Neem oil is a derivative of the Asian evergreen tree. A natural steroid, it causes insects to lose their appetite and lose interest in laying eggs.
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Derived from the bark, leaves and fruit of a common Asian evergreen tree, neem or neem oil contains the active ingredient azadirachtin as well as several natural steroids. When applied to insects and the plants they are eating, neem oil causes many insects to feed less, grow more slowly, molt less, and lose interest in laying eggs.
Neem’s effects are strongest on young insects, particularly those that grow rapidly such as squash bugs, Colorado potato beetles and Mexican bean beetles. Neem can also provide good control of aphids and many small, leaf-eating caterpillars.
Neem is best used preventively, before pests become a serious problem. For example, if your garden is always plagued by squash bugs, begin spraying plants with neem every seven to 10 days as soon as the first adults are spotted. Egg laying and overall vigor will be reduced, but you will still need to handpick adults and remove egg clusters regularly to get good control. Employ the same strategy when using neem oil to manage leaf-eating beetles, spraying weekly when the first adults and egg clusters are seen. Neem is best applied in warm weather.
Shake neem products well before using to emulsify the neem oil. Using lukewarm water also helps to get a good mix. Thoroughly wet both sides of leaves as well as places where insects may be hiding. If honeybees are expected to visit the plants, exclude them with an old sheet or piece of row cover for 24 hours after applying neem.
Mix only as much concentrate as you will need. If not used within a few days, dispose of unused solution by spraying it on grapes, cucurbits, or other plants at risk for powdery mildew (neem has slight activity against this disease). Store neem products in their original containers on a high shelf, out of the reach of children and pets, in a dark place where temperatures will not go below 40 degrees Fahrenheit. Under good storage conditions, neem products last about two years.
More information on neem is available from Cornell University.