United States 

Department o 

Agriculture 

Forest 

Service 

Northern 

Region 

200 East Broadway 

P.O. Box 7669 

Missoula, MT 59807 

Report 04-9 April 2004

MOUNTAIN PINE BEETLE, BLISTER RUST, AND THEIR INTERACTION 

ON WHITEBARK PINE AT TROUT LAKE AND FISHER PEAK 

IN NORTHERN IDAHO FROM 2001-2003 

John Schwandt and Sandra Kegley 

 

INTRODUCTION 

Whitebark pine stands in the Selkirk mountains have 

been steadily declining for many years due to a 

combination of white pine blister rust (Cronartium 

ribicola) (BR) and mountain pine beetle 

(Dendroctonus ponderosae) (MPB). BR has been 

present for many years causing a great deal of branch 

mortality and topkill in mature trees, and has greatly 

reduced survival and establishment of regeneration 

because small trees can be quickly girdled. BR has 

also contributed to mortality of large trees but over 

the past several years, mortality of mature trees has 

increased dramatically due to increasing populations 

of MPB. Ground surveys conducted in three areas of 

the Selkirks in the fall of 2000 documented a loss of 

45%-82% of the whitebark pine primarily due to 

MPB (Kegley et al. 2001). In 2001, surveys in five 

additional areas found 17%-94% whitebark pine 

mortality primarily due to MPB (Kegley et al. 2004). 

 In 2001, Trout Lake and Fisher Peak areas had 

relatively low MPB populations and beetles appeared 

to prefer trees with severe BR infection (Kegley et al. 

2004). We revisited those areas in August 2003 to 

determine if the status of MPB and its relationship 

with BR had changed. 

METHODS 

In August 2003, whitebark pine was examined for 

MPB attack and BR infection at Trout Lake and 

Fisher Peak on the Bonners Ferry Ranger District, 

Idaho Panhandle National Forests. Whitebark pine 

was examined in random strip surveys. Data 

recorded for each tree included diameter (d.b.h.) and 

a description of any bark beetle activity or blister rust 

infection. 

 Bark beetle activity was classified as current, last 

year, and older MPB attack, or unknown or 

secondary bark beetle mortality. Partial MPB bole 

attacks (strip attacks) and unsuccessful attacks 

(pitchouts) were also recorded. BR severity was 

classified as light for trees that had only dead 

branches or small branch cankers (red flags), 

moderate for trees with multiple large branch cankers 

or less than 30% top kill, and severe for trees with 

bole cankers or more than 30% top kill. BR rating of 

some trees was difficult because the crowns could not 

be seen clearly due to adjacent trees or crown fading 
2 

from MPB attacks. It is unlikely they would have 

been severely infected, but since it was not possible 

to accurately determine the presence or absence of 

smaller cankers, these were placed in a separate 

category. Older dead whitebark pine (snags) were not 

recorded. 

Four whitebark pines were felled at Fisher Peak to 

compare the amount of blister rust infection seen 

from the ground with what was actually present in the 

crowns. Once on the ground, every branch was 

closely examined for cankers, rodent chewing, and 

evidence of secondary bark beetles. 

BR infection on regeneration was also tallied in 

random strip surveys in areas with whitebark pine 

regeneration. Sampled trees were at least 4 feet tall, 

so that they would have had sufficient exposure to 

infection, and less than 6 inches d.b.h. so they would 

be small enough to be considered regeneration. They 

were rated as not infected, infected or killed by 

blister rust and tallied in one inch diameter classes. 

Logistic regression was used to explore the 

relationship between MPB attack, d.b.h. and BR 

condition. Pearson’s chi square test was used to 

determine the probability of MPB attack on trees 

with different BR conditions. Data was then pooled 

to compare light and non-infected with those 

moderately and severely infected, and to compare 

severely infected trees with all others. 

RESULTS 

MPB populations increased at Fisher Peak and Trout 

Lake from 2001 to 2003 (Table 1). In 2001, 71% of 

the whitebark pine trees examined at Fisher Peak and 

84% at Trout Lake were alive. In 2003, the amount 

of live whitebark pine decreased to 44% and 52%, 

respectively. Total dead from MPB attack increased 

at Fisher Peak from 27% in 2001 to 54% in 2003 and 

at Trout Lake from 14% to 44%. 

Table 1. Whitebark pine with mountain pine beetle (MPB) or blister rust infection by area 

and year. Blister rust infection was determined from the ground with binoculars. 

Location Fisher Peak 

2001 

Fisher Peak 

2003 

Trout Lake 

2001 

Trout Lake 

2003 

# WBP examined 139 200 200 198 

Ave. d.b.h. 14.2 in. 15.1 in. 16.9 in. 15.5 in. 

