Ecological Entomology (1980) 5 , 205-21 1
Taxonomic isolation and the accumulation of herbivorous insects:
a comparison of introduced and native trees
EDWARD F. CONNOR, STANLEY H. FAETH, DANIEL SIMBERLOFF and PAUL A. OPLER"
Department of Biological Science, Florida State University, and Tall Timbers Research Station,
Tallahassee, Florida, and * Office of Endangered Species, U.S. Fish and Wildlife Service,
Washington D.C., U.S.A.
ABSTRACT. 1. Evidence from leaf-mining insects on Fagaceous hosts suggests
that range expansions of insects onto introduced trees often involve species that
feed on native hosts closely related to the introduced host.
2 . An examination of the herbivorous entomofauna of British trees illustrates
that the size of the entomofauna is partially determined by the taxonomic
isolation of the host tree.
Introduction
The rapidity and facility with which most
phytophagous insects shift host-plants or
expand host ranges is largely unknown
(Strong, 1979). However, we do know that
some herbivorous insects are host-plant
specific, others are broadly polyphagous, and
that host switches may be very rapid and
frequent (Strong, 1979; Futuyma, 1976;
Morrow, 1977; Zimmerman, 1960; Opler,
1974; Futuyma & Gould, 1979). Host ranges
of phytophagous insects may be restricted
by characteristics of the host or the herbivore,
involving geography, ecology, and plant
chemistry (McClure & Price, 1976; Smiley,
1978). Some herbivores feed on a few host
species locally, but throughout their geographic
range consume many (Hsiao, 1978). Others
restrict feeding by habitat, plant growth form
(trees, shrubs, herbs, etc.), plant phenology, or
a combination of these factors (Futuyma,
1976; Morrow, 1977; Futuyma & Gould,
1979; Feeny, 1970, Opler, 1978). Host plant
selection and use involves the discrimination
Correspondence: Dr E. F. Connor, Department
of Zoology, Oxford University, South Parks Road,
Oxford O X 1 3PS.
of proximal chemical cues presented by the
plant in the form of either differences in
nutritional quality or attractants and repellents
(Lipke & Fraenkel, 1956; Schoonhoven, 1972;
Thorsteinson, 1960; Auclair, 1965; Hsiao,
1969; Jermy, 1966). Whatever determines the
distribution of these specific cues among
plants may ultimately determine host ranges
of herbivorous insects. Related species of
plants often present very similar cues and may
be included in the host range of a herbivore
more often than are taxonomically and therefore chemically unrelated plants (Ehrlich &
Raven, 1964; Feeny, 1975; Southwood,
1961a, 1972; Holloway & Herbert, 1979). We
present evidence to suggest that host-plant
switches and range expansions are likely to
occur between related host-plants and that
accumulation of herbivore species on introduced plants is in part determined by the
presence of closely related native plants in
the locality of introduction.
Although some herbivores are very hostspecific, the emerging pattern is that many
insects thought to be host-specific are really
oligophagous, feeding on a few closely related
hosts. Leaf-mining insects on oak trees in
eastern North America are an example,
0307-6946/80/0800-0205 $02.00 0 1980 Blackwell Scientific Publications
14
205
206
Edward F. Connor e t al.
feeding o n a few t o several hosts most often
within the same subgenus of Quercus. For
instance, the leaf-mining weevil Tachygones
leconti (Coleoptera: Curculionidae) feeds
widely on red oaks (subgenus Erythrobalanus),
but in 3 years of sampling we discovered only
one leaf-mine and one adult o n a white oak
(subgenus Lepidobalanus). The pattern for
leaf-mining insects on oak in California is
similar for oligophagous species, i.e. their host
ranges tend not t o span subgenera, but
California leaf-miners are in general more
host-specific than are their eastern counterparts (Opler, 1974). Cornell & Washburn
(1 979) also illustrate that cynipid gall wasps
(Hymenoptera: Cynipidae) are oligophagous
within oak subgenera, in both the eastern and
western U.S.A. Burdon & Chilvers (1974)
report a similar pattern for insects feeding o n
different subgenera of Eucalyptus in Australia.
However, this is not t o imply that an
insect’s host-plant range o r host switches and
range expansions will always involve closely
related species. The moth genus Hedylepta
(Lepidoptera: Pyralidae) colonized banana
(Musa sp.) introduced in Hawaii. There are no
native Musaceae in Hawaii and Hedylepta is
believed t o have fed originally on another
monocot, Prirchurdiu (Palmaceae) (Zimmerman, 1960). Yet, since its introduction into
central America, banana has recruited herbivores mostly from the closely related plant
genus Heiiconiu (Heliconiaceae) (Harrison,
1964). Futuyma & Could (1979) have also
demonstrated that an insect’s host list is at
least partially determined by factors other
than taxonomic affinities of t h e hosts.
