Insect and Disease Management Series Western Spruce Budworm 6.1 Management 2011 Lee Pederson Nancy Sturdevant Darren Blackford freemani Razowski Entomologists Hosts USDA-Forest Service The most chronic,  Douglas-fir destructive  All true firs defoliating of  Spruce Topics conifers in the  Western larch  During epidemics, pines and western Introduction 1 Northern Rockies. hemlock may also be fed on. Damage 2 Life cycle and behavior 2 Introduction Hazard rating 3 The western spruce budworm (WSB), On many of our forests east of the Permanent plots 6 Choristoneura freemani Razowski, is a Continental Divide, such as the Helena Budworm model 7 native insect that has co-evolved with National Forest, populations are Douglas-fir, spruce and true fir chronic, occurring over large areas Silvicultural control 7 forests. WSB populations are with relatively short periods between Chemical/biological 8 somewhat cyclic across many of our outbreaks. As forests have become control forests, especially west of the denser with proportionally more Effects on stand 9 Continental Divide. These forests, Douglas-fir and grand fir, WSB development and with the exception of the Bitterroot outbreaks have become more frequent succession and Lolo National Forests, usually and severe. Other Reading 9 have long periods between outbreaks where WSB defoliation is not found.

Key Points

 Multi-storied stands of Douglas-fir or true firs are most damaged.  Shade tolerant trees Management are defoliated heaviest in mixed stands. Encourage pines and larch because outbreaks usually  Heaviest defoliation and tree killing are occur in Douglas-fir, grand fir, and subalpine fir forests. seen in Douglas -fir stands on south- facing Avoid multi-layered stand conditions in which Douglas-fir slopes. or true firs are in overstory as well as understory.  Manage for resistant species. Accept some defoliation as a normal stand-development  Select for process. phenotypically resistant trees during harvest.

Page 2 Western Spruce Budworm 6.1 Management Damage WSB feeding can cause growth loss habitat, visual corridors, and in the form of topkill, deformities, campgrounds are examples of mortality, and reduction in seed potentially affected systems. In areas production. Tree mortality is usually of chronic WSB infestation, big concentrated in the smaller, game hiding and thermal cover may suppressed pole and sapling size be compromised, which could result Periodic outbreaks trees. However, mature trees can be in wildlife community instability. of western spruce killed with repeated severe However, WSB impacts in aquatic budworm have defoliation. In addition, trees ecosystems may actually improve always been part of weakened by repeated defoliation fish habitat. Defoliation in larger western forest may be predisposed to bark beetle diameter trees is concentrated in the dynamics. attacks and subsequent mortality. upper crowns and should not significantly affect canopy closure. WSB reduces seed and cone In turn, this should have little to no production directly by feeding on effect on stream temperature or seeds and cones, topkilling, and flow. Low levels of tree mortality in indirectly through the effects of these riparian areas due to WSB may defoliation. Even during light actually improve fish habitat by defoliation, significant losses in cone adding organic debris to streams. production can occur. A study in WSB larvae and adults may also fall Montana demonstrated that up to 71 into streams during dispersal, % of cones were infested during a enhancing the food supply of some period of light WSB defoliation. fish species. Although defoliation can significantly affect tree growth and survival, Recreation and its associated foliage recovery can be dramatic and economics may be affected if the within a few years tree crowns can public perceives its forest experience appear healthy. However, volume of to be diminished by defoliated, and Visual quality and live crown and wood volume may be possibly dying and dead trees. In recreation are the substantially reduced. campgrounds, recreation benefits two resources may be considerably reduced if budworm has the In addition to affecting the timber shading, esthetic quality, and privacy greatest potential resource, WSB may adversely impact screening is affected by heavy WSB to affect. other valued forest resources, such as defoliation (Rosenberger and Smith wildlife and recreation. Big game 1997).

