Neem extract is widely used to ward off a variety of insects, such as Japanese beetles, aphids, and scale, and also diseases such as powdery mildew. It's derived from the seed of a tropical tree and not considered harmful to beneficial insects, pets, or humans. But even an organic pesticide can have adverse effects on the environment.
Researchers at National Polytechnic Institute in Mexico studied the effects of neem oil on bean growth and rhizobium-nodule formation. Beans, like many legumes, have a symbiotic relationship with these soil-dwelling bacteria. The bacteria form nodules on the bean plant roots, which, in essence, make nitrogen fertilizer for the bean plants by utilizing nitrogen from the atmosphere.
In the study, bean plants were treated with neem oil extract, mata-raton extract (a plant extract used as an insecticide), and a chemical insecticide. Researchers found that the number of nitrogen-fixing rhizobium nodules in the neem-treated plants was 2.1 times lower than bean plants treated with the chemical insecticide, and 1.6 times lower than the mata-raton extract-treated beans. Results suggest using neem extracts on legumes can inhibit this symbiotic bacterial relationship and should be avoided.
For more information on this research, go to: Journal of Agronomy for Sustainable Development.