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KNAU and Arizona News

Invasive oystershell scale is threatening northern Arizona’s aspen groves

Oystershell scale
Danika Thiele/U.S. Forest Service, Coconino National Forest
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Tiny invasive insects known as oystershell scale, also known as mussel scale, are increasingly being seen in northern Arizona. They can potentially kill trees and other plant life and represent a major threat to the region's quaking aspen groves. This photo was taken during a collaborative field trip with the Northern Arizona University School of Forestry and the U.S. Forest Service on Dec. 7, 2021.

An invasive insect is threatening northern Arizona’s iconic quaking aspen trees. Foresters are aggressively trying to understand the effects of oystershell scale that’s expanded at an alarming rate in recent years.

The tiny grayish-brown parasites are about 2.5 millimeters long and attach themselves by the thousands to aspens and other plant life. They pierce the bark and suck out fluid and can eventually totally encrust and kill whole trees.

Oystershell scale is historically most common in residential and commercial areas in Arizona, but in recent years has expanded to the state’s forests. According to a recent survey by Northern Arizona University School of Forestry researchers, the organisms are now widespread in lower-elevation aspen stands above the Mogollon Rim.

Foresters consider it one of the leading threats worldwide to forest sustainability and say climate change is main driver in the accelerated spread of oystershell scale in northern Arizona’s aspen groves.

Scientists with the Coconino National Forest and NAU are considering several ways to combat the organism including predatory mites, insecticide sprays and biological controls, as well as clear cutting and the use of prescribed fire.

Invasive insects have caused the large-scale decline of several tree species in recent years like the American chestnut as well as ash and five-needle pine varieties.