Bt, the shortened name of Bacillus thuringiensis, is considered an organic pesticide for use against specific targeted insects. It is a naturally occurring bacteria that makes certain insects sick but has no effect on any warm blooded animal including humans. The Bt must be eaten (ingested) by the pest; it is not a contact insecticide. Therefore the Bt must be applied to a plant the pest insect will eat.
Since the 1960s, three strains have been discovered and made into fairly stable formulations viable for mass production that is marketable to the general public. Each strain will only kill certain pests.
Bacillus thuringiensis ssp. kurstaki (Btk) kills caterpillars of moths and butterflies. It is sold as the products Dipel or Thuricide.
Bacillus thuringiensis ssp. tenebrionis (Btt) kills only the Colorado potato beetle and elm leaf beetle larvae. It is sold as the product M-Trak or Novodor).
Bacillus thuringiensis ssp. israelensis kills mosquito, fly and fungus gnat larvae. It is sold at Gnatrol and as mosquito dunks or donuts to place in standing water in which mosquito larva will live.
The way Bt works, (its mode of action). The Bt bacteria produces a toxic crystal protein that once inside the pest, attaches to a cell wall of the insect. This crystal protein is called an endotoxin, (endo=inside, toxin=poison). This endotoxin causes the insect to stop feeding within a few hours of ingestion and die in one or two days.
Bt appears to be a wonderful organic solution to pest caterpillars, a few beetles, mosquitoes, flies and fungus gnat larvae; however Bt has its limitations. First it is most effective on newly hatched out larvae and not so effective on older stages of the larvae. Regular checking of plants to watch for the adult stage that will be laying eggs that then hatch into the larva will greatly help you put down the Bt at the right time to be the most effective. Second, Bt does not provide an immediate kill. It will take a while for the insects to eat the leaves sprayed with the Bt and a few days for the insect to get sick enough to die. Third, sunlight’s ultraviolet rays kills the bacterial spores within a few days after application making repeat sprays often necessary. Fourth, the pests can develop a resistance to Bt if over used as with any insecticide.
Knowledge is a key to pest control. Know the insect you are trying to control, its life-cycle, what and how it eats and when it is easiest to control. Also find out what eats the pest! You may already have beneficial insects on your plant waiting to eat the pest.