Woolly Elm Bark Aphid – Eriosoma rileyi

Woolly Elm Bark Aphid – Eriosomarileyi

Common Name: Woolly Elm Bark Aphid

Latin Name: Eriosomarileyi


The woolly elm aphid (Eriosoma americanum) is an associate of the Eriosomatinae (woolly aphid) subfamily. It assumes diverse forms based on its host and life stage. It is found on Saskatoon roots in its most damaging location and has a pale blue to black wingless body with a white fuzzy, waxy covering on the thorax. It is wingless with a red-orange body and can be seen on the underside of the leaf of American elm, typically accompanied by little white sticky ‘bubbles’ and a white cottony mass. Finally, it is a dark blue to black in its winged form, with transparent, black-veined wings.

Host plant:            

The woolly elm aphid feeds on two main hosts at different periods of the year. It is only seen in the Saskatoon (Amelanchier alnifolia) from late spring to fall and in the American elm (Ulmus americana) from early spring to late fall.


Woolly elm aphid is found in elm and serviceberry species in the eastern United States and Canada.

Damages caused by Woolly Elm Bark Aphid:

The above symptoms are caused by nymphs (juveniles) and adult aphids draining sap from American elm leaves and Saskatoon roots. Elms suffer only minor damage, with a modest reduction in their photosynthesizing capacity. However, the damage is more severe in Saskatoons, particularly in young plants, and can result in stunted growth, low berry yield, and death. Furthermore, when residing in American elm, the woolly elm aphid excretes honeydew, a sticky fluid that drips from the leaves and coats anything underneath. While honeydew is harmless to the leaves and vegetation below, it can be an annoyance since it is difficult to remove from property (i.e., cars, lawn furniture)

Life history and habits:

Woolly elm aphid eggs overwinter in elm bark fissures. Eggs hatch in the spring, and female aphids emerge. These females feed on the foliage, causing it to coil and form a protective cage. The females mature within the leaf curl and produce many females, most developing into winged aphids. From late June to early July, these winged aphids move from Elm to Saskatoon. When a winged female descends on a saskatoon plant, she deposits 14 female nymphs on the underside of the leaves. These little yellow aphid nymphs crawl down the plant to the roots beneath the soil’s surface. They will transform underground into light-grey to blue, soft-bodied aphids, establishing a colony that will generate two more generations of females. Winged adult females that migrate back to the elms are created in August and September. Male and female aphids mate here, and the females deposit the eggs that overwinter.