Thuricide is a commercial preparation of Bacillus thuringiensis — often abbreviated Bt — a naturally occurring soil bacterium used as a biological insecticide on crops. When caterpillars and certain other insect larvae eat foliage treated with it, protein crystals produced by the bacterium dissolve in their alkaline guts and destroy the gut lining, killing the pest within days. It is highly specific to its target insects and harmless to humans, mammals, birds, and most beneficial insects, which is why it is approved for organic farming. The same bacterial genes have been engineered into Bt crops such as insect-resistant maize and cotton. Resistance management — refuges of non-Bt plants — keeps it working.