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. They work in several ways. 1. as a feeding deterrent against a number of
insect pests. 2. to disrupt the molting process so the immature larvae do not
develop into adults. 3. some insect
larvae are killed by direct contact with the spray, 4. adult
insects are normally not killed but mating may be disrupted and 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.
