The most effective method to use to prevent an aphid infestation is a dormant season application of horticultural oil. This will smother overwintering eggs and greatly reduce the problem in the spring. This type of oil is applied when plants are dormant during February or March on a day when temperatures are above freezing. Because oils act on contact, adequate coverage is essential for good control. Spray thoroughly from the branch tips back about 18 inches. This is where aphids overwinter. Always read and follow label instructions, where you will find more detailed information about how to apply the product. Another oil-based insecticide, horticulture oil, can be applied during the growing season on aphids that are out in the open, not curled up in leaves. In some cases, use of systemic insecticide is successful for limiting aphids on shade trees such as aspens. Typically, it needs to be applied as a soil drench or soil injection, and then watered in. Multiple applications may need over the growing season, as its activity is only useful for a month.

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