Proven organic strategies for controlling spruce budworm.
The western spruce budworm, Choristoneura occidentalis, is the most widely distributed and destructive forest defoliator in western North America. In the Rockies, they most commonly infest Douglas-fir and white fir. Occasionally, they also attack Engelmann spruce, blue spruce and sub-alpine fir.
Adult moths are about 1/2 inch long and have a wing-spread of 1 inch. Moths of both sexes are similar in appearance, although the females are a bit more robust than males. Both sexes fly. The gray or orange-brown forewings are banded or streaked, and each usually has a conspicuous white dot on the wing margin. Eggs are oval, light green and overlap like shingles.
Larvae develop through six stages. Newly hatched larvae are yellow-green with brown heads. In the next three stages, larvae have black heads and collars and orange or cinnamon-brown bodies. In the fifth stage, larvae have reddish-brown heads marked with black triangles, black collars and pale olive-brown bodies marked with small whitish spots. Mature larvae are 1 inch long, with tan or light chestnut-brown heads and collars and olive or reddish-brown bodies with large ivory-colored areas.
Throughout most of its range, the Western Spruce Budworm completes one cycle of development from egg to adult within 12 months. Moths emerge from pupal cases usually in late July or early August; in the southern Rockies, adults often begin emerging in early July.
The adults mate, and within 7 to 10 days, the female deposits her eggs and then dies. Each female deposits approximately 150 eggs, usually on the underside of conifer needles. Eggs are laid in one to three-row masses containing a few to 130 eggs, with an average of 25 to 40 eggs per mass.
Larvae hatch from eggs in about 10 days. Larvae do not feed, but seek sheltered places under bark scales or in and among lichens on the tree bole or limbs. Here, they spin silken tents in which they remain inactive through the winter.
In early May to late June, larvae leave their hibernacula to search for food. They first mine or tunnel into year-old needles, closed buds, or newly developing vegetative or reproductive buds.
New foliage, which is normally the preferred food, is usually entirely consumed or destroyed before larvae will feed on older needles. Larvae become full grown usually in early July about 30 to 40 days after leaving their overwintering sites.
Larvae pupate in webs of silk they have spun either at the last feeding site or elsewhere on the tree. The pupal stage usually lasts about 10 days.
- Budworm populations usually are held in check by a combination of predators, parasites, adverse climatic conditions or inadequate food supply.
- Spiders, insects and a variety of birds are important predators.
- Adverse weather conditions, particularly sudden freezes in late spring, may kill large numbers of larvae.
- A major factor in ending long-term outbreaks appears to be starvation from inadequate or nutritionally poor food sources. However, this may not be a factor in urban situations.
- Cultural practices (thinning, watering and fertilizing), which promote tree vigor, may help trees better withstand repeated attacks.
- Spinosad Spray and Monterey Bt are two popular biological pesticides that have been used to reduce budworm numbers and are SAFE to use around people and pets.
- Botanical insecticides — derived from plants which have insecticidal properties — have fewer harmful side effects and break down more quickly in the environment than synthetic chemicals. However, they are still toxic and should only be used after other least-toxic options have been tried. Applications should occur during the two to three weeks immediately following bud break or flush of new growth. In most years, this occurs about mid-June.
- Azatrol EC is a broad spectrum organic insecticide that acts as an insect growth regulator and anti-feedant. When applied to nursery and landscape plantings it will disrupt the life cycle of this pest and deter them from feeding.
Note: Azadirachtin, the active ingredient in Azatrol, is extracted from neem seed and formulated to provide broad spectrum insect control with very low environmental impact. It is relatively NON-TOXIC to spiders, beneficial insects and honeybees.
Photo Credit: Natural Resources Canada