G. A. Hamilton and R. F. Whitcomb
also known from sandy areas of Ontario and New England forests (Hamilton 1982, 1995a),
although there are only five documented occurrences of Lepyronia gibbosa Ball, Philaenarcys
killa Hamilton, and Prosapia ignipectus (Fitch) in eastern Canada. Treehoppers, as their
name suggests, prefer woodlands. However, some feed on herbaceous plants in grasslands
and others on trees in savannas or along prairie rivers; none feed on grasses.
Leafhoppers are one of the most diverse families of any organism found in grasslands
(Ross 1970; Whitcomb et al. 1994; Hamilton 1995a). Oman (1949) made a conservative
estimate of 800 species of leafhoppers, including wind-transported microleafhoppers
(subfamily Typhlocybinae) in the prairies of the central states, Colorado, and Texas. An
additional 100 species have since been found on the Canadian prairies (Hamilton 2004a),
raising the estimate to at least 900 species. This makes the biodiversity of grassland
leafhoppers roughly comparable to that of cutworm moths (Lepidoptera: Noctuidae)
and ground beetles (Coleoptera: Carabidae), taxa that are frequently used as models for
biodiversity, although these have few prairie endemics (Ricketts et al. 1999). By contrast,
more than 470 species of leafhoppers are wholly endemic to the prairies. This is a larger
number of prairie endemics than in any family of plants: 337 species of prairie-inhabiting
Asteraceae, 258 of Poaceae, and 137 of Cyperaceae (Barkley 1977).
Leafhoppers are among the most abundant phytophagous insects in grasslands
(Osborn and Ball 1897). Local populations on a British grassland may exceed a million
individuals per hectare by mid-summer (Morris 1971). In the American Southwest, in a
single collection, more than 10,000 nymphs of Balclutha neglecta DeLong and Davidson
were obtained in 100 sweeps in a Bouteloua-dominated grassland (RFW, unpublished) and
in Alaska, 4,000 adults of Diplocolenus evansi (Ashmead) were taken in 100 sweeps in an
open meadow (H.H. Ross, unpublished).
Many species of grassland leafhoppers form intimate associations with dominant
or subdominant species of grass or forbs. For this reason, leafhoppers are an excellent
source of data for the assessment of grassland properties such as stand continuity and
integrity (Hamilton 2004b). The biodiversity and frequently bright colours of leafhoppers
are impressive; and their abundance, activity, and moderate size (usually 4–15 mm) make
them easy to find with various collecting techniques. By contrast, planthoppers are less
well studied. They are less diverse and, because they are tiny and relatively rare, are easily
overlooked. Individuals of some delphacid species are only 2 mm long. Many species of
this family look superficially alike and live near the root crowns of bunchgrasses. Some
of these species can be obtained only by vacuum collecting, and exacting techniques are
required to identify them. Thus, planthoppers are infrequently encountered by general
collectors and are even less frequently identified.
Grassland Types
The grasslands most familiar to us are associated with activities of human settlement, such as
those leading to the deliberate introduction of pasture or crop grasses or the weedy spread of
inadvertently or deliberately introduced exotics. These alien species often dominate deforested
land and follow transportation corridors. These species have few associated leafhoppers,,and
those that are found are mainly Eurasian or widespread native species. Therefore, they are of
little scientific or conservation interest and are excluded from this study.
By studying the insect faunas of native grasslands, we can begin to define grasslands
by the association of similar faunal suites with characteristic floral communities. From
an insect’s point of view, even very small patches of grass may be sufficient for food and
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
173
shelter. If there are enough patches to serve as “stepping stones” they can be conduits
for grassland insects to cross into other ecosystems. Therefore, all such mosaics of grass,
shrubs, and trees of any size are tentatively considered here as grasslands, although in
many ecological classifications trees are said to ‘dominate’ such grasslands.
Ephemeral grasslands, such as those that occur as glades in extensive forests, are
colonized mainly by widely dispersing bugs. In these transitory grasslands, the number
of endemic insect species is seldom more than one or two. Glades within eastern forests
of Canada and the northeastern United States give rise to local monocultures of native
perennial grasses such as poverty grass or wire-grass, Danthonia spicata (L.) Beauv. and its
specialists, Laevicephalus melsheimerii (Fitch) and Latalus personatus Beirne. Likewise,
grasses commingled with sedges and rushes in most wet meadows in forested areas or on
arctic tundra, where the fraction of grass species of the flora is much lower than in true
grasslands, have at most only a few associated leafhoppers. Most tundra and fen formations
have unique leafhopper faunas and are not really part of the grassland rubric. This is not
true of superficially similar wet areas of prairies that are dominated by salt-adapted sedges,
reeds and rushes, or alkaline fens that harbour prairie-adapted grasses. Many species of
leafhoppers specialize on varied monocotyledonous hosts on prairies, and these wetlands
are an intrinsic part of the grassland story.
Although most grassland-specific species of leafhoppers in Canada are inhabitants
of the Great Plains or intermontane valleys of the west, suites of several species also
occur on smaller grasslands outside the Great Plains. In the west, patches of grasslands
are interspersed in pine and aspen forests. In the east, suites of sand-adapted species of
grass, and their leafhopper specialists, persist on the dry, shallow horizons of sand dune
regions. These regions occur not only on the Atlantic shores and around large lakes, but
also on inland formations such as eskers and sand hills. Other grasslands persist on the
thin soil that crusts glacier-smoothed limestone plains called “alvars,” which have not
been colonized by trees. Alvars are home to the largest suites of grassland short-horned
bugs east of Illinois (Hamilton 1995a). Many of these species are characteristic of prairie
ecosystems (Bouchard et al. 2002), suggesting that alvars have remained treeless since at
least Altithermal times.
Life History Strategies
In all grasslands that we have studied, the associated insects have two distinct strategies.
One strategy belongs to the “generalists,” of which the most dispersive are termed “tramps.”
Generalist insects colonize a wide variety of grasses and forbs. Generalists tend to occur on
many of the plant species in a given association. The second strategy is that of the specialists.
These species occur either on a single host plant species or, at most, several related plant
species. Assemblages of grassland leafhoppers in a given vegetational formation often
consist of a few generalists and many specialists. Most grassland leafhoppers specialize
on grassland-endemic plants. Such species of bugs are considered to be themselves
grassland endemics. But in other cases, particularly if their hosts are unknown, species are
considered to be endemics if their populations are found in grasslands more than 95% of
the time (Ricketts et al. 1999). Thus, some species of leafhoppers associated with plants
not limited to native grasslands are nevertheless considered here to be prairie species if
their populations are largely limited to grasslands, including prairies (e.g., the flightless
Neocoelidia tumidifrons Gillette and Baker, which occurs on various goldenrods, Solidago
spp., is rarely found outside prairies).
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K. G. A. Hamilton and R. F. Whitcomb
Not all stands of common native grasses have an associated suite of leafhopper specialists
for reasons that are not always clear. For example, cord grasses (Spartina spp.) are widely
distributed across rivers draining into the Great Lakes in Ontario (Dore and McNeill 1980:
Map 187). However, the species of leafhoppers on cord grasses (Table 1) are restricted to
prairie and oak savanna west and south of Lake Michigan. The eastern extent of this fauna is
at the tip of southern Ontario at Ojibway Prairie in Windsor (see Chapter 9). Such distributions
appear to be the result of a combination of factors, which include, in addition to historical
features, reproductive potential, host specificity, phenology, and dispersal ability.
Reproductive Potential
On a regional scale, generalists have a competitive advantage over specialists because they
can locate and colonize a wide variety of plant species. Even more than typical generalists,
tramps adopt strategies that maximize reproductive potential. The dispersion that these
strategies encourage enables tramps to colonize grasslands over a wide geographical area.
Specialized sensory adaptations for locating a specific host (if they ever had them) have
been lost in the course of evolution.
The reproductive effort needed by specialists to maintain their breeding populations
should be lower than that of generalists because these species are sedentary and do
not suffer losses from dispersal. But dispersal is only one possible cause of population
depletion. Other factors, such as wildfire and floods, may account for a mortality rate
that is sufficient to favour high fecundity. Aflexia rubranura (DeLong ) is often found
in prodigious numbers on its host, prairie dropseed (Sporobolus heterolepis A. Gray), on
alvars that are frequently subject to spring flooding, and in oak savanna, where frequent
fire is important for maintaining grassland in the face of repeated forest invasion. We
believe that the high reproductive effort of some species may compensate for losses to fire.
However, this compensation runs counter to the normal strategies of specialist species of
insects. If such compensation can be verified, it would be a reversal in the general theory
for reduction of reproductive potential expected in K-selection (MacArthur and Wilson
1967). Such a circumstance should remind us that mortality in dispersal is only one of the
possible reasons for high mortality in life history strategies (Cole 1954).
