Colorado Forest Health

tussock mothIrene Shonle, Director CSU Extension in Gilpin County.  The 2015 Report on the Health of Colorado’s Forests from the Colorado State Forest Service is out. As always, there are fluctuations in insects and disease as well as a mix of good news and bad news around the State.

The good news is that the forests of Gilpin and Boulder Counties are in pretty good shape.  The Mountain Pine beetle that caused so much devastation in other areas (particularly the Western Slope) has now mostly abated. We will still see the normal, background activity of this native insect, but we can relax (for now, anyway). It’s never a bad idea to monitor your forests to see if you have a hotspot outbreak, but it is unlikely that we will see widespread outbreaks in the near future.

The most damaging and widespread insect in Colorado (for the past four years) is now the spruce beetle. A total of 409,000 acres of active infestation were found in high-elevation Engelmann spruce forests around the State. Although the new acres were slightly down from previous years, the overall footprint increased.

Blowdown events, combined with long-term drought stress, warmer temperatures and extensive amounts of older, dense spruce have contributed to this ongoing epidemic. So far, we are not seeing much popping up locally, although there is considerable activity near Rocky Mountain National Park.

The prize for the next biggest insect outbreak goes to the Western Spruce Budworm, with 312,000 infested acres. These outbreaks were all well to the south of us, but could move northwards in future years.

The closest insect of local concern is the Douglas fir tussock moth. Although the defoliation was only 26,000 acres statewide, this insect was found in Boulder County and along the Front Range. It is not of great concern since outbreaks are usually isolated and have not historically turned into major events. Further, ground surveys have documented the presence of a naturally occurring virus called “wilt disease” among the caterpillars, which has historically been a key indicator of imminent population collapse. This is one of its natural controls – others include wind dispersal and parasitic wasps.

However, since this critter has been seen in our general vicinity, it may be worthwhile to familiarize yourself with it.  The larvae can strip all the foliage from infested trees, leaving them weakened and susceptible to attack by bark beetles. The caterpillars are quite distinctive (see picture). Mature larvae are about 1-1¼ inches long and have dark tufts of hair behind the head and at the rear of the abdomen. Four cream-colored tufts of hair, or tussocks, with an orange-red colored band on top are located behind the head and at the rear of the larvae, giving these moths their common name. Another reason to learn to recognize the Douglas fir tussock moth caterpillar is that their hairs can cause skin irritation in humans, and a rash known as “tussockosis.” As gaudy as the caterpillars are, the adult male moths are small and unassuming; they are gray-brown to dark brown with feathery antennae and a wingspan of about 1 to 1¼ inches.  Fascinatingly, adult females do not have functional wings and stay on the pupal case from which they emerged, emitting a pheromone that attracts males; mating occurs shortly after emergence. Eggs remain dormant over winter and hatch from late May to early June, which coincides with bud break and new shoot growth on host trees.

Finally, we are literally above another outbreak that is on the verge of becoming widespread, expensive, and devastating.  The Emerald ash borer is poised to take out millions of ash trees in the metro area, with predicted impacts of more than $82 million in the Denver area alone. Luckily, we don’t have ash trees up here (our native mountain-ash is an entirely different species and not susceptible).

The CSU Gilpin County Extension Office is located at the Exhibit Barn, 230 Norton Drive, Black Hawk, CO 80422, 303-582-9106, Colorado State University Extension provides unbiased, research-based information about, horticulture, natural resources, and 4-H youth development. Colorado State University Extension is dedicated to serving all people on an equal and nondiscriminatory basis.

Irene Shonle can be reached at (303)582-9106 or by email:

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