The neem tree (Azdirachta indica), native to the Indian subcontinent and Southeast Asia, is the source of azadirachtin, the main active ingredient in commercial neem biopesticides that are usually marketed as superior horticultural oils and pesticide soaps. Organic growers spray these oils and soaps on fruits and vegetables as an alternative to artificial toxins and insecticides.
A solvent, usually alcohol or water, is used to extract an oil containing azadirachtin from crushed neem seed. The cake remaining after the oil is extracted is often sold as a fertilizer. Azadirachtin reduces the level of an insect hormone essential to the molting process, preventing larvae from developing into adults. Some immature larvae and nymphs die; others develop into crippled adults with distorted wings. Azadirachtin also gives insects indigestion, causing them to stop feeding and may prevent some insects from laying eggs.
The toxin affects more than 60 insects including beetles, aphids, leafminers, caterpillars, thrips, lace bugs, leafhoppers, mealybugs and whiteflies.
Biopesticides containing neem are registered for use by organic growers on herbs, fruits and vegetables. Check the labels of commercial products for specifics on how to apply.
The Organic Materials Review Institute says neem oil can be used for organic production until the day of harvest.
Azadirachtin is safe to use in combination with insecticidal soaps and superior horticultural oils containing Bacillus thuringiensis, a bacterium found in the soil and in the gut of butterflies, caterpillers and moths.
Neem biopesticides do not stay in the environment, and they are not highly toxic to humans or other mammals.
Effect on Beneficial Insects
Azadirachtin does little harm to ladybugs that eat aphids and wasps that pray on other insects.
The Toxicology Network reported a study concluding that worker bees were only affected by repeated spraying of high doses of neem biopesticides; small hives suffered to some degree, but hives with medium to large populations were unaffected.
Horticulturalists at Cornell University recommend multiple applications of foliar sprays because neem does not remain for long on plant surfaces. To suppress insects in their larval stage, apply neem biopesticides when temperatures are high, insect populations are low and early in the growing season. Apply to transplants before moving them to a field and spray rows of seedlings with neem oils or soaps mixed with large amounts of water.
Plants can absorb neem toxins in their roots where the poisons move upward through tissues throughout the plant. Neem is usefully applied with drip irrigation systems to saturate the roots of lettuce and other vegetables.
Drawbacks and Warning
Neem pesticides are only effective on insect larvae, not adults. Neem products do not last long; ultraviolet rays produced by bright sunlight and rain can degrade them. Neem products can damage flowers. Do not use neem products on plants that are stressed, wilted or newly transplanted.