Are natural enemies affected by Bt?

May 2012. The question was asked “does Bt (Bacillus thuringiensis affect natural enemies”.

The answer is no. Natural enemies are not affected by Bt. Bt only becomes active (toxic) in an alkaline environment, ie in the stomach of caterpillars. So, the caterpillars need to ingest the Bt spores and crystalline proteins, and that means eating the leaves. Large caterpillars can recover from small doses of the Bt. So, it is best to apply treat small caterpillars and have conditions where the residue lasts for some time. Lots of rain is not good for maintaining residues. Use a good rain-fast adjuvant with the Bt spray.

The one non-target effect of Bt on parasitoids is if the host caterpillar dies, then the parasitoid inside the caterpillar will also die.