Parasitoid host selection

The strategies used by insects in the host selection process, whether for food or for oviposition sites, may appear superficially to differ, but actually they have a great deal in common. While host selection for most parasitoids involves a single stage (i.e., the adult female) this is not always true. For example, the female tachinid Lixophaga diatraea, deposits eggs near host frass. The larvae must then locate and accept the host (Roth et al., 1978). 

A female must search at random within a habitat until one of the stimuli important in host selection is encountered. As with insects responding to sex pheromones, some parasitoids respond to air movements and fly upwind (anemotaxis) (Edwards, 1954). Odors stimulate a klinokinetic response in Mormoniella (Edwards, 1954) whereas odors may stimulate chemotaxis in other species (Read et al., 1970). However, there has been relatively little data on the long range orientation of parasitoids. The cues that allow a parasitoid to orient to a potential host community presumably act from a distance. The types of stimuli that meet this criterion include electromagnatic radiation, sound, or odors. 

The potential host community can be perceived by parasitoids through any of the above stimuli from the host, the food or shelter of the host, organisms associated with the host, or interactions between these factors. Plants appear to play an important role in potential host community location, partly because plants are the source of food for most hosts. It has been contended that the evolution of the parasitoid habitat in Hymenoptera may have stemmed from a previous plant-parasite relationship (Malyshev, 1968). If true, such a relationship would provide insight into the role of plants in the host selection process. The importance of plants in host community location is further supported by the observation that there is less tendency for parasitoids to select phylogen-etically related hosts than unrelated hosts found on the same plant (Cross and Chesnut, 1971). 

The great diversity of parasitoids and the complexity of parasitoid-host relationships is probably influenced by attempts of potential host insects to escape their predators, parasites and parasitoids. There is considerable speculation on the role that parasites (parasitoids) play in herbivore evolution. There are many examples where a host on different plants is attacked by different parasitoid species (see Vinson, 1981). As discussed by Zwolfer and Kraus (1957), and Vinson (1981), plants play an important role in the host selection process, probably by providing cues to the location of a potential host community. Theoretically, a host could escape a particular parasitoid by attacking a plant lacking those stimuli used by the parasitoid to locate the potential host community. This idea is supported by the observation that there is less tendency for parasitoids to select phylogenetically related hosts than to favor a range of hosts on a particular plant (Askew and Shaw, 1978 Cross and Chesnut, 1971). 

One outstanding question that comes to mind when one describes the role kairomones play in host selection is why the host continues to produce the kairomone. No doubt elimination of the kairomone has occurred many times during the evolution of a parasitoid-host relationship, but proving such an alteration is very difficult. Most, if not all, kairomones probably serve the host in some essential way. For example, the mandibular gland secretion that acts as... 

To meet these needs, parasitoids have evolved to respond to cues not only produced by the host, but also cues provided by the food and shelter of the host and associated organisms. While progress has been made in isolating and identifying the cues involved in host selection, most of the effort has centered on the short-range chemical cues of egg and larval parasitoids. Only a few studies have been conducted on the long-range cues. 

Luck, R.F., H. Podoler R. Kfir. 1982. Host selection and egg allocation behaviour by A. melinus and A. lingnanensis comparison of two facultatively gregarious parasitoids. Ecol. Entomol. 7 397-408. 

Toxic substances acquired from the host plaint may provide resistance to parasitoids (24), pathogens (25), and predators (45). By avoiding some toxins in plant material and selecting superior food tissues, insects feeding on variable hosts may become more susceptible to some enemies. Of course, other substances in preferred tissues may still be toxic to certain of these enemies, but this is less likely than it would be were plant compounds uniformly encountered by the host insect. 

Chapters in this volume consider how plants use chemicals to defend themselves from insect herbivores the complexity of floral odors that mediate insect pollination tritrophic interactions of plants, herbivores, and parasitoids, and the chemical cues that parasitoids use to find their herbivore hosts the semiochemically mediated behaviors of mites pheromone communication in spiders and cockroaches the ecological dependence of tiger moths on the chemistry of their host plants and the selective forces that shape the pheromone communication channel of moths. 

Fritz, R. S. (1982) Selection for host behavior modification by parasitoids. Evolution, 36, 283-8. 

Delphia CM, Mescher MC, De Moraes CM (2007) Induction of plant volatiles by herbivores with different feeding habits and the effects of induced defenses on host-plant selection by thrips. J Chem Ecol 33 997-1012 De Luca V, St Pierre B (2000) The cell and developmental biology of alkaloid biosynthesis. Trends Plant Sci 5 168-173 De Moraes CM, Lewis WJ, Pare PW, Albom HT, Tumlinson JH (1998) Herbivore-infested plants selectively attract parasitoids. Nature 393 570-573... 

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