Were host ranges of oligophagous and polyphagous insects most often comprised of
taxonomically and chemically related plants,
then the susceptibility of introduced plants t o
colonization by herbivores should be a
function of the presence of related plants
native in the locality of introduction - both
degree of relatedness and species richness of
related plants should be important. Two
classes of evidence are pertinent t o a test of
this hypothesis: (1) specific instances of
insects colonizing individual introduced plant
species, and (2) the relationships between
species-richness of herbivores associated with
introduced plants and taxonomic affinity of
the introduced and native plants.
Colonization of introduced plants
Direct evidence that degree of relatedness of
introduced and native plants is important
comes from Bush’s (1969, 1975) work on
colonization of introduced Prunus (Rosaceae)
by true fruit flies of the genus Rhagoletis
(Diptera: Tephritidae). Rhagoletis species feed
o n fruits of native Prunus in North America
and readily switched over t o introduced
Prunus having already reached sibling species
status. Our studies of leaf-mining insects o n
Fagaceae in North America, Australia, and
New Zealand yield a similar result.
Leaf-miner densities were estimated by
sampling five to ten leaf clusters per tree and
enumerating individual species counts and leaf
cluster sizes. North America data o n introduced
Quercus acutissima and Q.glauca were collected
in the vicinity of Tallahassee, Florida, in July
1978, and data o n introduced Castanea
crenata in Franconia, Virginia, in 1977.
Samples of Quercus introduced t o Australia
were collected in February 1979 at the Royal
Botanic Garden in Sydney, N.S.W., and in the
town of Mt Wilson, N.S.W. All samples of
Quercus in New Zealand were taken in March
1979 in the vicinity of Auckland. Species lists
for each host were compiled from these
samples and extensive inspection of other
foliage.
Table I illustrates that Quercus acutissirna,
an oriental white oak, introduced into North
America in 1862 (Rehder, 1940), has
accumulated a leaf-miner fauna of fourteen
species in north Florida alone since its introduction there in 1967. Castanea crenata, the
Japanese chestnut, has accumulated a leafminer fauna of eleven species in Virginia since
its introduction in 1876 (Opler, 1979;
Rehder, 1940). Both species are members of
subgenera with con-subgeneric species native
t o eastern North America.
Q.suber, a European oak belonging t o a
subgenus with North American representatives
(Lepidobalanus), was introduced in 1826
(Rehder, 1940) and has a fauna of six leafminers in California (Opler , 1974). However,
in Australia where Q.suber was introduced in
1917 (Streets, 1962) and has no native congeneric relatives, it also has n o leaf-miners
(except for one introduced species of leafminer). Likewise Q.gZauca (subgenus Cyclo-
Insects on native and introduced trees
207
TABLE 1. Locality, introduction, and leaf-miner faunal records for Fagaceous plants
Taxa
Quercus
Le pidobalan us
acutissima
alha
hirolor
rohur
suher
Native
locality
Locality of
introduction
Date of
introduction
Related
species
in locality of
introduction
Orient
N.A.
N.A.
Europe
Europe
1862
1931
Consubgeneric
Confamilial
Confamilial
Confamilial
Confamilial
Consubgeneric
Confamilial
virginiana
N.A.
N.A.
Australia
Australia
Australia, N.Z.
Australia
N.A.
Australia, N.Z.
Erythrobalanus
coccinea
nigra
ruhra
N.A.
N.A.
N.A.
Australia
Australia
Australia
Cy clobalanopsis
glauca
Orient
N.A.
Australia
Castanea
crenata
pumila
den ta ta
Orient
N.A.
N.A.
1820
1917
1900
1865
No. of
leaf-miner
species where
introduced
-
12-14
1
1
1
1
6
1
24+
-
15
-
1
-
0
Congeneric
Confamilial
-
0
-
0
1853
Consubgeneric
-
-
-
-
10
15
balanopsis) introduced into North America,
before 1835 (Rehder, 1940), and into
Australia, as well as a number of other Quercus
species introduced into Australia and New
Zealand, have accumulated virtually n o leafminers (see Table 1). But, these species have
no closely related native plants in these
localities; no North American con-subgeneric
species for Q.glauca and n o congeners for the
Quercus introduced t o Australia and New
Zealand. What leaf-mining has occurred o n
introduced Fagaceae in Australia and New
Zealand has been entirely fromPhy Ilonorycter
rnessaniella (Lepidoptera: Gracillariidae) also
introduced from Europe in 1951 (Swann,
1973; Common, 1976; Wise, 1953; Delucchi,
1958). No native Nutho~agusleaf-miners have
been observed t o feed on introduced Quercus.
