The following article was gleaned from Cornell University College of Agriculture and Life Sciences Department of Entomology

General Description  Bacillus thurigiensis var israelensis

bacillus-thuringiensis-israelensis-btiAccording to Cornell University, there are over 100 species of bacteria that are thought to be pathonogenic to insects. So far very few of these have been studied enough to give us a working relationship with the microbe. But that is not the case with Bacillus thuringiensis. Since the 1960s this microbe has been developed as a microbial insecticide, of which several species are now available in laboratories world wide. When effectively applied to their primary hosts, caterpillars, some beetles and fly larvae (including fungus gnats), they stop eating, become limp, shrunken, die and decompose. These products have an excellent safety record and can be used on vegetables up to the day of harvest with no negative human responses. Because the bacteria must be eaten by the larvae to be effective, good spray coverage is essential.

_bacillus-thurigiensis-israelensis-bti-1Mode of Action

Diagram courtesy of Abbott Laboratories.

The toxic crystal Bt protein is effective when eaten by insects with a alkaline gut pH and the specific gut membrane structures required to bind the toxin. The insect must have the correct physiology and be at larvic stage of development. The microbe must be eaten in sufficient quantity. When ingested by a larvae, the protein toxin damages the gut lining, effecting a gut paralysis. The insects stop feeding and die from the combined effects of starvation and tissue damage. The larva usually die within a few days or up to a week. The active ingredients in BTI are delta-endotoxins i. e. Cry4Aa, Cry4Ab, Cry10Aa, and Cry11Aa as well as Cyt1Aa (Cytolysin) proteins.  The chemical structure of the active ingredients (insecticidal crystalline toxins), are proteins of known amino acid sequences.

 

Application |  Bacillus thurigiensis var israelensis

Microorganisms always are more effective when combined with other compatible beneficial microbes. In this case application along with Paecilomyces , Beauveria and Metarhizium is recommended. These additional fungus will be effective against the adult stages while BTI acts contrary to its nymphal stages. When applied together, it is very effective.

Human Safety | Bacillus thurigiensis var israelensis

Toxicity: Products based on Bacillus thuringiensis subsp. israelensis (Bti) have a very high safety record. The insecticidal activity is limited to the Nematocera within the order of Diptera. Susceptible are Culicidae and Simuliidae. Bti did not demonstrate measurable toxicity when tested on animals for Acute Oral, Dermal, and Inhalation. In 1999, the World Health Organisation WHO stated that “Bti is safe for use in aquatic environments, including drinking-water reservoirs, for the control of mosquito, black-fly and nuisance insect larvae”. (Source: The International Program on Chemical Safety; Environmental Health Criteria 217, Bacillus thuringiensis, World Health Organization, Geneva, Switzerland, 1999. ISBN 92 4 157217 5)

Genetic Modification | Bacillus thurigiensis var israelensis

There is one controversial issue relating to BTI. As mentioned above, Bti is very intensively researched. In fact it was one of the first bacteria to be genetically mapped. So that means we now know the assigned DNA fragments of chromosomes which produce the insecticide crystals inside Bti. In recent years these same fragments have been inserted into certain vegetable chromosomes, like corn.

The very convincing argument against this particular genetic manipulation, is that before the GM, the natural drama between the bacteria and the insect was outside the range of human contact. However, the same insecticide, now inside the corn, is ingested by us.

References

 Hoffmann, M.P. and Frodsham, A.C. (1993) Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.

Tanada, Y., and Kaya, H.K. (1993) Insect Pathology. Academic Press, Inc., San Diego. 666 pp.

Weinzierl, R., and Henn, Tess. (1989) Alternatives in insect management: Microbial insecticides. Cooperative Extension, University of Illinois, Circular 1295. 12 pp.