Dr. Dennis Ring

Bacillus thuringiensis (Bt) is a rod shaped, gram-positive bacteria that forms a spore and is found in the soil. Classification of this bacteria includes Bacteria (domain); Eubacteria (kingdom); Firmicutes (phylum); Bacilli (class); Bacillales (order); Bacillaceae (family). Bt was isolated in 1901 and named in 1911. It was used as a commercial biopesticide for the first time in the United States in 1958. It is placed in IRAC group 11, microbial disruptors of insect midgut membranes. Bt is toxic to caterpillars, some fly larvae, and some beetle larvae but not toxic to other organisms. A few strains of Bt are available in products used in the United States. Bt var. kurstaki is toxic to lepidopteran (butterfly, skipper, and moth) larvae; Bt var. aizawai is toxic to wax moth larvae; Bt var. israelensis is toxic to mosquito, midge, fungus gnats, and blackfly larvae; Bt var. galleriae is toxic to larvae of May or June beetles (white grubs); Bt var. tenebrionis (or var. San Diego) 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, identification of the injurious insect is very important. The correct strain must be applied to susceptible insects. Applications of Bt to insects that are not susceptible will be ineffective. Bt is most effective against young larvae and usually does not kill adults or other stages of an 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 part they attack and applications of Bt on the surface of the plant are ineffective against them. For example, the pecan nut casebearer bites the outside of nutlets and spits it out. This insect eats the inside of nutlets and does not eat the Bt. Bt as a biopesticide applied to plants is not systemic or translaminar and does not kill on contact. It is not toxic to beneficials and is listed as an organic insecticide.

Bt is rapidly deactivated by ultraviolet radiation. Applications made in the evening, on cloudy, or on rainy days last longer. However, heavy rains wash Bt off the plant. Applications become inactivated in one to a few days and may need to be reapplied in 3 to 7 days. Applications for leaf beetles may be effective for only one day. Applications of Bt do not result in continuous management of insects by reproduction of bacterial cells, and Bt is applied similar to chemical insecticides. Once a solution of Bt is prepared it should be used immediately; especially, if the water used to make the solution has a pH greater than 7 (basic).

The effectiveness of Bt may be reduced after two or three years of storage. Dry formulations last longer than liquid formulations. Bt products should be stored out of sunlight and in cool, dry conditions.

A crystalline toxin and spore is usually produced by Bt cells. The toxin is called a delta endotoxin. Bt products usually contain the toxin and spores (environmental resistant stage of the bacterium) but some products do not contain spores. Spores may become bacterial cells inside the insect. Once the insect eats the Bt the delta endotoxin is activated in the insect’s gut by enzymes and alkaline (basic) conditions of the gut. A specific pH is required to activate the endotoxin. The endotoxin disrupts the cell walls of the gut. Bacterial cells enter the body of the insect. Infected insects stop feeding in a few hours and die in a few hours to weeks (frequently 2-3 days). Different strains of Bt have different endotoxins and kill different insects. The endotoxin is not activated in the gut of humans.

In summary, Bt is a microbial biopesticide that is very specific to certain insects. It causes insects to stop feeding in a few hours and usually kills insects in a few days. It must be eaten and kills larvae. It does not last long on the plant, may require frequent applications, is considered organic, and is not toxic to beneficials.

Dr. Dennis Ring