Conversely, populations of leafhoppers in some tallgrass prairies with extensive plant
cover are surprisingly small. Numerous reasons can be adduced. One simple explanation
is recent burning, which we have often observed to decimate local insect populations. A
second possible explanation is sampling bias. Sweeping is much more inefficient in dense
grass thatch or in old-growth grass clumps than in short or diffuse grass stands typical of
shortgrass prairie, resulting in undersampling of insects that are thatch dwellers. However,
populations in many old-growth prairies that have not been burned recently seem to be
low even when sampled by vacuum collecting. These observations suggest that biological
factors are involved. Perhaps incidence of attack by predators and parasites in extensive
grasslands severely reduces population levels of prairie bugs. One often finds large
populations of leafhoppers that are almost 100% parasitized. By contrast, isolated grass
patches (such as those in alvars or small prairie reserves) may have higher populations of
leafhoppers because their enemies may be more sporadic.
Host Specificity
Leafhoppers occupy the full spectrum of resource exploitation. The least restricted are
polyphagous, feeding equally well on woody or herbaceous plants. Some leafhoppers are
facultative, preferring herbaceous hosts, but will accept woody plants when their preferred
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
175
host plants have dried up. Others are generalists on woody plants, or on herbs, or on a
combination of grasses and sedges. Many more leafhoppers exhibit various degrees of
specialization, feeding on species of a single plant family (oligophagy), on a few closely
related genera of plants (stenophagy), or even on a single host species (monophagy)
(Hamilton 1983a, 1985).
When more than one stenophagous species with a restricted host range nevertheless
feeds on more than one plant genus, the plant genera may have a close phyletic relationship.
Both wheat grasses (Agropyron spp.) and wild ryes (Elymus spp.) are common hosts for no
fewer than five stenophagous leafhopper species (Table 1). Unsurprisingly, members of these
two grass genera are sometimes combined as Leymus spp. Similarly, little bluestem (which
was formerly classified as an Andropogon species before its transfer to Schizachyrium), is
colonized by several species of leafhoppers that also feed indiscriminately on both big and
little bluestem (Table 1).
Leafhopper specialists exhibit various patterns of host selection. Some feed on various
hosts, but oviposit on only one. This specialization is a type of monophagy. A pattern of
specialization on a single plant species (hyperspecialist) is sometimes accompanied by
physical adaptations to the host plant. The cicada Okanagana synodica (Say) is the most
striking example in the Canadian homopteran fauna. It is unusual in having a black body
barred with yellow (Fig. 2, right) that resembles the dappled shade cast by the sagebrush
shrubs where it sings on hot summer days. Thus, if an association between insect and plant
species is of sufficient long standing, morphological characters may evolve to reinforce the
specialization.
Patterns of leafhopper host specialization are most easily discernible in arboricolous
faunas. Canadian leafhopper species that feed on trees are usually restricted to related
plants, mostly a single host species or a genus (Hamilton 1985). Of 174 tree-feeding species
of leafhoppers native to Canada, only 16 are generalists on trees. A mere six species are
oligophagous, whereas 11 times as many are stenophagous. Of the latter, 34 species feed on
a single genus of trees, and 26 species feed on only closely related tree species within one
genus. This leaves 85 leafhopper species that are apparently monophagous because they
have been found on only a single native tree species.
Although it is difficult to study host relationships in grasslands, our work spans more than
40 years, includes thousands of host records, and is therefore robust. We have found in most
situations that almost pure stands, which we value highly for the purposes of ascertaining the
probable host of grassland-specialist leafhoppers, are rare on typical prairie because small
soil and moisture differences in microhabitats shift the balance of optimal conditions from
one grass species to another. Natural pure stands are most frequently found in peripheral
grasslands where grass patches are isolated or where there are few species of grass. Also,
pure stands sometimes occur on the most challenging soils or in zones of lake, pond, and
slough shores where moisture and salinity gradients form a cline. Natural depressions that
retain standing water offer a selective force that separates grass species that otherwise may
be closely associated. Buffalo grass, Büchloe dactyloides (Nutt.) Engelmann, is more flood
tolerant than blue grama, Bouteloua gracilis (HBK.) Lag., its codominant in shortgrass prairie.
But most pure stands of grass have been created by human activities. Plant material centres
in which plots of native grasses are set out for seed production, or conservation plantings
where (wisely or unwisely) grass monocultures have been planted, or highway rights-ofway often afford an opportunity to study host specificity. Although we normally have found
substantial populations of leafhopper specialists on these more or less artificial pure stands,
these populations were no higher than those on relatively pure stands in native grasslands.
Common Name
Leafhopper Specialists
POOIDEAE
Cool-season grasses
32 Leafhopper species
Ammophila breviligulata Fern
Beach grass
1 sp.: Paluda gladiola (Ball)
Poa spp.
Bluegrass
3: Auridius auratus (Gill. & Bak.), A. ordinatus (Ball), A. sandaraca Hamilton
Festuca spp.
Fescue
1: Orocastus (Cabrulus) tener (Beamer & Tuthill)
Deschampsia flexuosa (L.) Trin.
Hair-grass
1: Rosenus acutus (Beamer)
Koeleria macrantha (Ledeb.) Schultes
June grass
8: Amblysellus acuerus DeL. & Hm., Amblysellus punctatus Osborn & Ball, A.
wyomus Kramer, Athysanella obesa Ball & Beamer, A. robusta Baker, Auridius helvus
(DeLong), Memnonia maia Hamilton, Rosenus cruciatus (Osborn & Ball)
Stipa comata Trin. & Rupr.
Needle-and-thread
grass, speargrass
2: Orocastus (Cabrulus) labeculus (DeLong), O. (s.s.) perpusillus (Ball & DeLong)
Stipa spartea Trin.
Porcupine grass
1: Commellus colon (Osborn & Ball)
Oryzopsis asperifolia Michx.
Rice-grass
2: Latalus latidens (Sanders & DeLong), L. remotus Hamilton
Puccinellia nuttalliana (Schultes) Hitchc.
Salt-meadow grass
2: Deltocephalus serpentinus Hamilton & Ross, Laevicephalus saskatchewanensis
Hamilton & Ross
Hordeum jubatum L.
Wild barley
1: Psammotettix knullae Greene
Leymus spp. (Agropyron + Elymus spp.)
Wheat grass and wild
rye
8: Athysanella attenuata Baker, Attenuipyga (Dorycara) minor (Osborn), A. (D.)
platyrhynchus (Osborn), Commellus comma (Van Duzee), C. sexvittatus (Van Duzee),
Hebecephalus occidentalis Beamer & Tuthill, H. rostratus Beamer & Tuthill, H.
truncatus
Agropyron smithii Rydb.
Wheat grass, western
1: Mocuellus caprillus Ross & Hamilton
Agropyron trachycaulum (Link) Malte
Wheat grass, slender
1: Mocuellus americanus Emeljanov
CHLORIDOIDEAE / PANICOIDEAE
Warm-season grasses
52 Leafhopper specialists
Andropogon/Schizachyrium spp.
Bluestems
3: Laevicephalus unicoloratus (Gillette & Baker), Hecalus flavidus (Signoret), Stirellus
bicolor (Van Duzee)
K. G. A. Hamilton and R. F. Whitcomb
Scientific Name of Host
176
Table 1. Host grasses Poaceae and their 84 known or suspected specialist leafhoppers in Canada.
Andropogon scoparius Michx.
(=Schizachyrium scoparium)
Bluestem, little
7: Athysanella incongrua Baker, Chlorotettix spatulatus Osborn & Ball, Flexamia
dakota Young & Beirne, F. delongi Ross & Cooley, F. graminea (DeLong),
Paraphlepsius lobatus (Osborn), Polyamia caperata (Ball)
Bouteloua gracilis (HBK.) Lag.
Blue grama
4: Athysanella bifida Ball & Beamer, A. sinuata Osborn, Flexamia abbreviata (Osborn
& Ball), Flexamia flexulosa (Ball)
Buchloë dactyloides (Nutt.) Engelmann
Buffalo grass
1: Athysanella texana (Osborn)
Spartina gracilis Trin., S. pectinata Link
Cord grasses
5: Destria crocea (Beirne), Neohecalus lineatus (Uhler), N. magnificus Hamilton,
Paraphlepsius solidaginis (Walker), Pendarus magnus (Osborn & Ball)
Sporobolus heterolepis A. Gray
Dropseed, prairie
2: Aflexia rubranura (DeLong), Memnonia panzeri Hamilton
Sporobolus cryptandrus (Torr.) A. Gray
Dropseed, sand
4: Athysanella occidentalis Baker, Dicyphonia ornata (Baker), Unoka dramatica
Hamilton, U. gillettei Metcalf
Sorghastrum nutans (L.) Nash
Indian grass
1: Flexamia reflexa (Osborn & Ball)
Eragrostis spectabilis (Pursh) Steud.
Love grass, purple
1: Flexamia areolata (Ball)
Muhlenbergia richardsonis (Trin.) Rydb.
Muhly, mat
6: Athysanella secunda Blocker & Wesley, Flexamia decora Beamer and Tuthill, F.
serrata Beamer & Tuthill, Laevicephalus poudris Tuthill, Lonatura teretis Beamer,
Memnonia anthalopus Hamilton
Muhlenbergia cuspidata (Torr.) Rydb.
