The European elm scale (Eriococcus spurius, previously Gossyparia spuria) is a scale insect introduced to North America. This insect feeds on elm trees by sucking sap from branches and leaves. European elm scale is distinct from other scale insects because of the white waxy fringe.
To minimize the effects of European elm scale, keep elm trees well watered and healthy. Prune out any dead or dying elm wood between October 1 and March 31 only.
To remove overwintering female scales before they can lay eggs, pressure wash branches after leaf drop in the fall or before bud break in the spring. On smaller trees, you can dry brush the branches instead of pressure washing to remove the scales.
Photo Credit: City of Edmonton
Appearance: Mature females are up to 2/5 inch (10 mm), oval and brown with a white, waxy fringe.
Hosts: Elm, especially American elm.
Scale feeding can cause stunted, chlorotic (yellowed) leaves, premature leaf drop and branch dieback.
Black sooty mold growing on honeydew secretions can blacken infested limbs.
Scales overwinter as second instar nymphs in bark cracks and crevices.
Females mature in late May, mate and begin laying eggs.
Egg hatch may last up to the end of July, as crawlers start appearing in late June.
Crawlers feed on leaves throughout the summer, and then migrate to branches before leaves drop in the fall.
One generation per year.
Apply horticultural oils in late fall and early spring to smother overwintering nymphs.
Apply systemic treatments of imidacloprid in the fall or dinotefuran in spring.
Spray treatment for crawler stage:
Insecticidal soaps and horticultural oils.
Insect growth regulators (pyriproxyfen and buprofezin are available only to licensed applicators).