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Local agencies partner to study invasive species threatening Arizona’s aspens

Local agencies partner to study invasive species threatening Arizona’s aspens

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Oystershell scale

A group of local experts are teaming to treat aspen stands infested with a tiny insect that is quickly becoming a major threat to the iconic tree species.

Oystershell scale is considered by many experts to be an emerging invasive species issue with the potential to severely damage or even destroy northern Arizona's aspen over the coming years unless successful treatment techniques can be developed. The Kaibab National Forest is teaming with USDA Forest Service Forest Health Protection, Northern Arizona University and the Arizona Elk Society to better understand the oystershell scale, according to a Kaibab National Forest press release.

With only about 2,000 acres of aspen in these areas, tree specialists and researchers are eager to gain as much information as possible on how to effectively manage oystershell scale and preserve aspen on the landscape.

"Oystershell scale is one of the most damaging insects to aspen we have recently found," said Michael Sedgemen, silviculture forester for the southern two districts of the Kaibab National Forest. "We were already seeing a steady decline in Southwest aspen stands due to a number of environmental factors. With the rapid increase in activity of oystershell scale over the last few years, aspen could permanently be removed from the landscape if solutions aren't found."

Research into the biology and management of the species is considered critical for the future of aspen. The treatments are not only intended to help limit the spread of the insects in specific locations but also to inform research to aid in the long-term preservation of healthy aspen stands across the Southwest. \Aspen are considered a keystone species, meaning that it is critical to the survival of other species in the ecosystem and to the long-term sustainability of the ecosystem itself.

Oystershell scale are tiny, armored insects that live under protective covers on soft twigs or bark of their host plant. Mature scales are about one-eighth-inch long and are the general shape of an oyster's shell. The insects' hard, protective coverings, which are constructed of wax, shed skins and other substances, are exceptionally difficult to penetrate, making treatments such as insecticide spraying more challenging on a large scale.

The tiny insects feed on their host plant with mouthparts that are several times longer than their bodies, enabling them to consume large areas of plant tissue. Oystershell scales can quickly overwhelm their host, even though they may not immediately be noticeable due to blending in well with the underlying bark. As the number of insects increase, an extensive crust of scales develops that can entirely encircle the trunks of host trees, and injury symptoms including dying limbs, tree tops and whole trees rapidly ensue.

Kaibab National Forest managers are documenting expanding acres of heavily scale-encrusted trunks and dead or dying trees in many aspen stands on the forest's south zone.

"Aspen research and management have a long history in Arizona. Given the previous research, we have an understanding of the role of native insects and pathogens, and ungulates, on aspen reproduction," said Kristen Waring, professor of silviculture at Northern Arizona University's School of Forestry.

The presence of oystershell scale in northern Arizona's forests has been a relatively recent phenomenon, according to Amanda Grady, forest health entomologist with the Forest Service's Southwestern Region. While they have long been a problem in some ornamental tree species in urban areas, they were only observed across local wildlands within the last several years.

Two years of intensive monitoring has indicated that oystershell scale is a more extensive problem, affecting many areas in central Arizona, including lands on the Apache-Sitgreaves and Prescott National Forests.

Oystershell scale isn't the only threat facing northern Arizona's aspen stands. For the last 15 years or more, forest managers across the western United States have increasingly observed the rapid decline of aspen. Factors such as drought, conifer encroachment, ungulate browse, fire suppression, insects and disease likely contribute to the decline of aspen and lack of successful regeneration.


As part of the research effort, a study being conducted on both the Kaibab and Coconino National Forests seeks to quantify the effects of various silvicultural treatments, which are treatments applied to change the condition of trees and stands, on oystershell scale infestations.

Pre-treatment data is being collected to describe each stand and its infestation. Unique treatments will then be implemented within each documented stand. Finally, pre- and post-treatment conditions will be quantified and compared in order to determine the most effective silvicultural treatments and to inform best management practices.

The area to be treated on the Kaibab covers 21 acres within aspen exclosures, which are small plots of fenced aspen stands, in Spring Valley about 7 miles north of Parks just northwest of Government Hill and south of RS Hill. About half of the 21 acres will be treated by removing overstory aspen trees, known as a clearfell treatment, while the remaining untreated acres will serve as research control plots. The clearfell treatment is intended to remove infested trees while also stimulating regeneration of aspen.

The team of experts plan to begin implementation of the study's treatments in Spring Valley as early as the beginning of April. Residents and visitors may notice aspen felling within specific aspen exclosures as well as ongoing site visits by researchers and forest health specialists to monitor treatments. Different prescriptions are being implemented within aspen stands on the Coconino National Forest so that treatment results can eventually be compared.

"Many community members value aspen so it's good for them to be aware of this emerging threat and know that we are actively researching how aspen can be maintained on the landscape, which in this case may involve silvicultural intervention. Active forest management is often critical to maintaining the forests that we all treasure," said Connor Crouch, a Ph.D. student in the School of Forestry at Northern Arizona University.

In the meantime, members of the public can help researchers and forest managers by leaving all cut aspen in place within the aspen exclosures. Kaibab National Forest specialists plan to pile and burn the infested aspen in order to limit spread to other stands.

While aspen is popular as firewood, the material cut as part of these oystershell scale studies should not be moved to other locations in order to prevent broader infestations.

Research findings from the current silvicultural treatment studies are anticipated to be publicly available in fall 2021, according to Crouch.


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