A fast-moving infestation of tiny insects threatens the survival of Arizona's aspen trees
FLAGSTAFF — As flurries started to descend on the forest floor, a team of researchers examined a stand of sickly quaking aspen trees off U.S. Highway 180, just north of Flagstaff. To an untrained eye, the trees might have looked normal. But up close, the picture was different: The usually matte white bark was covered with thousands of tiny dark notches, giving the trunks a dull appearance, darkened, almost black.
Kristen Waring, a professor of silviculture and applied forest health at the School of Forestry at Northern Arizona University, knew what to look for. She and two graduate students, Connor Crouch and Kelsey Pemberton, quickly pinpointed problem areas.
What looked like an inanimate object was actually a tiny, sap-sucking insect called oystershell scale. Its diminutive size belies the greater threat it poses to aspens and trees throughout Arizona. Colonies of the invasive creatures can encase mature trees, leading to fatal infestations. Eventually, oystershell scale populations can balloon to the point where they can kill entire stands of trees.
Waring and her fellow researchers say the threat to aspens, the most widely distributed tree in North America, is so great that there could be a future where aspens no longer grow in the Southwest. The problem is particularly acute in this region, where warmer temperatures and less precipitation already stunt aspen regeneration.
"My biggest concern is that we will lose aspen out of the southwestern United States," Waring told The Arizona Republic. "That, for me, I think means that 'scale' might just be kind of the nail in the coffin in a battle we've been fighting for a long time."
For the past two years, Waring and her collaborators have been working with the U.S. Forest Service to avert that dire outcome. So far, their research has been confined to pens, or exclosures, meant to protect aspens from herbivores, like elk and cattle. The experimental exclosure Waring and her team recently visited had a section that had been cut in a process called clear-felling.
Clear-felling is different from clear-cutting because aspen trees can sprout new shoots from their roots. In this process, trees in one section of the plot are cut and stacked high into a burn pile, allowing the aspen to regenerate through root suckering, a process in which trees sprout new stems and shoots after being damaged.
These treatments temporarily reduce the tree density, which Waring and her team are hoping will mitigate oystershell scale outbreaks. It's just one strategy, among many, that they're deploying in the battle to control the millimeter-sized insect that can literally suck the life out of trees.
"They have this tiny … long, piercing, sucking mouthpart that gets inserted into the bark of aspen or other hosts," said Crouch, who was the lead author of a spring study into oystershell scale. "It inserts that into the host cells and just sucks fluid from those cells."
Spreading across the landscape
Since researchers with the U.S. Forest Service first observed oystershell scale on aspen trees near Parks over a decade ago, the insect, OSS for short, has spread across the Mogollon Rim. At the time, scientists were just starting to see outbreaks outside of urban settings. Their numbers in natural settings were previously so low that they weren't considered a problem, Waring said.
Some reports indicate OSS had been observed in lower-elevation forests as early as the 1990s. Mary Lou Fairweather, who worked for the Forest Service for over 32 years and is now semi-retired, began studying the mortality of young aspen trees at this time. She did not see OSS in regeneration areas, where fire burned through and led to recovery. She did observe OSS on aspens in a few forested areas, but mainly at lower elevations.
After almost 20 years, the insect has expanded in geography and population. In 2017, regional Forest Service staff noticed widespread infestations of an aspen-killing insect in the Coconino National Forest. Shortly after, further research also found it in the Kaibab National Forest.
That triggered a collaboration between federal agencies and NAU to study the insect. The Forest Service provided the initial funding. NAU and Forest Health Protection, a division with the U.S. Department of Agriculture, took on the research, and what they found worried everyone involved.
OSS has been observed in much of the Southwest. What's more concerning for researchers is the pest's expansion has been sudden and unexplained. They suspect it has to do with a warming climate.
"We believe that the scale probably took off a few years ago because of how much the climate already changed in Flagstaff," Waring told The Republic. "Because this is such a new problem for aspen, the previous work has focused on what were the current problems at the time, and nobody anticipated that 'scale' would become this huge problem causing mortality. And so because of that, we do have more questions than answers."
To help answer some of those questions, Waring and her collaborators are studying various methods, including clear-felling, pre-commercial thinning, fire, natural predators and fungus to see how the insects respond. What they're looking for most is a way to control OSS populations.
"If we know more about how to control 'scale,' then that will help us to maintain aspen in some areas, in addition to identifying potential refugia and protecting those areas," Waring said.
Arizona is a harbinger
Oystershell scale has been in America for centuries. Researchers believe the insects hitched a ride on apple trees transported from Europe in the 1700s. Since then, they have mostly afflicted orchards and ornamental trees in urban areas, where homeowners and arborists can easily treat trees with pesticides, scrubbing and pruning.
