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Syrphid fly larvae attack woolly apple aphids.

Syrphid fly larvae attack woolly apple aphids.

The woolly apple aphid overwinters as a nymph on the roots of apple trees, but can also overwinter on the aboveground part of the tree in protected areas on the trunk or main limbs.

In the spring, overwintering aphids produce live young that migrate up and down the tree. Adult aphids are a reddish-brown to purple color but are usually concealed beneath a white, cottonlike ­substance secreted from the aphid’s abdomen.

Galls form on the plant where aphid colonies feed. Roots of infested trees have large, abnormal swellings. Continued feeding can kill roots and affect tree growth or even kill young trees.

Since syrphid fly larvae are good predators of woolly apple aphids, Dr. Elizabeth Beers, entomologist at Washington State University, and her colleagues compared the attractiveness of six common annual plants to syrphid flies. Alyssum was the most attractive, followed by mustard, marigold, buckwheat, ­zinnia, and cosmos.

Alyssum has other qualities that make it a good candidate for attracting syrphids into orchards. It is low growing, with an average height of less than nine inches, compared with 32 inches for mustard, and it survives well in hot, dry weather, but can tolerate some shade.