What is insecticidal soap?

All soaps are made of salts and fatty acids, but not all soaps are good for killing insects. Some soaps can be highly destructive to plants and are useful as herbicides.

Insecticidal soaps are specifically formulated to kill insect pests while having a few adverse effects to people, plants and the environment.

Insecticidal soaps work on contact in two ways. First, they wash away the protective coating (the cuticle) on the surface of the insect’s body. Once inside, the soap will break the cell membrane and the cell will die.

Insecticidal soaps are most effective on small, soft-bodied insects such as aphids, spider mites, thrips, whitefly and mealybugs. They are only somewhat effective in controlling larger insects such as caterpillars and leafhoppers.

Insecticidal soaps are generally non-toxic to birds, mammals and people. Soap may be applied to food and non-food plants and may be used the day of harvest if necessary. Soaps can be used with some insecticides and have been shown to improve their performance. Be sure to check labels for compatibility.

There can be drawbacks to the use of insecticidal soaps.

Insects must come in contact with the soap before it dries in order to be controlled. Therefore, thorough coverage to both tops and bottoms of leaves is essential.

Do not apply insecticidal soaps directly to water or use near a water source.

Multiple applications are needed to control most insects.

Hard water is not effective for mixing soap sprays, so use softened or distilled water for best results.

Tender young growth of evergreens and shrubs in the spring can be sensitive to insecticidal soaps. Don’t apply until hardened off.

The use of horticultural oil is another approach to control unwanted insects.

For a century, oils have been used to control unwanted insects, fungi and mites on fruit trees and woody ornamentals.

Today’s horticultural oils are lighter and more highly-refined than dormant oils of the past. They may be made from petroleum products or plants.

Often called ultra-fine or summer oils, they can be applied to actively growing plants and plants in full leaf throughout the growing season.

Horticultural oils block the insect’s breathing holes, causing it to suffocate. Oil interferes with the cell metabolism in soft-bodied insects like aphids, scale, lacebug, woolly adelgid, mealybugs, whitefly and some caterpillars.

Oils can disrupt the feeding patterns of certain disease-carrying insects and can even kill overwintering insect eggs and fungal spores.

Horticultural oils are very safe to use and pose minimal safety threats to the applicator and the environment when used according to label directions. Insects rarely develop resistance to oils.

Oils tend to evaporate quickly leaving little toxic residue. Oils are inexpensive, easy to mix, and need no special spray equipment.

There can be drawbacks to the use of horticultural oils.

Horticultural oils have no residual effects, so the target pests must be present to be controlled and coverage must be thorough.

Some plants are sensitive to oils.

Plants should not be treated when wilting or under drought stress or under excessive heat and humidity conditions.

Horticultural oils are nonselective; therefore, beneficial insects may be killed along with pests.

Horticultural oils are toxic to fish.

Here are some tips for plants’ pest control.

Know your pest before you treat.

Read the label. Is your target pest and plant listed? Are you mixing at the proper rate?

Thorough coverage is a must.

Avoid treating plants in direct sun.

Don’t treat moisture or drought stressed plants; you may burn the leaves.

Avoid treatment when the relative humidity is 90 percent or above. Apply when temperatures are between 40- and 90-degrees Fahrenheit, or as stated on the label.

Treating bedding plants with soaps or oils while they are in flower may result in spotting of flowers.

Consider testing a few plants before full-scale application.

Native plants presentation:

Barbara Beck, Richland Township, Bucks County, Bird Town Leader, will present about the relationship between native plants, birds and insects, and Maggie Wallner, Conservation Ecologist, Lehigh County Conservation District, will present about recommended plants and trees for high-water table areas at the Salisbury Township Environmental Advisory Council meeting, 7 p.m. June 19, meeting room, Salisbury Township Municipal Building, 2900 S. Pike Avenue. The meeting is open to the public.

“Growing Green” is contributed by Diane Dorn, Lehigh County Extension Office Staff, and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613.