by Emily
Jack-Scott (Garfield County Master Gardener Apprentice)
The spruce
beetle (Dendroctonus rufipennis) is
currently the leading insect responsible for killing trees in Colorado, having usurped
mountain pine beetle in 2012 (2017 Report, CSFS). These bark beetles have swept
through hundreds of thousands of acres of forested lands in the Colorado
Rockies in recent years. Since 2000, close to 2 million acres have been
impacted across Colorado. Counties most heavily impacted between 1996-2018
include Hinsdale, Mineral, Saguache, Gunnison, Conejos, and Rio Grande; each
experiencing hundreds of thousands of affected acres. (2018 Report, CSFS)
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| Figure 1 Tree mortality in Colorado caused by mountain pine beetle vs. spruce beetle. Spruce beetle overtook pine beetle as the lead insect pest in 2012. Credit: 2017 Report on the Health of Colorado's Forests (CSFS) |
Spruce
bark beetles are a native species, endemic to the Rocky Mountains. In Colorado,
the beetles favor high alpine (above 9,000’) Engelmann spruce, but will also
attack Colorado blue spruce and Norway spruce, and at lower elevations. At
usual low endemic levels the beetles target dead trees, from windfall events or
the like. But once they rise to epidemic levels they will attack live trees,
initially favoring larger diameter trees (over 16” diameter), progressively
targeting smaller spruces down to 3” diameter. Larvae overwinter under the bark
of infested trees, emerging as adults and flying to new host trees between May
and July the following year. Once they find a new host, adults chew through a
host tree’s bark to tunnel around in the tree’s cambium and outer-most sapwood
just beneath the bark, creating elaborate tunneling patterns known as galleries.
It is in these galleries that they will lay their eggs, which will hatch into
larvae in the fall and start the life cycle over again (2018 Report, CSFS; CSFS
Quick Guide).
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| Figure 2 Tree mortality caused by spruce beetle. Credit: USDA Forest Service, Region 6, State and Private Forestry, Forest Health Protection. Source: William M. Ciesla collection; Fort Collins, Colorado. |
What to look for:
Signs include the small dark brown
beetles or white creamy larvae themselves. Symptoms include frass (insect poop,
appearing as a fine sawdust from boring activity) collecting in the furrows of
bark along the trunk, thin streamers of sap running down the trunk, visible
small holes in the trunk with or without pitch tubes (see Figure 3), increased
woodpecker activity, and/or the yellowing and eventual dropping of needles. Unlike
pines attacked by mountain pine beetle the needles do not turn a vibrant red
before falling off. Rather they fade to a sickly green before drying out
entirely and falling off over time (CSFS Quick Guide).
 |
| Figure 3 Pitch tubes Credit: USDA Forest Service |
Spruce
beetle is not confined to forested areas, and therefore should be on the radar
of gardeners and landscapers in the mountains. Spruce beetle can sometimes
favor trees in landscaped and urban settings, which may be under additional
pressures and adverse growing conditions. Other factors that can make trees
more at risk are drought stress, recent fires, increasingly mild winter low
temperatures, and abundance of spruce in an area (Spruce Beetle UAF).
The
Colorado mountains have not only experienced these stressors in recent years,
but most recently incurred historic avalanches during the 2019 winter. These
avalanches resulted in the disturbance, uprooting, and death of countless
spruce, serving as magnets for spruce beetles. This will likely increase spruce
beetle pressure in forests and yards of the high mountains that were otherwise minimally
impacted over the last few decades (see map below of recent spruce beetle
activity in Colorado).
What you can do:
Options
for prevention are limited. Pyrethroid insecticides can be sprayed on tree
trunks during flight windows (May-July), and very new research is confirming
that certain formulations of MCH[S1] [EJ2] pheromone packets (namely MCH-AKB)
has efficacy deflecting beetle attacks. These pheromone packets release a scent
that sends a false signal to beetles that a tree of forest stand has already
been infested by spruce beetles, so new beetles pass over such trees (Hansen et
al. 2019). Once trees have been attacked, they should be felled and either
completely removed from a location (including chips and slash), or should be
cut and stacked in an area with full sun and covered completely with clear
plastic. (Spruce Beetle, CSFS)
Sources
2017
Report on the Health of Colorado’s Forests. Colorado State Forest Service.
2018
Report on the Health of Colorado’s Forests. Colorado State Forest Service. https://csfs.colostate.edu/media/sites/22/2019/03/FINAL-307714_ForestRpt-2018-www.pdf
CSFS Quick Guide – Spruce
Beetle FM-2014-1. https://csfs.colostate.edu/media/sites/22/2014/02/Spruce-Beetle-QuickGuide-FM2014-1.pdf
Hansen, E.M., Munson, A.S.,
Wakarchuk, D., Blackford, D.C., Graves, A.D., Stephens, S. and Moan, J.E.,
2019. Advances in Semiochemical Repellents to Mitigate Host Mortality From the
Spruce Beetle (Coleoptera: Curculionidae). Journal of economic
entomology.
Spruce
Beetle – Trees/Forests at Risk. University of Alaska Fairbanks. http://sprucebeetle.open.uaf.edu/2-module-2/
Spruce
Beetle. Colorado State Forest Service. https://csfs.colostate.edu/forest-management/common-forest-insects-diseases/spruce-bark-beetle/
Despite
beetle threat, Aspen-area avy debris to remain. Aspen Times. July 2, 2019. https://www.aspentimes.com/news/despite-beetle-threat-aspen-area-avy-debris-to-remain/
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