Mechanism of Tobacco Budworm Resistance to Bt Proposed

How does a tobacco budworm develop resistance to Bt? An Agricultural Research Service scientist may be closer to answering that question, based on tests she conducted on budworm cells.

ARS research physiologist Marcia Loeb of the Insect Biocontrol Laboratory in Beltsville, Md., used tobacco budworm gut cells, cultured in the laboratory, to help understand how this insect--a major caterpillar pest of cotton, soybeans and tomatoes--becomes resistant to the natural toxin produced by the bacterium Bacillus thuringiensis (Bt).

Bt has been one of the most promising biocontrol products of recent years in combating pests that attack a variety of crops. But some subgroups of tobacco budworm (Heliothis virescens) are becoming resistant to Bt.

Bt toxin causes mature budworm gut cells to swell, burst and die. In culture, Loeb found that as the toxin kills, remaining cells produced cytokines, substances that signal budworm gut stem cells to multiply and rapidly differentiate to form new mature cells. If a low dose of Bt is present, more new cells will be produced than those killed.

In Loeb’s experiments, when the Bt toxin was washed from the cultured gut cells exposed to low doses of Bt, the ratio of cell types returned to normal and the culture recovered. According to Loeb, this suggests that if an insufficient dose of Bt is “washed out” of the insect’s gut during normal evacuation, the insect’s gut will heal and the insect will survive.

This could explain why low doses of Bt toxin don’t kill insects. To be effective, the dose of Bt must be high enough that the cell replacement process can’t be completed before the insect dies. If the insect is able to produce high enough numbers of new gut cells to replace those that have been killed, the insect will survive.

This information will be useful to applicators of Bt toxin for pest control, as well as other scientists studying this toxin.

ARS is the chief scientific research agency of the U.S. Department of Agriculture.

Scientific contact: Marcia Loeb, ARS Insect Biocontrol Laboratory, Beltsville, Md., phone (301) 504-8103; fax (301) 504-5104; mloeb@asrr.arsusda.gov.