European Elm Scale Description:
European elm scale (Gossyparia spuria) is a pest that feeds on elm tree twigs, branches and leaves, causing premature leaf drop and leaf yellowing. Heavily infested branches may die.
Hosts:
This scale affects all elm tree species. American elms and some of the newer elm cultivars are highly susceptible to European elm scale. ‘Valley Forge’, ‘Princeton’, ‘Patriot’ and ‘Prairie Expedition’ are also very susceptible. Resistant cultivars include ‘Accolade’, ‘Frontier’, ‘Triumph’, ‘Red Tip’, ‘Frontier’ and ‘Commendation’.
Biology & Symptoms:
Female scales are small, reddish-brown insects with white, waxy coverings. In June and July, eggs hatch into tiny, yellow-orange nymphs. By late summer, they attach themselves onto branches and twigs. The nymph and adult stages cause damage.
Females overwinter in cracks or crevices while males overwinter in small, white cocoons. In spring, females produce large amounts of honeydew. This sticky substance is usually first noticed when it drips onto cars or outdoor furniture. Sooty mold grows on honeydew, turning branches black. It does not infect plants, but it can reduce sunlight and stunt their growth.
Management:
Repeated infestations of European elm scale can kill trees and control options are limited. Horticultural oils applied during the dormant season may reduce scale populations.
They can also be effective when applied before spring bud-break, although elms may be too tall or large for homeowners to treat by themselves. Insect growth regulators can disrupt the life cycle of scale in the crawler stage. Some systemic insecticides applied through the soil may also help manage scale, although the scale seems to be developing resistance to them in some areas.
European elm scale has some natural enemies, including mites, spiders and predatory plant bugs. Parasitic wasps attack female scales.
Keep elm trees healthy to help them tolerate scale damage. If rainfall is scarce, provide irrigation during periods of drought.
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