Limit Pests With Horticultural Oils

From The Fresno Bee, Dec. 17, 2009, by Elinor Teague:

Horticultural oils are among the gardener's best nontoxic insect-control weapons. An annual application of winter-weight horticultural oil in late fall or early winter, after deciduous trees and roses have dropped their leaves and entered dormancy, can significantly reduce insect problems next summer. The oils suffocate soft-bodied aphids, immature whiteflies, some immature scale insects, mites, caterpillars and caterpillar pupae, and also smother some overwintering insect eggs.

Most of the horticultural oils available are either paraffin or petroleum-based. Newer products use cottonseed oil as a base. These oils kill the insects or smother the eggs on contact; after the oils have dried, they have no residual effect that could harm beneficial insects or pets, aquatic animals and humans. Spraying in winter further reduces the chance that bees or other valuable pollinators could be affected.

You'll need to use some sort of pressurized sprayer to get the best results. The small sprayer tanks with an adjustable nozzle work just fine. These are available at garden centers, nurseries and home supply stores.

The oils should not be applied if temperatures are below freezing, just before or after a rainstorm, or during foggy conditions.

Wait to spray until all the leaves have fallen from the trees and bushes; insects often hide themselves and lay their eggs on the undersides of the leaves and in leaf axils. They also hide and lay eggs in bark and branch crevices. A thorough drenching of all the bark surfaces is necessary. Clean up any leaf and twig debris underneath the trees and then spray the soil as well.

Read labels to verify which trees can be treated with oils. Citrus and avocado trees are not sprayed in wither when they are not dormant.

Lime sulfur is an inorganic fungicide and miticide that can be applied in winter on roses and fruit trees to control for powdery mildew, black spot, apple scab and other fungal problems.

Lime sulfur also reduces soil pH levels (very important in our area which has very alkaline, very high pH soil and water levels).

Lime sulfur can be mixed with winter-weight horticultural oils--one of the very rare instances that chemical can be mixed. Premixed formulations are available or you can make your own mix, following directions carefully.