The February 12, 1991 issue of the Wall Street Journal reports that the U. S. Department of Agriculture is recommending that home gardeners use a cooking oil spray to control aphids, white flies and spider mites.
“Mix one tablespoon of dish washing detergent to one cup of oil (soybean oil was used in the trials, but the implication is that other kinds are suitable), then mix between 1-2½ teaspoons of the oil-plus-detergent with one cup of water. The detergent causes the spray to emulsify in the water so that it can be sprayed.
Spray directly on plants every 10 days. Eggplants, carrots, lettuce, celery, watermelons, peppers and cucumbers have been successfully protected by the spray, but it burns leaves of squash, cauliflower and red cabbage leaves.
Researchers claim that the oil spray is only about one-third as costly as commercial pesticides with equivalent effectiveness.” [Thanks to both HortIdeas and Central American Development Foundation for referring us to the article. The latter added a note, “Do not use palm or coconut oil because if not used promptly they will gel within 24 hours.]