Aspects of Interaction Between Plant Genotypes and
Biological Controll
By JOHN M. BERGMAN AND WARD M. TINGEY
Dept. of Entomology, Cornell Univ., Ithaca, NY 14853
Althoug-h the combined effectiveness of resistant cultivars and biological control has been studied in a few
instances, the interactions between plant resistance and
arthropod predators and parasites remain poorly known.
Most investigations of plant resistance to insects have focused on the pest-plant interaction; however, as resistant
cultivars become more widely used in pest management
their compatibility with biological control agents will become an important consideration.
Plant resistance and biological control are generally
considered compatible pest management strategies (Casagrande and Haynes 1976, Kennedy et al. 1975, Schuster
and Starks 1975, Schuster et al. 1976a, b, Pimentel and
Wheeler 1973, Starks et al. 1972). The use of these two
control methodologies introduces unrelated mortality effects, thus reducing the rate of genetic response in the
pest population to selection pressure. Together, they also
provide density-independent
mortality in times of light
pest density and dynamic density-dependent mortality in
times of pest increase. Even low levels of plant resistance
can diminish the intrinsic increase rate of a pest population, thus providing a relative advantage to natural enemies (van Emden 1966, Debach 1974, Starks and Berry
1976).
Numerous studies, however, indicate that predator
and parasite performance may be altered by the host
plant of the prey (Morgan 1910, Seamons and McMillan
1935, Flanders 1939, 1942, Smith 1942, Simmonds 1944,
Smith 1957, Gerling 1966, Cheng 1970, Kuo 1977). Although different species of host plants produce the
greatest range in responses, cultivars of the same species
also can differentially affect predator or parasite success.
Painter (1951) discussed two general ways in which
plant resistance can influence the performance of natural enemies. First, reduction in prey populations may affect the success of some predators and parasites, if prey
density falls below the optimum searching capacity of the
natural enemy. Secondly, host plant-induced changes in
prey physiology and behavior may modify the success of
natural enemies. Although not cited by Painter (1951),
allomonal or toxic resistance factors and morphological
defense mechanisms might also limit populations of beneficial arthropods that come into physical contact with
the host plant or that use the plant for incidental feeding.
In this review, we discuss these relationships and their
signiticance in the development and implementation of
specific crop resistance mechanisms.
Direct Influence of Plant Host
Volatilt's (md Plant Growth CIUlracteristics
As early as 1938, Thorpe and Caudle demonstrated
olfactory attraction of Pimpla ruficollis (Gravenhorst), a
I R""oived
for publication May 8, 1979. A publication of tho Cornoll
Univ. Agric. Exp. Stn., Now York Stato Collogo of Agric. and Lifo Scion«5,
a Slatutory Collogo of S,U.N.Y.
'Accordin)! to Arnaud (1978), tho N•• rctic spec;" of Ly<klla parasitic
on O. nubilalir IS L. IhI1lrlp.roniHorting.
parasite of the European pine shoot moth, Rhyacionia
buoliana (Schiffermi.iller), to volatile components of the
host plant. In recent years, it has become clear that directed orientation of predators and parasites to host
plants of their prey is a relatively common phenomenon
(Doutt 1964) and differential attraction occurs among
crop varieties. Franklin and Holdaway (1966), for example, reported differential attraction of Lydella grisescens Robineau-Desvoidy,2 a parasite of the European
corn borer, Ostrinia nubilalis (Hubner) to different maize
hybrids, while Mansour (1975) observed greater oviposition by the aphid predator, APhidoletes aPhidimyw (Rondani) on Brassica oleraceae var. geminifera compared to B.
oleraceae var. gongylodes. Host-plant volatiles influence
prey selection by Pemlenus pseudopalliPes (Loan), a parasite of the tarnished plant bug, Lygus lineolam (Palisot de
Beauvois) (Shahjahan 1974, Shahjahan and Streams
1973). Similarly, Read et al. (1970) demonstrated olfactory attraction of Diaeretiella rapae (M'lntosh), a parasite
of the cabbage aphid, Brevicoryne brassicae (L.), to allyl isothiocyanate, a sulfur-containing glycoside of wide-spread
occurrence in Cruciferae.
