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 that alternate between two host plants: elm trees (Ulmus spp.) in spring and early summer, and serviceberry (Amelanchier spp.) for the remainder of the growing season. They are named for the distinctive white, woolly waxy coating that covers colonies on serviceberry roots.

On elm trees, their feeding causes leaves to curl tightly around the colony, which is largely a cosmetic issue but can stress trees during heavy infestations. On serviceberry, root colonies are more difficult to detect and can weaken plants over time if left unchecked.

Damage to the elm is most noticeable in spring and early summer. Natural predators — ladybugs, lacewings, and parasitic wasps — typically bring populations under control by mid-summer. Cool, wet springs that slow predator development tend to produce the worst infestations.

On elm trees: look for leaves that have curled or rolled tightly lengthwise. When unrolled, dense colonies of small, green to dark olive aphids are found inside. Sticky honeydew may coat leaves and surfaces below, and black sooty mould can develop on honeydew deposits.

On serviceberry: white, wool-like masses on and around roots, sometimes extending into the surrounding soil. Affected plants may show reduced vigour, yellowing leaves, or poor fruit production.

Winged forms appear in mid-summer to migrate from elm to serviceberry — small, dark-winged insects may be noticed near elm trees during this time.

Tightly curled or rolled elm leaves, often throughout the canopy in spring
Green or dark aphid colonies are visible inside leaves when unrolled
Sticky honeydew on leaves, branches, parked vehicles, or paving below
Black sooty mould developing on surfaces coated with honeydew
White woolly masses found at the base of or on the roots of nearby serviceberry plants
Yellowing leaves or early leaf drop on heavily infested elm branches
Reduced vigour or poor growth on serviceberry plants near infested elms

Monitor elm trees closely as leaves open in spring — treat before leaves curl to maximize effectiveness of contact sprays
Avoid excessive nitrogen fertilization, which encourages the soft new growth that aphids prefer
Maintain tree health through proper watering, mulching, and annual inspection
If serviceberry plants are nearby, inspect roots each spring for woolly white masses
Encourage natural predator populations — avoid broad-spectrum insecticide use that kills beneficial insects
Remove and bag heavily infested leaf clusters where accessible to reduce overwintering egg populations