Is the Biological Control of Insects a Natural Phenomenon?

Abstract

Classical biological control (natural enemy introductions) has long served as a paradigm for the role of predators and parasitoids in insect herbivore population dynamics, and it is widely held that there is no fundamental difference between successful biological control and the action of native natural enemies (natural control'). We ask if biological control is really a natural phenomenon. To address this, we focus on two aspects of biological control that potentially influence the ability of natural enemies to exert top-down control on victim populations: (1) control is most often attempted against insect herbivores occupying habitats that have been substantially simplified in diversity and structure, and (2) the enemy-herbivore-plant food web in biological control systems is often composed entirely of exotic species that share few evolutionary or ecological links with the native biota, further simplifying the web in which the enemies operate. Using the insect life table literature, we found no differences in the frequencies of all types of natural enemies acting as key factors in the dynamics of exotic and native insect populations. However, top-down control, when it occurs, is more frequently due to a single parasitoid species for exotic insect herbivores on exotic plants in cultivated habitats. whereas for native insect herbivores on native plants in natural habitats it is more frequently due to a suite of generalist predators. We also found from the historical record of biological control that success rates are substantially greater in exotie. simplified, managed habitats than in natural habitats, particularly when involving parasitoids. We suggest that biological control is not strictly a natural' phenomenon, because it overestimates the extent to which parasitoids exert top-down control on insect populations, and it results most often from the formation of a single strong link in simplified food webs, in contrast to the natural control' that results from multiple links in more complex food webs.