European elm scale—Gossyparia spuria =Eriococcus spurius
This scale insect (Eriococcidae) is a pest only on elms, Ulmus spp.
The flattened, oval-shaped, young nymphs are orangish to yellow when on leaves and vary in appearance on bark. Females and males occur on bark mostly at the crotches of twigs, in cracks, and on the lower surface of limbs.
The mature female is brownish to dark red or gray. Her body is surrounded by a feltlike or waxy, white fringe that covers the underside and sides. On top the body, the cover extends partially inward as toothlike extensions.
Male nymphs develop into elongate, whitish puparia. These cocoonlike, nymphal skins are usually found in groups isolated from the females. Adult males are rarely seen; they have a reddish body, one pair of translucent to whitish wings, and two filaments projecting from their rear end.
The scale overwinters as brownish, second instars on bark. Second instars gradually change in appearance and become wax covered as they mature into adults during February to April. A mature female may produce up to 400 eggs. Several eggs per day hatch into crawlers (mobile, first instars) from May through August. Nymphs feed on bark or the underside of leaves; those on leaves migrate to bark in the fall. The insect has one generation per year.
Feeding by European elm scale can cause leaf yellowing and premature drop of foliage. The scale excretes copious honeydew, which attracts ants, makes a sticky mess, and promotes the growth of blackish sooty mold. When abundant over several years, the scale causes plant decline and the death of small branches.
Keep trees healthy so they are more tolerant of insect feeding. American and European elms are adapted to summer rains; provide adequate irrigation in areas of summer drought.
Introduced parasites, primarily Baryscapus coeruleus and Coccophagus insidiator, attack European elm scale at some locations in California. Parasite presence is evidenced by the round emergence holes of adult wasps in mature female scales. Increase the effectiveness of biological control by controlling honeydew-seeking ants and avoiding the use of persistent, broad-spectrum insecticides.
If the scales are intolerable, thoroughly spray infested bark with horticultural oil during the dormant season. A systemic insecticide may also provide control, but European elm scale in some locations apparently has become resistant to neonicotinoid insecticides (e.g., dinotefuran, imidacloprid); where neonicotinoid insecticides have repeatedly been applied to elms they may no longer control European elm scale.
If spraying horticultural oil or another contact insecticide during the foliage season, monitor scale crawlers with several traps of double-sided tape wrapped around twigs. In spring or early summer, apply oil when crawler abundance declines after it has peaked. Because female scale density peaks in northern California at about 540 degree-days above 52°F accumulated from March 1, begin once-a-week monitoring of crawlers by about 400 degree-days if a foliar-season contact spray is planned.
If scale-infested trees will be sprayed and are large or tall, hiring a pest control company with the equipment and experience to effectively spray trees can be a good idea. Discuss in advance with the company how they plan to control your pest problem.
See The European Elm Scale in the West, Pest Notes: Scales, and The Scale Insects of California: Part 2: The Minor Families for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).