What limits the reproductive success of insect parasitoids in nature?

An enduring debate in the study of insect herbivores and parasitoids surrounds the question of whether the realized lifetime reproduction of adult females is constrained by the time available to locate suitable oviposition sites (termed “time limitation” or “host limitation”) or by the supply of mature eggs (termed “egg limitation”). One school of workers has emphasized the importance of time limitation, while the other school has suggested that egg limitation should not be ignored. A major reason for the long duration of this debate is the lack of empirical evidence regarding the reproductive success of insects in their natural environment. We developed a novel technique to measure egg limitation in proovigenic insects (which do not mature eggs as adults). Using modified capture trays, females are collected soon after they die, and the number of mature eggs in their ovaries can be counted. We used this techniques to estimate lifetime reproductive success and the risk of egg limitation in two parasitoid species of the genus Anagrus (A. erythroneurae and A. daanei, Hymenoptera: Mymaridae) which parasitize the eggs of the western grape leafhopper, Erythroneura elegantula (Homoptera: Cicadellidae), a highly damaging herbivore of California grapes. We quantified lifetime reproduction and the risk of egg limitation in populations from agricultural fields and from wild grapes and related it to host and food (nectar) availabilities. We discuss the implications of our results for general theory and for improving the use of these parasitoids as biological control agents.