United States 

Department of 

Agriculture 

Forest 

Service 

Northern 

Region 

200 East Broadway 

P.O. Box 7669 

Missoula, MT 59807 

Numbered Report 06-07 May 2006

Determining stand susceptibility to Western Spruce Budworm and 

potential damaging effects. 

Beverly Bulaon, Entomologist 

USDA Forest Service, Forest Health Protection 

Northern Region, Missoula, MT 

Nancy Sturdevant, Entomologist 

USDA Forest Service, Forest Health Protection 

Northern Region, Missoula, MT 

Introduction 

Western spruce budworm, Choristoneura 

occidentals Freeman, is the mostly widely 

distributed and destructive defoliator in the 

western United States (Fellin and Dewey 1986). 

Budworm plays a critical role in forest nutrient 

recycling and regulating stand composition and 

structure. Its primary hosts are Douglas-fir, 

Engelmann spruce, and true firs. Current forest 

stand conditions and climate in western Montana 

and northern Idaho are especially favorable to 

budworm. Historical aerial detection maps show 

high outbreak frequencies in the combination of 

forest type and stand conditions found in our 

Region (Kemp et al. 1985). 

Due to a complexity of factors influencing 

budworm populations’ outbreak frequency, 

duration, and damage are highly variable among 

stand, and hosts (Van Sickle 1985). Large 

populations can persist if stand conditions are 

favorable and host material is available. 

Prolonged budworm epidemics are found to 

cause marked reduction in tree girth and height. 

Zhang and Alfaro (2002) examined cores from 

host trees experiencing ten years of continuous 

defoliation by the 2-year cycle spruce budworm 

(Choristoneura biennis Freeman) in north central 

British Columbia. They found the growth loss 

period could last for 10 to 16 years, with radial 

increment loss from 16-21%. However, 

diameter growth returned to normal 3-5 years 

following the end of the outbreak period. 

Understanding budworm preferences and 

subsequent damaging affects would help 

determine appropriate management for future 

prevention or suppression. Managers could 

benefit from techniques to determine if lands are 

susceptible to budworm and to what degree. The 

most efficient approach would be to evaluate 

areas at stand or landscape scales using hazardrating techniques. 

Hazard-ratings are used to evaluate the 

likelihood of insect infestations. These ratings 

are functional at different scales, and may be 
2

used for setting treatment priorities. Risk refers 

to stand vulnerability in the event of an insect 

outbreak, while hazard defines a stand’s 

infestation potential. Bousfield, Wulf, and 

Carlson (1986) developed a hazard-rating system 

for WSB for forests in western Montana and 

northern Idaho. This rating system is a 

generalized modeling system that integrates 

stand and site characteristics that influence 

budworm activity. 

The nine variables they proposed are: 

• Percent crown cover 

• Percent host crown cover 

• Percent climax host crown cover 

• Relative stand density 

• Coefficient of variation of host tree 

height 

• Mean host tree age 

• Site quality for spruce budworm 

• Regional climate 

• Surrounding host type 

If the model is accurate, the product of 

multiplying stand variables will determine the 

quality of budworm habitat. Accurate 

predictions and early detection provide time to 

prepare for insect outbreaks if and when they 

occur. 

Starting in 1976, widespread budworm outbreaks 

occurred in western Montana and northern Idaho. 

Budworm activity has been at very low levels 

since 1990. In 2001, aerial detection surveys 

found small groups of budworm activity in 

scattered areas in the region. By 2002 and 2003, 

more activity had been detected in new locations, 

while previous locations had expanded. Number 

of acres defoliated by western spruce budworm 

increased more than two-fold between 2004 and 

2005. In 2005, a total of 453,739 acres 

containing some level of budworm activity were 

recorded. We may be in only the early stages of 

the current outbreak. As noted earlier, budworm 

outbreaks in Montana and north Idaho have 

lasted for more than 20 years. 

Objectives 

The objectives of the study are: 

1. Determine the cumulative impact of 

budworm defoliation on mature host trees 

(Douglas-fir, Grand fir, Engelmann 

spruce, and subalpine fir) in terms of tree 

growth, mortality, and stand composition 

during the current outbreak. 

2. Refine and validate the current western 

spruce budworm hazard-rating system. 

3. Evaluate impact of past harvesting on 

budworm population trends and 

defoliation. 

