It either kills susceptible larvae or severely inhibits their growth.
Young larvae are more susceptible to the toxin than older larvae.
The conventional insecticides used are broad spectrum and kill beneficial and pest insects and require careful handling because of the potential for toxicity to humans using such materials. In comparison, Bt has a very narrow spectrum of control and has never been proved to be harmful to humans or other vertebrates. In fact, the insecticide formulations of Bt have been approved for organic farming. Because Monarchs are in the family Lepidoptera they can be intoxicated if they feed on pollen containing the Bt protein. Milkweed is ubiquitous in Wisconsin, but is typically found in line fences, roadside ditches, pastures, CRP and public lands.