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Gypsy moth caterpillars were responsible for defoliating 23,000 acres of Wisconsin forest last year.

For a time last summer, Wisconsin’s cherished camping season fell to the mercy of an insect. Gypsy moth caterpillars—voracious, prolific pests that hatch and feed on deciduous trees—broke out in huge numbers, causing extensive tree damage and turning campsites into a fuzzy, squirmy mess. At Wisconsin’s Rocky Arbor State Park, the infestation was so extreme that managers shut down the park for two weeks in an attempt to keep them from spreading.

Officials are having to resort to such measures partly because other weapons—namely the bacterial insecticide Bt—have proved less than ideal in stemming the gypsy moth’s march across the northern United States. While applications of Bt have helped slow the moth’s spread, they have hardly stopped it. Last year, the insects defoliated 23,000 acres in Wisconsin alone. Experts predict conditions will favor another outbreak this summer.

New research, however, is beginning to explain why Bt may not work as well as hoped. In one landmark study, a group of CALS scientists has overturned one of the central assumptions about how Bt works—that it kills insects directly by causing the equivalent of blood poisoning—potentially opening the door to new ways to improve Bt’s potency.

In the laboratory, the team removed microbes that live naturally in a gypsy moth caterpillar’s gut and then administered Bt. Without other microbes to contend with, they expected the bacterium to be more lethal. Instead, they found that it didn’t work at all, suggesting Bt kills by teaming up with a microbe already present in the caterpillar’s gut.

“Ultimately, this is a very simple story,” says bacteriology professor Jo Handelsman, one of the lead investigators. “You need two bugs to kill an insect instead of one.”

Although researchers don’t yet understand how the two microbes interact to kill gypsy moths, the finding implies that insecticides could be made more effective by combining Bt with other microbes. It also creates a surprising new path for future research to explore.

“We didn’t believe it at first because it was so antithetical to what we were imagining was going on,” she says. “When your hypothesis is so wrong the opposite is true, you just have to laugh. But those are the moments we love in science, because the bugs are always more interesting than our imaginations.”

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