Nephotettix spp.
(Homoptera: Cicadellidae)
Green leafhopper
Photo by Merle Shepard

Why might a species be invasive in one country but not a big problem in its native land?

As an example, consider a plant that is a major weed in the U.S. but in its native land it may be a minor pest. Many weeds have chemical defense systems that make them taste bad. In the weed's native land the animals that feed on the weed often evolve along with the weed and become resistant to the weed's defense systems.

In its native land, the weed may have insects that eat its roots, maybe another that bores into its stems, while other insects may eat its leaves, and still others eat its seed. Additionally, there may be rodents that also feed on its seeds.

If this weed then starts growing in a new country, there may be few if any animals that are attracted to this weed as a food source. One way to control the weed is to go to its native land and look for what are called "natural enemies," that is diseases of the plant and insects that prefer that weed species as food. These natural enemies are then tested to see if they would attack plants in the U.S. other than the intended weed. If there are no problems, then the natural enemy might be released on the weed.