Potential host community location

A particular habitat can be organized into a number of animal and plant communities. Although the orientation of a parasitoid to the plants and local 

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 literature on potential host-community location by parasitoids (Vinson, 1981), like that for phytophagous species (Beroza and Jacobson, 1963), is full of anecdotal evidence of plant odors attracting insects. However, plant odors consist of complex mixtures (Schoonhoven, 1968) of which few have been isolated and identified as releasers of parasitoid behavior. 

The role of host-associated organisms in the orientation of parasitoids to potential host communities is of particular interest. One of the best examples is provided by Greany et al. (1977) who showed that, in rotting fuit, a fungus which is often associated with tephritid fruit fly larvae produces acetaldehyde, an attractant of the parasitoid, Biosteres Opius) longicaudatus. 

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Long-range cues important in potential host community location and host location may emanate from the host, its food, shelter, or associated organisms. Short-range cues important in host location, examination, ovipositor probing, drilling and acceptance usually come from the host. Further, the same source and chemical may act as a pheromone in one context and a kairomone or synomone in others. 

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). 

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