European Elm Scale- Tree dripping profusely
European elm scale is one of the most problematic and destructive pests in the Front Range.
Scales are red to light brown with a white outline and are found on twigs and branches. The younger stage is yellow and can be found on the underside of leaves. Leaves often turn yellow and fall prematurely. Black sooty mold grows on the copious amount of honeydew produced by these insects, causing the branches and trunk to turn black.
Small nymphs spend the winter in bark crevices and cracks in twigs and branches. When spring arrives, scales mate and eggs begin developing within the female. They hatch and emerge from the female for several weeks throughout June and into July. Nymphs settle near leaf veins on the undersides of leaves and feed for the remainder of the growing season before going back to overwintering locations.
Yellow leaves, twig dieback, premature leaf drop, sticky honeydew, and black sooty mold are a result of European elm scale infestations. Repeated infestations can result in death of the tree.
European elm scale is not easy to manage. Dormant oil sprays are effective at reducing scale populations. Various insecticide sprays are effective at killing small crawlers later in the summer. Systemic neonicotinoid treatments have been effective in the past. However, it seems that they are losing efficacy and this may be attributed to resistance in populations throughout the Front Range. Insect growth regulators (e.g. pyriproxyfen) have been having good results when targeting the crawler stage.