United States 

Department of 

Agriculture 

Forest 

Service 

Northern 

Region 

200 East Broadway 

P.O. Box 7669 

Missoula, MT 59807 

Numbered Report 07- 09 December 2007

Establishment Report: Permanent Plots to Monitor Douglas-fir Mortality 

Due to Western Spruce Budworm and 

Douglas-fir Beetle Interactions 

Brytten Steed, Nancy Sturdevant, Ken Gibson, 

Forest Entomologists 

USDA Forest Service, Forest Health Protection 

Northern Region, Missoula, MT 

Introduction 

Western spruce budworm (Choristoneura 

occidentalis Freeman) (WSBW), a native insect, 

is consistently the most widely distributed and 

destructive defoliator of coniferous forests in 

western North America. Found throughout the 

intermountain West, it is the most prevalent 

insect defoliator in Montana. Host tree species 

include Douglas-fir (Pseudotsuga menziezii

[Mirb.] Franco), true firs (Abies spp.), spruce 

(principally Picea engelmannii Parry ex 

Engelm.), western hemlock (Tsuga heterophylla

[Raf.] Sarg.), western larch (Larix occidentalis

Nutt.), and rarely pines (Pinus spp.) (Fellin and 

Dewey, 1986). 

WSBW outbreaks exhibit no particular pattern or 

trend in the West, although abnormally warm 

and dry weather may precipitate population 

outbreaks (Kemp and others, 1985). In the 

USDA Forest Service (USFS), Northern Region, 

the current outbreak has been building since 

2001 (Bulaon and Sturdevant, 2006) (Fig. 1). In 

Montana alone 1,158,619 acres of visible 

defoliation were observed in 2006, up 

significantly from 453,739 acres mapped in 2005 

(Gannon and others, 2007). 

Visible Defoliation by Western Spruce Budworm in the 

Northern Region

0

1000

2000

3000

4000

5000

19481951195419571960196319661969197219751978198119841987199019931996199920022005

Acres Affected (thousands) Figure 1. Annual visible defoliation by WSBW in the USFS 

Northern Region (R1) from 1948 to 2006 based on aerial 

detection surveys. Not all areas in Region are flown every year, 

adding minimally to the overall annual variation. 
 2 

When severe, chronic outbreaks occur, damage 

may include top-kill, growth loss, and host 

mortality, especially regeneration failure. Trees 

weakened by defoliation also may be more 

susceptible to Douglas-fir beetle (Dendroctonus 

pseudotsugae Hopkins) (DFB), the most 

destructive bark beetle of Douglas-fir in North 

America. Whereas WSBW feeding can cause 

dramatic visual changes in a tree’s appearance, 

more Douglas-fir mortality in Montana is 

attributable to DFB. 

Recent observations in heavily defoliated stands 

on the Helena National Forest, suggest there may 

be a relationship between heavy defoliation by 

WSBW (>90% total crown) and subsequent 

attack by DFB (Sturdevant and Kegley, 2006). 

Validity of this relationship has not been tested 

in the Northern Region. The importance of 

consecutive years of defoliation also is unknown. 

Our establishment of permanent plots in several 

areas of current WSBW-caused defoliation will 

allow us to monitor Douglas-fir mortality caused 

by defoliation and/or bark beetle attack. We may 

also be able to determine defoliation 

characteristics most attractive to DFB. 

Methods 

Three locations with high current WSBW 

defoliation were identified using 2005-2007 

aerial detection survey (ADS) maps and ground 

assessments (Fig. 2). Because ADS identifies 

defoliation only when significant enough to 

become apparent from the air, defoliation not 

visible in the lower canopy might be missed. In 

addition, other defoliators or diseases might 

create similar crown thinning. Thus, actual 

presence and intensity of WSBW activity was 

confirmed from the ground. All locations chosen 

have a history of both WSBW and DFB activity. 

