Dormant sprays control harmful insects

It’s time to catch sleeping insects before they emerge and chew their way through the garden.

Dormant or horticultural oils are a highly refined petroleum-based mineral oil or, in a few products, vegetable oil that is sprayed on woody plant materials to kill overwintering insects and their eggs.

“Basically the oil works through contact,” says Ben Kappen of Northwest Plant Health Care. “It smothers either the insect that is (hiding) in the bark cervices or on the bark or its eggs and egg sacs.” The oil plugs up the insects’ breathing mechanisms, and they suffocate.

When they were first used in the late 1880s, the oils were so crudely refined that they regularly would damage plants if the plant had broken dormancy. Today, these oils are so highly refined that they can be used with caution and in different formulations throughout the growing season.

In our area, horticultural oils are particularly effective on soft-scale insects like lecanium, cottony maple, juniper, pine needle and oyster shell, as well as aphids and spider mites.

How to use

Apply dormant oils before deciduous woody plant buds show any sign of green. New green tissue can be easily damaged if the spray is put on after this.

Keep in mind that plants will break bud at different times over several weeks. If you miss this window, read the label directions carefully for applying it after bud break.

Dormant sprays should be applied on a dry day when the temperatures are going to be above 35 degrees for three to four hours after application. The spray needs to dry before freezing temperatures return. Frozen spray can damage buds.

Spray the trunks and the large scaffold limbs, paying special attention to the unions where the branches meet the trunk.

“You are spraying to almost the runoff point,” says Kappen. “Because (the oil) doesn’t have any residual or systemic qualities, if you don’t contact the insect directly, it’s not going to do any good.”

The only way to do that is soak the plant. For some insects it is also necessary to spray branch tips and conifer needles where these bugs prefer to hide.

Hide and seek

When spraying dense conifers like arborvitae and Alberta spruce, be sure to get down inside the plant to get hidden insect populations. If you don’t, the insects inside the tree will cause damage when they come out.

If you have large trees, it may be wiser to hire a spray service that has the necessary equipment to reach higher branches.

Use with caution

A word of caution: Do not spray Colorado blue spruce or other sensitive evergreens with a blue-green color. The spray permanently removes the waxy covering on the needles that creates the blue cast to the otherwise green needles.

Don’t spray plants that appeared stressed or were hit hard by insects last year, as they tend to be sensitive to dormant sprays.