If your garden is overrun with insects, caterpillars and other undesirable bugs, you may need to use an insecticide to save your valuable plants. Different types of insecticides handle infestations differently, but all have the same objective: to reduce insect populations without harming plants. For the most beneficial insecticide effects, consider the type of pest you are trying to control as well as the crop you are attempting to protect.
Biological insecticides, also known as microbial insecticides, are living organisms that attack a very narrow range of pests. This selectivity makes them a desirable method of caterpillar control because beneficial insects, people, pets and wildlife are not affected by their action. The active ingredient in most microbial insecticides is Bacillus thuringiensis (Bt). Caterpillars eat the spores and toxins are released in the gut. According to the University of California Department of Integrated Pest Management, "Commercial formulations...contain Bt spores and crystalline endotoxin. When these are eaten by a susceptible caterpillar, the crystalline endotoxin is dissolved, releasing toxins inside its gut; gut paralysis follows, and the caterpillar stops feeding. Death usually results from toxemia, bacterial infection, starvation or predation and may follow in a few to 48 hours or longer."
There are several different strains of Bt, and each is effective against different insect species. The kurstaki variety is used against a wide variety of butterflies and moths, while the israelensis variety is used to kill the larvae of blackflies and mosquitoes. Another variety, Bt galleriae, combats a species of moth that infests beehives.
Horticultural oils are a safe and effective insecticide. Primarily used to control scale, mites and aphids on ornamentals and trees, oils suffocate insects by "blocking the air holes (spiracles) through which insects breathe, causing them to die from asphyxiation," according to W.S. Cranshaw and B. Baxendale of the Colorado State University Extension. They further note that "in some cases, oils also may act as poisons, interacting with the fatty acids of the insect and interfering with normal metabolism. Oils also may disrupt how an insect feeds." Horticultural oils are made from petroleum-based oils, vegetable oils (mainly cottonseed and soybean) and extracts from the seeds of the neem tree.
Insect growth regulators reduce insect populations by "suppressing the development of eggs, metamorphosis, and adult formation," according to the University of Connecticut Integrated Pest Management. Common growth regulators that affect juvenile insects include fenoxycarb, kinoprene, methoprene and hydroprene. Another type of growth regulator prevents the development of chitin, the insect's hard exoskeleton. These include diflubenzuron and azadractin. Insect growth regulators are effective against whiteflies, scale, aphids and some caterpillar species.