What is Bt (Bacillus thuringiensis) biopesticide and when does it work?

Bt is a natural soil bacterium that produces a protein toxic only to certain caterpillars, and it is harmless to people, bees and most beneficial insects, which is why it is a popular biopesticide. The catch is that the caterpillar must eat the sprayed leaf, so Bt only works on actively feeding larvae and it works far better on small young caterpillars than on big ones. So you must spray early, soon after eggs hatch, ideally guided by trap counts, and cover the leaf surfaces the pests are eating. It is used against caterpillar pests like diamondback moth on cole crops, bollworm, leaf eating caterpillars and others, but it does nothing for sucking pests, mites or beetles. Bt breaks down quickly in sunlight, so spray in the evening, add a sticker if the label allows, and repeat every few days during egg hatch. Note that this sprayable Bt is different from the Bt gene inside Bt cotton seed. Used early and repeatedly it gives good caterpillar control without harming natural enemies. Choose the right Bt strain and dose with your KVK.