Muhly, prairie
2: Flexamia stylata (Ball), Lonatura megalopa (Osborn & Ball)
Distichlis stricta (Torr.) Rydb.
Salt grass
4: Athysanella kadokana Knull, Lonatura melina (DeLong), L. salsura (Ball),
Memnonia brunnea (Ball)
Spartina patens (Ait.) Muhl.
Salt hay
2: Amplicephalus littoralis (Ball), A. simplarius (Osborn & Ball)
Calamovilfa longifolia (Hook.) Scribn.
Sand reed grass
3: Athysanella. terebrans Gill. & Bak., Flexamia grammica (Ball), Laevicephalus
exiguus Knull
Muhlenbergia asperifolia (Nees & Mey.)
Scratch grass
1: Flexamia inflata (Osborn & Ball)
Bouteloua curtipendula (Michx.) Torr.
Side-oats grama
1: Laevicephalus minimus (Osborn & Ball)
Panicum virgatum L.
Switch grass
4: Chlorotettix fallax Sanders & DeLong, Flexamia atlantica (DeLong), Graminella
oquaka DeLong, G. pallidula (Osborn)
177
1: Flexamia prairiana DeLong
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
Bluestem, big
Andropogon gerardi Vitman
178
K. G. A. Hamilton and R. F. Whitcomb
Some exotic hosts, such as crested wheat grass, Agropyron cristatum (L.) Gaertn., appear
to be unappealing to native insects, although the only specimen of the Aristida specialist
Attenuipyga balli Oman collected in Canada was taken from a pan trap in a crested wheat
grass stand. In some regions, however, introduced grasses provide an alternative host that
supports generalists but also, occasionally, specialists. Tall fescue (Festuca arundinacea
Schreb.) is accumulating a makeshift fauna as it is planted on roadsides from the Carolinas
to Canada. The suites of specialists that colonize such grasses vary from region to region. In
the southern United States, bermuda grass, Cynodon dactylon (L.) Pers., and weeping love
grass, Eragrostis curvula (Nox), have “stolen” a few specialist species, whereas farther
north, occasional small populations of native wheat grass specialists, such as Attenuipyga
platyrhynchus Osborn, are found on quackgrass, Agropyron repens (L.) Gould. In cases
where exotic grasses are planted extensively on a regional scale, they even have the potential
to steal specialists from their normal but increasingly rare hosts. Exotic grasses almost
always prove to have somewhat different phenologies from the dominant grass hosts of
their native leafhopper specialists. The populations derived from these transfers, however,
are often extirpated. For example, Elymus specialists that have colonized quackgrass may
be unable to reach maturity if this host plant undergoes an unusually dry summer.
Insects that feed on widely scattered perennials must be more proficient at dispersal
than colonists of dominant grasses. There is a parallel in tropical jungles. The immense
diversity of plants in such ecosystems leads to a reduction in insect host specificity as
compared with savannas at the same latitude (Ribeiro 2003). Thus, it should not be
surprising that populations of leafhoppers of the same species become less host specific in
southeastern localities of North America (Whitcomb et al. 1987a), where floral diversity
increases at the expense of dominance.
Why, therefore, are there any insects that specialize on single native host species or
small sets of species? Specialists have a vastly different strategy from that of generalists.
They stake their existence on tracking a single plant species or a small set of related plant
species. Their host species are almost always perennials because perennials assure a
constant food supply. Where conspecific plant colonies are stable and adjacent, as is usually
the case in Canada, phytophagous insects that feed on them may become closely attuned to
the physiology and phenology of a single host species at the expense of losing their ability
to breed on other species. Feeding on inappropriate hosts, although often successful in the
short term, can have disastrous consequences in the long term. For example, leafhoppers
that reproduce successfully on an inappropriate host during the summer may fail to enter
diapause (Whitcomb and Coan 1990: 680).
The degree of specialization of most species of grassland leafhoppers falls somewhere
between hyperspecialism and oligophagy. On the Canadian prairies, most of the small
number of species of leafhoppers that are widespread in both grasslands and forests are
generalists, whereas those adapted to grasslands (Hamilton 2004a) are mostly stenophagous
or even monophagous. Specialists are often coloured like grasses or woody plants, for
example, grass green as in Memnonia panzeri Hamilton, dappled white and gray as in
Empoasca subgenus Hebata on sagebrush and Ceratagallia cinerea (Osborn and Ball) on
winterfat, or twig brown as in Prairiana cinerea (Uhler). Only a few are hyperspecialists,
some of which are more or less shaped like the seeds of the host, such as Attenuipyga minor
(Osborn) on wheat grass, or are patterned like seeds, such as Flexamia areolata (Ball) on
purple love grass, Eragrostis spectabilis (Pursh) Steud.
More than half of all species of leafhoppers endemic to Canadian grasslands (132 of
223) have extremely narrow feeding habits, usually specializing on a grass or woody shrub.
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
179
Most monophagous prairie leafhoppers (85 species) feed on grasses, family Poaceae (Table
1). Another 53 species attack plants in eight other families, primarily composites, sedges,
and willows (Table 2). At least 10 of these are stenophagous or oligophagous in the United
States, having acquired one or more alternative hosts at lower latitudes.
Some species of leafhoppers have been able to extend their ranges by transferring
to closely related host species. For example, Flexamia abbreviata (Osborn and Ball),
occupies most of the entire range of blue grama, its only host in Canada. However, beyond
the periphery of its eastern range limit in the United States, it is represented by a biotype
that is adapted to hairy grama (Bouteloua hirsuta Lag.). Also, in southwestern grasslands,
F. abbreviata colonizes several grama grasses that are not utilized elsewhere as a food
source (Whitcomb et al. 1987a). From these considerations, we must revise our notions of
“specialist” and “monophagy” to state that these designations must be used for a particular
biotype and not for the species as a whole. Often, a presumably new biotype is generated
at the northern periphery of the leafhopper host range. This latitudinal shift in host range
often occurs at latitude 40°N or somewhat farther south. For example, Laevicephalus
exiguus Knull feeds on grama grasses on the southern Great Plains but has shifted to
feeding on sand reed grass, Calamovilfa longifolia (Hooker) Hackel, at Sauble Beach on
Lake Huron. Similarly, Driotura robusta Osborn and Ball is monophagous on gumweed
(Table 2) throughout the Canadian plains, although it is reported on aster, erigerons, or both
farther south (Whitcomb et al. 1987b).
Although such range extensions are more often latitudinal, they are sometimes
longitudinal. For example, Flexamia inflata (Osborn and Ball) has two distinct biotypes,
a western one from Saskatchewan and Washington State on scratch grass, Muhlenbergia
asperifolia (Nees and Meyen) Parodi (Table 1), and an eastern one from Manitoba to
Ontario, which is usually on rushes (Juncus spp.). Similarly, Paluda gladiola (Ball), which
is a widespread feeder on a cool-season grass, Calamagrostis canadensis (Michx.) Beauv.,
has a separate biotype on the Atlantic coast on beach grass (Ammophila breviligulata
Fernald), a warm-season perennial. In regions of the United States in which the suite of grass
species contains a diversity of warm- and cool-season grasses, some species of leafhoppers
shift from cool- to warm-season grasses in the summer. For example, Gillettiella fasciata
Ball and Beamer breeds abundantly on cool-season grasses in the spring in northern New
Mexico, whereas in the summer individuals shift to Muhlenbergia species.
Dispersal
Many species of grassland leafhoppers are not restricted to the Great Plains; a few others
may at first appear to be, but in fact are not. For example, the small and slender Balclutha
neglecta is a characteristic and abundant insect throughout the Great Plains, but it may be
found in many other sites as far northeast as Lake Nipigon in the boreal forest of northern
Ontario. Apparently, this species is wind dispersed, as are many of the tiny leafhoppers
of the subfamily Typhlocybinae. Still others, such as Ceratagallia uhleri (Van Duzee),
suddenly appear as populations of adults in locations where breeding populations are
absent, such as mountaintops in Colorado (Hamilton 1998). These transient populations are
clearly migrants, leaving drought-stricken hosts in search of more favourable sites in cooler,
moister areas. By contrast, eastern leafhoppers such as the aster leafhopper, Macrosteles
quadrilineatus (Forbes), are long-distance dispersers, invading northern latitudes and high
altitudes each summer from stable populations at low elevations in the south.
Conversely, many species of grassland leafhoppers are found in only part of the range
of their host, especially in the southwestern United States, where they are adapted to regions
Scientific Name of Host
Common Name
Leafhopper Specialists
Artemisia cana Pursh, A. tridentata Nutt.
Sagebrush
8 spp.: Ballana ortha DeLong, Carsonus aridus (Ball), Empoasca medora
DeLong, E. nigroscuta Gillette & Baker, Idiocerus canae Hamilton, Norvellina
columbiana (Ball), Texananus extremus (Ball)
Artemisia frigida Willd.
Pasture sage
5: Acinopterus viridis Van Duzee, Frigartus frigidus (Ball), Prairiana cinerea
(Uhler), Stragania rufoscutellata (Baker), S. atra (Baker)
Artemisia gnaphaloides Nutt.
