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According to a recent Wall Street Journal report, entomologists at the USDA research lab in Phoenix, Arizona, were able to control garden pests with a cheap, nontoxic, homemade insecticide. The ingredients: dish detergent and oil!
Researchers found that a spray of soybean oil protected cotton from aphids and whiteflies, spider mites and beet army worms. It's also been used successfully on eggplants, carrots, lettuce, celery, watermelon, pepper and cucumbers. The mixture tends to burn the leaves of squash, cauliflower and red cabbage, though. Burning (phytotoxicity) might also occur with broccoli, collard, and squash; more research is needed.
How does it work? Mineral and plant oils are considered physical poisons that interfere with insect respiration. However, other modes of action may also occur. Soybean and cottonseed oil, among others, demonstrate repellant activity to several insect and mite species.
Here's the recipe:
Mix one tablespoon of dishwashing detergent with one cup of cooking oil (the detergent causes the oil to emulsify in the water). Mix one to two-and-a-half teaspoons of the mixture with each cup of water.
Spray on plants every 10 days with a pump sprayer. Morning and late-afternoon applications are better with regard to phytotoxicity.
According to the researchers, thirty cents worth of the homemade mix is as effective as a dollar's worth of store-bought insecticide. Not to mention safer!