Using spray oils for insect control

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ANN ARBOR, Mich. - For pests that overwinter as eggs, spray oils are one of the most important sprays you can apply to fruit trees.

Reports from throughout the Midwest have indicated an increase in scale populations in apple orchards. This very likely is at least partially due to a decline in the use of oil.

Although there are more miticides to choose from than ever before, growers should think twice about abandoning the early season oil sprays, according to Gary Thornton, Michigan State University fruit pest management agent.

Why use oils? Horticultural spray oils, commonly referred to as “dormant oils,” superior oils, or 70-second oils, can be effective control materials for insects that occur early in the season.

Insects such as rosy apple aphid and European red mites overwinter in the egg stage and will be suffocated and killed when oils are applied.

Oils fit well into an integrated pest management program. Since they are applied when little else is active, they will have minimal effect on mite predators or other beneficials and are a good resistance management strategy, Thornton said.

An application of oil at the right time will often lead to fewer applications of other insecticides or miticides later in the season.

What, when to use. Several highly refined spray oils can be used in tree fruit without fear of phytotoxicity.

Paraffinic spray oils and are appropriate for insect control. These oils have a high percentage of unsulfonated residues, which make them safe for tender leaves.

Vegetable oils and mineral oils have not shown themselves to be effective pest control agents and should be avoided.

For European red mite control, oils are best applied at the tight cluster to early pink stage. Blossom damage can occur when oils are applied at full pink in warm weather, Thornton said.

Use correctly. Oils work by suffocation and work because they can get into cracks and crevices where insect eggs are laid. To get the best effect from oil you must have lots of water on the tree in order to get optimal coverage.

No one likes to spray dilute sprays, but for oil treatments, dilute applications will provide the best control, Thornton said.

“Keep in mind that at tight cluster time a dilute application takes roughly 60 percent of the full dilute rate of water. For example, a 400-gallon equivalent tree will be sprayed dilute with roughly 240 gallons of water at tight cluster,” he said.

Precautions. Oils are highly refined petroleum products and, as such, will have some compatibility problems.

Oils should not be used within 14 days of a pesticide containing sulfur. If applied closer than 14 days, phytotoxicity can occur with symptoms such as leaf tip burn showing up on trees.

Many fungicides are compatible with oil. Be sure to check the pesticide label if you are unsure of the compatibility with oil.

Temperature also plays a role in the safety of oil sprays to the tree.

Oil sprays will make plant tissue more susceptible to cold injury and should be applied when temperatures are going to be above 40 degrees.

Leaf tip burn and blossom damage can occur when there are sub-freezing temperatures after oils sprayed at half-inch or later.

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