However, the ornamental Quercus individuals
are planted in cities where they are isolated
from the montane Nothojugus forests.
Table 2 shows that despite accumulating
fourteen species of leaf-miners, Qacutissima
also shows total abundances of leaf-miners
slightly less than half that observed on native
oaks in the same region of North America,
But these reduced abundances are due largely
0
20+
-
N.A.
-
Confamilial
Confamilial
Confamilial
No. of
leaf-miner
species where
native
11
-
t o absence of Bucculatrix sp. (Lepidoptera:
Lyonetiidae), the single most abundant
species o n Quercus in North Florida. Castanea
crenata also shows total abundances of leafminers approximately half that of native
Castanca dentata and C.pumila with a fauna
of eleven species (see Tables 1 and 2). The
extremely high abundances of leaf-miners
o n Quercus spp. in Australia and New Zealand
are entirely because of the introduced Phyllunorcyter messaniella (Swann, 1973; Common,
1976; Wise, 1953). It is also interesting t o
note that Phyllonorycter messaniella feeds
largely o n oaks in the subgenus Lepidobalanus
where native, and that where introduced the
same preference is maintained. Although
P.messaniella feeds o n Castanea and Carpinus
in Europe and an even broader array of host
species in New Zealand, we observed highest
densities o n white oaks (Lepidobalanus) and
complete avoidance of red oaks (Swann, 1973;
Common, 1976; Wise, 1953; Delucchi, 1958).
Another instance of a depauperate herbivore
fauna o n an introduced tree is that of Rhizophora mangle (Rhizophoraceae) in Hawaii.
R.mangle was introduced to Hawaii in 1902
(Neal, 1965) and possesses an extremely
208
Edward F. Connor et al.
TABLE 2. Abundance of leaf-miners o n Fagaceous plants in native and introduced localities
Native localities
Taxa
Quercus alba
Q.virg’niana
Introduced localities
No. of mines
per 1000 leaves
% leaves
mined
Locality of
introduction
No. of mines
per 1 0 0 0 leaves
% leaves
1 5 2 . 9 8 (4);
7 7 . 2 5 (4)
58.75 (3)
10.48 ( 4 )
6.79 (4)
5.26 ( 3 )
Australia
Australia
New Zealand
Australia
Australia
N.A.
Australia
Australia
Australia
N.A.
Australia
New Zealand
Australia
Australia
4.56
89.4
110.0
7.7
9 1 1 1 .6 5
3 5 .5 8
1 7 7 2 0 .7
1 1 0 2 .9 4
0
0
15.7
6 7 1 0 .5
0
0 .4 6
8 .4 3
10.2
0 .7 7
9 8 .5 4
3.17
100.0
67.32
0
0
1.57
9 7 .0
0
0
Q.nigra
Q.acu tissima
Q. bicolor
Q.suber
Q.glauca
Q.robur
Q.coccinea
Q.rubra
Castenea denfata
Cpumila
Ccrena ta
69.0 (36)
4 9 . 0 (32)
* Numbers in parentheses indicate
0
(1)
(2)
(1)
(1)
(1)
(5)
(2)
(1)
(1)
(4)
(1)
(2)
(1)
(1)
mined
(1)
(2)
(1)
(1)
(1)
(5)
(2)
(1)
(1)
(4)
(1)
(2)
(1)
(1)
-
N.A.
2 4 .0 ( 3 2 )
-
the number of trees sampled.
depauperate herbivore fauna (Simberloff, in
prep.). In south Florida where it is native, five
macrolepidopterans (four host-specific) and a
tree crab are among the herbivores (Simberloff
& Wilson, 1969; Beever e t al., 1979), but
R.mang2e lacks lepidopteran and crustacean
herbivores in Hawaii. N o members of the
Rhizophoraceae are native t o Hawaii and only
two other species have been introduced.
However, no other members of the Rhizophoraceae are native t o south Florida either,
so that this difference cannot be completely
attributed t o taxonomic isolation. This comparison is confounded further by Rhizophora’s
much smaller geographic range in Hawaii than
in south Florida. Because of the species-area
relationship one would expect fewer herbivores
o n Rhizophora in Hawaii than in south
Florida independently of its taxonomic
affinity t o native plants. In addition, the comparison for Rhizophora may be complicated
by the fact that the entire Hawaiian entomofauna is depauperate. To factor out these confounding problems data are needed on
Rhizophora introduced t o areas with and
without related native plants and possessing
nondepauperate entomofaunas. The geographic range of Rhizophora in these areas
of introduction should also be similar. In the
Fagaceous
examples
mentioned
above
however, these confounding problems are
absent. Plant species with restricted geographic
ranges
in
countries
of
introduction
accumulated sizable leaf-miner faunas if
closely related plants were present.
