WHITE MOUNTAINS — Oh, no. Arizona’s dwindling aspen groves face yet another threat.
The US Forest Service last week warned that tiny, Oystershell scale insects are sucking the life out of aspen groves throughout Northern Arizona.
The tremulous, quaking aspen groves throughout the west have already dwindled, thanks to a century of wildfire suppression, drought and booming elk populations. The bizarre, sap-drinking insect that builds its own little castle represents the latest threat to the iconic tree of the mountainous west.
“Oystershell scale is one of the most damaging insects to aspen we have recently found," said Michael Sedgemen, a forester for the Kaibab National Forest. “We were already seeing a steady decline in Southwest aspen stands due to a number of environmental factors.”
Infestations have been documented on the Kaibab, Coconino, Apache-Sitgreaves and Prescott national forests.
The thousands of species of scale insects date back to before the dinosaurs and feed on a host of different plants. The females have no legs, but settle down on the bark of their preferred host plant – protected by a waxy, scale-like armored coating. The short-lived males can generally crawl and fly – driven by the need to mate. The females have mouth parts two or three times as long as their eighth-of-an-inch bodies, allowing them to suck sap out of the tree. The scale insects often can produce a drop of sweet fluid from the sap, which is the basis for a complex relationship with ants. Many ant species protect the scale insects, sometimes fending off predators and sometimes taking the females down into their colonies at night to milk the drop of sweet nectar. The relationship has apparently persisted for 200 million years, according to evidence found in fossilized amber.
Remarkably, the newly hatched young can become either male or female, depending on still poorly-understood environmental cues. They may molt into four different forms in the course of their complicated lifecycle.
All of this makes them a big challenge when they reproduce in such numbers that they cover a tree’s limbs and bark, sucking out enough fluid that they can kill the tree. Homeowners trying to save ornamental tree can use soaps or oils to remove the insects or simply scrape off the hunkered down females. Those sorts of labor-intensive approaches won’t work when the insects besiege a whole aspen grove.
The Kaibab’s once-extensive aspen groves have dwindled to about 2,000 acres, now heavily affected by the infestation.
The Forest Service has teamed with researchers from the University of Arizona to determine what sorts of treatments can slow the spread of the infestation, part of an ongoing effort to rescue the poplar with its rich fall colors and trembling leaves. The flutter of those leaves on the stem probably increases the ability of the leaf to catch and harvest the energy of the sunlight.
The experimental treatments to limit the spread of the scale insects will include thinning some groves, physically rubbing the armored insects off the tree and other treatments – with careful monitoring to figure out what works.
"Ultimately, we hope to find techniques that forest managers can use to reduce mortality and decline of aspen from oystershell scale," said Connor Crouch, a Ph.D. student in the School of Forestry at Northern Arizona University.
Aspen have for decades struggled against a host of other problems.
The century-long effort to stamp out wildfires may have played a lead role, since the fast-growing aspen are often the first trees to return after a fire. Wildfires clear away the overshadowing pines, allowing the aspen to glory in their time in the sun for the century it takes the pines to return. Ironically, now that high-intensity crown fires have overcome fire suppression efforts, the fires burn so hot neither the aspen nor the pines are returning in many areas.
Severe droughts in the past 50 years have also likely played a role. Studies in Colorado have demonstrated that a serious drought can damage the ability of the mostly shallow roots of the aspen to efficiently take up water, even when the rain or snow returns.
The explosion of elk populations in the west have also likely played a role. The elk love to browse on the shoots of the young aspen, limiting reproduction. Interestingly, aspen groves recovered in Yellowstone National Park when the reintroduction of wolves caused a sharp decline in the elk population.
All those factors likely worked in combination to kill off mature aspen and prevent the return of the saplings. Many aspen reproduce by growing clones from the roots of a founder tree. Sometimes, a forest of genetically identical clones can cover a whole hillside. This technically makes some aspen groves the oldest, largest single living thing in the world. But it also makes many trees so genetically linked that a single pathogen or insect species can devastate a whole mountain.