Within each of these locations, we chose three 

sites (stands), one to three miles apart. Stands 

were predominantly Douglas-fir with breastheight diameters (dbh) of eight inches or greater. 

Although we did not examine individual trees for 

existence of root disease, stands were to have 

little to no apparent root disease affects (i.e. no 

visible mortality centers nor thinning crowns not 

due to defoliation). 

At each site we established 10 variable-radius 

plots using a basal area factor of 20. Distance 

between plots was at least three chains (~200 

feet). However, if the plot at three chains did not 

contain more than seven Douglas-fir trees greater 

than 8 inches dbh, the distance was increased 

another chain. If no appropriate stand was 

located within seven chains we continued the 

transect in a more promising direction. 

Figure 2. General position of the three locations of study plots. 

Plot center was staked, a GPS coordinate was 

recorded, and numbered tags were nailed at dbh 

to in-plot trees. Data recorded for each tree 

included tag number, tree species, dbh (±1 inch 

using Biltmore sticks), canopy position 

(dominant, intermediate, understory, open 

grown), overall crown defoliation (<25%, 26-

75%, >75%), defoliation of current year growth 

(<50%, >50%), WSBW-caused mortality (very 

few green needles remaining and no bark beetle 

frass apparent), and bark beetle activity (current, 

last year, or older successful DFB attack, strip or 

unsuccessful attack, or attacks by secondary 

beetles). 

Data gathered on the 10 plots at each site was 

entered into the FINDITS program to obtain 

summaries of stand conditions. Details on this 

program and its calculations made can be found 

in Bentz (2000). Note that we changed 
 3 

defoliation levels represented by FINDITS 

defoliation codes 12 & 13. Current growth 

defoliation was represented by ‘User Codes’ 31 

and 32. 

Results: Stand Condition Summaries

Summary data on whole-tree and new-growth 

defoliation are shown in Figures 3 and 4, 

respectively. Results indicate that levels of heavy 

(>75%), whole-tree defoliation were 

significantly different among the three sites [Pvalue = 0.014, multiple response permutation 

procedure (MRPP) analyses (Petrondas and 

Gabriel, 1983). Simultaneous multiple 

comparisons to differentiate plot areas could not 

be conducted with so few replications.]. Flesher 

Pass (FP) (Helena NF) displayed the greatest 

levels of heavy defoliation, followed by Bridger 

Canyon (BC) (Gallatin NF) and Madison (MD) 

(Beaverhead NF). Data on defoliation of new 

growth confirms that most trees at FP were under 

current high defoliation (>50%) (or dead), 

resulting in fewer lightly defoliated trees. 

However, light defoliation of new growth was 

not significantly different at the 3 sites (P>0.05) 

using MRPP). 

Whole-tree Defoliation Levels

0%

25%

50%

75%

100%

Light (<25%) Moderate 

(26-75%)

Heavy (>75%)

Defoliation Level

Ave % of all DF Trees 

MD

BC

FP

Figure 3. Average percentage of Douglas-fir trees at each of 

three whole-tree defoliation levels. Error bars are 95% 

confidence intervals. (MD=Madison, BC=Bridger Canyon, 

FP=Flesher Pass) 

New-growth Defoliation Levels

0%

25%

50%

75%

100%

<50% >50%

Defoliation Level

Ave % of all DF Trees 

MD

BC

FP

Figure 4. Average percentage of Douglas-fir trees in two newgrowth defoliation levels. Error bars are 95% confidence 

intervals. (MD=Madison, BC=Bridger Canyon, FP=Flesher Pass) 

Stand summary data indicates FP sites are 

unique in having more DF per acre as mortality 

(55 average [range 29-105] versus 3 and 2 [range 

0-6] for BC and MD). Mortality due to 

defoliation alone was highest at FP, although 

constituted the largest proportion of total 

mortality at BC (23 and 4 average DF per acre, 

respectively). No defoliation-caused mortality 

was recorded at MD. Similarly, mortality due to 

current (2007) and last year (2006) DFB activity 

was only present at FP (16 and 5 DF per acre, 

respectively), although older DFB-caused 

mortality was present at both FP and MD (2 DF 

per acre each). Although BC has a history of 

DFB activity, it was interesting to note that none 

was detected in our survey despite the many 

years of defoliation. 

