Ouray’s Fir Forest Infestation Is Here to Stay
by Samantha Wright
Oct 07, 2013 | 2005 views | 0 0 comments | 35 35 recommendations | email to a friend | print
MEET THE BEETLES – At an outreach session in Ouray last Thursday, U.S. Forest Service Entomologist Roy Mask showed an image of what a fir engraver beetle infestation looks like, from the inside out. (Photo by Samantha Wright)
MEET THE BEETLES – At an outreach session in Ouray last Thursday, U.S. Forest Service Entomologist Roy Mask showed an image of what a fir engraver beetle infestation looks like, from the inside out. (Photo by Samantha Wright)

‘They Have Got Us Outnumbered,’ Says USDA Entomologist

OURAY – What’s eating Ouray’s forests? About 75 concerned residents crowded into an information session at the Ouray Community Center on Thursday, Sept. 26, to find out the answer to that question, and to learn what, if anything, they can do about it. 

U.S. Forest Service Entomologist Roy Mask, a Service Center Leader at the USDA Forest Service Gunnison Service Center, was the meeting’s primary presenter. He told area residents that there are “two main players” that are infesting fir forests in and around Ouray – the fir engraver beetle and Douglas fir beetle. 

Each beetle is endemic to the area and exceedingly species-specific; the fir engraver beetle (scolytus ventralis) attacks white fir trees almost exclusively, while the Douglas fir beetle (dentdroctonus pseudotsugae) specializes in the Douglas fir tree. 

A geo-referenced map showing the results of a recent USDA aerial survey of the Ouray area revealed just how extensive the recent infestation has become. The map was covered with large clusters of pox-like orange blobs, or “polygons,” indicating patches of newly infested forest blooming like cancer in all directions around Ouray.

Passing around vials of fresh beetle specimens taken from white fir and Douglas fir trees in the Amphitheater Campground near Ouray that very afternoon, Mask said, “There is an awful lot of it going on. You are not going to make this go away. It’s not possible. They have got us outnumbered.” 

Barring dramatic cooling of the climate, Mask said, the infestation will likely continue unchecked in the coming years.



Fir engraver beetles are tiny – about the size of a grain of rice. The Douglas fir beetle species is a bit larger and can grow up to a quarter-inch in some cases. While there are some other important differences between the two species, their M.O. is basically the same. 

Females initiate the attack. Using their chewing mandibles, they bore their way through crevices in the tough outer layer of bark on their tree of choice – generally one that is drought-stressed or otherwise compromised – and into the juicy, living inner bark layer called phloem, which functions as a pipeline through which soluble nutrients generated by photosynthesis are passed downward to the rest of the tree. 

Just inside the phloem is the cambium layer – where tree ring growth occurs. In between the phloem and the cambium is the sweet spot where the beetles like to hang out.  

Once they get in there, female fir engraver beetles start building corona-shaped egg galleries around the trunk of the host tree, depositing eggs in niches on both sides of the gallery. When the small, white, legless larvae hatch from the eggs, each chews a new trail radiating vertically away from the egg gallery, scoring the living layers of phloem and cambium, and thwarting the tree’s ability to grow. The larvae’s journey ends in pupation, which occurs in the inner bark at the end of the larval galleries.

Adding insult to injury, the beetles also introduce fungi that travel beyond the bark and into the xylem layer of the tree, plugging up the cells that conduct water. 

“You are basically girdling it and strangling it at the same time,” Mask said. “That’s how the beetles are killing trees.” 

Douglas fir beetles have a slightly different pattern of infestation; the females create vertically oriented egg galleries that run lengthwise along the tree trunk, and the larvae radiate out horizontally around the tree’s circumference, etching a pattern on the inner layer of bark, but not on the cambium layer.

Both beetle species complete their life cycle from egg to larvae to pupa to adult in a single year’s time. The beetles also have “phenomenal reproductive potential,” Mask said. 

Infestation happens in the early summer. The beetles overwinter in the host tree as larvae and adults. When things warm up the following spring, their metabolism picks up, and they start their feeding pattern again. Most of the population flies out and colonizes new hosts in May and June. 

The beetles have adapted over the eons to survive severe winters in the San Juans. The only thing that could kill them off en masse would be a sustained cold snap of at least 10 degrees below zero in the fall or spring, when the bugs are most vulnerable. And Mask said, “we just don’t have those kinds of conditions here.” 



White firs and Douglas firs exhibit quite different symptoms of infestation, but in both cases, it takes a while for the tree to respond to what is going on. Thus, it is likely that the crown (needles and branches) of a freshly infested tree will remain green for some time, Mask said. 

“The tree doesn’t know that it’s dead yet, even though it’s got beetles inside of it,” he explained.

