Bt

Bacillus thuringiensis, Bacillus thuringiensis var. kurstaki, Bacillus thuringiensis var. san diego

Bacillus thuringiensis, also referred to as Bt or B.t., is a very common, naturally occuring soil bacterium. It is a natural insecticide, active only against the larvae of certain insects, and safe against even against their adult forms. Various commercial formulations are readily available.

There are numerous subspecies of Bacillus thuringiensis, each with a different target host, though only a few are widely used. Var. kurstaki is effective against the larvae of most butterflies, moths, and spruce budworm. Var. san diego, also known as Bt var. tenebrionis, is used against Colorado potato beetles. The effect isn't immediate: death takes hours to days.

Bt is safe against most other organisms, though some beneficials may be effected. Side effects in mammals are possible though very rare and typically minimal.

The timing of the application is important: thoroughly spray all affected plants before adult stages appear, in morning of evening. Bt degrades rapidly after application: it is good only for several days, though will last a year or more on the shelf.

Finally, Bt should not be overused: resistance to Bt has already been reported.