Spruce Budworm in the Bighorn Mountains

In recent years, spruce budworm activity has increased in all counties in the Wyoming Bighorn Mountains and foresters and land managers are growing concerned. Based on aerial surveys, in Washakie County, the number of acres affected jumped from 130 in 2018 to 1,700 in 2019 – a thirteen fold increase – and in the Bighorn National Forest, over 6,500 acres were affected in 2019, up from 4,700 in 2018.

This forest pest is native to forests in the western U.S. and Canada, including the Bighorn Mountains, and is always present although it usually does not cause visible tree mortality.  Historically, populations occasionally have had epidemic events such as what we are seeing in localized portions of the Bighorns.

The sprucebudworm is about an inch long, in both the larva and moth stages. In the Bighorns, it generally infests Douglas fir, subalpine fir, and occasionally Engelmann spruce.  The moth larva feed on the buds and new foliage of the host trees.

While they only occasionally kill trees, repeated defoliation decreases tree growth, causes dead tops, results in a few dead trees, and stresses the defoliated trees, making them susceptible to being killed by bark beetles.  Budworm larva also destroy the buds that produce cones, thus reducing seed production in budworm-infested areas. While older, larger trees show the most damage, the budworms will then drop to understory trees where they reproduce. Thus, one treatment option is to thin understory trees.

Tree crown foliage becomes sparse in individual infested trees.  Where the insect is epidemic, the affected forest will have a reddish tint.  Several areas in the Bighorns have infestations that have been active for enough consecutive years that trees are being killed.

Control of this insect pest is generally limited to reducing density of host tree species to decrease stress on individual trees in infested timber stands and adjacent un-infested stands considered susceptible to infestation because of overstocking.  Aerial application of insecticides and microbial organic pathogens have been used in the past; however, this control method is expensive and is only effective if done as a coordinated effort by all forestland owners in the budworm infested area.

Public land managers have started planning and implementing harvest and understory tree thinning in some of the budworm infested timber in the Bighorns, with the intention of reducing budworm impacts.

Without actions by human, budworm populations typically “crash” naturally after a few years because of a combination of specific weather (temperature and precipitation) events; predation of the larva by native birds and other wildlife; reduction in available food sources; and natural parasites and pathogens that eventually develop in large populations.

Wildlife advocates credit spruce budworm with creating new snags (dead trees) and downed woody debris in areas that were deficient of wildlife needs, and in opening up conifer stands to provide forage for big game, and diversity for all wildlife.

The current focus of Bighorn Basin Firesmart and other agencies’ work is in the wildland urban interface (WUI), reducing wildfire risk to homes and properties.

If you’re a landowner in Wyoming who thinks you have spruce budworm on your property, contact your local State Forestry office (see sidebar for contact links and phone numbers). State foresters can then evaluate the situation and make recommendations. If you live outside of Wyoming, contact your state’s forestry office or extension service.