Study Sites 

Annual aerial detection maps from 1985 to 1991 

were used to locate potential study sites. Sites 

were separated into three defoliation categories: 

high (seven or more years of consecutive 

defoliation prior to plot establishment), medium 

(four to six years), and low (three years or less of 

defoliation). Study locations were then 

randomly selected from each category. At four 

locations, stands were selected that were 

previously treated as either shelterwood or 

selection harvests. They are being examined to 

determine if past harvesting practices affect 

budworm populations and associated damage. 

After conferring with forest personnel, stands 

were chosen that did not have any proposed 

management activities for the next decade. 

Permanent plots were established in 33 stands in 

western Montana and northern Idaho in 1992, 

1993 and 1994. An equal number of stands were 

selected in each defoliation category, low, 

medium and high. Stands were representative of 

forest types susceptible to budworm defoliation. 

Most are composed of dense stands of mature 

Douglas-fir with little or no regeneration; or 

mixed species stands of Douglas-fir and pine, 

dominated by Douglas-fir. Each stand has 10 

variable-radius plots 2 chains apart along 

transects, using a BAF of 40. A 1/100 acre fixed 

plot was established at the center of each 
3

variable radius plot. Stands are varying in 

aspect, slope, and elevation. 

Methods 

Stand information (aspect, slope, elevation, 

acreage, stand structure, and vegetation) was 

recorded during plot establishment using Region 

One Stand Exam Process (1985). Budworm 

impacts were estimated through periodic remeasurements of growth (diameter-at-breast 

height and tree height) and crown condition 

(crown ratio, crown class, top-kill, and branch 

dieback). Since plot establishment, sites have 

been re-measured at about five-year intervals. 

Flight traps (baited with attractant pheromones) 

were hung at the center of each plot annually, to 

monitor population levels through 2004. In 

2005, pheromone traps were placed only on plots 

with high defoliation estimates in 2004. In each 

plot, defoliation for permanently tagged trees and 

regeneration was rated annually (based on an 

ocular method designed by Hostetler 1986). 

Data for each stand variable were collected and 

calculated to rate site susceptibility per site. 

Hazard-ratings were grouped as: 20 or less are 

low, moderates are 21 – 30, and greater than 30-

high. Ratings are reflective of site conditions for 

the duration of the study since no treatment or 

notable disturbance occurred. An analysis of 

variance was run for each stand variable and 

hazard-rating value per site, to determine which 

of the nine variables strongly influences hazardrating outcomes. Changes in growth loss and 

crown condition from 1992-1994 were 

subtracted from 2003 inventories as a means to 

examine differences over the years, and between 

high and low hazard sites. 

Results 

Hazard-rating 

Of the nine independent variables, three 

significantly influenenced hazard-rating values 

(Table 1). Percent climax crown cover, relative 

density, and site index values were highly 

correlated to hazard-rating values. These 

variables are crucial in determining accuracy of 

the rating. This correlation found that canopy 

(host foliage material) and stand density 

(between tree competition) strongly influenced 

hazard-rating outcomes more than other 

variables (Table 1). 

Table 1. Correlation analysis of hazard rating versus 1994 permanent plots. 

Dependent 

variable 

Independent 

variable 

Dependent 

mean 

Independent 

mean Correlation R-squared F value P value

Hazard-rating Current defoliation 25.42 6.086 0.1084 0.0118 0.7 0.4055 

Hazard-rating Total Defoliation 25.42 5.156 0.257 0.0661 4.17 0.0455 

Hazard-rating Trap catch 25.79 1.019 0.0075 0.0001 0 0.958 

Hazard-rating Percent host crown cover 26.38 92.85 0.2958 0.0875 2.78 0.1062 

Hazard-rating Percent climax crown cover 26.38 78.61 0.517 0.2673 10.58 0.0029 

Hazard-rating Relative density 26.38 59.73 0.4832 0.2335 8.83 0.0059 

Hazard-rating Average host age 26.38 117.8 0.256 0.0655 2.03 0.1646 

Hazard-rating Site index 26.38 1.387 0.4761 0.2267 8.5 0.0068 

Hazard-rating Regional index 26.38 1.145 0.274 0.0751 2.35 0.1358 
4

For the period 1990-2003, sites were rated using 

variables defined by Bousfield et al. 1986. There 

was no difference in current defoliation intensity 

of high-hazard versus low hazard stands. 

