European elm scale,
All scale insects undergo incomplete metamorphosis and have three life stages: egg, nymph, adult. These insects can reproduce sexually or asexually. Female European elm scales are up to 10 mm (1/8 – 1/2 inch) long and surrounded by a white waxy fringe. Adult males can be winged or unwinged, with fully developed legs and antennae but no mouthparts. Unlike males, females are wingless and turn a dark red-brown color as they grow in the spring. They also produce copious amounts of honeydew, which can cause black sooty mold growth and blackening of limbs.
- The European elm scale is a scale insect and major pest of large leaf elms.
- This insect was introduced to North America from Europe. American elm trees are highly susceptible to European elm scale attacks.
- Some elm cultivars are more resistant to infestations. These include Accolade, Frontier, Homestead, Triumph, Red Tip and Commendation.
- European elm scales and mealybugs are similar in appearance due to the presence of white waxy covers.
European elm scale. European elm scales are important pests in Colorado due to their widespread distribution and destructive capacity. These insects secrete honeydew, which can lead to black sooty mold growth. While the European elm scale can be very destructive, some elm cultivars are resistant to their infestations. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
European elm scales packed into bark cracks. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
European elm scale nymphs on foliage. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
European elm scale females and cocoons produced by males. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Life history and habits
Egg laying occurs two or three weeks following the first appearance of fully formed elm leaves and continues through the summer; an individual female can lay up to 400 eggs.
Eggs hatch in June and July. European elm scale nymphs (crawlers) are small and yellow in color. After emerging from the egg, nymphs begin feeding on the primary leaf veins on the undersides of leaves from mid-June through August. Nymphs return to twigs in August and September where they overwinter in bark crevices.
Severe scale infestations can cause yellowing and shedding of leaves, as well as dieback of twigs and branches. The honeydew secreted by females can be a nuisance when it adheres to parked cars, sidewalks, and patio furniture underneath infested elm trees. Honeydew can also lead to black sooty mold growth on branches infested by European elm scales.
Drought stressed and poorly nourished trees are more susceptible to attack. Therefore, elm trees should be sufficiently watered and fertilized if it is feasible. Dead or dying elm wood should be removed between October and March.
Confirming that infested branches harbor live scales is critical – dead scales may resemble live ones and adhere to the host plant. In these cases, chemical management is not necessary and should only be used when scale insects on a plant are viable. To determine whether a scale insect is living, use a small sewing needle and hand lens. Flip over the waxy coating and look to see whether the insect is rounded, which is indicative of living scale. Dead scales will bleed when they are uncovered.
Estimates can be conducted by counting a minimum of 25 insects and multiplying the number of live insects by 4. When a high proportion of dead insects are present, pesticide applications should be avoided to help protect natural enemies.
Natural enemies of European elm scale include parasitic wasps, mites, spiders, and predatory plant bugs. Studies have indicated that removal of natural enemies causes a rebound in scale infestations, sometimes causing them to exceed their numbers from previous years. Therefore, insecticides should only be used when there are a high proportion of live scales attacking the tree.
Apply horticultural oils in the late fall and early spring to smother nymphs overwintering in bark crevices.
Overwintering females can be removed before laying eggs. This is done by pressure washing branches in the fall after leaves drop or in the spring before bud break. Instead of pressure washing, branches of smaller trees can be dry brushed.
Resistance to certain insecticides has been observed in many parts of Colorado, specifically those containing the active ingredients imidacloprid, dinotefuran, and clothianidin. As such, there are few chemical options available for managing European elm scale.
More recently, a class of insecticides called Insect Growth Regulators (IGR’s) has been developed and provides effective management for European elm scales. This class of insecticide works by halting the development of nymphs and preventing them from molting, which is necessary for their progression into the adult stage. Therefore, the timing of application is important for this type of insecticide.
Regardless of the insecticide being used, licensed commercial pesticide applicators should be hired when elm trees are too large for homeowners to treat themselves.
Bismarck (n.d.). European Elm Scale. Bismarck. Available https://www.bismarcknd.gov/1993/European-Elm-Scale#:~:text=The%20European%20elm%20scale%20(Eriococcus,of%20the%20white%20waxy%20fringe.
Colorado State University. (n.d.). European Elm Scale. Colorado State University Extension – Plant Talk Colorado. Available https://planttalk.colostate.edu/topics/insects-diseases/1400-24-european-elm-scale/#:~:text=What%20are%20european%20elm%20scales,of%20the%20newer%20elm%20cultivars.
Held, D. (2019). Controlling Scale Insects and Mealybugs. Alabama A & M and Auburn Universities – Cooperative Extension. Available https://www.aces.edu/blog/topics/lawn-garden/controlling-scale-insects-and-mealybugs/
US Department of Agriculture (n.d.). European Elm Scale: Elm branch dieback and shiny, sticky leaves. US Department of Agriculture: Forest Service. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5350728.pdf