Western spruce budworm, Choristoneura freemani
The western spruce budworm is native to North America and can defoliate Douglas-fir, true fir, and spruce trees. The moths are 13 mm (1/2 inch) long and have a wingspan approximately 1 cm (1 inch). Female moths are slightly larger than males. Both sexes have gray or orange-brown wings with bands or streaks, and the wing margins of some individuals have a white dot. The eggs are about the size of a pinhead, oval shaped, and overlap like shingles.
The appearance of larvae changes as they mature through six instars. After emerging from the eggs, larvae are yellow green with brown heads. The next three instars have orange or cinnamon bodies and black heads. The fifth instars have white spots on pale olive brown bodies and black triangles marking red or brown heads. The sixth and final instar reaches lengths of 25–32 mm (1-1¼ inches) with tan or light brown heads and olive or red brown bodies. Mature larvae also have ivory markings in certain areas.
Pupae are broader at the head and narrower at the tail end. They measure 13-16mm (1/2-5/8 inch) in length and change color as they develop. Initially, they are brown to yellow or green but later turn red-brown.
- The western spruce budworm is a native insect of North America that defoliates Douglas-fir, true fir, and spruce trees. Entire trees can be defoliated when outbreaks occur over successive years, and budworm larvae can also feed on reproductive structures.
- Forest management practices are the most effective strategy for controlling outbreaks of western spruce budworm. This includes reducing the density of suitable host trees in the forest and removing young trees located under the canopy of mature trees.
- Natural enemies of western spruce budworm include parasitoids and predators. Both can help limit budworm populations in forests when densities are low.
- Preventative applications of Bacillus thuringiensis or insecticides can be used to protect individual trees near homes, businesses, or other recreation sites.
Adult of western spruce budworm. Note the grey wings with brown bands. Image credit: William M. Cielsa
Caterpillar of western spruce budworm. Image credit: Dan West, CSFS
Pupa of western spruce budworm. Image credit: Dan West, CSFS
Defoliation due to feeding on western spruce budworm. Image credit: Dan West, CSFS
Life history and habits
Eggs are laid on the underside of needles in masses of about 25-40 eggs and hatch about 10 days later. Newly emerged larvae do not feed and instead construct a silken overwintering chamber called a hibernaculum. Overwintered larvae become active the following spring when leaf buds begin expanding on host trees, at which point they begin feeding on buds and flowers before boring into expanding buds. As they mature, larvae exit buds to feed on foliage and shoots and produce webbing on the branch tips for shelter. They feed in high numbers and can destroy most or all the new growth by mid-summer.
Pupation occurs in the silken web used for feeding or on twigs or branches. Adults emerge 10-15 days later. There is one generation produced each year, and the full life cycle is usually completed within 12 months but can take longer at higher elevations.
Larvae become full grown approximately 30 to 40 days after emerging from overwintering hibernacula. When fully grown, larvae of the western spruce budworm pupate in webs of silk for about 10 days until they emerge as adult moths.
Feeding of western spruce budworm larvae causes discoloration of foliage from the top of the tree downward and can cause significant injury to ornamental conifers. Since larvae only partially feed on needles, the needles are left intact but desiccate and turn a red-brown color. Entire trees can be defoliated when infestations are present successive years.
In addition to feeding injuries on foliage, heavy feeding on flowers and developing cones of host trees can reduce seed production. This species does not restrict its feeding to a single cone, and often second or third stage larvae feed on multiple developing cones which causes them to desiccate and drop from the tree. Cone production can be disrupted for multiple years under persistent heavy defoliation.
Ongoing forest management is the most effective strategy to reduce injury from western spruce budworm. Mixed conifer forests with a high proportion of Douglas-fir, true fir, and/or spruce trees are most susceptible to outbreaks of western spruce budworm. Reducing the proportion of suitable host trees while increasing non-host trees such as pine and aspen will help limit outbreaks. Areas with abundant host trees in the understory are especially susceptible to outbreaks since larvae can drop from the canopy of larger trees. Removing young host trees located under or directly adjacent to the canopy of older trees should be considered. Creating wider spacing between trees will also reduce the number of western spruce budworms dispersing to nearby hosts.
Western spruce budworm populations are regulated by natural enemies, including parasitoids and predators. Parasitoids of western spruce budworm include over 40 species of flies and small wasps, with four or five species being the most common. Both invertebrates and vertebrates feed on western spruce budworm, including spiders, ants, true bugs, certain beetle larvae, small mammals, and birds. Natural enemies of the western spruce budworm are responsible for significant mortality when budworm populations are low but have minimal impact when budworm populations are high.
When outbreaks of western spruce budworm occur in large, forested landscapes, preventative aerial applications of Bacillus thuringiensis (Bt) can provide some control. Sprays containing Bt or insecticides can also be applied to individual high-value trees near homes, businesses, or recreation sites. However, these treatments can be too costly to apply on the landscape scale, and overuse of insecticides can have negative environmental effects. It is recommended that preventative sprays be done by a certified applicator.
CSFS. 2016. Western Spruce Budworm. Colorado State Forest Service – Quick Guide Series. Available https://csfs.colostate.edu/wp-content/uploads/2014/02/Western_Spruce_Budworm_QG_10May2016.pdf
Fellin, D., and J. Dewey. 1982. US Department of Agriculture: Forest Service – Forest Insect & Disease Leaflet. Available https://www.fs.usda.gov/foresthealth/docs/fidls/FIDL-53-WesternSpruceBudworm.pdf
Kuhns, M. (n.d.). Western Spruce Budworm in Utah: Identification, Management, and Control. Utah State University – Extension. Available https://extension.usu.edu/forestry/rural-forests/forest-management/spruce-budworm