For Emerald Ash Borer, the target is the adults as they nibble on leaves and the young larvae as they feed in the cambium just under the bark. In spring, the trick is to apply the treatment before this happens, so the tree has time to carry the product throughout itself. “Can you apply treatments in the fall? Yes,” says Dr. Whitney Cranshaw, Professor of Entomology with Colorado State University, “but it’s just puddling around the roots not being taken up. The label on the pesticide allows fall treatment but it doesn’t mean it’s best. That has to happen when trees first begin to turn color.”

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