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Western Spruce Budworm
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| Western spruce budworm damage |
| DNRC |
Host: Douglas-fir, grand fir, subalpine fir, spruce, and larch. (Will occationally feed on pines during outbreak.)
Distribution: Throughout range of susceptible trees with most notable outbreaks currently in the Helena and Bozeman regions.
Identification: Silken masses of chewed needles; needle loss; branch and crown dieback.
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| Western spruce budworm feeding |
| DNRC |
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| Western spruce budworm feeding |
| DNRC |
Ecology: Western spruce budworm prefer warm, dry sites and stands with dense, multil-layered canopies of suitable host trees.
Western spruce budworm populations may occur cyclically with long periods between outbreaks, or populations may be chronically present with only short durations between outbreaks.
Life Cycle: Caterpillars feed on needles in the spring, feeding first on old needles and buds until the new foliage emerges. Later in the summer, they spin silken shelters in which they feed and pupate. Coppery brown moths emerge in August, mate, and lay eggs on the underside of needles. Eggs hatch in late summer and the young caterpillars migrate to bark crevices for the winter.
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| Western spruce budworm |
| Forestry Images |
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| Western spruce budworm larvae |
| S. Tunnock |
Management
Silvicultural manipulations: Fire exclusion has promoted dense, multi-storied stands consisting of tree species favored by western spruce budworm. Therefore, management activities that emulate the effects of fire are often beneficial in reducing the impact of western spruce budworm. Even-aged management practices reduce canopy layers and may disrupt dispersion of caterpillars, thus limiting populations. Reducing overstory tree density will also enhance the vigor of residual trees by alleviating competition for light, water, and nutrient resources. Vigorous trees will more effectively rebound after defoliation subsides. When thinning, retain trees that appear to be less impacted; these trees may have advantageous genetic traits. If possible, convert to non-host, early seral species by retaining ponderosa pine and western larch during thinning and planting these species.
Natural predators: Aquatic habitat conservation, snag retention, and coarse woody debris accumulation may enhance the survival of birds and insects that prey on western spruce budworm.
Insecticides: Various chemical and bacterial treatments are currently registered for western spruce budworm control. The bacterial insecticide, Bacillus thuringienses kurstaki (commonly referred to as Btk) is naturally occurring and only affects moths, butterflies, and skippers. Insecticide applications must be done at critical times and require monitoring of insect development. As always, closely read and follow the label! Please contact the DNRC Forest Pest Management Program coordinator or a licensed forest pesticide applicator to discuss this option.
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| Forest Insect and Disease Leaflet (FIDL) | Field Guide - Identification | Management Guide | ||
Additional Common Forest Insects in Montana
| Douglas-fir Beetle | Red Turpentine Beetle |
| Douglas-fir Tussock Moth | Spruce Beetle |
| Fir Engraver | Western Pine Beetle |
| Mountain Pine Beetle | Western Spruce Budworm |
| Engraver Beetles (Ips species) |
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