Felt Scales on Oaks

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Felt scales are a family of scales that include azalea bark scale (Acanthococcus azaleae), crape myrtle bark scale (Acanthococcus lagerstroemiae), European elm scale (Gossyparia spurius), and oak eriococcid scale (Acanthococcus quercus). These scales all have a felted covering that is most apparent when adults are producing eggs. Female oak eriococcid scales are producing eggs now. They produce a fluffy felted egg case with with 100 or more eggs that will hatch in the coming weeks. The crawlers (first instar nymphs) move to new growing branches to feed. They will mature in midsummer and produce a second generation.

Oak eriococcid scales on a willow oak twig. Photo: AG Dale

Oak eriococcid scales on a willow oak twig. Photo: AG Dale

Willow oaks and other species can become heavily infested with oak eriococcid scale and this time of year much of the trunk and branches will be covered with the felted egg cases that look like the head of a cotton swab. These scales also produce honeydew which will coat the leaves and surfaces below the trees and can foster black sooty mold.

Little is known about the management of these scales. However, as with other scales targeting management efforts toward the crawlers is best. Horticultural oil or insecticidal soap applied thoroughly once eggs hatch should help reduce the population. Other insecticides for scales are in this publication.

Two lady beetle larvae that mimic mealybugs on a willow oak trunk. Photo: SD Frank

Two lady beetle larvae that mimic mealybugs on a willow oak trunk. Photo: SD Frank

It is important to pay attention to whether the scales are moving or not. There are lady beetle species that are scale predators and mimic the appearance of felted scales and mealybugs as larvae. These lady beetle larvae will be crawling around the bark on trunks and branches this time of year, sometimes in great numbers. Please do not mistake these for pests. Lady beetle larvae move. Scales and mealybugs do not.