With a warming climate, scientists believe OSS has been able to expand its range. Aspen trees are already stressed in Arizona. They're at the southern end of their range and higher temperatures with less precipitation has added pressure in an already hostile environment. As a result, trees are weaker and perhaps less able to fight off infestations.
"When there's just a few oyster shell scales on the tree, that doesn't really impact the tree all that much," Crouch said. "Problems start to happen when you have them all over a tree, or they're completely surrounding a tree and can sort of cut off its nutrients."
Research into aspen mortality shows that 50-95% of the trees in aspen stands at lower elevations in Arizona have died in recent years due to things such as grazing, prolonged drought, diseases and native pests. The disappearance of aspen has been so pervasive that scientists have named the loss with an apt acronym: Sudden Aspen Decline, or SAD. Researchers have observed SAD less frequently in recent years, most likely because so many trees have already been killed, Fairweather said.
"Another primary factor in the decline of aspen is that the trees are very old and have not been able to regenerate. Aspen is a relatively short-lived species and regenerates primarily through root suckering," adds Fairweather. "Regeneration has been limited over the last century due to fire suppression, which allowed other species to dominate aspen forests, and also due to browsing by livestock and elk and deer."
An emerging pest species like OSS only compounds that loss, which means what aspen trees remain could die at a faster rate. Given Arizona's already hot and dry climate, researchers like Crouch say that what happens here can be a harbinger for what could happen to aspen throughout their range in the future as temperatures climb elsewhere.
"Being on the southern edge of the species’ range, we expect aspen in Arizona to experience the effects of a warming climate earlier and more severely than more northerly parts of the species’ range," said Crouch. "So between climate change and oystershell scale, we hypothesize that what we’re seeing for aspen in Arizona may be a sign of things to come for aspen in more northerly parts of its range as the climate continues to warm."
Challenges to control the spread
Waring says one of the best ways to control OSS is to stop it from spreading into new stands. That has been hard since there are so many unknowns about how to manage it.
One of the more harmful characteristics of OSS is its perniciousness. At only a couple of millimeters in length, they are small, so they're hard to see. They usually occur in one area of a tree initially, a branch or stem, making them pretty inconspicuous at first.
Once they're established, they work their way through to the trunk. But by that time, they could have laid thousands of eggs and it's likely already too late to save the tree.
As an animal that gradually expands its range and population before it's detected, OSS is what scientists call a sleeper species. That essentially means that they establish themselves before causing widespread damage to their host.
Their lifecycle and biology also make them hard to control. On average, the insect's entire life spans just a year. Eggs are laid under the female's hard oyster-like shell, which gives them their name, in late fall. There, they overwinter. The eggs hatch in the spring, feed on trees in the summer, and then disperse in early fall to breed, after which a new generation repeats the cycle.
By the time one generation is treated, another is born. And research has found that, given the right conditions, females can produce more than one brood per year.
Their knack for dispersing also makes control difficult. In addition to crawling to other host trees, OSS can be carried by strong winds or on other hosts such as animals, furthering their reach. Arizona's strong spring winds make this mode of transport especially effective when females are laying eggs.
What's more, while oystershell scale has had the most detrimental effects on aspen, they can feed on a wide array of hardwood tree species, such as willow and cottonwood. So while treatment and research has been targeted toward aspens, the insects are able to feed on other hosts, including shrubs.
All of these challenges make treatment elusive and hard to formalize. While one solution might be right for one area, it could be useless in another. For instance, while clear-felling might work in an aspen exclosure, it might not be right for a riparian tree species.
Even more concerning, Crouch’s research indicates that OSS prefers tall regeneration, trees that are generally taller than 4 feet, with a diameter of at least two inches. They're taller than stems, Crouch said, but not yet considered overstory. This is an underrepresented age group in Arizona's aspen forests. One of the main goals of the exclosures is to protect aspen from herbivores so new generations of trees can grow.
An OSS infestation can threaten that recovery by killing off trees in this age class, which means aspen stands could lose entire generations of trees and might not be allowed to reach their full potential.
"As the biggest, oldest trees die, we need younger, healthy trees to replace them. And so if you've got four size classes … we need trees in all the size classes to replace these older trees," Waring said. "So if you've only got the smallest size class, and then the oyster shell scale takes out the tall regeneration and the saplings, for example, then you've got a problem because you're not recruiting from the small regeneration into the overstory."
Ways to treat the infestation
Similarly dire situations are being documented in trees species across the country. Eastern hemlocks are plagued by woolly adelgid aphids. Pine trees are being decimated by bark beetles. Ash trees are ravaged by emerald ash borers. And American elms were almost wiped out by Dutch elm disease.
The long-fought battles with these pest species have offered Waring and her colleagues some guidance on where to start when it comes to managing OSS. In urban areas, ornamental trees are easily treated with pesticides and fungicides. If a homeowner has a tree in their front yard, they can scrub the scale off with a scraper, said Amanda Grady, an entomologist with Forest Protection Health.