Prairie sage
1: Mesamia ludovicia Ball
Balsamorhiza sagittata (Pursh) Nutt.
Balsamroot
1: Ballana chelata DeLong
Chrysothamnus nauseosus (Pall.) Britt.
Rabbitbrush
1: Ballana remissa DeLong
Eriogonum spp.
Fleabane
1: Norvellina rubida (Ball)
Grindelia perennis A. Nels.
Gumweed
1: Driotura robusta Osborn & Ball
Helianthus spp.
Sunflower
2: Mesamia nigridorsum (Ball), M. straminea (Osborn)
Solidago spp.
Goldenrod
1: Neocoelidia tumidifrons Gillette & Baker
River birch
2: Oncopsis incidens Hamilton, O. juno Hamilton
Atriplex spp.
Atriplex
2: Aplanus albidus (Ball), Norvellina clarivida (Van Duzee)
Eurotia lanata (Pursh) Moq.
Winterfat
1: Ceratagallia cinerea (Osborn & Ball)
180
Table 2. Host plants (other than grasses) and their 53 known grassland leafhoppers in Canada.
ASTERACEAE
Betula occidentalis Hook.
CHENOPODIACEAE
K. G. A. Hamilton and R. F. Whitcomb
BETULACEAE
CYPERACEAE
2: Hardya dentata (Osborn & Ball), Stenometopiellus cookei (Gillette)
Carex spp.
Sedge
3: Deltocephalus lineatifrons Oman, Paraphlepsius continuus (DeLong), P.
turpiculus (Ball)
Eleocharis spp.
Spike-rush
5: Limotettix bisoni Knull, L. uneolus (Ball), L. urnura Hamilton, L. elegans
Hamilton, Dorydiella kansana Beamer
Juncus balticus Willd.
Baltic rush
2: Cicadula longiseta (Van Duzee), Pasaremus concentricus (Van Duzee)
Scirpus validus Vahl
Great bulrush
2: Limotettix uneolus (Ball), L. utahnus (Lawson)
Creeping juniper
1: Texananus marmor (Sanders & DeLong)
Sea-milkwort
1: Erythroneura carbonata McAtee
Populus angustifolia James
Narrow-leaf poplar
1: Empoasca angustifoliae Ross
Populus deltoides Bartr.
Cottonwood
1: Idiocerus moniliferae Osborn & Ball
Salix exigua Nutt.
Wolf or sandbar willow
10: Empoasca albolinea Gillette, E. digita DeLong, E. exiguae Ross, Idiocerus
freytagi Hamilton, I. ramentosus Uhler, I. raphus Freytag, Macropsis feminis
Hamilton, M. rufescens Hamilton, M. rufocephala Osborn, Macrosteles major
(Dorst)
Golden currant
1: Idiocerus interruptus Gillette & Baker
JUNCACEAE
PINACEAE
Juniperus horizontalis Moench
PRIMULACEAE
Glaux maritima L.
SALICACEAE
SAXIFRAGACEAE
Ribes aureum Pursh
181
Prairie sedge
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
Carex filifolia Nutt.
182
K. G. A. Hamilton and R. F. Whitcomb
with highly contrasting host phenologies (Whitcomb et al. 1994). Because of glaciation,
most species of leafhoppers in Canada have not been in one area long enough to develop
regional adaptations to climate, in contrast to more mobile ground beetles (Carabidae).
A large percentage of northern species seem to disperse too slowly to have filled
their host’s geographical range in the 10,000 years since deglaciation. The glaciation of
Canadian grasslands has provided a natural experiment that demonstrates this dispersal
rate (Hamilton 1999a: Fig. 2). As the Wisconsin glaciers retreated (see Chapter 1), a vast
landscape became available for leafhopper recolonization. This natural process provides
an opportunity to determine the dispersal rates of species that had been displaced by the
glaciers. Half of the 24 species of arctic leafhoppers have dispersed across open tundra
at a rate of less than 1 km/year from their glacial refugium in the first 5,000 years since
deglaciation, and 30% have not crossed the 10-km-wide Mackenzie River valley in 12,000
years. One of the factors involved in slow dispersal is univoltinism. For example, almost
all species of leafhoppers in Yukon grasslands (including 17 endemics) are abundant only
from late June to August during the short growing season (Hamilton 1997). Only Cuerna
septentrionalis (Walker) overwinters as adults and is active whenever the weather permits.
This species has been observed (KGAH, unpublished) climbing on a fence in Winnipeg,
Manitoba, on a warm day in February!
Leafhoppers farther south have a much greater opportunity to disperse, but other factors
such as mountain barriers, phenological constraints, and behaviour may prevent them from
doing so. For example, the polyphagous but flightless leafhopper Errhomus calvus Oman
appears to have dispersed northward from the north side of the Columbia canyon only 150
km since deglaciation, at an average rate of 15 m/year (Hamilton and Zack 1999) and has
invaded only the most southerly parts of the Okanagan Valley of British Columbia.
As noted earlier, leafhopper specialists must perforce develop dispersal mechanisms
and host-finding sensory abilities if their host occurs as scattered colonies. Insect specialists
of grasses and shrubs in sloughs, for example, usually feed on plants that grow in widely
separated hollows and therefore can be colonized only by flight. Wetland-adapted plants,
such as manna grass, mat muhly, spike-rush, or wolf willow (Table 2), are the most usual
hosts for these insects. Plants adapted to saline conditions, (e.g., salt grass, salt-meadow
grass, and wild barley; Table 1), are particularly common today in roadside ditches, but
originally occurred in natural depressions such as salt pans on the prairies. These species of
plants have small but distinctive endemic faunas on the Great Plains. Similarly, plants that
grow only in widely separated sand hill areas of the Great Plains generally support only a
few species of bugs, but wherever these grasses grow, we usually find at least one of the
associated species of leafhopper.
How do leafhoppers and other bugs traverse enormous distances and find tiny pockets
of grasslands in isolated valleys of the Yukon, on limestone outcrops around the Great
Lakes, and on sandy ridges as far east as the Maritimes? Do they simply scatter to the
winds, raining down on the unyielding forest in untold millions until a single gravid
female chances upon a clump of grass? This strategy accounts for the wide geographical
distribution of tramp species that are not choosy about their food plants. It is also the way
some other insects, such as Delphacidae, have traversed thousands of kilometres of open
ocean over the millennia to populate the Hawaiian Islands (Zimmerman 1948). However,
such a strategy would seriously deplete the gene pool of a host-specialist leafhopper, whose
individuals stand little chance of finding a host that occurs only in isolated stands.
Bugs such as Delphacidae that have the greatest dispersal abilities and a highly
vagile lifestyle are overrepresented in peripheral grasslands. The grassland fauna of the
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
183
Atlantic coast is dominated by Delphacidae, whereas leafhoppers represent only 33% of
the fauna, as compared with 80% on the Great Plains. The grassland fauna of Homoptera
(Auchenorrhyncha) on isolated patches of bluestem grassland in boreal forest areas of
central Minnesota and northwestern Michigan comprises only a single Delphacidae,
Delphacodes parvula Ball, a wide-ranging specialist on little bluestem.
Distribution maps of Delphacidae sometimes show large disjunctions. Many of these
disjunctions result from a lack of sampling in intervening sites, but some cannot be so
easily explained. A curious delphacid, Parkana alata Beamer, occurs in Arizona and
Utah but has also been discovered in two remote localities in Canada, one deep in an
interior valley of British Columbia at Kamloops, and the other far up in the foothills of
Alberta near Calgary. Other wide disjunctions present evidence of niche consistency such
as Elachodelphax hochae Wilson that has been taken in the Peace River district and in a
number of Yukon sites. This species is also known from two Great Plains localities, each
of which is an island of aspen parkland in a sea of grasses (the crests of Cypress Hills
and of Moose Mountain in southeastern Alberta and Saskatchewan, respectively). Both P.
alata and E. hochae are strange-looking insects that any collector of tiny bugs would be
delighted to find in a net or trap. Why then have they never been collected anywhere else
in the thousand-plus kilometres of suitable grasslands that separate the prairie populations
from their sources of origin? We do not know, but it seems likely that the current isolated
populations were part of a single widely distributed population at a time when temperatures
were higher than at present.
Leafhoppers, like Delphacidae, are frequently flightless. Individuals of most Canadian
Delphacidae have shortened (brachypterous) front wings and tiny or absent hind wings. It is
thought that flightlessness in Delphacidae is an adaptation to increasing egg production in
which energy normally expended in producing flight muscles is conserved (Denno 1994).
Flightlessness is therefore most adaptive when host plants are low growing and plentiful.
Brachypterous forms of specialists that inhabit dominant and subdominant grasses often
build up large populations on their host patch.
Flightless Delphacidae normally produce winged individuals when population
density increases, resulting in the production of dispersing forms (Denno 1994). No
similar environmental factor seems to determine macroptery in leafhoppers. Instead,
the proportion of winged individuals is more or less fixed for each species. The spring
generation may have a higher percentage of winged individuals, as in the early-season
populations of Aflexia rubranura in Illinois, where this species is double brooded (R.