Life Cycle and Behavior Eggs hatch in late summer and first protective shelter through the sixth instars migrate to over-wintering instar. They pupate in these silken sites. The larvae molt to the second shelters and emerge as adults by instar and overwinter in silken August. Eggs are laid in a mass shelters under bark scales. Larvae containing up to 130 eggs, on the emerge in spring, April to June, and underside of a needle. mine buds and old needles until bud flush. As the buds flush, larvae web Insect dispersal occurs during the new needles together to feed in a adult and larval stages of 6.1 Western Spruce Budworm Page 3 Management

Life Cycle and Behavior (continued) development. Horizontal dispersal, and density. Good WSB habitat from tree to tree and from one consists of dense, multiple layers of stand to another, occurs mainly climax host species. The climate of Good WSB during the second larval instar and these stands may influence the habitat consists adult life stages (Carlson et aI. probability of an outbreak, but of dense, multiple 1988). Frontal systems and stand conditions will determine the layers of climax associated winds can carry duration and intensity. The upper host species. populations from one drainage to story provides a good food source another. This may account for and refuge from predation and sudden population increases in parasitism. The lower canopy layers areas previously uninfested. Vertical intercept WSB spinning from the distribution is more prevalent in the upper layers and provide sanctuary older, larval stages which are not as from the predators on the forest buoyant on wind currents, but also floor. Dense, even-structured stands Understory layers occurs throughout all larval stages. will limit the diversity of bird provide food, shelter, Through management, we can predators (Langelier et al. 1986) and and a moderated negatively influence this stage of may reduce the efficacy of insect climate which WSB development, mainly through parasites. Temperature fluctuations promote WSB interrupting dispersal by reducing and precipitation patterns can population growth. the number of canopy layers greatly affect WSB behavior and through harvesting. population development (Kemp et al. 1983). Climatic influences are an WSB habitat quality is determined integral part of the hazard rating by stand structure, composition, system developed for budworm.

Western Spruce Budworm Life Cycle

Page 4 Western Spruce Budworm 6.1 Management Hazard Rating Risk rating can Stand-level hazard are more likely to experience be used in Hazard rating for and diseases significant damage and/or tree combination can help set priorities for treatment mortality if insect populations are with hazard based on stand susceptibility to present and the weather is favorable. rating to project insects and diseases. Forest insects the probability require three things to cause The model we recommend for stand of a WSB significant impact to resource values: level data in the Northern Region is outbreak and susceptible hosts, insect populations, the Generalized Indexing Model the level of and favorable weather conditions. developed by Carlson and Wulf damage that will Hazard rating systems measure (1985). Please refer to this result. susceptibility of forested areas to a publication for information on using particular insect by evaluating the the model. amount of susceptible host. High and moderate hazard forested areas

Hazard Rating for Western Spruce Budworm Carlson and Wulf (1985)

Character Descriptive Variable Index Value Range Species Composition % host crown cover and % climax host 0-2.4 crown cover 0.6-2.4 Stand Density Total % crown cover 0.8-1.6 Crown Class Structure Coefficient of variation of host tree heights 0.9-1.7

Stand Vigor Basal area stocking/average maximum basal 0.9-1.6 area and incidence of stress-inducing pests

Stand Maturity Basal area weighted mean host tree age 0.3-1.3

Site Climate Habitat type group 0-1.5 Regional Climate Geographic location 0-1.2 Surrounding Hosts Type % host type in surrounding 1,000 acres and 0.6-1.7 Continuity adjacent to the stand

6.1 Western Spruce Budworm Page 5 Management

Hazard Rating (continued)

Landscape hazard rating been validated. Data sources for For conducting hazard rating on a use in this model may include in- larger scale, a system has been de- ventory data where basal area, host veloped (Randall and Bush 2010). composition and trees per acre data As of 2011, this model has not yet are available.

