Neem is a botanical insecticide extracted from the Neem tree (Azadirachta indicda), which is native to India. Extracts of neem are used for numerous medicinal and cosmetic purposes. As a garden insecticide, it is sprayed on leaves as a horticultural oil to suppress populations of insects that chew or suck on leaves; it is especially useful for organic gardeners who wish to avoid chemical pesticides.
The active ingredient in neem is azadirachtin, which is called a growth regulator because it interferes with the ability of insects to molt, preventing them from growing into adults. Its bitter taste discourages insects from eating for a few hours. Neem does not prevent adults from laying eggs and is best used on low populations of pest insects. The EPA first registered neem for use as an insecticide in 1985. Neem pesticides are not generally toxic to mammals.
Although the toxic ingredient in neem will not directly harm ladybugs, lacewings, wasps and other bugs that eat insects or bees needed for pollination, horticultural oil can still smother them. Neem spray is best applied in the morning or late afternoon so it can dry before the beneficial insects become active in the heat of the day.
Neem is more effective against insects that chew and less effective against those that suck. It is said to be useful against army aphids, army worms, cabbage loopers, corn ear worms, corn borers, cutworms, flea beetles, flies, fungus gnaps, leaf hoppers, leaf miners, mites, spruce budworms, tent caterpillars, thrips and whiteflies, among others. Neem works slower than chemical pesticides; its effectiveness is reduced by cool weather.
For a spray, add 5 tsp. of neem oil and ½ tsp. of insecticidal soap to 1 quart of water. For larger amounts, multiply the ingredients in the same ratio. Mix the soap with the warm water and stir vigorously while you slowly add the oil; shake or agitate the sprayer to maintain the mix. Neem begins breaking down immediately, so it should be used within eight hours after it is mixed.
Once an infestation has started, drench the plants thoroughly with neem spray, including the leaves and soil at the crown. Insects like to hide on the bottoms of leaves, so the spray should cover all of the leaves. Spray the plants weekly until the insects disappear. To prevent infestations of insects, neem should be sprayed on plants once every two weeks.
Horticultural oil can injure plants that are sensitive to oils, including black walnut, hickories and maples among trees and fuchsias, hibiscus, impatiens and roses among flowers. It should not be applied to plants stressed by drought or in humid weather. Household detergents are not a recommended substitute for horticultural oils, which are more refined and better tolerated by plants.