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Watch for lecanium scale

Jeffrey Hahn, Extension Entomologist

Lecanium scales, Parthenolecanium spp., have been commonly reported this year on a variety of hardwood trees, including oak, crabapple, ironwood, hickory, and maple. Look closely for them as appear as 1/8 inch reddish brown helmet shaped insects. They are typically clustered together along branches. Adults are covered by a waxing shell and move very little while newly hatched scales, called crawlers, are mobile but lack the waxy covering.
Lecanium scales. Photo: Dan Potter, Univ. of Kentucky

Scale insects use piercing-sucking mouthparts to feed on sap in the phloem layer of plants. The damage caused by this feeding varies. In most cases, especially on large healthy trees, little to no damage occurs. As scale infestations grow larger and more persistent, branch dieback can occur, and under extreme situations, plant death. Soft scales, like lecanium scale, are also prolific honeydew producers. Honeydew is a clear, sticky waste material. Its presence in a landscape can be very annoying.

There are several options for management. In most cases, trees can tolerate lecanium scale feeding and should be ignored. There are a variety of predators and parasitic insects that help minimize scales numbers. But they need time to build their numbers so patience is necessary.

However, if scale numbers are building too quickly and it is necessary to use insecticides to reduce their numbers, there are some options. It is important to know that direct insecticide treatments are not effective against adult scales because of their waxy covering. Instead, they are most vulnerable in the crawler stage. Different scale species hatch at different times of the year. Lecanium scales hatch in June and July and are vulnerable to insecticide sprays at that time.
Branch dieback on an ironwood due to
lecanium scale feeding.  Photo: Jeff Hahn,
UMN Extension

If you want to ensure crawlers are present before treating, there are a couple of methods for detecting scale crawlers. First you can shake an infested branch over a sheet of white paper or a paper plate and watch for tiny yellowish colored actively moving insects. If you see insects with wings, they are not crawlers. You can also place double-sided sticky tape around infestation and they check the tape for signs of crawlers.

Once you know crawlers are present, you can consider different insecticide options. If you want to use a low impact insecticide to protect natural enemies, consider insecticidal soap or horticulture oil. Both have to cover the crawlers directly as there is no residual. Repeat applications may be necessary. Horticultural oil can also be sprayed during late winter as a dormant application. There are also a variety of residual insecticides, such as permethrin, that can also be used if management is desired.

For more information about scales, including lecanium scale, see Scale insects on Minnesota trees and shrubs.

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