Classical biological control is the intentional introduction of natural enemies to control pest populations. The biological control agents are usually imported from the natural range of an invasive species. Because biological control agents are themselves exotic species, great care must be taken to ensure they do not become exotic pest species in their own right. Forest Service researchers mostly study biological control for insect pests and invasive plants. Biological control of plant pathogens usually relies on competition that results in protection of the infection site. Forest Service nursery researchers have found some reduction in root disease can result.
We also study the natural enemies that regulate populations of some high priority native pests, such as southern pine beetle.
Biological control is one of the few tools proven effective in controlling widespread invasive plants. Successful biological control agents can provide continuing and expanding control while reducing dependence on pesticides. However, because ecosystems are complex it is important to consider the effects on all the other organisms within the community, not just the pest and biological control agent. For example, gall flies imported to control spotted knapweed, a noxious weed in western North America, supported enhanced populations of deer mice, which can carry hantavirus to humans.