Native to Europe, this insect arrived to the US in 1884. It swiftly moved from coast to coast, and is now found wherever elms (Ulmus spp.) grow. The scale feeds on the fluids from the tree, and generally is found on the bottom sides of leaves near the midrib. As with any scale, it can be difficult to control. With advances in insecticides, particularly systemic ones, suppression is possible.
Monitoring is crucial. The easiest sign of this pest is sooty mold on the honeydew excrement. This will give the tree a black appearance and can sometimes be found under the tree. Crawlers in June and July become active, and are a conspicuous yellow/orange color. Horticultural oil and systemic insecticides are both useful tools.
Other Treatment Practices
- Do not over fertilize, as it can lead to increased scale populations.
- Maintain tree health by watering during droughts and mulching whenever possible.
A sprayed insecticide is fastest when applied to the active crawler stage. This can only occur if monitoring is appropriate. Systemic insecticides work well. The focus should be on suppression of this insect, not eradication. Scales can be difficult to control.
Signs of Damage
- Yellowing of leaves and potential twig dieback
- Foliage can also be stunted or chlorotic
- Black sooty mold growing on honeydew in spring and summer
- Yellow/orange crawlers may be found on the underside of leaves from late June to July
- Adult females are small (10 mm), oval, and grayish with a white fringe
Photo: Utah State University
Trees At Risk
Any species of elm (Ulmus spp.), some zelkova (Zelkova spp.), and some hackberry (Celtis spp.)
- Overwinters as an immature in bark cracks
- Females mature early in the Spring and lay eggs in late May
- Eggs hatch in late June through July(1029-1388 GDD)
- Crawlers like to be next to the midrib on the underside of leaves
- Females prefer twig forks
- One generation per year
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.