The microbial insecticide Bt is a concentrated formulation bacterium that kills the caterpillar stage of moths and butterflies. The active ingredients are spores and toxic crystals produced by the naturally occurring bacteria, Bacillus thuringiensis. As early as 1960, Bt was produced and sold as an insecticide. Currently, it is registered and sold for use as an insecticide in over 50 countries for use against leaf-eating caterpillars.