If pesticides are necessary, there are a handful of organic varieties available. Bacillus thuringiensis ("Bt") is a naturally occurring bacterium that is lethal to most leaf-eating caterpillars on trees, shrubs, flowers and vegetables. According to gardening writer Jeff Ball, it is harmless to all other insects, animals and humans. It comes in a powder form for use as a dust, or, when diluted with water, as a spray. Organic chemists have formulated varieties of Bt to kill mosquitoes or potato beetles as well.