Hazard Criteria for Western Spruce Budworm/Douglas-fir Tussock

Criteria Attribute Low (.5) Moderate (2) High(3) BA all species A <80 ft2/acre 80 < BA < 100 ft2/acre >100 ft2/acre (ft2/acre) % of total BA that is: Engelmann spruce, subalpine B fir, <50% 50% < % BA < 80% >80% grand fir, and Douglas-fir >5” DBH Trees per acre C <50 /acre 50 < TPA < 100 /acre >100 /acre >5” DBH

Directly Calculated Hazard Values and Hazard Rating Multiplicative Index

Hazard Calculated Values Hazard Rating

Extremely Low 0 0

Low <2 1, L

Moderate 2-17 2, M

High >18 3, H

High-hazard stands are those in trees, the preferred hosts of which a large amount of Douglas- Douglas-fir beetle. Douglas-fir fir and true fir defoliation is beetle may attack and kill trees that expected once an outbreak of WSB experienced repeated, heavy occurs. Mortality of small- and mid defoliation from WSB. Moderate- -size trees often occurs under these and low-hazard stands may conditions but seldom occurs in experience less defoliation and larger trees in Montana and . defoliator-caused growth loss, top High hazard stands contain larger kill, or mortality.

Page 6 Western Spruce Budworm 6.1 Management

Hazard Rating (continued)

Interpreting Hazard Rating High-hazard stands are those in When high-hazard stands are inter- which a large amount of Douglas-fir mixed with low-hazard stands, de- and true fir defoliation is expected foliator populations may not be as once an outbreak of WSB occurs. active. Low-hazard stands may Mortality of small- and mid-size have host species, but may be com- Habitat Types trees often occurs under these con- prised of smaller trees or single- ditions but seldom occurs in larger storied stands and/or are not con- Western spruce trees in Montana and Idaho. High tiguous across the landscape to al- budworm defoliation hazard stands contain larger trees, low defoliator populations to re- occurs in 46 habitat the preferred hosts of Douglas-fir main at high levels. Defoliators types described for Montana (Fellin et al. beetle. Douglas-fir beetle may at- may still cause significant mortality 1983); across much of tack and kill trees that experienced in the host components of low- Montana and part of repeated, heavy defoliation from hazard stands in a landscape, but Idaho. WSB. Moderate- and low-hazard losses will be lower than in a land- stands may experience less defolia- scape where high-hazard stands They fall in these tion and defoliator-caused growth dominate habitat type series— loss, top kill, or mortality.  Douglas-fir  spruce  grand fir Data Considerations insure that a representative sample  western red When using hazard ratings in a of current forest conditions exists. cedar landscape level analysis, it is Hazard ratings calculated for a  western hemlock  lower elevations assumed that available data are a sampled area assumes that forest of the subalpine representative sample of landscape conditions in the sample area are fir series. conditions. A spatially balanced homogeneous; if excessive inventory across the geographic variability in any of the parameters area of interest can be used to used to calculate hazard ratings derive estimates of hazard. occurs within the sampled area, the However, small forested areas, or hazard rating may not accurately areas with unique characteristics, reflect the hazard. If that is the may not be represented or have case, further stratification of the insufficient data for analysis. Users geographic area of interest into should carefully evaluate available various dominance types, size data for a particular analysis area to classes, and densities may be

Permanent Plots In total, we have established 360 measurements, stand information permanent plots located throughout has been collected 2-3 years the region in areas at risk to WSB. following an outbreak or at least Plots were established in low, once every 5-10 years. Defoliation moderate and high defoliation areas. is recorded via Aerial Detection Stands were selected on the premise Survey annually when present. of representing diverse conditions Impact data on individual trees and across stand and site conditions. stands are measured from plots, Since recording the initial including effects on succession. 6.1 Western Spruce Budworm Page 7 Management FVS-linked Budworm Damage Model Model description The damage model allows users to The WSB damage model, an specify defoliation patterns by tree extension to the Forest Vegetation species, size class, crown third, and Simulator (FVS), predicts topkill, needle age class (Crookston et al. height growth, diameter growth, and 1990). For the user's evaluation of tree mortality impacts of WSB the specified defoliation patterns, defoliation on its spruce and fir host there are two output tables available: tree species (Crookston 1991). This a cumulative damage summary and model is formed from two periodic damage tables at each FVS components of the Western Spruce cycle boundary, displaying the results Budworm Modeling System of budworm defoliation on FVS (Sheehan et al. 1989, Crookston et al. model trees. 1990): foliage dynamics and damage. Cultural manipulation of stands offers the Western Spruce Budworm Control Strategies greatest hope for preventing outbreaks or Silvicultural Alternatives reducing Even-aged management systems more or less favored. Removal of the impacts. mimic stand replacement fires and overstory should be made within 10 are an applicable system in areas of years following initial entry. historically long fire intervals. Multiple use applications of even-age Developing two or three-stored systems are suited to provide stands with one or two levels of increased water yields, diversity of regeneration beneath a nearly closed Intermediate habitat (particularly browse habitat) canopy will promote and support Treatment required by game and non-game WSB infestations. Understory Objectives wildlife (Gibbs 1978). Clearcutting regeneration will be climax species. (Schmidt, 1983) and seed tree systems promote Should a WSB infestation develop in

