By Dr. Paul W. Ivey, Contributor
BACTERIAL PATHOGENS used for insect control are spore-forming, rod-shaped bacteria in the genus Bacillus. They occur commonly in soils and most insecticidal strains have been isolated from soil samples.
Bacterial insecticides must be eaten by target insects to be effective because they do not kill on contact. Insecticidal products composed of a single Bacillus species or subspecies may be active against an entire order of insects or they may be effective against only one or a few species.
Bacillus thuringiensis (Bt) products are produced commercially in large industrial fermentation tanks. The bacterial cells usually produce a spore and a crystalline protein toxin called an endotoxin as they develop.
The best-known and most widely used Bt insecticides are formulated from Bacillus thuringiensis var. kurstaki. (Btk) isolates that are pathogenic and toxic only to larvae of the butterflies and moths (Lepidoptera). The most common trade names for these commercial products include: Biobit, Condor, Cutlass, Dipel, Full-Bac, Javelin, M-Peril, Thuricide, and MVP.
These products are commercially successful and widely available as liquid concentrates, wettable powders, and ready-to-use dusts and granules. They are used to control many common leaf-feeding caterpillars, including pests on vegetables, especially the "worms" that attack cabbage, broccoli, cauliflower, and other leafy vegetables and corn earworm larvae in field corn. Some products are used to control Indian meal moth larvae in stored grain.
Bacillus thuringiensis var. aizawai is another Bt that kills caterpillars. It produces slightly different toxins and is the active ingredient in the products Certan, Agree and Xentari.
Bacillus thuringiensis var. israelensis (Bti) kills the larvae of certain flies and mosquitoes. The main targets for this Bt are the larval stages of mosquitoes, black flies, and fungus gnats. Bti products that are available commercially include: Vectobac, Teknar, Bactimos, Skeetal, and Mosquito Attack.
Another group of Bt isolates, including those from Bacillus thuringiensis var. san diego and Bacillus thuringiensis var. tenebrionis, are toxic to certain beetles. Bacillus thuringiensis var. san diego, sold under the trade names M-Trak, Foil and Novodor, is registered for use against larvae of the Colorado potato beetle.
HOW DOES BT WORK?
Bt has to be eaten by insects to be effective. When Bt is ingested by a susceptible insect, the protein toxin is activated by alkaline conditions and enzyme activity in the insect's gut. If the activated toxin attaches to specific receptor sites, it paralyses and destroys the cells of the gut wall, allowing the gut contents to enter the insect's body cavity. Poisoned insects may die quickly from the activity of the toxin or may stop feeding and die within two or three days from the effects of septicaemia (blood-
poisoning).
Bt does not reproduce and persist in the environment in sufficient quantities to provide continuing control of target pests. The bacteria may multiply in the infected host but because few spores or crystalline toxins are produced, few infective units are released when a poisoned insect dies. Consequently, Bt products are applied much like synthetics insecticides. Bt treatments are inactivated within one to a few days in many outdoor situations and repeated application may be necessary for some crops and pests.
Dr. Paul W. Ivey, entomologist, Faculty of Science, College of Agriculture, Science and Education (CASE).