In addition to volatile cues, plant growth characteristics may alter performance of natural enemies. For example, height of the host plant appears to influence parasitism of the Nantucket pine tip moth, Rhyacionia
frustrana (Comstock), although the effect is not clearly
distinguishable from the influence of various plant organs (Eikenbary and Fox 1968).
Toxic and Nutritional Factors
Many species of predators and parasites use plant
juices, nectar, and pollen as sources of food and water.
For example, three major hemipteran predators of Heliothis zea (Boddie) [Orius insidiosus (Say), Geocoris pallens
St:'l.l,and Nabis americoferus CarayonJ, consume juices and
pollen of corn and cotton (Dicke and Jarvis 1962, Ridgway and Jones 1968). Performance of predators and
parasites with similar patterns of feeding behavior might
be impaired on cultivars containing broadly-toxic allelochemic resistance factors.
Nectar is an important source of food and water for
numerous natural enemies as well as their prey. Cotton
genotypes lacking extrafloral nectaries are effective in
reducing infestations by Heliothis zea and the tarnished
plant bug and are currently being used in pest management programs for these pests. Schuster et al. (1976b)
reported a reduction in predator densities on nectariless
genotypes but concluded that the decrease was due to
smaller prey populations rather than to elimination of
nectar as a food source for predators.
Foliage Morphology
Epidermal characteristics can also alter the behavior
and success of natural enemies. Way and Murdie (1965)
and Chandler (1968) showed that cultivars of brussels
sprouts with glossy leaves were more attractive to predators and parasites than those with waxy foliage. Downing
275
ESA
276
and Moilliel (1967) concluded that
by large leaf veins and pubescence
the greater abundance of predatory
cultivars 'Mcintosh' and 'Spartan'
Vol. 25, no. 4
BULLETIN
protection provided
was responsible for
mites on the apple
than on 'Delicious'.
Foliar pubescence, a widespread mechanism for plant
defense against arthropods, may pose special hazards for
natural enemies, because of its relatively non-selective action. The hooked trichomes of the common bean, Phaseolus vulgaris L., for example, confer pest resistance by
mechanical impalement of a wide variety of small, softbodied insects (Poos and Smith 1931, McKinney 1938,
Richardson 1943, Fluiter and Ankersmit 1948, Johnson
1953, Varis 1972, Pillemer and Tingey 1976). Glandular
trichomes on foliage of wild Medicago species and many
solanaceous plants, on the other hand, confer resistance
by the secretion of sticky exudates that either entrap and
immobilize arthropods or contain toxic, deterrent factors
(Thurston et a!. 1966, Gentile et a!. 1968, Stoner et a!.
1968, Thurston 1970, Gibson 1971, Aina et a!. 1972,
Gibson 1976a, b, Shade et a!. 1975, Tingey and Gibson
1978). The range in activity of secretory plant hairs is
even greater than that of non-glandular types and includes the Acarina and 4 orders of Insecta, i.e., Coleoptera, Lepidoptera, Homoptera, Thysanoptera.
In view of such remarkable non-specificity, cultivars
with high densities of hooked or glandular trichomes
might be expected to effect considerable mortality of
small, soft-bodied predators and parasites. Rabb and
Bradley (1968) provided evidence for this negative aspect of plant defense by pubescence. They demonstrated
that secretory trichomes on tobacco foliage severely limited egg parasitism of the tobacco horn worm by Telenomus sphingis (Ashmead) and Trichogramma minutum Riley.
Parasites became entangled in gummy exudate of the trichomes or accumulated exudate on their bodies to the
extent of impairing movement. Elsey and Chaplin (1978)
reported a similar effect of tobacco trichomes on egg
parasitism of the tobacco budworm, Reliothis virescens
(F.), while Milliron (1940) found that glandular trichomes of Nicotiana glutinosa L. trapped the parasite, Encarsia formosa Gahan.
Non-glandular, simple trichomes that interfere spatially with host selection and feeding or ovipositional behavior (Parnell et a!. 1949, Schillinger and Gallun 1968)
are less likely to affect natural enemies directly, although
May (1951) suggested that mobility of aphid parasites
and predators might be restricted on highly pubescent
cotton genotypes.
