The Basics by Michael Kelly - Bacillus thuringiensis

The Basics – Bacillus thuringiensis

Bacillus thuringiensis (Bt) is a naturally occurring bacteria, common in soils, that causes disease and starvation in certain insects, notably caterpillars. Discovered in the 1900s, it has been available as a product for use in organic growing since the 1960s and multiple tests have shown it to be safe for use on crops.  It is a good example of a natural, targeted pesticide that is lethal to a specific range of insects, but not to other beneficial insects, animals, birds or humans.

Bt works because of its active ingredient - a crystal protein which messes with the digestive system of the insects and starves them to death.  It is the genes of this active ingredient that has been genetically modified in to some crops such as corn, causing some controversy (somewhat unfairly) for Bt itself as an organic control.  Bt generally comes in powder form, and is added to a watering can or sprayer to be sprayed on to the leaves of brassica plants.  The timing of the spraying is crucial – Bt is a stomach poison for insects, so the caterpillars have to actually eat it for it to work.  The time to spray is after the eggs have hatched in to caterpillars but before they start to pupate (turn in to a cocoon).  Make sure to spray on the underside of leaves too.  The best time to spray the plants is early morning and evening, since the product is susceptible to sunlight.  Bt is generally safe, but you should still follow all the safety directions on the label about mixing and cleaning up afterwards.