The neem tree (Azadirachta indica) is a fast growing shade tree native to Southeast Asia and India. Compounds (Azadirachtin) extracted from the seed have been found to possess pesticidal qualities; the greatest concentrations of these substances are found in the seed.
Also extracted from the seed kernel is the oil. Neem oil, like other horticultural oils, works by suffocating the insect, coating its body and blocking the breathing openings. Products are available which contain neem oil alone or with azadirachtin. In tests involving the control of aphids, products containing both had a greater efficacy than either ingredient alone. Neem oil can also prevent the germination of some fungal spores such as powdery mildew.
Azadirachin, the active ingredient in many neem products, actually consists of more than 25 closely related compounds. This ingredient works in several ways. 1. It can act as a feeding deterrent against a number of insect pests. 2. It can disrupt the molting process so that the immature larvae do not develop into adults. 3. Some insect larvae may also be killed by direct contact with the spray. 4. Adult insects are normally not killed but mating may be disrupted. 5. Azadirachtin also has a repellent effect on certain insects and mites. Insect pests affected by azadirachtin include aphids, beetles, caterpillars, lace bugs, leafhoppers, leafminers, mealybugs, psyllids, thrips and whiteflies. Generally, neem will have less of a detrimental effect on beneficial insects (parasites/predators) compared to the broad spectrum pesticides.
Multiple applications of neem are generally recommended. Frequent
spraying is more effective because neem does not persist on plant surfaces.
Like other botanical insecticides, it is quickly broken down by sunlight and
washed away by rain. For smaller plants, neem seems to work well as a soil
drench; the product is absorbed by the roots and translocated systemically
throughout the plant.
Another product sold as a fertilizer is neem cakes. They are the residual seed meal remaining after extraction of oil from seeds.
