Exotic or non-native pests species can become a big problem when introduced to a new environment because they have no natural predators. There are many invasive weeds in the U.S., many of which have flourished because of no natural enemies to help keep their populations in check. One way to combat these invasive pests is to identify where they were introduced from (their native range) and find natural predators to be introduced to help control this pest. Often times, these natural biological control agents are insects and can be used to help control plants or other pest insects.
Many people have the common misconception that these introductions are ALWAYS a bad idea because this natural predator can get out of hand. While there are some examples of just how bad this can be, often times there is years and years of research and careful testing that leads up to the decision to introduce natural predators. Things like choice tests, oviposition tests and host tests help determine if the natural predator is specific to just the pest of interest or can extend its range to other non-target species. Also, the natural enemy, although not native, may have already found its way to the new environment and have become established on its own. In this case, no new introductions are needed because the insect is already present. However, populations can be augmented to help increase control of the pest, especially in times when pest populations have increased too rapidly or to too high a number.
These links provide information on two international research projects focused on potential biological control of invasive plants.