Panzer, pers. comm.) Long-winged individuals comprise no more than 10% of the
population there, but can easily ensure habitat connectedness, although patches as wide
as 36 m may be devoid of the Sporobolus host plants (Panzer 2003). These long-winged
forms are rare (<0.3%) elsewhere.
Females of many species of leafhoppers disperse widely only when they are sexually
immature. By the time they become gravid, they have lost the ability to fly (Waloff 1973).
This peculiarity may account for the presence of brachypterous females in so many
monophagous species of grassland leafhoppers (Table 3). In some genera, such as Commellus
and Extrusanus, males are usually flightless, whereas at least 10% of females can fly. The
proportion of flying adults tends to increase in more northerly grasslands (Hamilton 1995b).
The reason for flightless males is not clear. Males are more frequently capable of flight than
females, with 50 to 90% of females being brachypterous (Hamilton 1999b).
Wing morphology gives important clues to a species’ dispersal strategy. Flightless
morphs may be divided into brachypterous (the front wings are reduced to scale-like
184
K. G. A. Hamilton and R. F. Whitcomb
appendages not covering the abdomen) and submacropterous (the front wings are only
slightly reduced in length, exposing at most the tip of the abdomen, whereas the hind
wings are distinctly smaller or reduced to tiny, strap-like remnants). Individuals with
the shortest wings probably feed the lowest on a plant (Hamilton 2000), a strategy that
suggests an adaptation to a cryptic lifestyle and low powers of dispersal. Conversely,
leafhopper species that have long, narrow wings (e.g., the aster leafhopper; see Hamilton
1983b) are more likely to be migratory than are species with shorter wings. Similarly,
the long wings of Flexamia grammica (Ball) suggest that this species flies actively to
seek out its widely dispersed host, sand reed grass, whereas other species of Flexamia,
which are usually short winged, can reach their dominant or subdominant grasses without
relying on flight.
Biogeography
Range Determinants
Host specificity is a primary determinant of leafhopper ranges, but many other factors limit
the ranges of species of leafhoppers. Among these factors are diel temperature regimes,
seasonal phenology, and historical development of the grasslands in which these species
live. These factors contribute to making regional endemism of leafhoppers highest on the
northern edge of the Canadian prairies, in the Aspen Parkland Ecoregion (Hamilton 2004a).
Many other peripheral grasslands in Canada and the northern United States are colonized
by endemic species of leafhoppers. For example, two undescribed oak-feeding species
of leafhoppers of the genus Eutettix have been found in a single oak barren (savanna) in
New Hampshire (KGAH, unpublished). Other notable examples are found in the interior
valleys of British Columbia and the Peace River district of Alberta (Hamilton 2002) and
in an alkaline fen in Michigan (Bess and Hamilton 1999). Some of the most localized of
these species of endemic leafhoppers occur in areas outside glaciated lands, in the Yukon
(Hamilton 1997), on the Queen Charlotte Islands (Hamilton 2002), in serpentine barren
grasslands of Maryland (Hamilton 1994b), and on coastal grasslands (e.g., Blocker and
Wesley 1985; Hamilton 2009).
Temperature
Leafhoppers, unlike ground beetles, do not seem to be niche hyperadaptive. The
commonest species tend to occur wherever their host is found and not just in conditions
of particular humidity, soil conditions, or shelter. For example, Commellus sexvittatus
(Van Duzee) ranges from subarctic Yukon (Hamilton 1997) to sand dunes in Michigan
(Hamilton 1994a). Of all the environmental factors limiting the dispersal of leafhoppers,
only temperature regimes clearly govern the northernmost limits of the ranges of
Canadian species (Hamilton 1997). This limitation is probably mediated by a reduction
in the length of growing season and a consequent reduction of available degree-days for
development, known to be a major factor in spittle bug distribution (Hamilton 1983a).
Northernmost limits of ranges thus tend to follow latitudinal climate zones. Few species
of grassland leafhoppers are found in the arctic, subarctic, or boreal zones. There are
dramatically more leafhopper species in the transitional or hemiboreal zone that covers
much of southern Canada. Another large suite of grassland insects is represented only
from the austral zone. The grasslands of the austral zone are represented in Canada only
by oak savanna on the southern end of Vancouver Island and its offshore islands on the
west coast, in the southern Okanagan Valley of inland British Columbia, and in isolated
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
185
Table 3. Flightlessness and host associations of 111 monophagous grassland leafhoppers in southern Canada and
(flightless females only) adjacent Pacific Northwest states. Cool-season and warm-season grasses are designated
as C3 and C4, respectively. Pinumius, Telusus, and Twiningia are omitted because their hosts are not known.
% Winged
Female
Male
Host
Plant
0
(22 spp.)
0
C4
20-70
100
1-10
(39 spp.)
0
Woody
C4
1: Carsonus
18: Errhomus [only 1 recorded from Canada]
1: Aflexia
4: Lonatura; [oligophages only: Attenuipyga (s.s.)]
1-10
13: Flexamia; 11: Athysanella
Forb
1: Driotura
Sedge
[oligophages only: Extrusanus]
C3
100
20-70
(5 spp.)
2 spp.: Unoka
1: Dicyphonia
Forb
<1
Leafhopper Monophages
2: Amblysellus, Mocuellus
1: Orocastus (s.s.)
C4
5: Memnonia
Forb
1: Neocoelidia
1-10
C3
1: Commellus
20-70
C4
1: Destria, Neohecalus, Polyamia
100
C3
1: Auridius; [oligophages only: Attenuipyga
(Dorycara)]
100
(54 spp.)
3: Orocastus (Cabrulus), Rosenus, 2: Amplicephalus,
Deltocephalus, Latalus; 1: Chlorotettix spatulatus,
Laevicephalus saskatchewanensis, Paluda,
Psammotettix; [oligophages only: Elymana, Gypona,
Hebecephalus, Sorhoanus]
C4
Sedge
Spike-rush
3: Graminella, Laevicephalus [part]; 2:
Paraphlepsius; 1: Chlorotettix fallax;
[oligophages only: Pendarus, Stirellus]
1: Hardya, Stenometopiellus; [oligophages only:
Cicadula]
2: Limotettix; [oligophages only: Dorydiella]
Forbs
3: Ballana, Norvellina; 1: Aplanus, Ceratagallia,
Erythroneura, Mesamia
Woody
6: Idiocerus; 4: Empoasca; 3: Macropsis; 2: Oncopsis,
Stragania, Texananus; 1: Frigartus, Macrosteles;
[oligophages only: Gyponana, Prairiana]
186
K. G. A. Hamilton and R. F. Whitcomb
Table 4. Phenology of 216 grassland leafhoppers in southern Canada, with polyphages designated by an
asterisk (*). Northern leafhoppers have one generation per year, in July.
Broods
In Winter
Adults
Prairie Endemics and Their Genera
2
As nymphs
June, August
36 spp.: Amblysellus (5), Athysanella (13),
Diplocolenus (1), Driotura (1), Hardya (1),
Mocuellus (4), Psammotettix (4), Rosenus
(3), Sorhoanus (2), Stenometopiellus (1),
Stirellus (1)
Many
As adults
Continuous
7: Ceratagallia
August to
April
5: *Cuerna (3), Erythroneura (1),
Idiocerus (in part: 1 willow feeder)
May
1: Errhomus
June
23: Attenuipyga (4), Ballana (7), Carsonus
(1), Memnonia (6), Norvellina (3),
Oncopsis (2)
Mid-June to
mid-July
1: Neocoelidia
June to July
26: Auridius (4), Extrusanus (1),
Hebecephalus (8), Hecalus (2), Orocastus
(4), Paluda (1), Prairiana (3), Stragania
(2), Telusus (1)
1
As nymphs
In south
(migrant)
As eggs
1: *Exitianus
August
1: Aplanus
July
23: Commellus (4), Destria (1), Elymana
(2), Empoasca (in part: 4 sagebrush
feeders), Graminella (3), Gypona (1),
Lonatura (4), Macropsis (3), Pendarus (1)
Mid-July to
mid-August
1: Gyponana
July to
August
43: Amplicephalus (2), Deltocephalus (7),
Flexamia (14), Frigartus (1), Idiocerus (in
part: 6 cottonwood, currant, and sagebrush
feeders), Latalus (6), Mesamia (4),
Neohecalus (2), Pinumius (1)
August
51: Aflexia (1), Chlorotettix (2), Cicadula
(1), Dicyphonia (1), Dorydiella (1),
Empoasca (in part: 3 tree feeders),
Laevicephalus (9), Limotettix (6),
Macrosteles (3), Paraphlepsius (8),
Polyamia (2), Texananus (7), Twiningia
(3), Unoka (2), *Xerophloea (2)
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
187
grasslands of southern Ontario, from Ojibway Prairie in Windsor (Hamilton 1995a, 2002)
to the Rice Lake Plains east of Toronto (KGAH, unpublished).
