SOURCES OF VARIATIONS IN PATTERNS OF PLANT INSECT ASSOCIATION

MODELS of evolution in parasite-host or predator-prey systems assume that particular pairs of species either always interact or never interact1. But, in nature, a particular parasite species may interact with a particular host species at some times (or in some places) but not at others, even if host availability is not variable. These differences could stem either from variation among conspecific hosts in their resistance to parasitism or from variation among conspecific parasites in their tendency to attack particular host species. We report here that both of these effects exist, that they are probably due to genetic variation both in parasites and in hosts, and that they interact to produce spatial variation in ecological relationships. We studied three potentially interacting species, a herbivorous insect and two of its potential host plants, at each of two sites. At one site, a plant species was avoided by the insects because of a combination of local insect preference and local plant resistance. This result shows that the diet is a property neither of the parasite nor of the host, but of the parasite-host interaction.