Forest Insect Defoliators
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Western Spruce Budworm
Choristoneura occidentalis (Freeman)

Hosts: Douglas-fir, true firs, and spruce

Figure 3. Adult western spruce budworm with pupal case.Symptoms/Signs:  Adults are a mottled orange brown and have a wing span of 22 to 28 mm. Eggs are white to light green in color and are laid in shingle-like masses on the underside of needles. Newly hatched larvae are cream colored with brown heads. Full-grown larvae are 25 to 32 mm long with brownish head and body and prominent ivory colored spots. Pupae are 12 to 16 mm long, broad at the head and tapering toward the tail. Signs of feeding include current foliage being partially or fully chewed over the entire tree; expanding buds mined and evidence of feeding on second and third year shoots possible; and shoots webbed into feeding shelters giving the tree a reddish-brown appearance.

Figure 4. Egg mass of western spruce budworm on underside of needle. Note shingle-like feature.Biology:  Eggs are laid in July and August and hatch in about 10 days. The initial larvae do not feed, but spin silken shelters (hibernacula) under bark scales or lichens where they will hibernate. The next spring larvae mine old needles until the buds swell and then bore into the buds and feed on the expanding needles. Later they web the growing tips together and feed on the new needles. The larvae pupate in June and July. Adults begin to appear in July and start egg laying for the next generation.

Effects:  Defoliation by western spruce budworm can cause growth loss after 1 or 2 years. Repeated heavy defoliation (4 or 5 years) can cause a significant decrease in growth, tree deformity, top-killing, and ultimately tree mortality, particularly in seedlings and saplings.

Figure 5. Late instar larva of western spruce budworm feeding on foliage.Similar Insects and Diseases:  The white fir needleminer, Epinotia meritana, feeds inside the needles of true fir. Mined needles are a bleached yellowish color and do not have a chewed appearance. The mature larvae are much smaller (8mm) than western spruce budworm and yellowish green to cream colored. The adult moths have wings that are dusty gray with alternating bands of dark scales. Wing spread is 11 mm. Repeated defoliation by white fir needleminer can cause branch mortality, deterioration of tree crowns and increased susceptibility to fir engraver.

References:  19, 23

Figure 6. Defoliation of Douglas-fir tree caused by western spruce budworm.Figure 7. Near view of western spruce budworm defoliation to Douglas-fir.