Neem oil works best under certain conditions: hot weather and immature, light insect populations. It doesn't provide a quick fix solution -- the oil must be reapplied consistently. Neem oil breaks down in sunlight after several days and easily washes off in rainy weather. "I think [farmers] want to see the kill of an insect within 24 hours of application, which should not be expected from neem," says Koul.
"Figure out what pests you are trying to control before spraying anything so you can purchase a spray that will be effective," advises Ellis. "The vast majority of insects people see are either beneficial or benign and don't need any control at all."
Contact on the skin may cause dermatitis or inflammation for certain individuals. A 2008 study by George Washington University investigators found that one patient developed the disease after applying the oil for alopecia.
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