How to Manage Pests

Pests in Gardens and Landscapes

European elm scale—Gossyparia spuria

The European elm scale occurs only on elm. The mature female is a dark red, brown, or purple oval surrounded by a white cottony wax fringe on the sides of the body.

Life cycle

Damage

Scales encrust bark at the crotches of twigs and on the lower surface of limbs. In spring and summer, some crawlers settle and feed on the underside of leaves. Honeydew and blackish sooty mold may occur on the plant. Branches may die back.

Solutions

Keep trees healthy so they tolerate scale feeding. Provide adequate irrigation in areas of prolonged summer drought. On individual trees, apply a forceful stream of water to twigs, branches, and crevices with a hydraulic sprayer. Spray after the soft-bodied females have begun maturing in spring, but before elm leaves unfold to obstruct the water. Foliage usually begins pushing out when the clusters of seeds have matured and start to drop. Parasitic wasps can provide control in certain areas of California if ants are controlled around trees. To determine whether elm scale parasites are established in your area, inspect mature female scales during spring and summer to determine whether any scales have tiny round parasite emergence holes or have become mummified (hardened and slightly puffy due to immature parasites feeding inside). If populations are intolerable, narrow-range oil can be applied in spring or early summer as timed by monitoring crawlers.

Adult European elm scales
Adult European elm scales, center left is mummified with parasite emergence hole

Colony of Europena elm scales
Colony of European elm scales


Statewide IPM Program, Agriculture and Natural Resources, University of California
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