Aphids – Wooly Alder

Q: I have an elm tree that is covered with white stringy looking objects that move slowly. There are so many that I’m afraid they will hurt the tree. Do you know what they are?

A: Many owners of elms and maples are startled to find a tremendous population explosion of small, white, wooly insects covering leaves and stems of their trees in May.

These are wooly alder aphids. They are common each spring. Profuse amounts of sticky liquid fall from the insects crawling in a tree, covering walks, cars and decks below. The aphids suck sap from the tree and excrete ‘honeydew’. Surfaces on which the honeydew falls may turn black as sooty mold grows on the sweet liquid.

If they are really a problem for you, blast them out of small trees with a hose or spray with insecticidal soap, following label directions.

Larger trees can be treated with a drench that contains imidacloprid (Bayer Advanced Tree & Shrub Insecticide). Irrigate the tree regularly after drenching to make sure the chemical is absorbed by the roots.

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