Bt proteins are used to control pest insects on corn, cotton, rice, and other important crops. Many Bt proteins have been discovered and each one is toxic to a narrow group of insects. Increased use of Bt carries the risk that insects will adapt and become resistant to these valuable insecticidal proteins. In the past 12 years my laboratory has focused on the target sites (e.g. receptors) in the insect midgut that determine insecticidal specificity and how those receptors differ in insects that are either naturally tolerant or have acquired resistance to Bt. Our group also participates in interdisciplinaryprojects with colleagues at the University of Georgia to engineerBt d -endotoxins into soybeansand peanuts. Students working under my supervision have been using isolated toxin binding proteins, and reconstituted them into membranes to determine toxin function. We are also taking a genetic approach by identifying and cloning genes encoding toxin binding proteins in Manduca sexta and Heliothisvirescens . Genes encoding binding proteins have been expressed in cultured insect cells. Recently, we have used phage technology to genetically modify the insecticidal specificity of d -endotoxins. By combining molecular and biochemical tools the challenge of insect resistance to Bt toxins can be addressed. Control of pest insects is an ongoing challenge for farmers. Crop plants genetically modified to produce Bacillus thuringiensis (BT) insecticidal proteins now augment traditional chemical pesticide control strategies. In the 2001, about 25% of the US corn acreage was planted to BT corn. BT cotton usage was even greater with farmers in several states planting over 60% BT cotton. BT cotton provides excellent control of the tobacco budworm (Heliothis virescens) and pink bollworm (Pectinophora gossypiella). Control of the cotton bollworm (Helicoverpa zea) is adequate, but BT cotton does not control armyworms in the Spodoptera genus. Similar scenarios exist in India, China and Australia. BT cotton is effective against some, but not all lepidopteran pests. Insect resistance is the major threat to the sustained use of BT crops. The fact that insects have the ability to become resistant to BT proteins is well documented. One mechanism of resistance is by eliminating BT receptors from the midgut target site. This mechanism of resistance is complicated by the phenomenon called cross-resistance. It is critical that insect resistance be delayed or avoided. It is also important that novel BT and additional insecticidal proteins are developed for insect control
