Host evasion and defense

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 

Another interesting behavior is frass throwing by Pieris rapae which has been found by B. Usher (personal communication). She suggests that the removal of frass, which often serves as a parasitoid cue, provides an escape from parasitoidism. It is expected that other examples of parasitoid evasion will be found in the future. 

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