New weapon to control gypsy moths may replace toxic sprays

Thursday, November 29, 2001
By Environmental News Network

The gypsy moth in its caterpillar stageThe gypsy moth, brought to the United States from Asia in 1869 in a failed attempt to start a silkworm industry, has become a scourge, feeding on more than 300 species of trees and shrubs in the Northeastern United States and Southeastern Canada.

In an effort to contol the destructive moths, state and local authorities have sprayed pesticides over millions of acres of forest lands and whole towns, including residential areas and schools, drawing angry protests from residents and environmental groups.

A variety of natural agents are known to kill gypsy moths in nature, including more than 20 insect parasitoids and predators that were introduced over the last 100 years from Asia and Europe. But so far the moths have defied every effort to control them.

Now, a team at the University of Wisconsin-Madison has found that a combination of an antibiotic and a naturally occurring bacterium has exposed a chink in the armor of this insect that defoliates thousands of acres of forests, causing millions of dollars in damage every year.

By using the antibiotic zwittermicin A to alter microbes that inhabit the stomachs of gypsy moth caterpillars, the researchers are discovering an improved strategy to combat moth infestations.

They have found that the antibiotic zwittermicin A enhances the lethality of Bacillus thuringiensis (Bt), a bacterium routinely used to control gypsy moths and other insect pests. The combination of these two substances looks like a solution to halting infestations of moths.

"Zwittermicin has no measurable effect on gypsy moths directly, but when added to Bt, it increases its ability to kill insects," said Nichole Broderick, a UW-Madison graduate student studying the microbial ecology of the gypsy moth gut. "What's of interest to us is how this works."

Broderick's discovery that zwittermicin A amplifies the ability of Bt to kill gypsy moths may provide new biological insights that will permit science to devise even more deadly methods of controlling insect pests.

Bt, because it is a naturally occurring microbe that is nontoxic to humans and because it has proven to be the most effective substance to control gypsy moth caterpillars, is already the weapon of choice when infestations do occur.

But Ken Raffa, a UW-Madison professor of entomology, said Bt has had only limited success, and new strategies are needed to keep the gypsy moth in check. Widespread use of Bt has also raised fears that insect pests such as gypsy moths may develop resistance, and new techniques must be devised to maintain the effectiveness of such weapons as Bt.

Zwittermicin A is an antibiotic discovered in the laboratory of Jo Handelsman, a UW-Madison professor of plant pathology and a collaborator on the insect project. Used to coat the seeds of several crop plants to keep them free of soil pathogens, the antibiotic is produced by a bacterium known as Bacillus cereus. They know how to produce it, but the researchers are still theorizing about exactly how the antibiotic works to enhance the killing power of Bt.

"Perhaps zwittermicin A kills gut bacteria that keep the insect healthy. Perhaps it changes the internal chemistry. Right now, we just don't know," Broderick said.

In experiments, the antibiotic seems to suppress some of the bacteria that make their home in the stomach of the gypsy moth. By eliminating or suppressing the bacteria that normally occur in the insect's gut, zwittermicin A may create broader niches or opportunities for Bt to become established and perform its lethal work, said collaborator Robert Goodman, a UW-Madison professor of plant pathology.

What we are wondering now, Handelsman said, is "does zwittermicin A set the table for Bt to come in and do its work? We don't know, but our intent is to explore the question and gain a better understanding of the roles that these normally occurring bacteria play."

Figuring that out may help scientists find ways to manipulate them to better control the gypsy moth, universally recognized as the most devastating forest and shade tree insect pest in North America.