Durability of Bacillus thuringiensis transgenic corn for management of the European corn borer (Lepidoptera: Crambidae)

The European corn borer, Ostrinia nubilalis (Hubner), is the major insect pest of field and sweet corn in the upper Midwestern US. It is difficult to control because larvae move into protected places within the plant shortly after egg hatch. Methods of control that have been studied include broad-spectrum neurotoxic insecticides, which are lethal to non-target organisms, including beneficial insects and vertebrates, and host plant resistance, which is often insufficient to prevent economic damage by O. nubilalis. Recent genetic transfer technology have produced plants which express the insecticidal proteins from the bacterium, Bacillus thuringiensis Berliner. These proteins have narrow-spectrum invertebrate toxicity and degrade quickly in the environment. Given the safety and efficacy of this technology, growers, researchers, and industry personnel have a vested interest in both maximizing the benefits of transgenic corn, as well as preventing the evolution of insect resistance to such transformed crops. These goals motivated this dissertation research. Laboratory selection with B. thuringiensis was done on 4 colonies of O. nubilalis, with a maximum resistance ratio of 161-fold, demonstrating that this species can become resistant to one of the proteins from B. thuringiensis . Further evaluation of these insects on transgenic tissue, as well as efficacy trials using transgenic plants in the field and greenhouse, led to the refinement of a rating scale for leaf damage assessments. This scale allowed for discrimination of subtle differences between transgenic events and between colonies of insects feeding on them. Experiments were done to assess cross-resistance to other B. thuringiensis products and proteins, using the Cry1Ac-selected colony of O. nubilalis. Finally, a larval monitoring method for detecting field resistance to transgenic corn was evaluated, and elucidated a number of concerns with this type of monitoring. These studies have provided useful data necessary for the development of realistic and effective resistance management planning to extend the durability of this exciting new pest management tool.