Neem-based pesticides have recently become popular in the home gardening trade because of their low impact on the local environment. Most neem-based products breakdown very quickly and are completely natural. This makes it suitable for use on fruits and vegetables intended for human consumption and is an important preventative insecticide for organic gardeners. Many chemicals present in neem cannot be reproduced synthetically and must be obtained from neem trees.
Neem-based pesticides are derived from the neem tree, Azadiracta indica, which is native to lowland India and other southeast Asian countries. It has been spread by humans throughout the tropical world and is currently grown in the United States in the warmer climates of Florida and southern California. Neem trees naturally produce a chemical called azadirachtin in all parts of the tree that deter insects from feeding on it. These chemicals are extracted and used as pesticides to control many different types of insects and fungi.
Neem is a growth inhibitor. It does not directly kill insects, but rather prevents them from reproducing and maturing properly. It usually is used to prevent insect eggs from hatching or to prevent larvae from reaching adulthood. It reduces fertility in many insects leading to fewer eggs and killing their sperm.
Neem extract oil has the benefit of deterring many adult insects from feeding on plants once it has been applied. It essentially makes the plant taste bad to the insect. This only effects some adult insect species like grasshoppers and locusts but not on all types of insects like adult beetles.
Neem has been found to be useful in controlling spider mites, aphids, leaf miners and various beetle larvae, by inhibiting their development. The anti-feeding deterrent works on insects like grasshoppers, locusts and Japanese beetles.
Neem based insecticidal soaps and oils are sprayed onto the leaves on both on the top and underneath the leaf. The drawback of using these products is they tend to break down in the environment very quickly and only last a couple of weeks, depending on the product. This is also one of the strengths of using neem--because it breaks down so quickly, it does not contaminate the local environment and has no long-lasting negative effects.