Woolly aphids (family Eriosomatidae) occur on many hardwood and coniferous tree and shrub species in Minnesota, including elm, silver maple, ash, alder, apple, pear, pine, spruce, hawthorn, and juneberry (Amelanchier). They are small (2-4 mm [1/8 inch] in length), pear shaped insects, and are often covered with white waxy strands. The wax filaments give this pest a fluffy, cottony appearance, as though they are covered with wool. Woolly aphids are similar to true aphids, which are very common in Minnesota (see Yard and Garden Brief Aphids On Deciduous Trees and Shrubs, E448A).

Woolly aphids generally have two hosts: a primary host on which they overwinter, and a secondary host on which they spend much of the summer. Most woolly aphids share a similar life cycle, although some details of the life cycle may vary among species. They usually overwinter as eggs laid in bark of their primary host. In spring, the eggs hatch into females which give birth without mating. Each female can produce hundreds of offspring, so populations can grow rapidly.

After one or two generations on the primary host, winged females are produced, and they fly to secondary hosts. They remain on secondary hosts for the remainder of the summer, producing several generations of young aphids. In late summer or early fall, a different group of winged females flies back to a primary host where they give birth to tiny male and female aphids that mate. Gravid females deposit a single large egg (or eggs) into protected locations in the bark and then die. While woolly aphids generally have two hosts, many species can sustain themselves on their secondary host alone (see below).

Woolly aphids feed by inserting needle-like mouthparts into plant tissue and withdrawing sap. They feed on leaves, buds, twigs, and bark, but can also feed on the roots. Symptoms of feeding include twisted and curled leaves, yellowed foliage, poor plant growth, low plant vigor, and branch dieback. Physical injury may result when large numbers of woolly aphids attack young trees or unhealthy, stressed trees. Fortunately, severe woolly aphid infestations only occur periodically and are generally kept in check by natural enemies. In addition to the physical damage to the plant, accumulations of wax and shed skins are sometimes very conspicuous signs on the leaves, twigs, and bark.

As a result of feeding on the sap, woolly aphids produce a sweet, sticky waste product called honeydew. It often coats leaves, bark, and objects beneath the tree, giving them a wet, shiny appearance. Honeydew can be difficult to remove (see Yard and Garden Brief Removing Sap and Honeydew from Cars, H454R). An unsightly black fungus called sooty mold sometimes grows on honeydew. Sooty mold does not damage the plant, but heavily infected leaves can have a lowered photosynthetic rate. Eliminating honeydew at its source is the best way to control sooty mold (see Yard and Garden Brief Sooty Mold, P440S).

The most commonly encountered woolly aphids in Minnesota are discussed below.

Woolly Elm Aphid: The woolly elm aphid produces curled leaves that contain white, cotton-like masses as well as abundant honeydew. This species has two generations each spring on its primary host, American elm. In early summer, winged females are produced that fly to their aphid's secondary host, juneberry, and give birth. All aphids produced on juneberry migrate to the roots where they feed for approximately two months. In the early fall, winged females are produced from the root colonies. They climb up the plant and fly back to American elm. On the elm, they produce males and females, which mate. Females lay a single large egg in a bark crack and then die. Woolly elm aphids seldom cause significant injury to mature hosts, although large numbers can damage shrubs that are less than 3 years old.

Woolly Alder Aphid: Also called the maple blight aphid, this species produces dense, white, woolly masses on the leaves and twigs of its primary host, silver maple, and secondary host, alder. The infested leaves may fold lengthwise and cover the aphids inside. This species overwinters as an egg or nymph on either host. The eggs hatch, and the nymphs become active when new leaves appear in the spring. Although not damaging to the tree, the white, waxy threads can be mistaken for a fungus, causing the homeowner to believe the tree is diseased.

Pine Bark Adelgid: This insect is an adelgid, not a true woolly aphid, and is included here because of its great similarities in appearance and damage. Pine bark adelgid occurs on white, Scots, and Austrian pine, and is recognized by the white, cottony, snowy appearance that it gives to twigs and bark. These insects differ from others in this publication by feeding on stem tissues in addition the needles. Adult females overwinter and produce eggs in the early spring. As many as five generations are recorded in the Lake States.

Other Woolly Aphids: Several other woolly aphid species occur on deciduous trees and shrubs in Minnesota. The ash leaf curl aphid, Prociphilus fraxinifolli, feeds on ash, while Prociphilus corrugatans infests juneberry leaves. The woolly elm bark aphid inhabits American and slippery elm limbs and trunks. All these woolly aphids cause damage as described above.

MANAGEMENT
Natural biological controls, such as lacewings, lady beetles, hover flies, and parasitic wasps, normally keep woolly aphid populations below numbers that heavily damage trees. Try to tolerate damage or presence on trees and shrubs. Populations of woolly aphids rarely get to levels that harm plants despite the appearance of distorted leaves. If the infestation is small and if it is practical, prune out and destroy the infested branches.

Insecticides are effective in reducing aphid numbers. Systemic insecticides, such as acephate (e.g., Orthene) effectively manage woolly aphids. Contact insecticides and insecticidal soap are ineffective because they do not penetrate through curled leaves or wax. Affected leaves remain curled and distorted even when woolly aphids are successfully managed.

CAUTION: Read all label directions very carefully before purchasing and again before using an insecticide. Information on the label should be used as the final authority. Treat only plants listed on the label of the insecticide.

If you do not want to or cannot treat a woolly aphid problem yourself, contact a reputable landscape care service. They have considerable experience treating tree and shrub pests.

E453W
Revised 10/02