Antibiotic combo deadly to moths

By using an antibiotic to alter the zoo of microbes that typically inhabits the stomachs of gypsy moth caterpillars, scientists may have exposed a chink in the armor of an insect that annually defoliates thousands of acres of forests and that, so far, has defied every effort to control it.

In a series of experiments, a team of scientists has found that the antibiotic zwittermicin A enhances the lethality of Bacillus thuringiensis (Bt), an insect-killing bacterium routinely used to control gypsy moths and other insect pests.

“Zwittermicin has no measurable effect on gypsy moths directly, but when added to Bt, it increases its ability to kill insects,” says 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.”

Finding out, she says, may pave the way for improved strategies to combat gypsy moths, universally recognized as the most devastating forest and shade tree insect pest in North America.

A native of Europe and Asia, the gypsy moth was introduced to the U.S. in the late 1860s and has spread widely in the eastern United States. A voracious and indiscriminate feeder, the gypsy moth causes millions of dollars in damage every year and infestations can strip the leaves from trees and even broad swaths of forest. They are known to feed on more than 300 species of trees and shrubs.

Stopping the gypsy moth has been an intractable problem, however, and the insect has prevailed over most attempts to control it. Bt, because it is a naturally occurring microbe that is non-toxic to humans, and because it has shown the most efficacy in efforts to control gypsy moth caterpillars, is the weapon of choice when infestations do occur.

But Bt, says Ken Raffa, a UW–Madison professor of entomology, has had only limited success, and new strategies are required to keep the gypsy moth in check. Moreover, widespread use of Bt has raised fears that insect pests such as gypsy moths may develop resistance, and new methods will be needed, he says, to maintain the effectiveness of such naturally occurring weapons like Bt.

Thus, 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.

Zwittermicin is an antibiotic discovered in the laboratory of Jo Handelsman, a UW–Madison professor of plant pathology and a collaborator on the insect project. The antibiotic is produced by a bacterium known as Bacillus cereus, and is used to coat the seeds of several crop plants to keep them free of soil pathogens.

“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 says.

In experiments, the antibiotic seems to suppress some of the bacteria that make their home in the stomach of the gypsy moth. Numerous types of bacteria routinely colonize the intestinal tracts of most, if not all animals, including humans, but their functions within the host are little understood and the microbes may play a role in keeping their hosts healthy.

By eliminating or suppressing the bacteria that normally occur in the insect’s gut, it may create broader niches or opportunities for Bt to become established and perform its lethal work, says collaborator Robert Goodman, a professor of plant pathology.

One hypothesis proposed by Broderick is that some of the normally occurring gut bacteria found in the gypsy moth gut create an acidic environment that is not friendly to Bacillus thuringiensis. “It may create an environment that suppresses Bt” enabling the animal to escape an effective weapon, says Broderick.

What we are wondering now, Handelsman adds, 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 us find ways to manipulate them as a way to better control the insect.”