Personal observations suggested some stands 

that rated low or moderate, received more 

defoliation than some high hazard stands. 

Defoliation recorded from ground surveys 

showed no differences between sites because 

little defoliation occurred before 2003. 

However, this trend may change if the outbreak 

increases in intensity over the next few years, as 

predicted. In 2005, stands with hazard ratings of 

less than 20 (low hazard) had very little 

defoliation but highest trap catches (Appendix 

1). One exception occurred in an isolated stand 

on the Lewis & Clark NF. Most remaining 

stands with moderate or high hazard ratings 

(<20) were moderately or heavily defoliated in 

2005. 

Individual tree and stand conditions 

As of 2003, tree growth in high defoliation 

frequency stands has not been suppressed by 

budworm activity. Between 1990 and 2003, 

diameter and height growth were not 

significantly different between years, and 

between stands that were historically either 

lightly or heavily defoliated. No recorded 

mortality of large trees was directly attributable 

to budworm defoliation, but rather to root 

disease or bark beetles. However, only within 

the past few years has budworm defoliation 

intensified significantly on many permanent 

plots. Following the previous outbreak, stands 

that were defoliated for more than 7 years, 

appeared not to be visibly affected as expected. 

Regeneration was sparse throughout the 

permanent plots. Canopy openings were rare, 

and the few live seedlings found were in poor 

condition. Most dead seedlings died from 

overstory suppression or budworm feeding. The 

only concentrations of live regeneration were 

found in wide-open areas where large trees were 

at least 20 feet away. 

Treatments 

As expected, hazard-rating values between cut 

and uncut plots at Whitebird and Whitetail sites 

were different, compared to Titan Gulch and 

Jimmie New which were similar. However, 

there were noticeable visual differences in stand 

density between all treated and untreated plots. 

Defoliation ratings from 2003 show no 

difference in budworm preference for any of the 

plots. In 2005, defoliation on 2 of the 3 uncut 

plots rated was higher than the cut plots. Neither 

the Whitetail cut nor uncut plot were defoliated 

in 2005. Most trees on the fourth paired plot 

(Whitebird) were killed during the 2003 fires. 

During 2005, 21 % of trees in the uncut Titan 

Gulch stand were infested and killed by 

Douglas-fir beetle, Dendroctonus pseudotsugae

Hopkins, as compared to none in the adjacent, 

cut stand. 

Discussion 

The main purpose of this study was to determine 

current- and post-outbreak budworm affects on 

forest structure in western Montana and northern 

Idaho. If the hazard-rating system is accurate, 

then stands that receive high values have 

potential for significant damage if and when a 

budworm outbreak occurs. In this study, even 

after almost 20 years of continuous defoliation 

and a decade of low budworm activity, most of 

the mature trees in high-hazard sites appeared to 

have experienced little permanent damage. 

Mature trees on low-hazard sites appeared 

unaffected by defoliation. Budworm feeding can 

have pronounced effects on cone and seed 

production. Even when defoliation is light, up to 

71% of Douglas-fir cones in 13 stands in one 

study were infested (Van Sickle 1985). In this 

study, we did not measure affects of budworm 

on cone and seed production but suspect they are 

significant. Limited amounts of regeneration 

were found on most plots. 

Budworm-caused impacts are often short-term 

and mature trees can recover quickly after 
5

populations subside. Studies have shown 

budworm-caused mortality is usually light but 

can vary greatly between stands. Mortality is 

usually limited to smaller suppressed 

regeneration and pole sized trees, which collect 

falling larvae from the upper canopy. This mass 

aggregation can overwhelm less vigorous trees, 

causing severe branch die-back or loss in height 

(Ferguson 1988). Tree mortality following 

several years of consecutive heavy defoliation 

can ranged between 1 and 53 % at sites in 

Washington, Idaho, Oregon and British 

Columbia (Van Sickle et al. 1985). 

To date, we have not recorded mortality of 

mature trees from budworm on our permanent 

plots. We did record mortality resulting from 

heavy budworm defoliation on smaller saplings. 

Mortality of mature trees may increase in future 

years. Defoliation observed in areas outside 

several permanent plots was very severe and 

some mortality was seen. 

