Dormant oils: how to match spray with pest

Once upon a time, gardeners relied on winter-applied products called "dormant oils," and you may have heard longtime gardeners mention these mysterious things. So, you ask, what are they?

Dormant oils are also called horticultural oils. The term refers to winter-applied sprays for insect pests and diseases, used in cold weather before foliage begins to leaf out. (The trees — not the oils — are dormant.)

Horticultural oils are actually a kind of pesticide. They have been used since at least the 19th century to kill insects while they are wintering. They are manufactured from crude petroleum. Traditionally, these were used in winter during the tree's dormancy because the oils caused damage to leaf, bloom and fruit if applied when the tree was in active growth.

Many of today's horticultural spray oils are labeled "year-round," because they are super-refined and won't damage plant leaves. As a result, fruit growers can use them in summer without harming their trees, and some super-refined oils are even considered safe for use on house plants.

I never recommend using a pesticide without identifying the pest, the need for the treatment, and evaluating the least-toxic way of dealing with the problem. A dormant spray isn't meant to be an all-purpose winter splashing of pesticide around the garden. It's chosen for specific pests or disease problems. If you do not grow fruit trees, or if your trees have had no specific aphid problems, your home garden probably does not need dormant oil sprays.

But for trees that do need it, these oils are often used in the dormant season to control insect pests such as scale and certain aphids. (The aphids overwinter as eggs, and the spray seems to smother the eggs, preventing spring hatching. Oils are seldom effective against woolly apple aphids protected by their exterior coatings.) They may also inhibit egg-laying by pests like pear psylla.

The dormant spray used on fruit trees is often sold as superior-type oil, and it may be mixed with lime sulfur depending on the pest to be controlled. (Lime sulfur works against disease problems.) It's sprayed thoroughly to give good coverage on the trunk, branches, small limbs and shoots. Sprays containing copper may be used in the fall on trees such as flowering and fruiting cherry that are diagnosed with bacterial canker.

Any pesticide spraying must be done when temperatures are above freezing. And because dormant sprays are generally applied early in the season, they tend to be less disruptive to beneficial insect predators and parasites, which aren't in active life stages in midwinter. I'm not certain about their effect, however, on overwintering ladybug larvae tucked into branches.

Remember: Dormant oils, even when properly used, don't do it all.

What often confounds gardeners is that dormant oil sprays do not control many common fruit-tree insect and disease problems that appear on a tree's leaf, bloom and fruit.

Ubiquitous difficulties such as apple maggot, codling moth, apple scab and brown rot must be dealt with during the early and middle growing season.

Mary Robson is area horticulture agent for Washington State University/King County Cooperative Extension. She shares gardening tips every Wednesday. Her e-mail: gardeningtips@seattletimes.com.