What Is Neem Oil and Where Does It Come From?
Neem oil is pressed out of the seeds obtained from neem trees. The botanical name for this tree is Azadirachta indica A. Juss. The tree is a broadleaf evergreen that is indigenous to India. In addition to its use as an organic insecticide spray, this oil has been used medicinally and in the cosmetics industry.
How It Works, How and When to Apply It
One seller 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."
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).
The people who sell the specific product that I tested (which is called "70% Neem Oil"), supply the following instructions for application:
Mix the Neem oil at the rate of 2 tablespoons (1 ounce) per gallon of water. Mix the solution thoroughly.
Spray all plant surfaces (including undersides of leaves) until completely wet.
I used to the Garden Gorilla sprayer to apply Neem oil on my plants. This company puts out the easiest garden sprayer to use that I have tested so far.
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 on a 7 day schedule.
Pests Killed or Repelled by This Organic Insecticide
This organic insecticide kills some pests (after they have 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 are not 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.
Benefits of Neem Oil for Pest Control, and the Origin of the Name
Besides being an organic insecticide, using this product allows you to target pests, specifically, as opposed to beneficial insects (for example, 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 do not eat plant leaves).
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.