Species-richness of herbivores on introduced
trees
The second class of information pertinent to
this question is work of Southwood (1960,
1961b) and Strong (1974a, b) on the
herbivores of British trees. Both have shown
that the size of the herbivore fauna associated
with a tree is a function of the tree’s recent
abundance (either quarternary pollen records
or current geographic range). Strong (1974a,
b ) has further demonstrated that the time
elapsed since introduction of a particular tree
species t o Britain has n o bearing o n its subsequent accumulation of herbivores from the
native entomofauna. All species of introduced
trees accumulate sizable herbivore faunas
rapidly (100 years or less), and for their
geographic range have as many herbivores as
d o native trees (Strong, 1974a, b). However,
the size of the accumulated faunas is also
partly a function of the presence or absence
of related trees native to Britain.
Insects on native and introduced trees
We ranked the residuals from Strong’s
(1974a, b) species-area curves for insects
on trees introduced t o Britain, and we ranked
the degree of relatedness of the introduced
t o native trees. The degree of relatedness of
introduced t o native trees was based on the
systematic treatments of Clapham etal. (1962)
and Willis (1966) and were assigned the
following ranks: nonconfamilial - 4, confamilial - 3 , congeneric - 2 , and con-subgeneric - 1. Tied species were assigned the
average of the tied ranks. These ranks are
positively correlated (Spearman’s r = 0.652,
P = 0.066, n = 7). Hence, if closely related
native trees are present, introduced trees are
likely to accumulate larger faunas of
herbivorous insects, after accounting for the
relationship between herbivore species-richness and host-plant geographic range.
This pattern is not peculiar t o introduced
trees in Britain; a similar relationship exists
for native trees. Ranks of the residuals from
Strong’s (1 974a, b) species-area curve for
insects on native trees are correlated with the
ranks of degree of relatedness to other native
plants (Spearman’s r = 0.4164, P = 0.0485,
n = 17). Ranks were assigned as above. In
other words, trees that are the sole members
of genera o r families native t o Britain have
fewer herbivores than d o trees with consubgeneric, con-generic, o r confamilial relatives
native t o Britain. This must be due t o adaptive
radiation of herbivores onto related plants
and t o shared faunas among related plants.
However, the relationship is confounded by
the fact that most isolated and most
depauperate species are often small trees or
arborescent shrubs, so that this pattern may be
attributable to plant architecture (Lawton &
Schroeder, 1977; Lawton, 1978) and not
taxonomic isolation.
Lawton & Schroeder (1977) have
addressed the question of taxonomic isolation
with regard t o the remainder of the British
flora. In a fashion similar t o ours, they
examined the relationship between a plant
species’ residual from species-area curves and
the species-richnesses of the plant genus in
Britain, and found a significant correlation for
monocots only. They point out, however, that
the species-richness of a genus in Britain may
not be an accurate measure of its taxonomic
isolation there.
209
Discussion
Strong and co-workers (Strong, 1974a, b , c;
Strong & Levin, 1975, 1979; Strong e t al.,
1977) interpret the ‘rapid asymptotic’
accumulation of herbivores onto introduced
plants t o imply that biogeographic speciesrichness patterns of phytophagous insects are
determined largely independently of time,
after an initial period for colonization (up t o
300 years). Our results are consistent with this
interpretation. If the plant is taxonomically
and chemically distinct from other plants, its
herbivore species-richness should be low compared t o that of plants with sympatric close
relatives, whether it be introduced or native.
Such plants appear t o remain depauperate
regardless of their antiquity in a particular
region. That plants like Eucalyptus and
Casurina when introduced into the New World
and Quercus when introduced into Australia
and New Zealand have depauperate herbivore
communities (relative t o where they are native
o r relative t o common plants in the locality of
introduction) is then not so surprising. We
suggest that the proper null comparison is
between taxonomically and chemically novel
introduced plants and taxonomically and
chemically isolated native plants. For similar
geographic ranges these plants should have
similar herbivore species-richnesses.
Acknowledgments
We thank Miss Marjorie Sloan for providing
access t o oaks and D. Williams for assistance
in the field at ‘Bebeah’, Mt Wilson, N.S.W.,
Australia, and Ewen Young and Howard
Choat for hospitality and assistance in
Auckland, New Zealand. This manuscript
benefited from comments by J . H. Lawton,
D. R . Strong, L. G. Abele, D. B. Means, D. A.
Meeter and M. Auerbach. NSF grants DEB
76-07330, DEB 7 9 4 8 7 5 7 , and INT 78-1 3959
t o Daniel Simberloff supported this research.
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Accepted 2 9 March 1980