Gloomy scale (Melanaspis tenebricosa) is a problematic pest on urban landscape trees because the insect reaches extremely high densities in response to conditions common to urban landscapes, like drought and high temperatures. Shown here is an adult female (pink) with its dorsal armored covering, knonw as a “test,” removed. The ventral test is flat, white in the center and black around the edge, and attaches to the bark of the host tree. Intact dorsal tests of gloomy scales of various life stages are also shown. Photo by Matt Bertone, via Flickr)

By David Coyle, Ph.D.

David Coyle, Ph.D.

Few would dispute that scale insects are not exactly the “charismatic megafauna” of the insect world: They’re small, largely immobile, and often go unnoticed by the untrained eye. These insects feed by tapping their long mouthparts down into cells to extract plant fluids. Many different species of scale are common on trees, both in natural and more managed areas. In most cases—especially in natural areas—scales do little to no measurable damage. However, trees in urban landscapes are particularly susceptible to injury from scale insects for a variety of factors. To learn more about the implications of gloomy scale (Melanaspis tenebricosa), I spoke to the authors of a new article published in December in the Journal of Integrated Pest Management that highlights the ecology and management of this pest on landscape trees.

Michael G. Just, Ph.D., now a research ecologist at the U.S. Army Corp of Engineers’ Engineer Research and Development Center, is lead author on the paper and conducted research on gloomy scale while a postdoctoral research fellow at North Carolina State University (NCSU). His co-authors are Adam G. Dale, Ph.D., assistant professor of turfgrass and ornamental entomology at the University of Florida, and Steven D. Frank, Ph.D., professor and extension specialist in entomology and plant pathology at NCSU and head of its Insect Ecology and Integrated Pest Management lab.

Coyle: There are a lot of scale species. What makes gloomy scale stand out from the others?

Just: The fact that they don’t stand out. I mean, they can be pretty hard to detect on red maples, as they are a similar color to the host’s bark and they don’t move much. (Female adults do not move at all). Also, when compared to some other insect pests, they are not very flashy when it comes to tree damage. They will not denude a canopy in a single season. Instead, they are a chronic pest and it takes some time before host damage is easily spotted. They are also notable because after being described as one of the most important enemies of shade trees in North Carolina, there was a pause on gloomy scale research for almost a hundred years.

This red maple twig shows a heavy infestation with gloomy scale on its bark. (Photo by Adam G. Dale, Ph.D., originally published in Just et al 2020, Journal of Integrated Pest Management)

Why is gloomy scale such an issue in urban landscape trees? What kind of damage does it cause? And what can a homeowner do about it?

Dale: Gloomy scales are problematic on urban landscape trees because the insect reaches extremely high densities in response to conditions common to urban landscapes, like drought and high temperatures. As gloomy scale density increases, you’ve got more sap-feeding insects extracting nutrients from within the tree’s woody tissue, which results in branch dieback throughout the tree canopy and an overall decline in the host tree’s condition. Heavily infested trees literally appear “gloomy,” with a darkened, dying appearance—hence the name. The best thing for homeowners to do is plant red maples in areas that are less conducive to gloomy scale success (remember, drought and heat). This can be as simple as planting them in areas with a low amount of surrounding hardscape (less than 32 percent within an 80-foot radius of the tree). If the tree is already planted in a bad spot, we recommend frequent monitoring so that action can be taken before an infestation reaches damaging levels.

There are a lot of forest landowners in the Southeast who grow trees for profit. Should they be worried about gloomy scale affecting their hardwood productivity?

Just: Right now, gloomy scales are not a problem in forested areas, but their abundances have risen in forests during previous warm spells over the last century. Gloomy scales behave like invasive species in cities, with their pestilence driven largely by urban warming, which suggests they may become a problem in future forests as warming continues.

Two red maple street trees illustrate the impact of a gloomy scale (Melanaspis tenebricosa) infestation. Non-infested is shown at left; infested shown at right. The pest’s common name comes from an infested tree’s “gloomy” appearance. (Photos by Adam G. Dale, Ph.D., originally published in Just et al 2020, Journal of Integrated Pest Management)

Your article says that gloomy scales exhibit characteristics of sleeper species. What are those, and why are they important?

Frank: Characteristics of sleeper species are similar to those of invasive exotic species. These include rapid reproduction, phenotypic plasticity, disconnection from natural enemies, and encountering novel hosts in the new environment. In the case of sleeper species, the new environment comes to the insect in the form of climate change or urbanization instead of the insect moving to a new geographic location. (The same is true for plants and other taxa.) Sleeper species are important because they are often insects for which we have little background knowledge, since they were not previously pests. Therefore, when a change occurs such as warming and the sleeper species “awakens,” we are scrambling to learn as much as we can. Monitoring pests in urban heat islands gives us forewarning of sleeper species that could become forest pests with global warming.

Urban areas are generally warmer than undeveloped areas (i.e., the heat island effect). Can this elevated temperature impact gloomy scale population dynamics?

Frank: Urban heat islands have many direct and indirect effects on gloomy scale population dynamics. Urban warming can directly increase gloomy scale populations by increasing winter temps and thus winter survival of gloomy scales. Urban warming also increases gloomy scale fecundity directly and indirectly by affecting plant quality and makes gloomy scales grow larger. Finally, urban heat islands can extend gloomy scale distribution by allowing gloomy scales to live at latitudes and altitudes that would otherwise be too cold.

Gloomy scale probably won’t kill your trees. Heck, most folks probably won’t even notice it. But this species does act as a “canary in the coal mine” in urban settings as to the impacts of warming on our native flora. Did you know that one way to mitigate warming is by planting trees? Funny how it’s all tied together, isn’t it?

David Coyle, Ph.D., is an assistant professor in the Department of Forestry and Environmental Conservation at Clemson University. Twitter: @drdavecoyle. Email: dcoyle@clemson.edu.