Treatments for Calico scale, cottony maple scale, and Lecanium spp. scales

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.

In most cases, it is not necessary to use an insecticide for soft scales on street trees. The outbreak should subside in a year or two when parasites and predators catch-up with the scale. However, some of the calico scale outbreaks are severe and seem to persist many years, and in some places where mosquito sprays are used, the outbreaks tend to persist longer and are more severe. In an outbreak area, it may be desirable to treat an individual tree over a deck or driveway to avoid the sticky mess from dripping honeydew.

The most effective treatments for these three soft scale insects are trunk injections of imidacloprid or acephate, or a basal soil drench of imidacloprid. Other systemic insecticides like dinotefuron and clothianidin are being evaluated at this time and may also be effective. Unfortunately, imidacloprid basal soil applications and imidacloprid trunk injections at this time (late May) work very well to prevent the development of the next generation of scale insects this summer, but they do not work fast enough to prevent the rain of honeydew in early to mid-June.

In contrast, trunk injections of acephate will be more effective now, but may not work as well on the next generation of young scales that will be developing in July and August. For large trees, it is best to contact a professional landscaper or arborist for a trunk injection treatment. Homeowners also have the option of using imidacloprid products that can be purchased at garden centers, like Bayer Tree and Shrub Insect Control. This product is mixed in a bucket of water at the rate of 1 oz per inch of trunk circumference, and then poured around the base of the trunk. When trees have a trunk diameter greater than 12 inches (greater than 38 inches circumference), two drenches at least a week are recommended. Basal drenches can be made any time from late May to early July, but they will not be effective for at least two weeks after the drenches are made.

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