In one area on the Helena NF near several 

permanent plots, average defoliation for trees in 

an unthinned area was 83.5% compared to 41.9% 

for trees in a thinned area. Forty-two trees in the 

unthinned unit had greater than 90% defoliation 

compared to 11 trees in the thinned unit. In 

general, trees in thinned units appeared greener 

and to be more vigorous than trees in unthinned 

ones. Thirty-five trees in the unthinned unit 

were 100% defoliated. Of these, 18, or 50% had 

dry phloem and are probably dead. Seven trees, 

or seven percent, in the thinned unit were 100% 

defoliated by budworm and therefore have the 

highest probability of dying within the next few 

years. 

Analysis of the hazard-rating variables suggests 

that reducing stand density should decrease 

susceptibility to budworm defoliation. 

Treatments can be beneficial in areas where 

budworm populations have historically been 

very heavy. Carlson et al. (1985) found that 

defoliation in thinned stands was much lower 

than in unthinned stands. Dispersing larval 

mortality increases in thinned stands, due to 

increased distance between hosts. Vigorously 

growing stands typically withstand and recover 

from insect damage better than stressed stands. 

One of the unthinned stands has 21% of the trees 

infested and killed by Douglas-fir beetle versus 

the adjacent thinned stand. Negron et al. (2001) 

showed that increased stocking density of 

Douglas-fir was associated with increased stand 

susceptibility to Douglas-fir beetle. 

Brookes et al. (1987) suggests that budworm 

infestations persist in stands until the internal 

competitive stress between host trees and 

immigration of new larvae are both reduced. 

This allows for the re-establishment of natural 

enemy regulation which is essential in 

maintaining populations at endemic levels. He 

also suggests that epidemics will continue 

indefinitely in forests of advanced successional 

stage but may fluctuate periodically based on 

abnormal weather patterns. 

Based on this information, reducing stand 

vulnerability to budworm defoliation using 

silvicultural methods or prescribed burning can 

be accomplished. Reducing number of canopy 

layers and stand density would reduce stand 

susceptibility to budworm. Prescribed fire could 

also be used to reduce stand susceptibility by 

reducing the intermediate tree layer and stocking 

density. 

This report does not include the affects of the 

current budworm outbreak (2004-2005) to 

determine if differences can be found between 

sites with varying levels of defoliation histories, 

treatments, and effects; there are other factors 

that contribute to budworm host preferences and 

population sustainability that were not explored. 

Kemp et al. (1985) discussed several studies that 

found strong correlations between temperature, 

precipitation, and budworm infestations. 

Abnormally warm and dry weather patterns often 

precede budworm outbreaks. Moisture 

deficiencies and high stocking densities can 

reduce defensive chemistry of trees and 

consequently improve nutritional quality of 

foliage for larvae (Brookes et al. 1987). For the 
6

past seven years, much of Montana has been 

experiencing unseasonably dry weather. 

National drought monitors show precipitation is 

75% below average for some parts of the state. 

Therefore, budworm activity in west and central 

Montana is still expected to increase; ADS 

records and flight trapping indicate strong 

potential for current outbreaks to intensify and 

expand if weather conditions favorable to 

budworm persist. 

This study showed very little differences in tree 

growth (diameter and height) across permanent 

plots during the 1990-2003 sampling period. 

Therefore, following the end of the current 

outbreak, differences in tree growth and height 

can likely be attributed at least in part to 

budworm defoliation. Tree cores will be taken 

from sample trees on each plot following the end 

of the current outbreak. A follow-up to this 

report will include core sample information and 

possible defoliation effects after the current 

budworm outbreak subsides. 
7

References 

Bousfield, W.E., N. W. Wulf, and C.E. Carlson. 

1986. Rating susceptibility of stand to 

western spruce budworm: user’s guide 

and documentation to SBW-Hazard. 

USDA Forest Service, Cooperative 

Forestry and Pest Management, 

Missoula, MT. Forest Pest Management 

Report 86-7. 22 pgs. 

Brooks, M.H., R.W. Campbell, J.J. Colbert, R.G. 

Mitchell, and R.W. Stark. 1987. 

Western Spruce Budworm. USDA 

Forest Service, Cooperative State 

Research Service, Technical Bulleting 

No. 1694. 

Campbell, N. 1992. Region 1 Study plan: 

Impact assessment and long-term 

monitoring of Western Spruce Budworm, 

implications for management. USDA 

Forest Service, Region 1. unpublished. 