"So much of this period is gathering information on how we mitigate it, like these silvicultural treatments for management, in the forest setting, because we do understand quite a bit about scale management in value trees, especially in urban areas," Grady said. "But we have not evaluated how or had the chance, because it's so new, to mitigate oystershell scale in aspen in the forest setting."
Silviculture treatments aren't ideal in the forest for several reasons. For one, it would be time-intensive and costly to spray and cut each infested branch or tree. Second, spraying large stands of trees with chemicals could have unintended consequences for the surrounding area and its inhabitants. The effects of DDT on birds is a famous example of pesticides gone awry.
Thus far, the most promising solutions appear to be a combination of clear-felling and fire. Aspens naturally sprout new shoots when cut and oystershell can only live on a live host. When a branch is cut, the tree regenerates and the insects die.
This still leaves room for some populations to persist on lower shrubs and bushes, which is where fire comes in. Fire is a natural disturbance in Arizona's forests and early treatments in the Prescott National Forest have shown no OSS activity post-fire.
Other potential management strategies could include natural predators. Richard Hofstetter, a professor of forest entomology at NAU, and one of his graduate students, Kelsey Pemberton, are looking into which potential native insects feed on oystershell and what sort of effect they might have on infestations.
To do that, they're taking inventory of what lives on the trees using simple sticky traps that collect bugs. They're also collecting scales and monitoring infested branches every two weeks, except during the winter. In addition, they hope to determine if natural predators differ between natural and urban areas.
So far, they've found mites, ladybugs, thrips, and wasps, all predatory insects that feed on oystershell. Those insects also prey on other insects and there simply aren't enough of them to have a significant impact on oystershell populations.
Another natural tactic might be fungus.
"There's a fungus called Beauveria bassiana. And it's ubiquitous around the globe. It's an invertebrate pathogen. But what's interesting is the fungus can persist within a plant," Hofstetter told The Republic. "So the idea is that if we can inject the fungus into plants, and it persists within them, once the insect feeds, it gets infected and dies within a few days."
While no one solution seems to be a silver bullet, if each treatment is able to dent OSS populations, that would be encouraging. It likely means that all options would have to be used in concert for effective population management. What's needed most of all, Waring said, is more funding to conduct more research.
For instance, another potential disturbance could be cold weather. The researchers have found OSS mostly at elevations between 2,000 and 2,500 meters, or about 6,500 feet to just over 8,200 feet. They theorize this could be due to colder temperatures as elevation increases. They have also found OSS more prevalent on the north side of tree trunks, which means there could be a role that solar rays play as well.
It's still too early to definitively conclude anything without more studies. The team is working on proposals to get more funding, but to do that, they have to show that OSS is a major problem and that there is a risk of losing something. It's hard to do that with such little information.
"We're going to have some really good data to begin answering some of the questions. We're still in progress because it's such a new problem," Waring said.
'An iconic species in the Southwest'
What's at risk may be hard to quantify in numbers but the loss would be great. Aspen forests make up less than 1% of Arizona's forests, according to Grady, but its golden metamorphosis is famous throughout the West.
The fall color change brings in thousands of tourists each year to the high country. It's around that time of year that tourists flock to Arizona's national forests like the Coconino, Apache-Sitegraves, and Kaibab to see the changing colors, bringing in millions of dollars in revenue to local businesses.
"Just to point out why aspen is so special, especially in Arizona and other parts of the West, is that, unlike the eastern forests, we don't have very many hardwood species occurring in the forests. Aspen is our most prominent species that changes color in the fall," Fairweather said. "And so that's why aspen is such an iconic species in the Southwest."
In Flagstaff alone, a pre-pandemic report stated that over 5 million people visited the city between 2017 and 2018, generating almost $60 million in tax revenue. And with nearly 50% of the visitors coming for outdoor recreation not tied to hunting, a great deal of that tourism is likely due to the changing fall colors produced by the quaking aspen.
If infestations of OSS are allowed to persist, researchers say Arizona could lose this key tree species in wild stands forever. That means losing not only the beauty and source of revenue they yield but also the significant ecological services they provide, such as standing as a fire break in conifer forests or providing refugia for species that like wetter environments, says Crouch.
"I think when people hear aspen or think of fall in the western U.S., it's really a critical component of kind of that aesthetic and cultural experience," Crouch told The Republic. "From a non-human standpoint, it is hugely important for birds and mammals and other insects that really depend on aspen because it's kind of one of the few deciduous trees that can form dominant stands across the west."
Lindsey Botts is an environmental reporter for The Arizona Republic/azcentral. Follow his reporting on Twitter at @lkbotts and Lkbotts on Instagram. Tell him about stories at email@example.com
Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow The Republic environmental reporting team at environment.azcentral.com and @azcenvironment on Facebook, Twitter and Instagram.
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