CO-Horts Blog

Wednesday, May 13, 2015

Western Spruce Budworm Outbreak Potential

Posted by:  Mark J. Platten, Teller County Extension Director

The western spruce budworm (WSBW), Choristoneura occidentalis Freeman, is the most widely distributed and destructive forest defoliator in western North America.  An outbreak has been prevalent throughout Teller and several adjacent counties the past three-to-four years and is likely to continue into 2015.  Their primary host is the Douglas-fir, although they will also be found on white fir, Engelmann spruce, blue spruce, and subalpine fir. 

The budworm larvae emerge from their hibernacula in early May through late June and begin feeding on old needles until the new buds emerge and then they feed on the new growth, hence their name.  They emerge as tiny larvae, approximately 1/8-1/4 inch, with yellowish-green bodies and a brown head. 
Juvenile Larvae
As the new needles continue to lengthen, the rapidly developing larvae continue to feed.  It is during this phase that most of the damage occurs when they loosely web the new foliage together, feeding in relative protection from predators.  You may not even notice them until they drop, or hang, from the affected trees, attached by what appears to be spider-like threads. 

Webbing of New Tips

They go through six stages of growth with the final larvae between 1-1.25 inches in length, with tan or light-brown heads, and brownish-olive bodies.  Each mature body segment has two conspicuous pairs of white spots. 

Mature Larvae

This process of growth takes approximately 40 days, at which time the larvae pupates and the adult moths emerge 7-10 days later. 

Pupal Casing

Some of the first moths emerged last year in Teller County the middle of July.  

Western Spruce Budworm Moth

After mating, the females lay masses of overlapping, green eggs on the undersides of host tree needles.  The young larvae hatch in approximately 10 days and move to crevices under bark scales, or lichen, where they spin silken hibernacula and overwinter.  This completes their cycle, with one generation per year. 
The greatest impact to mature trees is reduced growth because new needles photosynthesize more efficiently than mature needles.  Multiple years of defoliation can lead to branch tip loss, top death, and even tree mortality.  Saplings and young stands directly beneath the mature host trees are especially affected when the larvae disperse from above.

Tip Defoliation

Even if the WSBW doesn’t kill your trees, the injury and stress will make your trees more susceptible to secondary infestations of Douglas-fir beetles and other insects/diseases, which may lead to the death of your trees. 

In most years, the natural predation via arachnids, parasites, climate, and birds will keep them in check.  Adverse weather conditions, especially sudden freezes toward the end of May when the larvae have just emerged, could kill a significant portion of the larvae.  Unfortunately, with our relatively mild winters over the past decade, this may not be likely.  With three plus years of fairly heavy outbreaks in the area, you may want to consider other measures.

Cultural practices such as thinning, watering, and fertilizing enhance tree vigor, which may help them withstand repeated attacks.  Chemical control is often used to protect high-value trees, much the same way as we protect against mountain pine beetles.  For more information on chemical use, please see Washington State University Extension’s Forest Health Note:  One successful control agent is a naturally-occurring bacteria, Bacillus thuringiensis, or B.t.  It is specific to larvae without having any adverse effects on the environment.  See the CSU Extension Fact Sheet for more information on B.t.

It is often cost prohibitive to spray your entire property, especially if you have large parcels of land; although, there are several subdivisions in the region that have conducted aerial spraying.  There tends to be a minimum number of acres required by the aerial operators.  In Teller County, it is 450 acres minimum with a cost around $55 per acre.   

Even with aerial spraying, only the top of the canopy is covered. Between spraying and predation, hopefully the outbreak can be put in check and most of the trees saved.  Whether conducting individual or aerial control, the best time to spray is the two-to-three weeks following bud break, generally occurring early, to mid, June.
For a list of forest contractors who may be qualified to spray your individual trees, please contact your local Extension office, or the Colorado State Forest Service


  1. Great article Mark! Thanks for the helpful tips and info.

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  3. Wouldn't B.t. also affect native species, especially if aerial sprayed?

    1. Since Bt occurs naturally and is relatively host specific, it is not nearly as detrimental as other control methods. For more information on Bt, please see our CSU Fact Sheet:

      Mark J. Platten