Bacillus thuringiensis (Bt) is a rod-shaped, gram-positive bacterium that forms spores in the soil.
Bt was isolated in 1901 and named in 1911. 1958 was the first time it was used as a commercial biopesticide in the United States. It was placed in IRAC group 11, microbial disruptors of insect mesenteric membranes. Bt is toxic to caterpillars, some fly larvae, and some beetle larvae, but is non-toxic to other organisms. Some strains of Bt are available in products used in the United States.
Kurstaki is toxic to Lepidoptera (butterfly, skip, moth) larvae; Bt var. tenebrionis (or San Diego var. tenebrionis) is toxic to Colorado potato beetle, elm leaf beetle, and willow leaf beetle larvae. However, Bt var. tenebrionis does not kill all leaf beetles.
Bt strains are very specific to the insects they kill. Therefore, it is important to identify harmful insects. The correct strain must be applied to the susceptible insects. Applying Bt to non-susceptible insects will be ineffective. Bt is most effective on larvae and usually does not kill adults or other stages of the insect. Insects must eat Bt for it to be effective, and good coverage is important. Some insects do not eat the outside of the plant parts they attack and applications of Bt on plant surfaces are ineffective against them.
Bt applied to plants as a biopesticide is not systemic or trans-layered and does not kill on contact. It is non-toxic to beneficial substances and is classified as an organic pesticide.
Bt is rapidly inactivated by UV light. Applications made at night, on cloudy or rainy days lasted longer. However, heavy rain washes Bt away. Applications will be deactivated within one to a few days and may need to be reapplied within 3 to 7 days. The application of Bt will not result in continuous treatment of the insects through bacterial cell multiplication, and the application of Bt is similar to a chemical pesticide. Once a Bt solution has been prepared, it should be used immediately. This is especially true if the pH of the water used to prepare the solution is greater than 7 (alkaline).
After two or three years of storage, the effectiveness of Bt may be reduced. Dry preparations last longer than liquid preparations. Bt products should be stored in a cool, dry environment, away from direct sunlight.
Bt cells usually produce crystalline toxins and spores. The toxin is called delta endotoxin. Bt products usually contain toxins and spores (the environmental resistance stage of bacteria), but some products do not contain spores. The spores may become bacterial cells inside the insect. After an insect eats Bt, enzymes, and alkaline (basic) conditions in the intestinal tract activate the delta-endotoxin in the insect's gut. A specific pH level is required to activate the endotoxin. The endotoxin destroys the cell wall of the intestine. Bacterial cells enter the insect's body. Infected insects stop feeding within hours and die within hours to weeks (usually 2-3 days). Different strains of Bt have different endotoxins and kill different insects. The endotoxin is not activated in the human gut.
In summary, Bt is a microbial biopesticide that is very specific to certain insects. It causes insects to stop feeding within a few hours and usually kills them within a few days. It must be consumed and kills the larvae. It does not last very long on plants, may require frequent use, is considered organic, and is non-toxic to beneficial substances.