Accounting for differential success in the biological control of homopteran and lepidopteran pests

One of the strongest patterns in the historical record of biological control is that programmes targeted against lepidopteran pests have been far less successful than those targeted against homopteran pests. Despite fueling considerable interest in the theory of host–parasitoid interactions, biological control has few unifying principles and no theoretical basis for understanding the differential pattern of success against these two pest groups. Potential explanations considered here include competitive limitation of natural enemy establishment, the influence of antagonistic parasitoid interactions, generation time ratio, and gregarious parasitoid development. An analysis of the biological control record showed that on average six natural enemies have been introduced per pest for both pest groups, providing no evidence of a differential intensity of competition. Similarly, use of a discrete time host–parasitoid model showed that antagonistic interactions that are common among parasitoids of Lepidoptera should not limit the success of biological control as such interactions can readily be counteracted by host refuge breaking. A similar model showed that a small generation time ratio (coupled with a broad window of host attack) and gregarious development can facilitate the suppression of pest abundance by parasitoids, and both were found to be positively associated with success in the biological control record. Of the four explanations considered here, generation time ratio coupled with a broad window of host attack appears to provide the best explanation for the differential pattern of success.