Grassland species of leafhoppers in Canada, including the specialists, often occur
in an enormous range of habitats and environmental conditions. The majority have wide
distributions across the continent (Maw et al. 2000). Many species likewise range far south
of their northern limits, inhabiting two or three different latitudinal zones. Some, such
as Psammotettix latipex (Sanders and DeLong), are found in arctic to austral situations
(Hamilton 1997). Exceptions to this generalization include cases in which severe stress
leads to extinction of local populations. For example, exceptionally cold temperatures or
other stressors may extirpate isolated populations of Athysanella magdalena Baker at high
elevations in mountains as far south as New Mexico. This species has enhanced levels of
macroptery to compensate for such losses.
Local patches of favourable habitat (even if small) are significant for plants. Sunwarmed south-facing slopes are particularly important in Canada and Alaska (Ross 1970).
Grasses and sage (Artemisia spp.) may flourish on such slopes, and their leafhopper
biota follows along. This may account for the survival of much of the grassland fauna in
deep valleys of the Yukon, including endemic species such as the sage-feeding Chlorita
nearctica Hamilton (1997). South-facing slopes become increasingly arid through
southern Canada into the United States. In the face of such aridity, leafhoppers favour
west-facing slopes, where conditions are optimal for host growth, and shun east-facing
slopes, where insolation occurs during the coolest part of the day. This may explain why,
in the west, some species of leafhoppers seem to be confined to Cordilleran grasslands
even when grassy passes have adjacent prairies on their eastern slopes, although the
Pacific Northwest grasslands themselves are sometimes invaded by leafhoppers from the
prairies (Hamilton 2002).
Phenology
Species of leafhoppers with two or more generations per year, or whose adults live for two
or more months, have the greatest opportunity to find new stands of their hosts. Species
belonging to 14 genera may be double brooded on grasslands in southern Canada, but
most (52 genera) have only a single generation per year (Table 4). The latter usually are
represented by adults mostly during June (15 genera with overwintering nymphs) or
August (17 genera with overwintering eggs). In favourable years, adults may appear as
early as mid-May or continue into October. Many late-season species of leafhoppers feed
on dominant grasses. These species tend to have longer adult lives than those that emerge
earlier and presumably are better at dispersing. The last of the early-season species of
leafhoppers are disappearing (the females are the last to go) at about the time that lateseason leafhoppers are maturing. Sampling in mid-July therefore often yields a portion of
both faunas, as well as species of the 11 genera that peak in mid-summer or the 9 whose
adults survive through both July and August.
Leafhoppers that feed on birches and willows take advantage of the spring growth
flush when the nutritional value of their host plants is high, as do many leafhoppers that
feed on grasses. Conversely, the adults of sedge- and rush-feeding leafhoppers always
appear late in the season. Presumably, the egg hatches of these species are delayed until the
high water tables of spring recede.
Three genera of grassland leafhoppers use another strategy. They overwinter as
adults so that they can lay their eggs early in the season such that their nymphs benefit
from early summer growth. Grass-feeding insects that use this strategy profit by escaping
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autumn and spring wildfires. This lifestyle is found also in certain willow-feeding
leafhoppers of the genus Idiocerus, but is most suited for polyphagous bugs that feed on
annuals. Flying adults are better suited for finding annual hosts that have survived the
winter as seeds. In the case of Cuerna, the large adults are often long lived, with only a
short gap between generations in mid-July (Table 4). Species of this genus overwinter
as adults. The small but robust adults of Ceratagallia are continuously brooded on forbs
throughout the summer. Their generations are seldom evident except by the abundance
of maturing nymphs. These insects are the best dispersers and have wide ranges across
Canada (Hamilton 1998).
Grasses are a much less reliable food source for specialists than woody plants in arid
or strongly seasonal areas. Grasses grow rapidly when conditions (especially moisture)
are just right. The timing of this growth flush is critical for both the grass and its insect
colonists. Any leafhopper specializing on such a fleetingly available host must be able to
adapt to the host’s seasonal growth cycles. In Canada, winter dormancy, the length of the
growing season, and the photosynthetic pathways of the grass are the principal seasonal
factors affecting leafhoppers. Cool-season grasses (tribe Poeae) with a C3 photosynthetic
pathway take advantage of an early start to maximize the length of their growing season.
By contrast, warm-season (chloridoid and panicoid) grasses make up for a later start
by vigorous growth even during relatively dry periods, using a different photosynthetic
pathway (C4 type) to minimize water usage and take advantage of heightened metabolism
conferred by higher temperatures (Farquhar et al. 1989). This phenomenon is familiar to
home owners who must cope with vigorous growth of crab grass, Digitaria sanguinalis
(L.), an annual C4 grass, at the very time that lawn grass (Poa pratensis L., a perennial
C3 grass) is drying up. Warm-season perennial grasses are usually dominants on prairies
because they use moisture more effectively and rely on a deep root system—3 m or more—
to garner scarce groundwater.
Leafhoppers residing in boreal to arctic areas use the early and sustained growth of
cool-season grasses to fit their adult lives into the short northern summer. But farther south,
where summers are longer and warm-season grasses are common, various life history
strategies are found. For example, the two species of leafhoppers that specialize on prairie
dropseed (a warm-season grass) occur on plains and alvars at different times of the year.
Memnonia panzeri overwinters as late-instar nymphs and feeds as adults on the first flush
of early summer growth. Aflexia rubranura, by contrast, overwinters as eggs and feeds on
the fully grown plant in late summer. Aflexia takes advantage of the seasonal growth of the
grass but at the expense of fire vulnerability. Wildfires in fall or spring (when grasses are
dry and most flammable) can wipe out Aflexia eggs and decimate their populations. Thus,
Aflexia is probably more prolific than Memnonia to survive, and this fecundity increases its
ability to repopulate adjacent patches of its host.
Not all prairie faunal and floral elements are limited to low elevations in southern
Canada and the United States. Cool-season grasses occur abundantly in the southwestern
mountains of the United States, provided that the elevation is high enough. In these
mountains, grasslands between the shrub steppes of the valleys and the high cool-season
grass zones are composed largely of warm-season grasses. In the far north, for example,
the Yukon, cool-season grasses predominate in the valleys and on mountainsides below the
tundra. A few cool-season grasses, such as Stipa neomexicana (Thurb.) Scribn., are adapted
to relatively warm climates of the southwestern United States and Mexico. Similarly, a
few warm-season grasses are adapted to far-northern climates and occur together with
cool-season grasses in the high-elevation grasslands of the United States and in northern
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
189
Canada. For example, Muhlenbergia richardsonis (Trin.) Rydb. has been found along with
one of its specialist species of leafhoppers on the shores of Lake Manitoba in the boreal
forest zone (KGAH, unpublished). This grass is so adapted to northern climates that the
few remaining patches in southern New Mexico (which occur in the fir forest zone of the
Sacramento Mountains) are dying out.
Specialists on grasses and shrubs in Canada are often limited to regions of the Great
Plains where their hosts are dominant or subdominant. Thus, leafhopper specialists on blue
grama, prairie sedge, spear grass, and wheat grasses (Table 1) are characteristic inhabitants
of western plains dominated by Bouteloua, Elymus, and Stipa (Hamilton 2004a). Similarly,
species of leafhoppers of bluestem, Indian grass, porcupine grass, prairie dropseed, and
switch grass (Table 1) are restricted to the eastern prairies dominated by AndropogonSpartina-Sporobolus associations and also differentiate between tallgrass prairie and oak
savanna (Hamilton 2005).
Warm-season grasses and their leafhoppers generally occur together in single Canadian
biotic provinces (Hamilton 2004b). By contrast, in USA grasslands, species of leafhoppers
and their grass hosts co-occur over a wide geographical range that encompasses various
ecoregions (Whitcomb et al. 1994), determined in large part by the amount and seasonal
patterns of precipitation. The more widespread the host, the more likely it is that the ranges
of its leafhopper specialists will not fill the entire host range. Ranges of leafhoppers probably
reflect gene pools whose wild types are adapted to regional phenology. Leafhopper faunas
in the southwestern states are particularly dynamic. Late summer rains are predictable in
both Chihuahuan and Sonoran grasslands, but winter and spring rains are either absent
(Chihuahuan) or unpredictable in some years, absent in others (Sonoran). Grass growth
cycles (and those of their leafhopper specialists) have extremely different seasonalities
in southwestern ecoregions, and the zones, influenced in one way or another by mountain
topographies, are much smaller than Canadian ecozones. This circumstance has resulted in
subdivision of the range of blue grama in New Mexico by Athysanella species (Whitcomb
et al. 1994). By contrast, in Canada, selection for specific timing for emergence and
development of leafhoppers that would coincide with the phenology of their grass hosts is
generally unnecessary. Periodic droughts affect only the most arid regions of the western
Canadian plains.
Historical Range Determinants
The leafhopper faunas of Canada and the southern United States differ more in one respect
than in any other: the degree to which glaciation has driven Holocene faunal changes. Coastal
grasslands probably have been least affected because maritime conditions buffer temperatures.
