Courtesy photo

Is it time to spray oil on my fruit trees?

Horticultural oils are an important tool in fruit tree care. Now isn’t the time to do that first routine spray. We’ll get to the best timing in a bit, but first, let’s take a look at what a delayed-dormant oil spray will and won’t do for your fruit trees.

Horticultural oils are highly refined petroleum products or plant-based oils used to reduce pest problems. They work by coating and suffocating insect eggs; soft-bodied adult pests like aphids, spider mites, thrips, mealybugs and whiteflies; and the larvae of some other insects. Oil sprays are also safer for beneficial insects when properly applied. Oil sprays are sometimes used in managing some fungal diseases by interfering with fungal growth.

There are a couple of things that horticultural oils won’t do. A horticultural oil spray won’t completely eliminate insect problems on your woody plants, and won’t have any effect on insects and mites that arrive on your tree after you’ve sprayed and the oil has dried. Why is this?

Your oil spray must make direct contact with your target pests, so any eggs, soft-bodied adults, or larvae that are protected in deep bark crevasses or under tight bud scales, or simply in areas that you missed and may not be exposed and won’t be affected by the spray.

Courtesy Meredith Seaver

Because the oil spray doesn’t leave a toxic residue and must directly contact the pest to be effective, insects and mites that happen to pass through the sprayed area after it has dried won’t be affected either.

All these “survivors” can continue to develop, mature, and lay eggs, building up the pest population on your trees and shrubs. A delayed-dormant spray can reduce the pest population, but diligent scouting and monitoring is important for continued good pest management through the season.

So, what about the best timing for those first horticultural oil sprays of the season? For most home orchards, the delayed-dormant oil spray at a 2% dilution (5 tablespoons oil to 1 gallon of water) is applied when trees are no longer dormant but before leaf-out and bloom. Watch for bud swelling and a tiny tip of green on the buds. At this time, overwintering pests are more active and eggs are closer to hatching, making them more susceptible to an oil spray.

A dormant oil spray may not be needed if you didn’t have problems with aphids, spider mites, and similar pests last year. If you’ve had chronic pest problems that you haven’t been able to control with horticultural oil, you can add an insecticide to your oil.

If you need to apply a delayed-dormant oil spray this year, keep these guidelines and cautions in mind.

• Some ornamental trees and shrubs, including many evergreens, are very sensitive to oil sprays. Read your product label carefully to make sure it’s safe for the plants you need to spray.
• Apply your delayed-dormant spray on a calm, clear, sunny day when temperatures are above freezing, ideally no colder than 40 degrees.
• Don’t apply an oil spray if temperatures will dip below freezing within 24 hours after your application.
• Thoroughly cover the canopy of the tree or shrub, making sure you get good coverage where cracks and crevasses in the bark may be shielding pests.
• If you are spraying apple or pear trees, flowering pear trees or crabapple trees that had problems with fire blight last year, you can mix copper with the oil to help control this serious disease of apples, pears and their ornamental relatives.
• Don’t mix oil and sulfur or apply and oil spray and a sulfur spray within two weeks of each other

Courtesy Meredith Seaver

Horticultural oil can also be applied after your trees have leafed out, to control soft-bodied pests that show up during the growing season, but there are different guidelines you’ll need to follow to avoid damaging your trees and shrubs.

• The dilution rate is different for oil sprays once your tree or shrub has leafed out. Mix your spray at a 1% dilution instead of at 2%.
• Don’t apply an oil spray if the daytime temperature will exceed 85 degrees.
• Avoid spraying drought-stressed trees and shrubs, as this can make it more difficult for leaves to regulate moisture loss.

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