Fellin, D.G. and J.E. Dewey 1986. Western 

Spruce Budworm. USDA Forest Service, 

Forest Insect and Disease Leaflet 53. 10 

pg. 

Ferguson, D.E. 1988. Growth of regeneration 

defoliated by Spruce budworm in Idaho. 

USDA Forest Service, Intermountain 

Research Station, Research Paper INT393. 13 pg. 

Kemp, W.P., D. O. Everson, and W.G. 

Wellington 1985. Regional Climatic 

patterns and western spruce budworm 

outbreaks. USDA Forest Service, 

Cooperative State Research Service, 

Technical Bulletin No. 1693. 31 pg. 

Ryerson, D.E., T.W. Swetnam, A.M. Lynch 

2003. A tree-ring reconstruction of 

western spruce budworm outbreaks in the 

San Juan Mountains, Colorado, U.S.A. 

Canadian Journal of Forest Research, 

33:1010-1028. 

Timber Management Data Handbook. 1985. 

Chapter 400. R-1 FSH 5/85. 

Van Sickle, G.A. 1985. Host responses. In

Brooks, M.H., J.J. Colbert, R.G. 

Mitchell, and R.W. Stark. Technical 

Coordinators. Western Spruce Budworm. 

USDA Forest Service, Canada-United 

States Spruce Budworm program, 

Technical Bulletin 1694, 198 pg. 

Williams, C., P. Shea, and G. Walton 1971. 

Population Density of Western Spruce 

Budworm as related to stand 

characteristics in the Bitterroot National 

Forest. USDA Forest Service, Pacific 

Southwest Research Forest and Range 

Experiment Station, PSW-72, 9 pg. 

Zhang, Q.B. and R.I. Alfaro. 2002. Periodicity 

of two-year cycle of spruce budworm 

outbreaks in central British Columbia: a 

Dendro-Ecological Analysis. Forest 

Science, 48 (4): 722-731. 
8

Appendix 1. Hazard-rating values, defoliation and trap catches for each location 

Site name1

 National 

Forest2

Hazardrating3Ave. defoliationmature trees 

(2005) (%)4

Ave. number of 

Moths trapped 

(2005) 5

Jerry Creek BeaverheadDeerlodge 47.00 72.0 12 

Jimmie New (Cut) BeaverheadDeerlodge 25.71 67.0 5 

Jimmie New 

(Uncut) 

BeaverheadDeerlodge 23.72 83.0 16 

Cat Creek BeaverheadDeerlodge 38.31 32.0 14 

Titan Gulch (Cut) BeaverheadDeerlodge 30.54 76.0 18 

Titan Gulch 

(Uncut) 

BeaverheadDeerlodge 29.66 81.0 18 

Half Moon Park BeaverheadDeerlodge 26.96 41 

State Mine BeaverheadDeerlodge 34.53 0 3 

Moffet Mountain BeaverheadDeerlodge 46.69 56.0 15 

Blake’s Fork Clearwater 13.05 

Giant White Pine Clearwater 9.79 

Mission Mountain Clearwater 34.15 

Skyline Drive Clearwater 17.44 

Cabin Gulch Helena 20.69 64.0 31 

Gates of the 

Mountains 

wilderness 

Helena ----- 

Pole Creek Helena 19.19 0 20 

Avalanche Creek Helena 13.39 4.0 26 

Little Shanley Lolo 10.06 

Brewster Lolo 36.21 

Cotttonwood Lolo 21.19 

Buckhorn Saddle Bitterroot 15.75 

Guide Saddle Bitterroot 19.15 

Anaconda-Pintler 

Wilderness 

Bitterroot 35.54 

Miller Gulch Lewis and Clark 29.74 60.0 40 

 
9

Lion Creek Lewis and Clark 23.21 

Logging Creek Lewis and Clark 31.29 

Whitetail (Cut) Lewis and Clark 8.68 0 5 

Whitetail (Uncut) Lewis and Clark 53.78 0 11 

Seven Devils Nez Perce ----- 

Whitebird (Cut) Nez Perce 16.05 

Whitebird (Uncut) Nez Perce 53.78 

Squaw Saddle Nez Perce 4.48 

1

 Names of sites and associated treatments. 

2

 National Forest that each site is located within. 

3

 Calculated hazard-rating value using Bousfield et al 1986 method. 

4 Defoliation averages for selected sites only. 

5 Pheromone trap catches for selected sites only.