Much of the Great Plains grassland biota likely found refugia on the emergent Gulf Coast.
Some species of leafhoppers may have moved northward from their southern refugia through
glacial-era grasslands on the exposed Atlantic plain, but not farther than Long Island (a
glacial feature). The steeper Pacific coast, separated from montane grasslands by north–south
trending mountain ridges, would be less suitable for grassland refugia. Nevertheless, both
Vancouver Island and the Queen Charlotte Islands remained ice-free during the Wisconsin
(Hendrix and Bohlen 2001) and have retained unique endemic species of leafhoppers.
However, many species of grassland leafhoppers seem to have survived on south-facing
slopes and arid basins of the Cordillera (Hamilton 2002), and, as noted earlier, on the south
slopes of the southern Rocky Mountains of northern New Mexico and Colorado.
South-facing slopes of prairie coulees are also unusually dry microenvironments that
support arid grasslands and their biota. The major west-to-east valley system of the South
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K. G. A. Hamilton and R. F. Whitcomb
Saskatchewan, Qu’Appelle, and Assiniboine rivers that extends from southern Alberta
to the glacial-era delta at Spruce Woods Forest Reserve in central Manitoba thus serves
as a major corridor for western species of leafhoppers expanding their ranges eastward
(Hamilton 2004b). Cooler sites in the same valleys provide habitats for eastern prairie
species expanding their ranges westward.
Spruce returned to eastern Canada at least 1,000 years after deglaciation, and other
tree species lagged far behind (Hamilton and Langor 1987: Table 9). This long period
without trees probably allowed grasses, for example, salt-meadow grass, Puccinellia
nuttalliana (Schult.) Hitchc., which is adapted to wet soils, to invade the region around
James Bay. The probable invasion route of grasses is still marked by patches of prairie
grasses such as slender wheat grass, mat muhly, and even switch grass that persist along
the Albany River system (Dore and McNeill 1980: Maps 186, 162, 241). In this way,
Deltocephalus serpentinus Hamilton and Ross probably followed its host, salt-meadow
grass, 1,000 km from the Great Plains to James Bay. This and eight other leafhopper
species of prairie origin are now isolated in many glaciated, boreal sites far north of the
prairie (Hamilton 1997).
The persistence of some leafhopper specialists cannot be accounted for by these
refugia. Some specialists, particularly flightless species, such as Aflexia rubranura, must
have survived in northeastern localities. This species invaded a present-day island in
Lake Huron not later than 9,000 years ago, when rising lake levels cut it off from the
mainland (Lewis and Anderson 1989; Hamilton 1994a). It must, therefore, have inhabited
southern Ontario at the time of deglaciation and moved northward as the ice cap melted.
A periglacial grassland (Catling and Brownell 1995), induced by summer sunshine under
a permanent high-pressure system over the ice cap (Hamilton 2002), seems to be the
best explanation for this and other relict grassland species and biotypes still found in and
around southern Ontario.
The present biotic provinces of the prairies emerged after the Altithermal, when
temperatures dropped to levels comparable to those that prevail today. Longer summers
permitted multiple broods of specialist leafhoppers, enabling them to expand their ranges.
The zone in which bi- or multivoltinism was a viable strategy shrank after the Altithermal,
and the zone in which univoltinism was the optimal strategy expanded. Throughout all
climatic shifts, ecoregions on mountain slopes moved uphill and downhill. At the same
time, both northern and southern prairie faunas experienced strong stressors, both natural
and invasive, which are the subject of the next section.
Stressors
Grasslands and their faunas are now undergoing the greatest environmental changes and
pressures since the last glaciation, thanks to human activities such as farming, housing, and
transportation corridors (see Chapter 1). Loss of most natural grasslands on a continental
scale seems inescapable. If leafhoppers survive, must we accept substantial degradation
of biodiversity as inevitable? We argue that if properly managed grassland reserves are
established, they should have a full complement of characteristic leafhopper specialists.
If one considers not only human-induced stress, but also natural hazards, such as
wildfire, flood, and drought (realizing that these stressors are increasingly dangerous as a
landscape becomes more highly fragmented), one might wonder how highly specialized
species of grassland leafhoppers will survive at all. But this pessimistic expectation
is countered by certain recent studies that show amazing resilience of leafhoppers to
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
191
disasters, including flood, fire, drought, and environmental degradation, as detailed in
the following subsections.
Flooding
In narrow river valleys with annual floods, such as those in southern Wisconsin, grasslands
are effectively stripped of all but their most highly dispersing species of leafhoppers
(KGAH, unpublished). However, leafhoppers appear to survive in wider flood plains. The
Red River in Manitoba overflows its banks regularly and at times catastrophically, with a
flood plain that is >70 km wide, yet its vicinity has a well-developed leafhopper fauna at
St. Charles Rifle Range near Winnipeg (see Chapter 10 for a description of this study site).
This fauna is little different from that found at Grosse Isle 20 km to the north, at the crest
of the valley and thus above the flood plain. Evidently, leafhoppers can recover from floods
in much the same period as they can recover from fire.
Fire
Fire is an intrinsic natural force that maintains grasslands. However, annual burning by
prairie managers to discourage tree growth and encourage blossoming of forbs in small
reserves can have disastrous consequences for the invertebrate fauna if no section of the
managed prairie is left unburned. Sadly, grassland fragments often lack diverse leafhopper
specialists because prairie managers prefer to burn the entire reserve to promote showy
flowers and butterflies over grasses and leafhoppers. However, it is not necessary to
sacrifice biodiversity for showiness. Where grassland reserves are large enough, they can
be subdivided so that the entire reserve need not be burned in a single year. Alternative
management practices, such as grazing, mowing, and hand clearing of brush, will suffice
on smaller sites in most years, and these activities do not affect leafhoppers.
Large leafhopper faunas are often found at small sites where fire management has been
infrequent at best. For example, the tiny intact prairie remnant at Grosse Isle, Manitoba,
supports 30 prairie-endemic bugs, rivalling or exceeding the faunas of much larger prairie
sites elsewhere in Manitoba and Minnesota (Hamilton 1995a). The infrequency of fire on
that site, combined with its moist peripheral areas, likely contributes to this unusually high
species richness.
Prairie fires usually have hot and cool patches. When a fire occurs in extensive
grasslands, there are many skips, each of which is a possible refugium for faunal elements.
Life history strategies of leafhoppers take advantage of such fire patterns; thus, populations
rebound quickly (Panzer 1988, 2002, 2003; Panzer and Schwartz 2000). Adaptations to fire
may include oviposition close to the ground where heat is least uniform. Alternatively, the
niche breadth is probably wide enough in most species to permit oviposition in moist areas
that are apt to be skipped by fire.
In areas of frequent fires, selection for high fecundity may occur to compensate
for a high proportion of fire-kill. Preliminary results from studies at the St. Charles
Rifle Range site (KGAH, unpublished) indicate that most species of leafhoppers
become dramatically more abundant within two or three years after a burn off. This
time frame may reflect, in part, increased host fitness resulting from mineral recycling,
or decimation of predators and parasites. Effectively, fires may restart predator-prey
cycles, in the earlier stage of which the prey may outbreed the predators. After four
or five years, the fauna reverts to its pre-burn population density. Farther south, where
insects have more annual generations, the faunal turnover takes only one or two years
to revert to equilibrium (Panzer 2003).
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K. G. A. Hamilton and R. F. Whitcomb
Drought
The success of specialist species of leafhoppers depends very much on the regional success
of their host plants. Nutritious and luxurious stands of grasses provide better opportunity
for successful colonization and permanence of a colony once it is established. Succulent
growth, such as that provided by annuals, or by perennials regrowing after fire, is certainly
attractive to colonizing leafhoppers. But at any one site, growth flushes come and go
during each growing season. After each generation, leafhoppers are in the air, often by the
millions. A succulent patch of new growth can acquire a substantial leafhopper population
literally overnight.
Drought is less dramatic than fire but perhaps more devastating to leafhoppers when
it causes large areas of grassland to wither. Surprisingly, one finds numerous instances of
large populations of specialist leafhoppers on drought-stressed host grasses. The stressed
hosts are likely occupied not by choice but by necessity. Some small sites with severely
stressed grasses, such as Ontario alvars, may have a leafhopper superabundance in both
species richness and total populations. The alvar species are specialists confined to their
vegetational island by their non-dispersive life history strategy. In the long term, periodic
droughts favour the survival of alvars because wildfire and dieback are essential for
maintaining such isolated grasslands in a surrounding sea of trees.
Whatever the degree of specialization, leafhoppers will accept unusual hosts if there
is a stark choice of feed or die. If a grassland patch becomes unsuitable in the course of
normal heat stress, leafhoppers move to more succulent plant species, even if they do not
usually feed on them. Enormous numbers of leafhoppers that normally feed on grasses or
forbs commonly gather on sedges and rushes around a water hole.
Generalist leafhoppers usually feed on the most succulent herbaceous plant available
(Tonkyn and Whitcomb 1986). Such plants are often annuals, which have vigorous growth
flushes in the early part of the season. However, as the season progresses, plants in the
early stages of growth are no longer available. Generalists must then fend for themselves.