development of seral plant the overstory, vertically dispersing  Maintain tree vigor communities. Climax species larvae will be intercepted by the to enhance survival invariably become part of the understory. These understory layers and recovery overstory and as the stand develops, provide food, shelter, and a increased amounts of suppressed, moderated climate which promote climax species develop in the WSB population growth (Schmidt et  Alter the stand understory. Intermediate treatments al. 1983). In addition to the physical and may be necessary to modify stand development of budworm biological density, tree spacing, and favor seral populations, the genetic quality of parameters to reduce species. the seral species will be lost by the budworm habitat second or third cut. Thus, the future Shelterwood cuts should be used management could be relegated to  Capitalize on natural with care but can be used in areas climax species and chronic WSB resistance of where the probability of WSB infestations. When other individual trees and infestation is slight. Dependent on management objectives lead to species the site, residual overstory, and creating multi-storied stands, number of entries made to remove consideration should be given to the overstory, the climax species are maintaining healthy stands through

Page 8 Western Spruce Budworm 6.1 Management

Silvicultural Alternatives (continued)

intermediate cuttings and fire, and less damage as the residual trees, by creating conditions that are will have an impact on the outbreak favorable to natural enemies of by the simple removal of the food WSB. source.

Even-aged Manipulating stand density and Uneven-age management can management tree spacing, if properly timed and occur in stands of irregular or systems mimic applied, can increase growth on uneven-age structure, on fragile stand other host and non-host trees. The sites, steep slopes, high water replacement fires foliage food value, with regard to tables, very dry sites, or sites that and are an WSB, appears to be better in foliage would be adversely affected by applicable system from stressed trees. Therefore, complete removal of forest canopy. in areas of reducing stress within stands should The multiple use applications of historically long reduce the food quality. Increasing uneven-age silviculture are best fire intervals. tree spacing will reduce interception suited to travel influence zones, of larvae by the understory and water influence zones, watershed increase WSB vulnerability to protection, scenic areas, wildlife predators, parasites, and erratic habitat requiring high forest cover weather conditions. Favoring non- and vertical diversity in vegetation host trees and host trees showing (Gibbs 1978).

