|Squash bug eggs on the underside of a leaf|
Neem oil (Azadirachta Indica) is pressed from the fruit and seeds of the Neem tree, which is native to India. It has a plethora of traditional medicinal uses as well as farming applications. In traditional medicine, including Ayurvedic medicine, neem has been used to treat inflammation, fever, skin irritations, eczema, psoriasis, ring worm, head lice, leprosy, athlete's foot, and even as a spermicide. It is both antibacterial and anti fungal and has immunostimulant properties. It is generally recommended that neem NOT be taken internally. For external uses it is best to dilute neem in a carrier oil of some kind as it is quite potent (and also doesn't smell great!). I'm not going to expound on medicinal uses in this post so if you're interested in using it medicinally please do your research and make sure you are using/mixing correctly.
Onto farming applications. Neem is a wonderful companion for organic farmers. It has been proven as an organic biopesticide. While it is harmful to many "pest" bugs, such as squash bugs, mealy bugs, aphids, Japanese beetles, cockroaches, flies, thrips, mites, termites, mosquitos, and the list goes on, it is NOT HARMFUL to beneficial insects such as butterflies and honeybees!
The fatty acid composition of neem oil can vary widely depending on the method of processing. I have heard reports of leaf burn almost exclusively from people who bought neem pesticide sprays from their garden center. We have not experienced this, but formulate our homemade neem spray using neem oil intended for medicinal use in humans. We use the Now Solutions brand pure and natural neem oil. If you're concerned about leaf burn, my neighbor recommended only spraying the undersides of the leaves (this is where the bugs lay their eggs - squash bugs anyway). We've been spraying everywhere, just to cover all the bases. :)
Here's our recipe:
5-10ml neem oil (5 is probably fine as a preventative measure, 10 if you already have an infestation)
1-2ml dish soap (just a squirt, I never measure)
26 oz. warm water
Mix in a spray bottle and you're ready to go!
Sources for this post:
Esoteric Oils - Uses of Neem Oil
Wikipedia - Neem Oil