Plant taxonomy classifies the trees from which Neem oil is derived as Azadirachta indica A. Juss.
What Is Neem Oil?:
Neem oil is pressed out of the seeds obtained from neem trees. In addition to its use as an organic insecticide spray, it has been used medicinally and in the cosmetics industry. One purveyor of the product (Dyna-Gro) explains how it works as an organic insecticide as follows: "It disrupts insects' hormonal balance so they die before they molt to the next life stage."
How Does It Work?:
The National Pesticide Information Center states, "Neem oil is made of many components. Azadirachtin is the most active. It reduces insect feeding and acts as a repellent. It also interferes with insect hormone systems, making it harder for insects to grow and lay eggs."
According to the EPA, "Azadirachtin acts in the following ways: It deters certain insects, such as locusts, from feeding and it interferes with the normal life cycle of insects, including feeding, molting, mating, and egg laying." Just how much Azadirachtin a product you buy off the shelf contains is not, however, always readily apparent (the label may refer to "other ingredients," then fail to specify).
Organic Insecticide: Pests Killed or Repelled:
This organic insecticide kills some pests (after they've eaten leaves sprayed with it), while it repels others with its strong smell. The product is used to control many pests, including whitefly, aphids, Japanese beetles, moth larvae, scale and spider mites. Because it kills mites -- which aren't insects but, instead, related to spiders and ticks -- it is listed as a "miticide." Sprays containing clarified hydrophobic extract of neem oil are also used as fungicides against rust, black spot, mildew, leaf spot, scab, anthracnose, blight and botrytis.
Organic Insecticide: How to Apply:
According to the people who sell the specific product that I tested ("70% Neem Oil"), "Mix 70% Neem Oil at the rate of 2 tablespoons (1 ounce) per gallon of water. Thoroughly mix solution and spray all plant surfaces (including undersides of leaves) until completely wet." Garden Gorilla puts out the easiest garden sprayer to use that I've tested so far.
Organic Insecticide: When to Apply:
When applied as a preventative, neem oil should be applied on a 7 to 14 day schedule, say the manufacturers of 70% Neem Oil. To control a pest or disease already present, they recommend an application of neem oil on a 7 day schedule.
Benefits of Neem Oil for Pest Control:
Besides being an organic insecticide, using this product allows you to target pests, specifically, as opposed to beneficial insects (e.g., bees and lady bugs). By definition, "pests" are the insects eating your plants, and this product, properly applied, kills an insect only if it ingests the sprayed foliage (bees and lady bugs don't eat plant leaves).
Origin of the Name:
Neem oil and the tree from which it is derived are so called from the from Sanskrit, nimba.
A Success Story -- And a Failure:
One day in May this year, I noticed that the ninebark shrub I had just planted the prior fall was covered with aphids. I sprayed 70% Neem Oil on the foliage (following the mixing directions cited above) every 7 days for 3 weeks, after which period I found no more aphids on the plant.
In July, however, I had less success using the same product to fight a pest invasion. Upon finding whitefly on my black hollyhock, I began treating the plant with Neem oil. I can't honestly say that the organic herbicide was of much help in dealing with my whitefly problem.