A simple soap-oil spray formula recommended by the University of Hawaii College of Tropical Agriculture and Human Resources (CTAHR) is useful for controlling aphids, mealybugs, whiteflies, scale and citrus blackfly.
Aphids are very small insects with soft bodies that live off the sap in plant leaves. They’re black, green, brown and pink or absent of color. There are over 4,000 species of aphids. They travel by wind or as stowaways on plants and animals. Most plants tolerate light aphid feeding, but as the aphid population grows, infested leaves wilt and turn yellow. A single aphid can produce tens of thousands of offspring in a few weeks.
Mealybugs are soft-bodied wingless insects that appear as white cottony masses on the leaves, stems and fruits of plants and trees. They feed by inserting long sucking mouthparts called “stylets” into plants and drawing sap from the tissue. Mealybugs are a common greenhouse pest that affect ornamentals, houseplants, avocados and fruits.
Mealybugs and other sucking insects create honeydew which encourages the growth of sooty mold.
With whitefly infestation you may see a spiraling pattern of eggs, oval-shaped nymphs, and adult whiteflies together on the same plant. Adults look like tiny white moths. Nymphs produce a white, waxy filament-like material. As with mealybugs, whiteflies feed with sucking mouthparts and excrete honeydew which leads to growth of sooty mold.
Scales are another type of sucking insect that insert tiny, straw-like mouthparts into bark, fruit, or leaves on trees and shrubs. Some scale can seriously damage the host while other species do no apparent damage even when abundant. The presence of scale can be missed as these insects don’t resemble other bugs. Scale insects, mealybugs and aphids are members of the same order (Homoptera). A fellow gardener here in Hawaii says scale is difficult to spot on his citrus trees as the insects are essentially the same color as the green stems and leaf undersides they inhabit.
The citrus blackfly is a highly harmful insect that can reduce a citrus tree to non-productivity more quickly than any other known citrus pest. They change color as they mature. Adult black citrus aphids are black and shiny, and like other aphids, they can be winged or wingless. Nymphs are reddish-brown. Females lay eggs in spirals. The greatest harm is caused by the sucking of plant sap, which removes water and nutrients.
Soap Oil Concentrate Recipe by CTAHR
A simple soap-oil spray formula can be used to control aphids, mealybugs, whiteflies, scale and citrus blackfly. Combine 1 tablespoon of mild dishwashing liquid such as Ivory, Joy, or Dr. Bronner’s Castile soap (not Dawn) with 1 cup of vegetable oil such as peanut, safflower, corn, soybean or sunflower. Dilute this formula with 1 cup of water for every 1-2 teaspoons of concentrate and shake well. Spray plants thoroughly in the morning or late afternoon – especially the undersides of leaves. Spray once a week for two or three weeks.
Fellow gardeners mentioned benefits of replacing 1/4th of the vegetable oil with the more effective but more expensive Neem oil. If Neem oil is incorporated into the recipe it may be best to spray only in the late afternoons or evenings to avoid harming bees. There are differences of opinion on the impact of Neem oil on bees, but there is agreement that it’s best to avoid spraying soap-oil solutions when bees are most active. Using hot water when diluting your soap-oil concentrate can improve dispersion. Clean your sprayer after use to avoid clogging.
Special thanks to Charles Berube, Ty McDonald, Nan Rhodes and Leslie Schmitz of Kailua-Kona for their help improving this article. All errors are mine.