Description and Life Cycle
Spruce budworms are back! Last year, the Flathead Valley was hard hit by these caterpillars. These insect pests primarily attack Douglas firs. In May, they enter developing buds, the habit that gives them their name. As the new needles lengthen, the rapidly growing larvae continue to feed. It is during this stage that most of the damage occurs. They web the new foliage together loosely and feed inside, where they are somewhat protected from predators and other enemies.
In the late larval stages (Figure 3), budworms have brownish heads and brownish-olive bodies. Each body segment has two conspicuous pairs of white spots. About 40 days after feeding begins in the spring, usually about the end of June, the larvae pupate inside feeding webs or on foliage. Adults emerge a week or so later and the cycle is complete. There is one generation per year.
Control
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 such as thinning, watering and fertilizing, which promote tree vigor, may help trees better withstand repeated attacks.
Chemical control often is used to protect high-value trees from defoliation and associated damage. The materials listed below are registered for western spruce budworm control and have been used with success in Colorado. They can be applied both from the ground and aerially. In either case, time spraying to occur during the two to three weeks immediately following bud break or flush of new growth. In most years, this occurs about mid-June.
Insecticides for control of budworm
Bt (Bacillus thuringiensis) which goes under the trade names Dipel, Thuricide, Biobit and Foray (aerial spraying); and carbaryl (trade name Sevin).
Article reproduced from the Montana State University Extension Service Flathead County Newsletter.