Tobacco Budworm Resistance to Bt Proposed
By Sharon Durham
March 8, 2001
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
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 Loebs 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 insects gut
during normal evacuation, the insects gut will heal and the insect will
This could explain why low doses of Bt toxin dont kill insects. To be
effective, the dose of Bt must be high enough that the cell replacement process
cant 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;