PARASITISM AND GENERATION CYCLES IN A SALT-MARSH PLANTHOPPER

1. In warm climates many insects exhibit discrete generations, in the absence of obvious factors that could synchronize their age structure. It has been hypothesized that parasitoid wasps might be responsible f or these oscillations in the host age structure, known as generation c ycles. 2. We examine the role of the parasitoid Anagrus delicatus in t he dynamics of the salt-marsh planthopper Prokelisia marginata. In par ticular, we evaluate the hypothesis that Anagrus contributes to the fo rmation of generation cycles in the hopper, in the subtropical climate of Florida. 3. Two kinds of evidence are presented. First, we constru ct continuous-time models of the system that incorporate what is known about Anagrus foraging behaviour, the life cycles of host and parasit oid, and their movements. We then examine the behaviour of these model s to determine if Anagrus has the potential to induce generation cycle s. We also show how spatial variation in the risk of parasitism can ge nerate 'pseudo-interference' in our continuous-time framework. Secondl y, we examine the temporal pattern of host and parasitoid abundance, a nd parasitism rates in the field. If Anagrus were contributing to the formation of generation cycles, we would expect to see a characteristi c pattern of abundance and mortality from parasitism. 4. Both our mode lling and empirical results suggest that Anagrus contributes to genera tion cycles in Prokelisia. Anagrus has the theoretical prerequisites f or generation cycles to occur and in the field generates a cyclic patt ern of parasitism that would help induce generation cycles in the plan thopper. 5. The existence of an adult host stage that is invulnerable to parasitism strongly influences the stability of our models. Without this stage Anagrus cannot fully stabilize the system, even if the ris k of parasitism varies greatly from patch to patch.