An organic gardener's first line of defense against insect pests is to build healthy soil, grow a diverse mix of crops to support predators and parasitoids, and choose crops that are resistant to pests. If insect pests become a problem, then naturally occurring insecticides may be used. Like synthetic insecticides, these materials can kill beneficial as well as pest insects and should be used with care.
Naturally occurring insecticides fall into three broad categories:
Botanical insecticides are made from chemicals produced by plants.
Microbial insecticides are made from living bacteria or fungi, or from products produced by bacteria or fungi.
Mineral insecticides are made from products extracted from the earth.
Botanical Insecticide Examples:
Neem oil is extracted from the neem tree, which is native to India. It deters feeding or disrupts the growth of many insects. It is not toxic to mammals and has little effect on insects that do not eat plants. Many neem oil formulations are commercially available.
Pyrethrins are refined from natural pyrethrum, which is extracted from a species of chrysanthemum. These chemicals disrupt the central nervous system, causing insects to quickly appear dead. Some individuals may recover from this "knock-down" effect. Pyrethrins break down quickly in sunlight, so they have little residual activity. They are particularly toxic to soft-bodied insects because they are absorbed through the skin. They are not effective against spider mites.
Rotenone is produced from the roots of several South American legumes. Rotenone is a nerve poison that paralyzes the insects that eat it. It is highly toxic to beetles, fish, birds, and pigs. Some people are allergic to it. Rotenone is effective for about a week before it breaks down.
Microbial Insecticide Examples:
Bacillus thuringiensis, commonly referred to as "B.t.," is marketed under the trade names Dipel, Thuricide, and others. It consists of spores and crystals produced by a soil-inhabiting bacterium. When certain insect species ingest these spores and crystals, the digestive tract becomes paralyzed and the insects stop feeding, get sick, and die in four to seven days. There are several strains of B.t., each specific to a different family of insects. The most frequently used strain is Bacillus thuringiensis kurstaki, or "B.t.k.," which only kills caterpillars. This is especially useful for controlling cabbageworms and other caterpillars that damage garden crops. It is not toxic to humans or other mammals.
Spinosad is sold under the trade name Entrust. It is produced by the fermentation of a soil-dwelling bacterium called Saccharopolyspora spinosa. It targets the nervous system, controlling caterpillars, flies, and thrips, but not beetles and plant bugs. It is not toxic to humans or other mammals but can kill bees.
Beneficial nematodes are microscopic, worm-like parasites of harmful insects. They are very specific, targeting the immature stages of root weevils, white grubs, mole crickets, and other soft-bodied, soil-dwelling insects. They can be difficult for home gardeners to use because they have a short shelf life. They are most effective in warm, moist soils.
Mineral Insecticide Examples:
Insecticidal oils include dormant oils, summer oils, and vegetable oils. Dormant oils are used in winter to kill overwintering stages of mites, scales, aphids, and other insects. They are applied as a 1 to 3 percent mixture of oil in water, before bud break, when temperatures are above 40 degrees F. Summer oils and vegetable oils can be applied to fully leafed plants to control aphids, spider mites, scales, psylla, mealybugs, and some caterpillars. Summer oils are applied as a 2 percent mixture with water. Vegetable oils are prepared by mixing 1 tablespoon of liquid soap with 1 cup of cooking oil, then adding 2.5 teaspoons of this mixture to a cup of water to spray. Some plants are sensitive to oils; spray a small area and wait a few days to check for damage before spraying thoroughly.
Finely ground sulfur can be used as either a dust or a spray to control spider mites. It may cause a chemical "burn" on tender foliage if the air temperature is 90 degrees F or higher. Sulfur also may cause an unpleasant taste if used on vegetables shortly before harvest.
Kaolin clay is a naturally occurring clay sold under the trade name Surround WP. When sprayed on plants it leaves a dry white film of microscopic interlocking particles. This film protects plants from some insect pests by acting as a physical barrier to feeding, and by irritating or repelling insects. It does not usually kill insects, so it it not a true insecticide.
