Non-selective herbicides that kill all plants can then be applied to these crops to kill the weeds but not the crop plants. When insect pests feed on these plants, they are exposed to the toxin. Organic farmers and gardeners have used Bt as a biological pesticide for many years because the toxin specifically kills only certain insects, but does not harm humans or other animals. Genes from some plant viruses have been engineered into plants to make them virus resistant. Examples include potatoes, tomatoes, papaya, and squash.
Virus diseases are otherwise very difficult or impossible to control.
New genes can improve crop quality and nutritional content. Canola has been engineered to produce oil high in unsaturated fatty acids. A new golden rice has been genetically engineered to contain increased levels of beta- carotene. Because rice is a primary food in developing countries, this new rice can potentially improve the nutrition of billions of people. Some research is aimed at eliminating or disrupting genes for proteins that cause life-threatening allergies in foods, such as peanuts.