Neem oil is extracted from seeds of the neem tree which grows in India, Pakistan, and Bangladesh. It has been used for over two thousand years as a medicinal plant. As a biopesticide it has found great favor among organic gardeners. Neem oil is not the most cost-effective product for large crop areas, but a little bit goes a long way in the typical home garden. It should be used as part of an Integrated Pest Management strategy.
How does it kill pest insects and not beneficial insects? Neem oil must be ingested for it to work, so only critters eating plants that have been sprayed will be affected. It’s not an instant “knock-down” effect like synthetic broad-spectrum pesticides, but it only takes a few hours and is just as effective (if not more) at preventing harmful pest infestations. When a pest larva ingests neem oil with the plant material, a compound called “azadirachtin” starts to act on the larva’s hormonal system and prevents it from molting to the next stage, resulting in death.
Of critical importance is the fact that neem oil does not lead to the development of resistance in future generations of pests, because it acts on the hormonal system, not the digestive or nervous system. This is a great advantage over many synthetic pesticides which do cause resistance and necessitate the need for more and more toxic chemicals.
Neem oil also acts as a feeding deterrent and an oviposition deterrent for some pest species. When they “taste” it, the compounds in neem oil reverse the signals that tell the pest to feed, so the pest moves on and eventually starves. Adult females of many pest species will avoid laying eggs on plants sprayed with neem oil, significantly reducing the likelihood of infestation.
Neem oil has a pungent but not unpleasant odor, somewhat like citrus or garlic. It is normally sold as a concentrate to be mixed in a spray bottle. It can be slightly irritating to the skin, so take precautions, and don’t let the spray blow back in your face. Spray in early morning or late evening because intense sun can burn leaves when they’re still wet with the oil.
Use neem oil when pests such as caterpillars are doing a lot of damage despite your best efforts at IPM techniques (including hand-picking pests like the tomato hornworm). It is also good for neutralizing or preventing diseases and viruses, if the plant is not too far gone.
Here is a list of plant-eating pests that can be controlled with neem oil:
- Orthoptera: grasshoppers, katydids, crickets etc.
- Coleoptera: wide range of beetles/weevils
- Hemiptera: leafhoppers, aphids, psyllids & some scale insects
- Lepidoptera: cutworms, borers & caterpillars
- Thysanoptera: thrips
- Diptera: Sciarid fly, fruit fly, buffalo/blow & march fly
- Heteroptera: sucking bugs – Green veggie bug, spotted fruit bug etc.
- Others: nematodes, snails, and also some fungi and pathogenic viruses
[via Real Farmacy]