The toxin produced by the bacterium Bacillus thuringiensis (Bt) is a popular insecticide used to control pest moths and butterflies, and in some GM pest-proof crops. In a study published in the open access journal BMC Biology, researchers show that its effectiveness against a number of susceptible Lepidopteran species depends on the presence of the normally "friendly" bacteria that colonise their guts. Without these bacteria, the Bt toxin can become impotent in some species.