Forest and Wildlife Extension

Forest Health Notes

A Series for the Non-Industrial Private Forest Landowner

Western Spruce Budworm
(Choristoneura occidentalis)

Hosts: Grand fir, subalpine fir, Engelmann spruce, Douglas-fir, western larch

Quick ID:

• Foliage with scorched or blighted appearance at tips
  
• Needles bound together with webbing at branch tips
  
• Defoliation caused by 1" or smaller caterpillars
  
• Caterpillars brown with green markings, white spots on sides, not hairy
  
• Topkill or death following defoliation

Field Identification

Tree: Affected trees will have foliage with a scorched or blighted (as by frost) appearance at the tips of branches. Early in the summer, when budworms are feeding on new foliage, needles will be bound together at the tips with webbing. Defoliation is most obvious in the upper crown and at branch tips. Tree decline, topkill, and death may follow severe defoliation.

Insect: Adult insects are orangish-brown triangular moths, a little less than an inch across. Egg masses on needles are pale green. Larvae go through several development stages, beginning as tiny green caterpillars with brown heads and finishing as 1" long, olive-brown caterpillars with pronounced ivory "warts" on either side. They are not hairy; if you have a defoliator that is hairy
it's probably Douglas-fir tussock moth. Pupae are golden-brown.

May be confused with: Frost damage, sawflies, Douglas-fir tussock moth.

Life cycle: Adults lay eggs in a shingle-like fashion on the underside of needles in midsummer. Eggs hatch in about 10 days. The larvae molt once, then hibernate for the winter on rough bark surfaces in the canopy and down the bole. In the spring, larvae re-emerge and begin to tunnel into older foliage, or float away on silken threads. After about two weeks, they move on to expanding buds and finish with the new foliage. Larvae spin webbing among the needles of expanding shoots, and feed until disturbed or the shoot is completely destroyed. When feeding on new foliage early in the summer, budworms construct a shelter by binding the needles together at the tips with webbing. They retreat to this shelter when disturbed. In midsummer, they pupate either in their feeding webs or in other webs in the foliage. Adults emerge from the pupae and fly to seek a mate.
At night, females emit pheromones to attract males; after mating, females begin to lay eggs a day later.

Dispersal redistributes budworms within the crown and between trees and stands. It may occur at any point in the life cycle, but most commonly it is the younger larvae that are carried by the wind to other trees. This can be significant as the tallest trees are preferred by the egg-laying adults, and larvae are blown to shorter, intermediate or overtopped trees. Adults are also dispersed by wind currents and over a long range by frontal systems.

Predisposing factors: Pure stands of firs and spruce are more susceptible to infestation and receive more damage than mixed stands that include other species the budworms don't eat, like pine, hemlock and western redcedar. Physiological stress in host trees influences susceptiblity to attack and concurrent damage to a high degree. Agents of stress can take many forms: drought, overstocking, disease, insect attack, poor site conditions, fire, harsh frost events, etc. Nonvigorous
trees are more susceptible to attack and receive more damage than vigorously growing trees. Dominant and codominant trees are more susceptible to infestation than are intermediate and overtopped trees, but overtopped and intermediate trees receive more damage when attacked. Stands managed under uneven-aged structure are more susceptible and suffer more damage than stands that are managed for even-aged structure. Budworms are also favored in stands that are overmature, as trees in those stands are usually declining in vigor. However, most tree mortality occurs in regeneration, sapling and pole-sized stands. Warm, dry spring weather and warm, dry sites in general favor budworms.

Impact: As western spruce budworm is a defoliating insect, it doesn't necessarily kill trees. However, severe defoliation will kill a conifer in one year, and repeated infestation year after year will severely weaken the tree, causing loss in growth and predisposing it to attack by bark beetles.

Management: Direct control of spruce budworm has been attempted on many occasions during this century, and still is ongoing in places. Chemical spraying operations, although occasionally highly successful in reducing budworm populations, have fallen out of favor as questions about undesirable impacts on the ecosystem have been raised Fine-tuning of insecticides against the budworm are ongoing. One successful agent that has been discovered is a naturally-occurring
bacteria, Bacillus thuringiensis, or B.t. It is specific to moths and butterflies and quite effective in knocking the population back without having any adverse effects on the environment.

Up until this century, true fir stands were limited to cooler, wetter areas that were less subject to fire. After the advent of fire suppression, true firs took over drier sites from pine. While this was not "abnormal" succession, it has lead to conditions that favor the spruce budworm. Whether or not we are entering a long-term period of global warming and climatic change, the Pacific Coast has always been subject to periodic droughts that can last for several years. Insufficient water reduces the tree's ability to defend itself and at the same time increases the nutritional quality of the foliage to spruce budworms. When all interrelated habitat characteristics, including diversity in crown-class structure, are analyzed collectively, budworm habitat has improved as the forest has changed from pine to fir.

To lessen the damage, agents of stress and other predisposing factors should be removed. It is not possible to control abiotic events such as drought, but management activities can lessen the severity of drought and other agents of stress:

On warm, dry sites, stand managers should choose species well-adapted for them such as pine and larch. That may involve planting these species and thinning or eliminating true firs. On good fir sites, encourage species diversity through thinning and planting. Western larch is more resistant to budworm than true fir, spruce or Douglas-fir, and western redcedar, western hemlock, and all pine species are not considered to be more than occasional hosts. Climax budworm host species (true firs and spruce) should not make up more than 1/3 of the stand. Use silvicultural treatments to maintain stands at a proper stocking level and increase stand vigor. At age 10, 300 to 600 seedlings/acre should be appropriate, depending on site conditions and seedling distribution. Thinning from above will not reduce stand susceptibility to spruce budworm. Manage even-aged patches rather than uneven-aged stand structure. Clearcutting, seed tree cuts or shelterwood cuts
may be used, depending on the site. If using a seed tree or shelterwood cut, remove the overstory within 10 years. Creating a mosaic of varying stand conditions over a large area may help. Cut intermediate and overtopped host species in a middle-aged stand. Harvest on time so the stand does not become overmature. Subject to other management constraints, a large harvest area is preferable to a small one as insects, both larvae and adults, are found with decreasing frequency as one moves from the edge to the center of a cut. Harvest scheduling should concentrate on cutting the most susceptible stands first and regenerating them to thrifty, fast-growing, shade-intolerant species.

Adapted from Brookes et al., Western spruce budworm, USDA Forest Service Cooperative State Research Service, Technical bulletin No. 1694.

Note: Use pesticides with care. Apply them only to plants, animals, or sites listed on label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

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Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.