Lecanium scale and cottony maple scale on honeylocust, silver maple and other street trees

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Localized outbreaks of Lecanium scale and cottony maple scale are continuing in some cities in southeast Michigan, northwest Michigan and in Saginaw and Midland counties where mosquito spray programs tend to boost the scale populations there to higher levels by suppressing parasites and predators. In these outbreak areas, the tree trunks and branches often look very dark from black fungi growing on the sticky honeydew that rains down from the scale insects. Small branches and twigs are covered with small brown bumps; the scale insects themselves. Cottony maple scales will produce white cottony egg sacs by mid-June.

Although a heavy infestation may cause some thinning of the canopy, it rarely results in tree death. The biggest problem is the honeydew dripping down on decks, picnic tables, lawn chairs and parked cars. Honeydew is the sugary, liquid waste excreted by scale insects. A considerable amount is excreted because scale insects need to ingest lots of sap from trees in order to get the amount of protein they need for growth and development. This creates a large excess of sugars that are excreted in the honeydew. Peak levels of honeydew are produced in late May and early June, and then subside during egg development and crawler emergence in June and July. Dripping honeydew may become a problem again in late summer and early fall as the scale insects grow larger. Treatments are discussed below.

Read this Lecanium scale bulletin from Vermont Forestry for more information: http://www.uvm.edu/~pmrc/lecanium_scale_info_sheet.pdf

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