First noted in New York in 1884, this non-native scale is now widespread in North America and is found on native and European elms, but also rarely on hackberry and Zelkova. Adult females are approximately 1/8 inch in length. The females produce a ring of white fibers around their black, oval bodies. They are often found in twig forks or rough/creviced areas of bark. By the end of June, females will lay eggs that hatch into bright yellow crawlers, which will disperse to the midrib and leaf veins on the underside of elm leaves where they will remain to feed. Crawlers are tiny and magnification is necessary to observe. Immatures overwinter in the cracks and crevices of the bark of elm trees, or at the base of buds and twigs. In late winter, males may form tiny white cocoons that look like grains of rice which are found attached to host plant bark. One generation occurs per year.
This insect can cause yellowing of foliage, premature leaf drop, and eventually dieback on its host. Honeydew and thus sooty mold are produced, particularly in the presence of a large population of this insect. Young trees or those planted under poor site conditions (trees that are otherwise stressed) are most susceptible to damage caused by this insect.
Look for female scales on the bark of twigs and small branches, particularly in forks or crotches. From May throughout the fall, look at the underside of elm leaves (near the leaf veins) using magnification to observe the tiny, yellow-bodied crawler stage. Yellowing leaves, dieback, or stunted growth may indicate the presence of this scale.
Asiatic elms may be less susceptible to this scale insect, and may be considered/selected for planting in an area where this insect has historically been problematic. Native and other introduced elms are considered to be more susceptible.
Natural enemies such as parasitic wasps and predatory insects have been reported as successful in managing this insect. At least four species of parasite are listed as associated with the European elm scale on the Invasive Species Compendium (ISC).
Abamectin (NL)
Acephate (NL)
Acetamiprid (L)
Azadirachtin (NL)
Buprofezin (NL)
Carbaryl (L)
Chlorpyrifos (N)
Clothianidin (NL)
Cyantraniliprole (NL)
Cyfluthrin (NL)
Dinotefuran (NL)
Gamma-cyhalothrin (L)
Horticultural oil (L)
Imidacloprid (L)
Insecticidal soap (NL)
Lambda-cyhalothrin (L)
Neem oil (NL)
Pyrethrin+sulfur (NL)
Pyriproxyfen (eggs) (L)
Spinetoram+sulfoxaflor (N)
Elms may be sensitive to acephate; read and follow all label instructions for proper use to avoid plant injury.
Active ingredients that may be applied systemically include: abamectin (injection), acephate (injection), acetamiprid (injection), azadirachtin (injection, soil drench), clothianidin (soil drench), cyantraniliprole (soil drench, soil injection), dinotefuran (soil drench), imidacloprid (soil drench), and neem oil (soil drench).
Make insecticide applications after bloom to protect pollinators. Applications at times of the day and temperatures when pollinators are less likely to be active can also reduce the risk of impacting their populations.
Note: Beginning July 1, 2022, neonicotinoid insecticides are classified as state restricted use for use on tree and shrub insect pests in Massachusetts. For more information, visit the MA Department of Agricultural Resources Pesticide Program.