BTI is a mixed formulation of two bacteria: Bacillus thuringiensis var. israelensis and Lisinibacillus sphaericus for the control of dipteran larvae (mosquitoes).
BTI works by producing proteins (delta endotoxins, known as "toxic crystals") that react with the gut cells of susceptible insects. The BTI proteins paralyze the digestive system and the insect stops feeding after a few hours. Occasionally the bacteria can reproduce within the insect.
The mode of action of BTI has two aspects that modulate its toxicity:
Inside the intestine, most insects have an alkaline environment that activates toxins.
Depending on the species of insect and the nature of its receptors, the bacteria's toxins may or may not be efficient in infecting it.