Lecanium scale may be the cause of thin-looking honeylocust trees and dripping honeydew

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.    

Guerin Wilkinson of GreenStreet sent me an email saying that he is getting calls about Lecanium scale problems on honeylocust trees in Canton, Plymouth and Northville, Michigan.  Last year we also had some reports from Troy.  Populations of Lecanium scale naturally go in cycles and rarely cause any harm to landscape trees.  However, the scale infestation sometimes becomes much worse when the trees are sprayed for other reasons (like the mosquito control program in Saginaw and Midland counties) because the insecticide kills predators and parasites that keep the scale insects under control, while the scales are resistant to the insecticides that has been sprayed for many years.  Usually honeylocust trees do not need to be treated with insecticide for scale insects, but sometimes mosquito sprays or a localized outbreak becomes severe enough to cause noticeable branch thinning and dieback.

Even more of a problem is the honey dew (the sugary excretions of scale insects and aphids) raining down on cars parked below the trees.  This can cause some aggravation.  In those situations, I recommend using an imidacloprid (Merit or Tree and Shrub Insect Control) basal soil drench for small trees (less than 15-inch trunk diameter) in May, or contact an arborist about imidacloprid trunk injections and imidacloprid soil injection for larger trees.  Lecanium scale can infest many different tree species.  Cottony maple scale causes the same problem on silver maple and honeylocust trees.

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