University of Wisconsin-Madison scientists have discovered that an obscure antibiotic makes a widely used biological pesticide more deadly to gypsy moth caterpillars.

The discovery could boost the natural pesticide”s potency, delay the appearance of insects resistant to it, and lead to new approaches for controlling insect pests, according to the researchers.

By itself, the antibiotic – zwittermicin A – was not lethal to gypsy moth larvae. But when researchers added tiny amounts of it to the type of Bacillus thuringiensis used to control pest caterpillars (Bt kurstaki), it greatly enhanced the pesticide”s toxicity.

“There”s definitely a synergy between zwittermicin and Bt kurstaki,” says entomologist Ken Raffa, who studies insects that attack trees.

In addition to Raffa, members of the UW-Madison team include entomologist Nichole Broderick and plant pathologists Robert Goodman and Jo Handelsman. All are with the College of Agricultural and Life Sciences.

Raffa doesn”t think the discovery will change current U.S. efforts to control gypsy moth. “Bt remains effective in controlling outbreaks of gypsy moth because we don”t have Bt-resistant populations,” he says. But Broderick cautions that there are Bt-resistant gypsy moth populations in Europe.

A few agricultural pests have become resistant to Bt products. Several scientists have approached the UW-Madison team about conducting laboratory tests with zwittermicin on Bt-resistant bugs.

It was Handelsman and her coworkers who first isolated and identified zwittermicin A from a strain of Bacillus cereus. They discovered the zwittermicin-producing strain while searching for biological control agents that would protect alfalfa and soybean seedlings from a fungus that attacks them.

“Zwittermicin A belongs to a novel class of antibiotics,” Handelsman says. It doesn”t resemble any of the antibiotics used in human and veterinary medicine. Different strains of B. cereus produce different amounts of zwittermicin A, which is also produced by B. thuringiensis, she says.

Neither B. cereus strains that produce zwittermicin nor zwittermicin itself killed gypsy moth larvae that ate it. However, when groups of gypsy moth larvae were fed a constant amount of Bt and different amounts of zwittermicin, gypsy moth mortality increased in direct proportion to the amount of zwittermicin in the diet.

Compounds that enhance the activity of pesticides are not uncommon, according to Raffa. “Often insects have a mechanism that allows them to resist the effects of a toxin and a synergist acts to disable that resistance mechanism,” he says. “Usually the amount of synergist needed to do the job is as great or greater than the amount of toxin. Zwittermicin A is very unusual in that small amounts are extremely effective at enhancing the Bt toxin.”

The discovery that zwittermicin enhances the action of Bt may lead to new methods for controlling insect pests.

“We believe that zwittermicin acts in the insect gut to make bugs more susceptible to Bt toxin,” Raffa says. “If we can understand the mechanism, we think it will give us a new target we can use in managing insects.”

A Beloit, Wis. native, Broderick began the research while an undergraduate student in a class taught by Goodman. She is now completing a joint entomology-plant pathology master”s degree. She is studying the bacteria that live in the digestive tracts of gypsy moths and other insects.

“Scientists know almost nothing about these bacteria and their role in insect health,” Broderick says. ”The information we learn certainly may be useful in controlling insects.”

The research was supported by state funding to the UW-Madison College of Agricultural and Life Sciences; grants from the University-Industry Relations Program, the Wisconsin Department of Natural Resources, the USDA”s McIntire-Stennis Program, and the McKnight Foundation; and Hatch funds from the College of Agricultural and Life Sciences.