1979
nense compared to S. dulcamara L. Finally, nutritionally
inadequate diets can impair the ability of prey to encapsulate developing endoparasites (El-Shazly 1972a, b), or
can adversely influence the development, fecundity, and
longevity of parasites (House and Barlow 1961, Zohdy
1976).
Secondly, natural enemies may be exposed to toxic resistance factors through their prey. Gilmore (1938a, b)
observed that Apanteles congregatus (Say) seldom parasitized larvae of the tobacco hornworm reared on darkfired tobacco culiivars compared to cultivars of burley
tobacco. He noted the greater nicotine content of darkfired tobaccos and suggested that toxicity was passed to
larval parasites through their prey. Adult parasites might
also be exposed to toxic plant substances through feeding on hemolymph exuded from a punctured prey.
Altered Prey Behavior
Confinement of prey on resistant host plants commonly leads to a disruption of normal behavior patterns.
On frego bract cotton, for example, the floral braCls are
twisted and flared, resulting in restlessness and abnormal
wandering activity of the boll weevil, Anthonomus grandis
grandis Boheman. Mitchell et a!. (1973) suggested that
this behavioral modification increases the probability of
contact with natural enemies of the boll weevil, and contributes to the expression of resistance. Another example
of interaction between parasite searching efficiency and
plant morphotype was reported by Pimentel (1961). Preyfinding and parasitism of the imponed cabbage worm,
Pieris rapae L. by APanteles glomeralus (L.) and Phryxt vulgaris (Fall.) were enhanced on open-leaved cabbage cultivars, whereas closed-leaved types provided concealment for prey and interfered with parasite efficiency.
Berlinger and Gol'berg (1978) concluded that the protection afforded the citrus mealybug, Planococcus citri
Risso by size and shape of grapefruit sepals was a significant factor in governing parasitism success by Anagyrus
pseudococci Girault.
Compatibility
of Resistant Cultivars and Biological
Control
Most reviews of plant resistance as a strategy in integrated pest control stress compatibility with other management methods. Relatively little information is available, however, defining the elements of compatibility with
biological controls.
A major impact of plant resistance is reduction in pre·
dator and parasite density resulting from reduced prey
Nutritional and Toxic Properties of Prey
populations (Pimentel and Wheeler 1973, Kennedy et a!.
1975, Casagrande and Haynes 1976, Schuster et a!.
The nutritional substrate offered by a host plant can
1976b). In general, these studies suggest that parasitism
indirectly influence predator and parasite fitness in sevrates and predator efficiency are not adversely affected
eral ways. First, prey confined to resistant hosts comon resistant cultivars, despite reduced prey abundance.
monly experience reduced growth rates, greater develHowever, it should be noted that the expression of varopmental time and mortality, and decreased fecundity.
ietal resistance may vary with pest density, a parameter
With such dramatic alterations of fundamental physiomodified, in part, by the action of natural enemies. Thus,
logical processes, the nutritional quality of the prey as
suppression of predator and parasite populations owing
food for predators may be affected. Landis (1937), for
to direct impact of nonspecific resistance mechanisms, or
example, reported reduced growth rates and greater . other adverse factors, could seriously limit the expresnymphal mortality of the pentatomid predator Podisus
sion of resistance, particularly on marginally resistant
maculiventris Say reared on Colorado potato beetle larvae
genotypes (van Emden 1966). Although this potential
fed foliage of Solanum atrofrurfrureum Sendter and S. carhazard further emphasizes the need for thorough assessolinense L., compared to those reared on larvae fed foment of the interaction between varietal resistance and
liage of the cultivated species, S. tuberosum L. Similar rebiological control, such studies are seldom feasible at
sults were obtained for a related predator, Perillus
early stages of germ plasm evaluation. As a practical almaculatus (F.), reared on larvae fed foliage of S. caroliternative, some workers suppress populations of natural
Influence
of Resistance
Factors Through Prey
Vol. 25, no. 4 1979
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277
BULLETIN
enemies in initial research trials, so as to avoid confusing
the impact of varietal resistance on pest performance
with that imposed by predators and parasites.
nate prey species minimized the rapid decline in parasitoid populations usually experienced during periods of
greenbug unavai]ability.
