Nichole Broderick thought she knew how Bt toxin worked. After all, the toxic crystal produced by the bacterium Bacillus thuringiensis has been known since at least 1911 and widely used as an organic insecticide since the 1950s. Scientists have even genetically engineered various crops to produce the pesticide. According to the accepted model, Bt toxin punches holes in an insect's gut. These pores either allow the bacterium to infect the insect's blood, the so-called hemolymph, or cause the insect to starve.

So when the University of Wisconsin-Madison graduate student fed the pesticide to gypsy moth caterpillars that had been cleared with antibiotics of other gut bacteria, she expected it to become even more lethal. "Initially I was testing the hypothesis that the gut bacteria were actually protecting the moth from Bt," she recalls. "I found that once they did not have a gut community [of bacteria], I could no longer kill them with Bt."