Western Spruce Budworm - Field Guide to Diseases and Insect Pests of Idaho and Montana Forests

Western Spruce Budworm

Choristoneura occidentalis Freeman

From: Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, USDA Forest Service Northern Region, Publication Number R1-89-54

Hosts. – Douglas-fir, all true firs, spruce, and larch. May be found on pines.

Distribution. – Wherever hosts are found in Idaho and Montana; however, significant damage is most often found on dry sites.

Damage. – Larvae mine buds and old needles in spring, then consume new foliage as it appears (fig. 96). After several years of heavy defoliation, branch dieback, top kill, and tree mortality can occur. Cones and seeds of all host species are also destroyed. Terminal and lateral new shoots of larch are severed.

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Figure 96 - Early instar Western Spruce Budworm larva mines buds and feeds in developing shoots.	Figure 97 - Later instar Western Spruce Budworm web needles together to make silken nests in which they feed.
Identification. – Look for larvae or pupae in silken nests of webbed, chewed needles (fig. 97) from June until August. Small larvae first seen in spring and early summer are light green to light brown with darker heads. Mature larvae have brown heads and bodies with prominent ivory-colored spots (fig. 98). They can be about one inch long when full grown. Pupae are three-fourths of an inch long and brown and are found in the nests from mid-July into August. Adults are mottled rust-brown and have a wingspan of about seven-eighths of an inch. Female moths lay eggs on needles in a shingle-like pattern in August. Larvae hatch and immediately seek a sheltered spot to overwinter.	Image Unavailable
	Figure 98 - Mature Western Spruce Budworm larva. Note prominent ivory- colored spots.

Similar damages. – Larvae of western blackheaded budworm cause similar damage, but larvae are pale yellow with black heads and are somewhat smaller. Other defoliating larvae may be present but none with distinctive coloration and spots of budworm larvae. On pines, they may be confused with sugar pine tortrix which are generally smaller. Cone damage is similar to that caused by several species of cone worms.

Link to Images in Forestry Images

References.

Anonymous. 1982. For. Insect & disease identification and management. USDA For. Serv.,Northern Region; Idaho Dept. of Lands, Insect and Disease Control; Montana Dept. of State Lands, Division of Forestry. 192 p.

Fellin, D. G., and J. E. Dewey. 1986. Western spruce budworm. USDA For. Serv., For. Ins. & Dis. Leaflet 53, 10 p., illus.

Furniss, R. L., and V. M. Carolin. 1977. Western For. Insects. USDA Forest Serv., Misc. pub. 1339, 654 p., illus.

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