Crop & Pest Report


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Begin Scouting for Scale Insects on Trees (07/18/19)

This week we’ve started to notice scale insects on several tree species, including maple, ash, hackberry and others.

This week we’ve started to notice scale insects on several tree species, including maple, ash, hackberry and others.  The cottony maple scale and the European fruit lecanium scale are two of the more common scales in our area (Figure 1).  What people most often notice is the reddish brown protective cover of the adult female scale; on the cottony maple scale, the covering puffs out as the “cotton” becomes visible.  However, the adults do not cause damage to trees, but rather it is the crawler stage that feeds on the underside of the leaves (Figure 2) next to the main-order veins that may damage or even kill twigs and branches.  The crawlers are also the life stage that is most susceptible to treatments.


Egg hatch occurs for only a brief period; therefore, monitoring for crawlers is critical for proper timing of insecticide applications.  Crawlers can be seen with the naked eye or with a 10X hand lens.  Another method for finding crawlers is to place a piece of white paper below a twig, then briskly tap the twig to knock the crawlers onto the paper.  They are soft-bodied and white to yellow in color. 

Several products are available for treating scales, but if populations are low, then treatments are rarely warranted.  Also, if natural predators (e.g., lady beetles, lacewings and others) are abundant, then insecticides with a low residual should be used; these include horticultural oil, insecticidal soap, or insect growth regulators such as pyriproxifen.  For high populations of scales, common insecticides available for treatment include those with the following active ingredients: acephate, bifenthrin, carbaryl, deltamethrin, lambda-cyhalothrin, malathion, permethrin and others.  The window for treating scale crawlers is 1-2 weeks, so properly timing the insecticide treatments is critical.  Soil-applied treatments of imidacloprid generally take three weeks or longer to reach the top of a large tree, so applying that chemical after crawlers are observed is unlikely to do anything to reduce their numbers this year.  However, applying imidacloprid this fall should help to minimize scale insects next year.


Joe Zeleznik

NDSU Extension Forestry Specialist


Janet J. Knodel

Extension Entomologist

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