Varying levels of resistance to insects occur naturally in crop plants and closely related species. Historically, plant breeders have developed resistance to insects in situations where it was perceived to provide economic advantages over the use of insecticides in conventional agriculture: in field crops with low value per unit area and high acreage, and in staple food crops of developing countries. Under these circumstances, the use of resistant varieties is the principal method of control for many key insect pests. The process of identifying new sources of resistance and moving genes for resistance into commercial varieties through conventional breeding can be slow, often taking 12 years or longer.