At MD there are fewer total (live and dead) DF 

trees per acre than at BC or FP (94, 156, 166, 

respectively). Yet, due to the larger QMD of 

those trees (18.5, 16, 15.1, respectively) the 

average DF basal areas are fairly similar (157, 

179, 179, respectively). Meanwhile, MD sites 

may be unique in having visible root disease 

effects on crown condition, although this was the 

only location where a root disease specialist was 

present with the crews. 
 4 

Only BC3 had additional WSBW host tree 

species (SAF and ES). Although defoliation was 

measured on these trees, no DFB activity would 

be expected. Lodgepole pine also was present on 

five of nine sites. However, trees per acre and 

basal areas of this species were low with only 

MD3 containing any significant numbers. A 

more detailed summary of individual site 

conditions is provided in Table 1. 

Discussion

We expected that levels of whole-tree defoliation 

would reflect defoliation history with heavier 

defoliation in stands experiencing more 

consecutive years of WSBW activity. However, 

aerial detection survey (ADS) data from 1999 to 

2007 does not support this assumption. (See 

Table 2.) 

According to ADS maps all FP sites have been 

subject to three years of WSBW activity and 

possibly up to five years. However, BC stands 

have experienced defoliation longer with seven 

years of WSBW activity, often at high levels. 

Meanwhile, MD defoliation has been fairly 

recent, recorded in one stand in 2006, with levels 

high in all three stands starting only in 2007. 

Based on ADS data we would expect higher 

whole-tree defoliation levels at BC, followed by 

FP and MD. 

We are not sure why whole-tree defoliation 

levels do not reflect the duration of the 

defoliation event. Differences in defoliation-level 

assessment due to point of view (ground versus 

aerial) or use of such a broad category for heavy 

defoliation (50-99%) may be factors. 

New-growth defoliation was >50% on more than 

half the live DF trees, reflective of 2007 ADS 

survey results mapping all sites as highly 

defoliated. 

Both defoliation intensity (whole-tree 

defoliation) and longevity (years of defoliation) 

may affect DF survival. Although no conclusions 

can currently be made with our limited data 

(three sites at one year), DF mortality levels are 

better reflected by defoliation intensity than 

longevity. 

Absence of current DFB-caused mortality on BC 

and MD does not necessarily indicate that trees 

are unsuitable for DFB colonization. In order for 

DFB-caused tree mortality to occur, DFB needs 

to be present and active in the area. 

Using ADS maps, we identified the nearest 

DFB-caused mortality detected during the 2005, 

2006, and 2007 surveys. (See Table 3.) Although 

BC had high levels of defoliation and a longer 

history of WSBW activity than FP, DFB activity 

has been further than one mile from the plots 

during all three years. At the other extreme, DFB 

activity has been less than four miles distant 

from MD plots during all three years, often with 

distances less than a half-mile. Without ground 

checks it is not possible to know if this DFB 

activity at MD is occurring in more heavily 

defoliated stands than those we measured or if 

our stands have just been missed to date. 

Although all FP plots contained some level of 

DFB activity in both 2006 and 2007, ADS 

surveys were not able to discern the mortality. In 

all three years mapped activity was over two 

miles from sites. However, it is possible that 

heavy defoliation masked DFB activity. From 

the air, a red-crown is the primary signature of 

DFB-caused mortality. If few needles remain, 

this signature would be absent. 