WBP alive 99 (71%) 88 (44%) 167 (84%) 103 (52%) 

Year 2003 MPB 

mass attack na 42 (21%) na 21 (11%) 

Year 2002 MPB 

mass attack na 36 (18%) na 54 (27%) 

Older MPB mass 

attack na 30 (15%) na 13 (7%) 

MPB pitchout na 1 na 0 

MPB strip attack na 3 na 6 

Ave. d.b.h. with 

MPB 14.9 14.6 20.0 15.7 

Ave. d.b.h. no MPB 13.9 15.8 16.4 15.4 

Unknown or 

secondary beetle 

mortality 

na 0 na 1 

Total killed by MPB 37 (27%) 108 (54%) 28 (14%) 88 (44%) 

WBP infected with 

Blister Rust 90 (65%) 133 (67%) 134 (67%) 143 (79%) 
3 

Average d.b.h. of trees attacked by MPB at Trout 

Lake was 15.7 inches compared with 15.4 inches for 

those not attacked. There was no significant 

relationship between tree d.b.h. and MPB attack at 

Trout Lake. At Fisher Peak, the average d.b.h. of 

trees attacked was 14.6 inches compared to 15.8 for 

trees not attacked. The probability of MPB attack at 

Fisher Peak increased with increasing d.b.h. up to 

16.2 inches and then decreased as d.b.h. reached 25.2 

inches. This made the average d.b.h. of trees 

attacked by MPB significantly smaller than those not 

attacked at Fisher Peak (p<.03). The opposite was 

found at both Trout Lake and Fisher Peak in 2001 

when MPB populations were much smaller (Table 1). 

In 2001, MPB attacked significantly larger trees than 

those that were not attacked at five areas in the 

Selkirks with varying MPB populations 

(p<.02)(Kegley et al. 2004). 

The proportion of trees with signs of BR infection in 

2003 was slightly higher at Fisher Peak and 12 

percent higher at Trout Lake (Table 1) than in 2001. 

Most trees sampled in 2003 at Trout Lake had light 

or moderate BR infections while most trees at Fisher 

Peak had light or none (Table 2.) Trees with severe 

BR infections were less than 15 percent at both sites, 

and the BR condition of 20 trees at Trout Lake and 

two at Fisher Peak could not be determined. 

Table 2. Number and (percent) of BR infection 

levels observed at each area in 2001 and 2003. 

(Percents exclude trees with unknown BR.) 

BR 

Cond 

Code 

BR Infection 

Level 

Fisher Peak 

2003 

Trout Lake 

2003 

0 No BR 

observed 63 (32%) 37 (20%) 

1 Light 

infection 82 (42%) 57 (32%) 

2 Moderate 30 (15%) 63 (35%) 

3 Severe 21 (11%) 23 (13%) 

 Total 196 180 

4 Unknown 2 20 

BLISTER RUST AND MOUNTAIN PINE 

BEETLE INTERACTIONS 

When both areas were combined and all BR 

conditions were analyzed for MPB attack, BR 

condition 4 (couldn’t see well enough to accurately 

rate BR) had over 90% successful MPB attacks 

which was significantly more than any other BR 

condition (none, light, moderate, or severe BR) 

(Table 3). Sixty percent of trees with no BR 

observed had successful MPB attacks which were 

significantly higher than the proportion of MPB 

attacks in trees with light or moderate BR infection. 

MPB attack success was also greater in trees rated 

clean than severely BR infected, but the difference 

was not statistically significant (p=.11). 

Since the relationship of successful MPB attacks was 

so strong in the BR condition 4, it was felt that it 

might be clouding other relationships. Therefore 

additional analyses were made between all BR 

conditions except the condition 4 trees. When we 

pooled the light and non-infected trees and compared 

them to the moderate and severely infected trees, we 

found more successful attacks in the clean and lightly 

infected trees, but it was not statistically significant 

(p=.07) (Table 4). 

Table 3. Number (percent) of trees attacked or not 

attacked by MPB by BR condition. 

Blister rust 

condition 

(code) 

Not 

Attacked 

MPT 

Attacked 

Unknown 

Mortality Total 

No BR 

observed 

(0) 4 (40%) 60 (60%) 0 100 

Light BR 

 (1) 73 (53%) 66 (48%) 0 139 

Moderate BR 

 (2) 54 (58%) 39 (42%) 0 93 

Severe BR 

(3) 24 (55%) 20 (46%) 0 44 

Could not 

accurately see 

 (4) 1 (5%) 20 (91%) 1 (5%) 22 

Total 192 205 1 398 
4 

Table 4. Percent of trees with none or light BR 

attacked or not attacked by MPB versus trees with 

moderate or severe BR. 

BR condition (codes) 

Not 

Attacked 

MPB 

Attacked 

None or light (0 or 1) 47% 53% 

Moderate or Severe (2 or 3) 57% 43% 

When trees with severe BR were tested against none, 

light, or moderate BR conditions, a similar pattern 

was observed; MPB avoided attacking the most 

severely infected trees, but the difference was not 

statistically significant (Table 5). This may due to a 

small sample size of severely BR infected trees. 

Table 5. Percent of trees with severe BR attacked or 

not attacked by MPB versus all other BR conditions. 