Pesticides provide only temporary Chemical and Biological Pesticides protection against budworm. Chemical control for WSB can be long) and before they reach 5th and Protection usually used to protect high-value trees 6th instar larvae when significant only lasts 2- 3 from defoliation, damage and damage results from continued years before mortality in the short term. Large feeding. Insecticides should also be populations forested areas can be aerially applied when most of buds on trees resurge. sprayed for short-term protection have flushed and are about an inch and individual trees can be sprayed in length. Chemicals currently using ground equipment. registered for use against WSB include carbaryl and a microbial Over the long term, chemical insecticide, Bacillus thuringiensis (B.t.) control is not effective in reducing var. kurstaki. B.t. is a naturally the impacts of WSB because of re- occurring host-specific pathogen invasion by the adult from that affects only the larvae of surrounding infested stands. lepidopterous insects feeding on Repeated applications during the foliage at the time of application course of an outbreak may be and a short time thereafter. B.t. is needed for adequate protection. used to spray in environmentally sensitive areas where chemical Timing of insecticide application is applications may be prohibited. critical for successful reduction of Carbaryl is a broad spectrum damage and mortality from WSB. carbamate that affects all insects on Insecticides should be applied in the foliage at the time of early summer when most WSB application (Blackford et al. 2007). larvae are in the 4th instar (3/4 inch 6.1 Western Spruce Budworm Page 9 Management WSB Effects on Stand Development and Succession Insects and diseases, as well as fire, away from true firs to include more have a natural role and function in resistant species such as Douglas- forest dynamics. Under endemic fir, larch and pines essentially In Douglas-fir conditions, insects and diseases are resetting succession. Over the short stands... nature's tools to keep a forest term, there would be losses in terms healthy. They work quietly to keep of recovery of volume per unit area, Natural cycles of stands thinned. Even periodic but probably minimal impacts on budworm weed out outbreaks of insects serve a useful other resources directly resulting individual large purpose. They weed out genetically from WSB. trees and inferior stock in a stand providing suppressed smaller, more growing room for resistant In pure Douglas-fir stands, which slower growing trees. comprise much of our east-side trees, resulting in a WSB habitat, natural cycles of WSB push toward climax. In unmanaged, mixed-species would weed out individual large stands, if fire and insects such as trees and suppressed smaller, slower WSB are allowed to play out their growing trees, resulting in a push natural roles, there would be a shift toward climax.

References and Other Reading

Blackford, D., L. Lazarus, L. Pederson, and N. Sturdevant. 2007. Western Spruce Budworm in the Intermountain West. USDA For. Serv. Northern and Intermountain Regions. http://www.fs.fed.us/r6/nr/fid/pubsweb/ westdef/wsb-brochure-r14.pdf

Carlson, C.E., W.W. McCaughey, and L. Theroux. 1988. Relations among stand structure. Dispersal of second instars of western spruce budworm, defoliation and stand height growth of young conifers. Can. J. For. Res. 18:794-800.

Crookston, N.L., J.J. Colbert, P.W. Thomas, K.A. Sheehan, and W.P. Kemp. 1990. User's guide to the western spruce budworm modeling system. Gen. Tech. Rep. INT-274. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 40 pp.

Crookston, N.L. 1991. Foliage dynamics and tree damage components of the western spruce budworm modeling system. Gen. Tech. Rep. INT-282. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 40 pp.

Fellin, D.G., R.C. Shearer, and C.E. Canson. 1983. Western spruce budworm in the Northern Rocky Mountains. Western Wildlands, A Natural Resource Journal 9(1):2-7.

Gibbs, C.B. 1978. Uneven-aged silviculture and management? Even-aged silviculture and management? Definitions and Differences. In: Uneven-aged Silviculture and Management in the United States. USDA Forest Service. Timber Management Research, Wash., DC.

Kemp, W.P. 1983. The influence of climate, phenology and soils on western spruce budworm defoliation. Ph.D. dissertation, Univ. of Idaho, Moscow, ID. 143 pp.

Langelier, L.A., and E.O. Garton. 1986. Management guidelines for increasing populations of birds that feed on western spruce budworms. USDA Forest Service, Ag. Handbook No. 653.

6.1 Western Spruce Budworm Page 10 Management

References and Other Reading (continued)

Rosenberger, R.S., and E.L. Smith. 1997. Nonmarket economic impacts of forest insect pests: a literature review. Gen. Tech. Rep. PSW-GTR-164-www. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Dept. of Ag.; 38 pp. http://www.fs.fed.us/psw/publications/documents/psw_gtr164/

Publication Citation

Pederson, L., N. Sturdevant, D. Blackford. 2011. Western spruce budworm management. Chapter 6.1 Forest insect and disease management guide for the northern and central Rocky Mountains. USDA Forest Service, Northern Region, State and Private Forestry. 10 pp.

Available from USDA Forest Service, Idaho Department of Lands, and Montana Department of Natural Resources.

Online at: https://fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5186684.pdf

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