The movement of humans around the earth has been associated with an amazing redistribution of a variety of organisms to new continents and exotic islands. The natural biodiversity of native communities is threatened by new invasive species, and many of the most serious insect and weed pests are exotics. Classical biological control is one approach to dealing with nonindigenous species. If introduced species that lack natural enemies are competitively superior in exotic habitats, introducing some of their predators (herbivores), diseases, or parasitoids may reduce their population densities. Thus, the introduction of more exotic species may be necessary to reduce the competitive superiority of nonindigenous pests. The intentional introduction of insects as biological control agents provides an experimental arena in which adaptations and interactions among species may be tested. We can use biological control programs to explore such evolutionary questions as: What characteristics make a natural enemy a successful biological control agent? Does coevolution of herbivores and hosts or predators (parasitoids) and prey result in few species of natural enemies having the potential to be successful biological control agents? Do introduced natural enemies make unexpected host range shifts in new environments? Do exotic species lose their defense against specialized natural enemies after living for many generations without them? If coevolution is a common force in nature, we expect biological control interactions to demonstrate a dynamic interplay between hosts and their natural enemies. In this chapter, I consider biological control introductions to be experiments that might yield evidence on how adaptation molds the interactions between species and their natural enemies. I argue that the best biological control agents will be those to which the target hosts have not evolved resistance. Classical biological control is the movement of natural enemies from a native habitat to an exotic habitat where their host has become a pest. This approach to exotic pests has been practiced since the late 1800s, when Albert Koebele explored the native habitat of the cottony cushion scale, Icrya purchasi, in Australia and introduced Vadalia cardinalis beetles (see below) to control the cottony cushion scale on citrus in California. This control has continued to be a success.
100 years of biological control of invasive alien plants in South Africa: History, practice and achievements