All plots will be revisited in summer 2008 to 

assess WSBW-caused defoliation levels and bark 

beetle activity. Continued monitoring beyond 

2008 is planned although visits will be based on 

presence of WSBW defoliation. Ultimately, 

individual tree mortality will be assessed to 

determine which tree and stand characteristics 

best predict tree death. However, to determine if 

defoliation levels influence DFB’s selection of 

host trees bark beetle populations must increase 

within the study areas shortly after defoliation. 

To increase our likelihood of capturing 

additional data on DFB activity in defoliated 

stands, we may establish additional plots at other 

locations in subsequent years. 
 5 

Literature Cited 

Bentz, B.J. 2000. Forest insect and disease tally 

system (FINDIT) user manual. GTR 

RMRS-GTR-49. Logan, UT. USDA 

Forest Service, Rocky Mountain 

Research Station. 2 pp. 

Bulaon, B.; Sturdevant, N. 2006. Determining 

stand susceptibility to western spruce 

budworm and potential damaging effects. 

Number Report 06-07. USDA Forest 

Service, Forest Health Protection, 

Missoula, MT. 9 pp. 

Fellin, D.G.; Dewey, J.E. 1986. Western spruce 

budworm. Forest Insect & Disease 

Leaflet 53. Washington, D.C. USDA 

Forest Service. 10 pp. 

Gannon, A.; Sontag, S.; Skov, K. 2007. Montana 

forest insect and disease conditions and 

program highlights – 2006. Numbered 

Report 07-1, USDA Forest Service, 

Forest Health Protection, Missoula, MT. 

73 pp. 

Kemp, W.P.; Everson, D.O.; Wellington, W.G. 

1985. Regional climatic patters and 

western spruce budworm outbreaks. 

Technical Bulletin No. 1693. USDA 

Forest Service, Cooperative State 

Research Service. 31 pp. 

Petrondas, D.A. & Gabriel, K.R. (1983) Multiple 

comparisons by rerandomization tests. 

Journal of American Statistical 

Association, 78: 949-957. 

Sturdevant, N.; Kegley, S. 2006. Spruce 

budworm defoliation and mortality on 

Flesher Pass. MFO-TR-06-10. USDA 

Forest Service, Forest Health Protection, 

Missoula, MT. 5pp.
 6 

Table 1. Western spruce budworm (defoliator) and Douglas-fir beetle (DFB) activity, as well as basic 

stand characteristics for the three sites at each of the three locations: Bridger Canyon (BC), Madison 

(MD), and Flesher Pass (FP). Data summary is from FINDITS (Bentz, 2000) and is given as trees-peracre unless otherwise specified. All Douglas-fir trees (DF) over 5” in diameter at breast height (live and 

recently dead) were used in calculations unless otherwise specified. 

BC1 BC2 BC3* MD1 MD2 MD3 FP1 FP2 FP3 

DEFOLIATION LEVELS of DF: 

 All foliage (whole-tree): 

Light Defoliation (<25%) 38.4 43.6 78.7 89.7 72.6 64.2 0.6 40.6 4.7

Moderate Defoliation (26-75%) 48.1 93.5 71.4 21.1 2.1 25.5 25.9 40.9 26.7

Heavy Defoliation (>75%) 36.9 48.2 0.9 51.3 101.6 119.1

 Current year foliage (new-growth): 

Defoliation <50% 78.9 51.9 74.4 32.8 38.5 23.8 22.2 7.1

Defoliation >50% 44.5 133.4 75.8 79.0 36.1 64.0 90.7 166.6 143.3

 Defoliation Mortality: 2.2 5.7 17.6 15.7 35.8

 

BARK BEETLE ACTIVITY on DF: 

Current 2007 DFB attack 9.1 10.4 59.7

2006 DFB attack 3.3 1.9 9.4

Older DFB attack 1.4 0.8 3.8 1.5 

Unsuccessful DFB attack 0.6 0.6 14.2

Current DFB strip attack 2.6 6.3

Current secondary beetle attack 1.1 8.3

 

