Woolly elm aphids

Identification

Size: 1–2 mm long

Color: Green to dark olive (on elm); covered in white woolly wax (on serviceberry roots)

Description: Woolly Elm Aphids are small sap-feeding insects that cause tightly curled, distorted leaves on elm trees in spring and summer. They also infest the roots of nearby serviceberry (saskatoon) plants as part of a two-host life cycle

Woolly elm aphids (Eriosoma americanum) are a native aphid species found in Edmonton and throughout Alberta. They use two host plants during their lifecycle: elm trees (Ulmus spp.) in spring and early summer, followed by serviceberry (Amelanchier spp.) for the remainder of the growing season. They get their name from the distinctive white, woolly wax that covers colonies found on serviceberry roots.

On elm trees, woolly elm aphids feed on developing leaves, causing them to curl tightly around the colony. The damage is usually cosmetic, although heavy infestations can place additional stress on a tree. On serviceberry plants, aphids feed on the roots, making infestations much harder to spot and potentially weakening affected plants over time.

In Edmonton, woolly elm aphid damage is generally most noticeable during spring and early summer. Ladybugs, lacewings, parasitic wasps, and other natural predators often help bring aphid populations under control by mid-summer. Cool, wet spring conditions can contribute to larger infestations when they slow the development and activity of these beneficial insects.

Woolly elm aphids can be identified by curled elm leaves, clusters of small green aphids, sticky honeydew, or white woolly colonies around serviceberry roots. Their appearance and signs of infestation differ depending on which host plant they are feeding on.

On elm trees: Look for leaves that are tightly curled or rolled lengthwise, particularly during spring and early summer. Carefully unrolling an affected leaf may reveal a dense colony of small green to dark olive aphids inside. You may also notice sticky honeydew on the leaves or surfaces below the tree, sometimes followed by black sooty mould growing on the honeydew.

On serviceberry: Look for white, wool-like masses on the roots and in the surrounding soil. Because these colonies develop underground, they can be harder to identify. Infested serviceberry plants may also show reduced growth, yellowing leaves, or poor fruit production.

In mid-summer: Winged woolly elm aphids migrate from elm trees to serviceberry plants. During this period, you may notice small, dark, winged aphids around elm trees as they leave their spring host.

Tightly curled or rolled elm leaves, especially during spring and early summer
Clusters of small green to dark olive aphids are hidden inside curled elm leaves
Sticky honeydew coating elm leaves, branches, vehicles, patios, or pavement beneath the tree
Black sooty mould growing on leaves or other surfaces covered in honeydew
White, wool-like aphid colonies around the roots or base of nearby serviceberry plants
Yellowing leaves or early leaf drop on elm branches with heavy aphid activity
Reduced growth or poor vigour in serviceberry plants, particularly when growing near infested elm trees

Inspect elm trees as new leaves emerge in spring so aphid activity can be identified before leaves become tightly curled.
Avoid excessive nitrogen fertilizer, which can encourage the soft new growth that aphids prefer to feed on.
Keep elm trees healthy with appropriate watering, mulching, pruning, and regular inspections.
Check nearby serviceberry plants each spring for white, wool-like aphid colonies around the roots and surrounding soil.
Protect beneficial insects such as ladybugs, lacewings, and parasitic wasps, which naturally help control woolly elm aphid populations.
Avoid unnecessary broad-spectrum insecticides that can kill beneficial insects along with aphids.
Remove heavily infested curled leaves where practical and dispose of them rather than leaving affected material around the tree.