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