Neem oil works as organic insecticide

Horticultural oils come from several sources and have long been used to control plant pests and diseases. All are considered organic pesticides and most are petroleum based but some come from vegetable sources such as soybeans, cottonseed or the neem tree.

Generally, horticultural oils are used for control of pests such as scale, aphids, whiteflies, spider mites and their eggs. They also may prevent some fungal infections, especially powdery mildew. Oils are considered safe to humans and have a low impact on beneficial pollinators such as bees. Oils kill susceptible insects mainly by smothering them but probably have other insecticidal effects as well.

In the past, petroleum-based oils came only as thicker, dormant winter-use preparations and thinner summer-use ones. Now they are highly refined and some may be used for winter and summer treatments, but in different concentrations.

Neem oil is obtained from neem trees grown in India and Southeast Asia. Neem products have been used for hundreds of years in cosmetics, medicines and as pesticides. The crude neem oil extracts are obtained from the tree's seeds and have a wide range of biological effects.

There are two main ingredients found in neem oil that are being marketed as pesticides - one a chemical termed azadirachtin and another termed "clarified hydrophobic extract of neem oil." Each is sold separately under several brand names. Azadirachtin preparations are difficult to find locally but are available on the Internet. Clarified neem oil is found on shelves of most local gardening centers and does not contain significant azadirachtin.

Azadarachtin products help control a wide range of insects by interfering with molting, reproduction and feeding behavior. It must be eaten by insects to be effective. The chemical is labeled organic and is non-toxic to animals and most beneficial insects.

Neem oil, like petroleum-based horticultural oils, coat and smother susceptible insects when they are contacted by the spray. It also repels some insects and prevents feeding. In addition, neem oil is labeled to prevent several fungal diseases such as black spot on roses, powdery mildew and various rusts. Neem oil has an excellent profile as an organic pesticide and is always a smart choice when a horticultural oil is indicated.

All of the oils, even the newer highly refined ones, may be damaging to certain plants. Their labels will list plants that should not be sprayed and should be followed carefully.