The program resulted in the introduction of 24 species of parasites in U.S. corn production areas, of which only six became established. While these beneficial insects have provided control of the corn borer in some years in some areas, their impact has been limited. Losses due to the corn borer vary each year with infestation levels, and they are generally unpredictable from one year to the next. Largely uncontrolled until the introduction of Bt corn varieties, the European Corn Borer has caused production losses that have ranged from 33 million bushels to over 300 million bushels per year. Bt corn varieties have been shown to provide a very high level of protection from the corn borer.
Plants sustain only minute damage as the corn borer larvae attempt to feed.
The benefits that growers will realize from planting Bt corn varieties depend on the level of infestation. In light infestation years the benefits may not be great, while in heavy infestation years growers will realize substantial yield increases. It is expected that in 10 of the 13 years from 1986 to 1998, corn borer infestations in the Corn Belt were such that corn growers would have realized a gain from planting Bt corn. Alabama growers planted Bt cotton varieties on 77 percent of the state's cotton acreage in the first year the new technology was available. The primary benefit to growers of adopting Bt cotton varieties is controlling insect pests that have grown resistant to commonly used insecticides.