Q: Has there been any research on the safety of using "BTi mosquito dunks" in horse water troughs and buckets?      

Judy, Bar Harbor, Maine

A: Mosquito dunks are a form of the solid-dwelling Bacillus thuringiensis bacteria (that’s the "BT" part) variety israelensis (the "i" part) that come in a solid form. As they float the dunks slowly release the BTi on the water surface. It then gradually settles to the bottom, where mosquito larvae eat it and die.

BTi dunks work by producing proteins (delta-endotoxin, the "toxic crystal") that react with the cells that line the gut of susceptible insects (there are only one or two insect orders that are susceptible to BTi, but mosquitoes are in one of those orders). The BTi proteins paralyze the insect’s digestive system by binding to specific receptor sites in the insect’s gut, and, as a result, the infected insect stops feeding within hours. Horses and other mammals lack those receptors, so their guts are not affected (as a point of interest, that’s similar to the reason why many deworming agents work). BTi-affected insects generally starve to death, although this can take several days