This fending may lead to conspicuous migrations. Local drought conditions may account
for reports of millions of swarming leafhoppers of the polyphagous genus Xerophloea in
Nebraska in 1920 and again in 1924 (Lawson 1931). Such drought-induced migrations must
be distinguished from the seasonal movements of some generalists, for whom swarming is
a natural, regularly occurring phenomenon.
When leafhoppers are tramps, their generation times are short, and once mature, the
insects usually migrate. In their second generation, they may find other annuals undergoing
a growth flush. Several tramp species winter south of Canada (e.g., the aster leafhopper)
and begin their first generation much earlier in the season than they could in more northern
latitudes. Such species move north with each successive generation (Chiykowski and
Chapman 1965), thereby increasing their chance of finding new growth.
Specialists that feed on hosts whose growth terminates each year before their own
life cycle is complete follow a similar strategy to that of tramps. They feed on alternative
hosts, but without dispersing from the vicinity of their usual host. For example, Ballana
chelata DeLong feeds on balsamroot (Balsamorhiza sagittata (Pursh) Nutt.) in the spring,
but by the time adults emerge, the balsamroot leaves have begun to wither in the summer
heat. Adults will then leave the host to feed on a variety of other forbs in the vicinity of
the balsamroot patch (KGAH, unpublished), maintaining sizeable populations of females
that probably oviposit on balsamroot before they die.
Usually, high populations of leafhoppers do not cause obvious stress to their hosts,
but anyone who has reared leafhoppers knows from hard experience that plants are
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
193
not an infinitely rich resource and that there is such a thing as too many leafhoppers
on a host plant. Leafhopper damage on grasses (e.g., black grama) is detrimental to
seed yield (R. Garner, pers. comm.) Because parasites normally keep overpopulation
under control, damage from large numbers is scattered. In cases of superabundance on
isolated patches of their host, conditions may occur in which competition, normally not
observed in sap-sucking insects, operates between closely related leafhopper species
(Hamilton and Zack 1999).
Habitat Fragmentation
Native grasslands today are broken into numerous isolated fragments, much as they once
were during Pleistocene glaciation. The principles of island biogeography (MacArthur
and Wilson 1967) should apply to such grassland patches; that is, they lose species until
they reach a point at which species richness is in equilibrium. However, grasslands appear
to have increased in ecological complexity through the Ice Age. The Hawaiian island
chain, which continues to expand in size and number (Zimmerman 1948), may be a better
analogue of prehistoric interglacial prairies. Such grasslands expand greatly each time world
temperatures ameliorate and their leafhopper fauna could diversify at the same time.
Retention of Canadian leafhopper faunas in undisturbed, though highly fragmented,
sites seems not to be a problem. Specialist leafhoppers can sometimes persist on very
small patches, and large differences in faunal suites have been noted between prairie sites
separated by as little as 6 km (e.g., Freda Haffner Kettlehole Preserve and Cayler Prairie
in Iowa). A thriving colony of Laevicephalus minimus (Osborn and Ball) was discovered
near Belleville, Ontario, on only four tufts of its only known host grass, side-oats grama,
Bouteloua curtipendula (Michx.) Torr. This leafhopper species also has persisted on
small isolated host bunches as far east as the shale barrens of western Maryland (RFW,
unpublished), whereas another side-oats leafhopper specialist, Flexamia pectinata (Osborn
and Ball), appears to have been unable to track this patchy resource east of the prairie
peninsula in Ohio (Whitcomb and Hicks 1988). Small grassland sites not subjected to
stresses such as fire and drought appear able to provide refuges almost indefinitely, because
endemics have survived in remote montane sites in the Pacific Northwest throughout
Pleistocene glaciation (Hamilton 2002).
The situation of grasslands on sandy or gravelly soils left by glaciation is much more
fragile. Erosion and revegetation of glacial deposits presumably fragmented the ranges
of many species of leafhoppers that feed on plants, which are, in turn, found primarily
on outwash substrates or on sand and loess. For example, Altithermal temperatures may
have permitted such western species as the flightless Neocoelidia tumidifrons to move
into alkaline bogs and sand ridges in eastern Ontario and Pennsylvania where the habitats
are unstable and decreasing in area. Winged grassland insects such as the leafhopper
Commellus comma (Van Duzee) and the spittle bug Prosapia ignipectus, by contrast, are
more mobile and can still be found in extensive relict grasslands as far east as upstate New
York and Maine (Hamilton 1982, 1994a).
Conclusions
Canadian grasslands differ in many ways from those of the United States. These differences
appear to be both proximal (short growing season and more limited choice of host plants) and
historical, superimposed on pre-existing patterns by glacial geography. Prehistoric origins
determined to a large degree the pre-settlement characteristics of northern grasslands.
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K. G. A. Hamilton and R. F. Whitcomb
Glacial scouring and redeposition, followed by meltwater coulees and outwash deltas,
laid the foundations for extensive postglacial grasslands in much of southern Canada.
Severe winter conditions during the height of glaciation, combined with cooler, shorter
summers, have limited the number of potential leafhopper hosts. One of the results of this
limited plant diversity has been the evolution of numerous species of leafhoppers that are
strict monophages adapted to dominant or subdominant grassland plant species. After the
Altithermal, short summers in the north have permitted a limited number of leafhopper
generations each season. Many species in Canada have only a single brood. Univoltinism
discourages dispersion but permits insects to more fully adjust to the seasonality of their
hosts. By contrast, long summers farther south, combined with the vast geographical extent
of the prairie, probably have fostered broader host and geographical ranges in the United
States. Comparatively small leafhopper ranges in the desert plains grasslands are accounted
for by distinctly different host phenologies in small ecoregions.
The leafhopper fauna of Canadian grasslands is unexpectedly biodiverse. The reasons
are not entirely clear, but this biodiversity can be partially explained by host specificity,
glacially driven habitat change, and persistent isolated grasslands. This situation contrasts
with isolated grasslands in forested areas farther south, which are mostly the result of
frequent but scattered local disturbance, such as wildfire, and usually harbour only widely
dispersing leafhoppers.
Monophagy is common in species of leafhoppers that feed on perennial grasses and
prairie shrubs. These plants, like trees, provide a generally stable, abundant food source
that is easily located by ovipositing females. On the other hand, no single grass clump is
sufficient for the long-term continuity of a leafhopper colony on the prairies. Whereas it is
possible for a single tree to host a leafhopper colony for many decades or even centuries,
this is not true for a single grass plant, which makes the fauna of very small prairie
fragments vulnerable over time to extirpation. However, some leafhopper populations
have developed compensatory mechanisms that make them remarkably resilient to such
stressors as fire, floods, and fragmentation of landscapes. Like snails, leafhoppers have
persisted for thousands of years in isolated grasslands much too small to preserve relict
populations of larger or more vagile animals.
Present distributions of species of leafhoppers in Canadian grasslands reflect
past distributions of both prairie ecoregions and glacial-age ecosystems. Cool-season
grasses naturally proliferated during deglaciation, and relict postglacial flora and
fauna associated with moraines and sand ridges are found as far east as the Atlantic
coast. Several warm-season grasses (e.g., mat muhly) became adapted to periglacial
grasslands. Similarly, the flightless leafhoppers Aflexia and Memnonia appear to have
come north to islands in Lake Huron with prairie dropseed, a warm-season grass, at
least a thousand years before the Altithermal. The presence of Memnonia panzeri on an
alvar near Almonte, Ontario, indicates that a periglacial grassland formerly extended at
least as far east as the vicinity of Ottawa. Prairies at their maximal (Altithermal) extent
probably never reached farther east than Windsor, Ontario, the easternmost site for a
tallgrass prairie remnant with a rich fauna of insect specialists (see Chapter 9). Thus,
even before the Altithermal, postglacial patches of grasslands have been an integral part
of the ecology of eastern Canada.
The future of many prairie species of leafhoppers is in doubt. Although they can
survive many stressors, their host plants cannot, and global warming may well decimate the
floras of many prairie preserves over the next century. If and when this happens, specialist
leafhoppers probably will not be able to find and colonize remote patches.
Leafhoppers (Homoptera: Cicadellidae): a Major Family Adapted to Grassland Habitats
195
Acknowledgements
Studies on the impact of fire on fauna at the St. Charles Rifle Range in Winnipeg were
conducted by R. Roughley in 1997–2000, and his Homoptera samples were donated to
the Canadian National collection in Ottawa. It is important for us to also acknowledge the
inspiration of H. H. Ross, who interested us, as students, in grassland leafhoppers. In this
chapter, we have attempted to synthesize the results of our joint studies and the personal
communications left to us by Ross, which represent more than 60 years of study. We have
worked for years independently on different aspects of this study: one of us (KGAH) on the
biogeography and systematics of leafhoppers in Canadian and northern USA grasslands,
and the other (RFW) on the ecology of more southerly USA grasslands, and, to a minor
extent, those of northern Mexico. Our very different perspectives have greatly enriched
this synthesis.
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