Another potentially undesirable interaction between
varietal resistance and biological control could lead to
pest outbreaks in companion crops. In alfalfa-cotton and
alfalfa-small grain ecosystems, e.g., alfalfa provides a reservoir of natural enemies of great importance in regulating pest abundance in companion crops (Hagen et al.
1971, Stary 1978). Non-specific resistance mechanisms
in such key reservoir crops might seriously upset the biological equilibrium of similar agroecosystems. To minimize such potentially disruptive effects, cultivars resistant by non-selective factors should be assessed for their
impact on populations of natural enemies and their projected hazards versus benefits prior to release for commercial production.
Other possible strategies to maximize the beneficial association of biological control agents and plant resistance
include management of plant species that provide supplemental nourishment for natural enemies, management of habitats that favor permanent establishment of
natura] enemies, and limitations on use of broadly-toxic
pesticides and cultivation practices disruptive to predators and parasites. Finally, by genetic manipulation of
natural enemies it may be possible to produce races to]erant to toxic, nutritional, and physical plant resistance
factors.
Combined Action of Resistance and Biological Control
In cases where plant resistance and biological control
fail to achieve desirable levels of control when used
alone, the simultaneous use of both strategies frequently
offers promise. Starks et al. (1972), e.g., demonstrated
that use of the green bug parasite, LysiPhlebus testaceipes
(Cresson), in plots of resistant barley and sorghum genotypes led to smaller greenbug populations and less plant
damage than that on susceptible genotypes with the parasite, or on resistant genotypes without the parasite.
Kennedy et al. (1975), likewise, suggested that use of
muskmelon cultivars resistant to the cotton aphid, Aphis
gossypii Glover, coupled with natural enemies might control the pest below economic levels.
Finally, Wyatt (1970) showed that parasitism of the
green peach aphid by Aphidius matricariae Haliday was
greater on resistant than on susceptible chrysanthemum
cultivars. He attributed this phenomenon to 2 mechanisms: either the parasites spent a greater period of time
on the resistant than the susceptible cultivars, or rate of
aphid population growth on susceptible cultivars exceeded the intrinsic increase rate of the parasite.
Strategies in Integrating Plant Resistance
Biological Controls
and
The results obtained through integration of resistant
cultivars and biological control will be subject to limitations inherent in use of either strategy alone, such as impermanence of plant resistance, and fluctuation in predator-parasite populations. With regard to the former,
however, development of pest biotypes capable of colonizing resistant hosts might be slowed by the diversification in selection pressure associated with partial control
by natural enemies.
As for the 2nd limitation, the combination of densitydependent and density-independent
selection pressures
may contribute to more efficient control throughout seasona] or yearly fluctuations in a pest population. In addition, several supplemental strategies might be considered. Adjacent trap crops can be managed to supplement
pest or alternate prey populations, thus providing food
sources for natural enemies during periods of prey-parasite asynchrony or low population density (Stary 1970).
An excellent example of this approach was provided by
Eikenbary and Rogers (1974), who achieved better control of the greenbug on sorghum when parasitoid species
were provided with an alternate prey, the sunflower
aphid, APhis helianthi Monell, commonly associated with
weed flora of sorghum fields. The presence of an alter-
Acknowledgment
We are indebted to Profs. M. J. Tauber and R. B. Root
for their helpful comments.
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Vol. 25, no. 4
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279
ESA BULLETIN
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COMMONWEALTH
AGRICULTURAL
BUREAUX
The Taxonomy, Distribution and Host
Preferences of African Parasitic Wasps
of the Subfamily Ophioninae
1.0. GAULD
Commonwealth Institute of Entomology, London
and
P .A. MITCHELL
This volume is the first in a series of monographs on the systematics
of economically important insects. The work covers all species of
Ophioninae of Africa south of latitude 200N including Madagascar
and associated islands. The Ophioninae are common parasites of a
variety of Lepidoptera and are of importance in the natural control
of a number of pests of agricultural crops.
A4, Soft Covers, 287 pp., 810 illus., 102 maps. 1978.
PRICE: Post free $43.20
Dicamptus bantu;
Head, dorsal.
Order from:
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Farnham House, Farnham Royal, Slough SL2 3BN, UK.