State foresters say pine needle scale infestation may be linked with heavy use of pesticides in war against pine beetles
By Bob Berwyn
FRISCO — Colorado forest experts are documenting an unprecedented outbreak of pine needle scale on conifer trees in Grand, Summit and Eagle counties that may be linked with the widespread application of pesticides used several years ago to try and kill mountain pine beetles.
The intensive use of those pesticides may have wiped out beneficial insects like predatory wasps and beetles that keep pine needle scale in check, said Granby-based Ron Cousineau, district forester for the Colorado State Forest Service.
“Most of the concentrated spray for mountain pine beetle ended about three, four or five years ago. That’s when we started seeing the buildup of pine needle scale,” Cousineau said.
Periodic needle scale infestations are common in Colorado mountain forests, but they typically have minimal impacts, very limited geographic scope and are short-lived, lasting one or two years. In contrast, local foresters say the current infestations have been widespread and have endured over the past four to five years in some areas.
There’s no clear indication that the pine needle scale outbreak is linked with climate change, but according to a U.S. Forest Service fact sheet, severe cold snaps are thought to limit outbreaks, and Colorado hasn’t experienced any significant winter cold snaps in the past few decades. That temperature trend was also thought to be a factor in the mountain pine beetle outbreak.
Most of the pine needle scale infestation is on private valley bottom lands. Foresters have not yet documented a heavy outbreak on public mountain forest lands in the backcountry.
“These infestations have become so heavy and persistent in some areas that we are seeing many trees die with no other insect or disease influence,” Cousineau said. “We have never seen pine needle scale become this damaging in this part of the state.”
Pine needle scales feed on needles, which can weaken affected trees and cause needle drop and dieback. Scale infestations also can lead to increased susceptibility to other insects or diseases.
During outbreaks, the scale-covered needles can give trees the appearance that they are spattered with white paint. The bugs themselves are so tiny that it takes a magnifying glass to see them in detail. Cousineau said he shook some of the affected tree branches above a paper plate, which enabled him to watch the bugs crawl across the plate.
The areas with the heaviest infestation of pine needle scale have been observed within or adjacent to locations that have been heavily sprayed to control mountain pine beetle over the past decade.
“In Vail, all the way through the valley, their spruce trees are getting heavily impacted,” Cousineau said, adding that there’s no simple simple solution to address the latest insect invasion.
“in honesty, we don’t have a magic bullet,” he said. The best thing people can do is promote healthy trees in those affected landscape settings in hopes that the trees can fight off the bugs, he said, adding that foresters are not encouraging people to apply more pesticides, because that might kill off even more of the beneficial bugs needed to maintain a natural balance.
In planted settings, proper tree care is a primary management strategy against scale insects, including pruning heavily infested branches. Road dust and air pollution are also thought be factors that can intensify a pine needle scale outbreak.
Cousineau said lodgepole pines generally keep their needles for three years. At the end of summer, those third-year needles turn brown and drop off naturally. The pine needle scale is attacking third- and second-year needles, so in some cases, the tree branches may only have needles on the tips, while the rest of the branches are bare.
Several natural enemies help control needle scale populations, including some predatory beetles and parasitic wasps. Impacts of scale infestations also may be mitigated using horticultural oils or insecticides, if applied at the appropriate time of year and precisely timed with the insect’s “crawler” stage.
“This is a tough insect to deal with. Spraying is not incredibly effective,” Cousineau concluded.