What is insecticidal soap?

All soaps are made of salts and fatty acids, but not all soaps are good for killing insects.

Insecticidal soaps are specifically formulated to kill insect pests while having 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 like caterpillars and leafhoppers.

Soap may be applied to food and non-food plants and may be used the day of harvest.

As soon as the leaves have dried, beneficial insects can be reintroduced.

Soaps can be used with some insecticides and have been shown to improve their performance.

Be sure to check labels for compatibility.

There are some drawbacks.

Insects must come in contact with the soap before it dries to be controlled.

Therefore, thorough coverage to 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; 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.

What is horticultural oil?

For centuries, various 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 refined than dormant oils of the past.

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 suffocation.

Oil interferes with cell metabolism in soft-bodied insects like aphids, scale, lace bug, 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 safe to use and pose minimal safety threats to the applicator and 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 are drawbacks.

Horticultural oils have no residual effects, so the target pest 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.

Tips for Using Insecticidal Soaps and Horticultural Oils:

Know your pest before your treat.

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

Thorough coverage is a must.

Avoid treating plants in direct sun.

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

Avoid treatment when relative humidity is 90 percent or above.

Apply when temperatures are between 40 and 90 degrees or as stated on the label.

Treating bedding plants with soaps or oils while in flower may result in spotting of flowers; test a few plants before full-scale application.

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