OTHER on DF: 

Undamaged Stand 2.2 

Unknown Mortality 2.2 0.9 

Root Rot affected (live) 1.3 4.6 

 

STAND CHARACTERISTICS: 

Trees per acre of DF (live) 123.4 185.3 152.3 111.7 74.7 89.7 69.3 172.6 90.7

Trees per acre DF (dead) 2.2 5.7 0 1.4 .8 4.7 31.5 29.2 105.0

%of stand is DF (trees/acre) 98.9 100 78.4 100 94.9 74.7 100 100 98.2

DF QMD (live and dead) 17.9 14.7 15.4 19.0 19.3 17.0 18.4 13.5 13.5

Basal area of DF (live) 196 192 146 184 140 126 126 146 94

Basal area of all species (live) 198 192 168 184 144 136 126 146 96

 Other tree species 

(live tree/acre; live basal area) 

LPP 

 (1.4, 2) 

 SAF 

 (22,6) 

ES 

 (15,10) 

LPP 

 (6,6) 

 LPP 

 (4, 4) 

LPP 

 (30,10) 

 LPP 

 (1.6, 2) 

 

*BC3 also contained the western spruce budworm host trees of subalpine fir (SAF) and Engelmann spruce (ES). High, medium and low 

whole-tree defoliation levels on these additional hosts were 0, 4.5, 16.5 and 0, 3.0, 3.0 trees per acre for SAF and ES, respectively. Newgrowth defoliation levels were 0, 21.1 and 0, 6.0 trees per acre of <50% and >50% for SAF and ES, respectively. Lodgepole pine (LPP) is 

not typically a host species so defoliation was not recorded on it. 
 7 

Table 2. Total acres affected by WSBW as detected by aerial detection survey for the reporting area in 

which sites are located, and level of WSBW activity at each site. Site defoliation levels are given as low 

(L) or high (H), corresponding to <50% or >50% visible crown defoliated, respectively. (BC=Bridger 

Canyon, MD=Madison, FP=Flesher Pass) 

Aerial Detection Survey Summary of Sites - WSBW 

 Gallatin* BC1 BC2 BC3 Beaverhead* MD1 MD2 MD3 Helena* FP1 FP2 FP3 

1999 0 0 0 0 0 ** 0 0 0 

2000 0 0 0 0 0 ** 0 0 0 412 0 0 0 

2001 0 0 0 0 1,309 0 0 0 

2002 19,934 H H L 23,500 0 0 0 4,567 0 0 0 

2003 56,004 H H L 15,224 0 0 0 29,653 0 0 H 

2004 73,009 H H L 36,801 0 0 31,173 

2005 124,487 H H L 60,818 0 0 0 145,039 H H L 

2006 208,787 H H H 150,194 0 0 L 413,384 L L L 

2007 n/a H H H n/a H H H n/a H H H 

 

* Reporting area includes all ownerships within the National Forest boundary. 

** Acres affected estimates are determined from ADS polygons and may be subject to errors. 

Blank cells indicate specified area was not flown. 

Table 3. Estimated distances (in miles) of plots from nearest Douglas-fir beetle (DFB) activity detected 

during aerial detection surveys (ADS) in 2005, 2006, and 2007. (BC=Bridger Canyon, MD=Madison, 

FP=Flesher Pass) 

 

 

Aerial Detection Survey Summary of Sites – DFB* 

 Gallatin Beaverhead Helena 

 BC1 BC2 BC3 MD1 MD2 MD3 FP1 FP2 FP3 

2005 2-3 1-2 3-4 <0.5 0.5-1 <0.5 2-3 2-3 >3 

2006 >10 >10 >10 3-4 0.5-1 2-3 >3 >3 2-3 

2007 8-9 9-10 >10 1-2 <0.5 <0.5 >4 >4 >3 

 

*ADS maps fading trees indicative of DFB activity that occurred the previous year.