Introduced from Europe, European elm scale is a serious pest of large leaf elms. Elm scale is one of the most widespread, prolific and destructive insects in Colorado. American elms are most susceptible to scale as are some of the newer elm cultivars. Elm cultivars showing a high level of resistance to scale include Accolade, Frontier, Homestead, Triumph, Red Tip and Commendation. Patriot, Prairie Expedition, Valley Forge and Princeton are very susceptible.
Heavy scale infestations cause premature yellowing and shedding of leaves and killing of twigs and branches. Small trees may be killed. Associated honeydew and black mold is a nuisance. Peak honeydew production in early June to early July adheres to parked cars, patio furniture, and sidewalks underneath infested elm canopies.
European elm scale has one generation per year. Scales overwinter in crevices on the bark of elm trees. Molting of females to the adult stage coincides with elm seed development in spring of the year. After molting, the adult grayish colored females move to the main branches. When the elm seeds begin to fall, the adult females form the characteristic white waxy sacks resembling a mealy bug. Egg laying begins 2 or 3 weeks after the first elm leaves are fully formed continuing into late summer. An individual female may lay as many as 400 eggs. The eggs hatch and the yellow scale crawlers move to the undersides of leaves feeding near the primary leaf veins from mid-June through August. In August and September scale crawlers migrate back to the twigs to overwinter.
Soil applied systemic treatments of insecticides including the active ingredients imidacloprid, dinotefuran, and clothianidin have been used for scale control. Control was excellent to begin with, however now these treatments are largely ineffective in many sites in Colorado.
Currently options are few for scale control. Dormant (horticultural) oils can help control overwintering scale life stages. Oil applications sprayed just before bud-break in the spring smothers the overwintering stages of elm scale. A newer class of insecticides termed insect growth regulators or IGR’s control the immature stages of insects. These insecticides inhibit insect molting thus not allowing them to turn into an adult and complete their life cycle. The crawlers continue to feed but do not overwinter on the elm twigs. This effectively breaks the insect life cycle for the following spring.
Most elms are too large for homeowners to treat (spray) themselves. Insect control applications should be completed by a commercial pesticide applicator licensed by the Colorado Department of Agriculture.