Neem oil is an organic pesticide and insect repellent that is extracted from the fruit and seeds of the neem tree (Azadirachta indica). It can be used to control a range of insects including sap suckers, caterpillars, whitefly, leaf miners and fungus gnats. As with all pesticides, unless you're spraying it on foliage and branches etc. to repel insects, it should only be sprayed directly on pests as it can harm beneficial insects if sprayed directly on them. It should not be used on anything that you plan to eat as animal studies have shown that it may cause infertility and has the potential to induce abortion (prolonged exposure). It is irritating if inhaled or if it comes into contact with eyes or skin.
Neem oil is available commercially or you can make your own if you have access to a neem tree. An infusion of neem leaves is also effective as an insecticide and as a pest repellent.