- Dark Bt-infected helicoverpa larva. Photo: R. Teakle.
Bacillus thuringiensis var. kurstaki
Bt is naturally occurring bacteria produces spores that contain a toxin. Larvae must eat Bt spores for it to be effective. Unlike NPV, Bt does not spread in the larval population. There are a number of different strains of Bt, and each is specific to an insect group, e.g. moths, beetles or flies.
The kurstaki strain targets helicoverpa and other moth larvae.
|Impact on pests|
Bt causes larvae to stop feeding soon after the Bt is eaten. Larvae become immobile and death occurs within 3-4 days due to septicaemia and starvation. Affected larvae turn black and shrivel up.
|Bt as a biopesticide|
Commercial formulations of Bt have been developed. They are nontoxic to humans and other mammals, bees, fish and other wildlife. Good coverage is essential as Bt must be ingested. The pesticide degrades with exposure to sunlight, and can be washed off by rainfall or overhead irrigation. Bt is often mixed with additives such as stickers or feeding attractants to increase its efficacy. See Pest Genie or APVMA for current pesticide registrations
Genes from the Bt organism have also been used to genetically modify cotton plants so that the toxin is expressed in the plant's tissues. When young helicoverpa larvae feed on a Bt cotton plant, the Bt toxin kills susceptible individuals. This technology has resulted in a significant reduction of insecticide use in cotton production.