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 . Due to its insect growth regulating properties, it is most effective against the immature stages of insects. 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.