The neem tree (Azadirachta indica) is native to Southeast Asia and India where it is a fast growing ornamental shade tree. For thousands of years, Indian farmers have been aware of the insecticidal properties of the neem tree. Its branches were hung in granaries to protect stored grain from insect attack. Historically, neem has also been used in India for both cosmetic and medicinal purposes. For example, oil extracted from the seed is used in soap, wax and lubricants and its twigs have been traditionally used as a toothbrush. Since the 1970s, scientists in Europe and the United States became interested in neem because of its insecticidal properties plus its low toxicity to mammals.
Substances with pesticidal properties are found in all parts of the neem tree. However, the greatest concentrations of these substances are found in the seed. Azadirachtin, the active ingredient in many pesticides currently available, is extracted from the seed kernels. Azadirachtin consists of more than 25 different but closely related compounds. In another extraction process, neem oil is extracted from the seed kernel.
Mechanisms of Action
Azadirachtin can act as a feeding deterrent against a number of insect pests including beetles. It reduces the level of the insect hormone Ecdysome by disrupting the insect's molting process so that the immature larvae cannot develop into adults. After treatment with neem-based pesticides, you may see insects with crippled, distorted wings. Or the immature larvae and nymphs remain in an immature stage and then die. Some soft-skinned insect larvae may be killed by direct contact with the spray. Adults are not killed by the growth regulating properties of azadirachtin but mating and sexual communication may be disrupted which results in reduced fecundity.
Neem oil works in a number of different ways. The oil forms a coating on the insect's body, blocking the breathing openings and suffocating the insect. It also has a repellent effect on certain insects and mites. Neem oil prevents the germination and penetration of some fungal spores. In one study, researchers discovered that a one percent neem oil treatment was effective in managing powdery mildew on hydrangeas, lilacs and phlox.
More than 60 insect pests may be affected by azadirachtin including aphids, beetles, caterpillars, lace bugs, leafhoppers, leafminers, mealybugs, psyllids, thrips and whiteflies. (Read the label of the specific product you are using before application). Due to its insect growth regulating properties, it is most effective against the immature stages of insects. For example, the immature larvae of many species in the lepidoptera family (moths and butterflies) are particularly sensitive to azadirachtin. Neem products may be registered for use on certain fruits, herbs and vegetables in addition to ornamentals. For edible crops, some neem-based products may be used up to the day of harvest.
Benefits of Neem
Neem does not persist in the environment and is degraded by ultraviolet light and rainfall. Many neem-products tend to have low mammalian toxicity. Because many neem products degrade quickly, they may have less of an effect on non-target beneficial organisms compared to some of the more traditional pesticides. Researchers found few effects on many insect predators including spiders, earwigs, and ants but flower fly larvae are very sensitive to neem sprays.
Neem-based materials are compatible with integrated pest management (IPM). Regular scouting helps to insure the early detection of immature stages of many insects. Azadirachtin is compatible with insecticidal soap, superior horticultural oil and Bacillus thuringiensis (Bt).
As mentioned previously, azadirachtin has a number of different modes of action. It is less likely that insects or pathogens will develop resistance to neem products compared to materials with a single mode of action.
Limitations of Neem
Botanical pesticides, such as neem, have limited persistence in the environment. Temperature, ultraviolet light, rainfall and other environmental factors can degrade neem. Repeated applications may be needed to achieve the desired result. Because it is an insect growth regulator, it is only effective against the immature stages of insects. Rescue treatments will not be effective. You will not see an immediate knockdown effect and insects may continue to feed. However, due to its repellant effects, insect feeding will be reduced. Phytotoxicity (damage to plants) may be of concern for certain formulations of neem-based products with flowers being particularly sensitive. Newly transplanted plants with limited root development or plants that are wilted or under stress should not be treated.
Leanne Pundt, University of Connecticut, Extension Educator, Commercial Horticulture