Crown fading typically occurs the year following infestation, by which time the beetles may have already moved on to another host. “By the time you see fading trees, it’s really too late,” Mask said.

In white fire trees, immediate signs of infestation include tear-like trickles of sap that seep out of the holes that the beetles have bored through the bark to get to the phloem inside. 

Infested Douglas fir trees have piles of boring dust (called frass) on the bark surface and eventually at the base of the tree. 

Endemic populations of both the fir engraver beetle and the Douglas fir beetle can infest broken tree tops and fallen trees as well as living trees, and will often colonize trees at sublethal levels, causing wounds and sometimes branch dieback from which the tree can sometimes recover. 



The unusual density of the forests near Ouray makes them particularly susceptible to beetle epidemics, particularly once their vigor has been compromised. And the fir trees around here are definitely compromised, Mask said. Not only are they drought-stressed; they are also still recovering from a Western spruce budworm epidemic that spread throughout the region in the 1990s. 

“It’s a connected world,” Mask said. “Years of defoliation by budworm sets up bark beetle infestation.” 

When trees become stressed, not much can be done to prevent infestation on a forest-wide scale. Even natural predators such as woodpeckers are unable to keep beetle populations in check once they reach epidemic proportions. 

On a forest-wide scale, the U.S. Forest Service utilizes several strategies to address beetle infestation, including “sanitation” (i.e. giving infested trees a free ride down to the lumber mill), controlled burns and thinning green stands.

On an individual basis, preventive trunk spraying is the only proven way to protect white fir trees from infestation. Three chemicals have been proven to be effective: carbaryl, permethrin and bifenthrin. “These will work 95 percent of the time, and have been rigorously tested,” Mask said. 

White fir trees need to be sprayed at least once a year, in the late spring or early summer before beetles fly out of their old host trees in search of new ones. 

Costs associated with spraying treatments vary with ease of access and the size of the tree, but typically range from $30 for a small tree to $60 for a large one, said Linda Corwine, whose Montrose Landscape Consulting and Spraying business treats trees in the area. For large treatment areas, Corwine charges by the gallon of insecticide rather than by the tree. 

While the sprays are toxic to fish and some “friendly” insects (such as bees), they are considered safe for mammals, Corwine said. 

It is also important for property owners to clean up beetle-attracting blown-over trees and fallen branches in a timely manner.

While fir engraver beetles appear to be thriving and multiplying in the stressed environment around Ouray, Mask said, “The good news for this beetle is that they do not overwhelm the entire forest at one time,” as is the case with the mountain pine beetle that has killed off vast swathes of evergreen forest in other parts of Colorado.

That’s because unlike the mountain pine beetle, the fir engraver beetle does not emit an aggregate pheromone to attract fellow beetles to an area. Rather, scientists believe that fir engraver beetles are able to detect favorable host trees by following the chemical signals emitted by the stressed trees themselves. 

Douglas fir beetles, in contrast, do communicate with one another via aggregate pheromones. The denser the stands, and the larger the diameter of the trees, the better the Douglas fir beetle likes it. Infestations generally occur in cycles; the beetles come in, take out the largest diameter trees in the densest stands, making way for younger trees to grow up, then the cycle repeats itself again. 

Individual trees and small groves of Douglas firs can be protected by using inexpensive anti-aggregate pheromone patches that can be attached around the perimeter of a property. There is no known anti-aggregate pheromone that will repel the fir engraver beetle. 

Freshly dead trees that are still holding all of their brown needles may still be infested with beetles and larvae. Once a tree has lost all its needles, most likely the beetles have moved on. One clue is to look for bore holes in the bark, revealing that the beetles have chewed their way out of the tree. 

Ideally, dead and infected trees should be cut down. If an infected tree is cut in the fall, it is important to strip and macerate the bark to destroy surviving beetle populations. “Debarking is not fun but that is where the eggs and insects are,” Mask said.

Mask recommended that residents focus on saving trees in so-called “high-value areas.” 

“You may not be able to save them all,” he said, “but you could save some of them.” 



All the extra rain this summer and fall has helped the fir trees around Ouray regain some of their vigor. However, Mask warned, “We are not out of the woods yet. There is always a hangover effect. The metabolism of trees is such that they don’t miraculously heal up. We had a phenomenal monsoon, but not enough to make the beetle infestation collapse. I wish I could tell you it’s all going to go away, but it will hang on for a while.”

The good news, he stressed, is that there will not be uniform mortality among the trees. The bad news? “As long as the forest continues to be stressed, the beetle will hang on.”  

More detailed information on tree descriptions and protection solutions can be obtained by contacting Ann Morgenthaler, City of Ouray, Community Development Coordinator, either by telephone at 970/325-7087 or email at morgenthalera@ci.ouray.co.us. swright@watchnewspapers.com or Tweet @iamsamwright

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