BR condition (codes) 

Not 

Attacked 

MPB 

Attacked 

None, light or moderate 

(0, 1, 2) 57% 43% 

Severe (3) 55% 46% 

SAMPLING BLISTER RUST FROM THE 

GROUND VERSUS ACTUAL CROWN 

INFECTION 

All four trees that were closely examined after falling 

had many small cankers that were not seen from the 

ground (Table 6). We did not see any cankers in two 

of the four trees from the ground even with careful 

inspection with binoculars. Typical flagging from BR 

infections on small branches was especially hard to 

see in trees that were fading due to MPB attacks. 

Very few cankers were actively sporulating, but most 

cankers could still be readily identified by 

characteristic branch swelling, discoloration, or 

resinosus (Figure 1). Most of the dead branches had 

cankers at the base and many had been chewed by 

rodents (Figure 2). A few branches girdled by BR 

also had galleries of small secondary bark beetles 

(Pityophthorus sp). 

Figure 1. Small swollen branch indicative of BR 

infection. 

Figure 2. Resinosus on rodent-chewed BR canker.
5 

RUST INFECTION IN REGENERATION 

About half of the regeneration tallied at both Trout 

Lake and Fisher Peak were infected with BR and 

about 5% were dead from BR (Table 7). Total 

infection percentage is much lower than the total 

infection observed in the mature trees nearby. The 

smallest regeneration trees had the lowest proportion 

of BR infection and the proportion of infected small 

trees tended to increase with diameter at both sites, 

but sample sizes of larger trees were quite small. 

Table 6. Blister rust condition as seen from the ground with binoculars compared to actual cankers found in the 

crown of felled trees. 

Tree# 

BR condition from 

ground 

# Cankers seen 

from ground 

Actual # 

Cankers MPB Condition 

1 Severe (3) 3 11 2002 Attack (fading) 

2 Moderate (2) 10 23 2003 Attack (green) 

3 None (0) 0 20 2003 Attack (green) 

4 None (0) 0 18 No attack 

Table 7. Infection levels on a sample of regeneration. 

 Fisher Peak Trout Lake 

d.b.h. Clean Infected Dead Clean Infected Dead 

<1 22 14 0 15 10 0 

1 19 17 3 26 25 4 

2 7 9 2 14 11 2 

3 5 9 0 1 4 0 

4 1 2 0 0 1 0 

5 0 1 0 1 0 0 

Total 54 (49%) 52 (47%) 5 (4%) 57 (50%) 51 (45%) 6 (5%) 

DISCUSSION 

In 2001, we found that MPB appeared to prefer trees 

infected with BR at relatively low beetle population 

levels at Fisher Peak (27% MPB-killed trees) and 

Trout Lake (14% MPB-killed trees). Two years later, 

MPB-caused tree mortality doubled at Fisher Peak, 

and more than tripled at Trout Lake and beetles 

appeared to prefer trees with little or no BR. These 

observations tend to support our earlier findings at 

four other areas that showed high MPB populations 

preferred trees with little or no BR (Kegley et al. 

2003 and 2004). Therefore, there appears to be a 

point where increasing MPB populations switch from 

attacking BR weakened trees to apparently healthy 

trees. The high correlation between MPB attacks and 

trees that could not be clearly rated for BR (BR 

condition 4) was primarily due to the fact these trees 

were fading or older dead, which masked most blister 

rust infections, not because MPB actually preferred 

these trees. 

Even though we have seen a significant relationship 

between MPB and tree diameter in other studies 

(Kegley et al. 2003 and 2004), we found no 
6 

relationship between MPB and d.b.h. at Trout Lake 

and a negative relationship at Fisher Peak in 2003. 

In 2001, the probability of MPB attack increased with 

increasing d.b.h. at these sites (Kegley et al. 2003). 

The apparent contradictory results on the relationship 

between MPB attack and d.b.h. between 2001 and 

2003 is somewhat puzzling but is probably due to the 

few large diameter trees sampled. This relationship 

should be examined further. 

Since different trees were measured during each year, 

the change in BR infection at Trout Lake may only 

reflect natural variation in the population of rust 

infected trees, rather than a real increase in infection 

level. We would not expect to see a large number of 

new cankers in just two years on mature trees. Even 

though the data indicates MPB attacked fewer 

severely infected trees, it was not statistically 

significant. But this may have been hampered by 

small sample sizes. 

The inability to detect small cankers on trees from the 

ground is of real concern. Many surveys have taken 

data on “clean” trees, and this data is now highly 

suspect. This is also very important to those 

interested in selecting uncankered ‘plus’ trees for 

inclusion in a genetics program. Trees selected for 

breeding are climbed to collect cones, so are 

examined more closely for cankers at that time. 

However, some cankers near the tips of branches are 

likely to be missed even by climbers. 

The amount of infection observed in the regeneration 

is similar to that seen in other places (unpublished 

data, Hadfield and Flanagan 1996). Regeneration 

trees, especially the smaller diameter classes, have 

not experienced as many (if any) periods of high BR 

infection, as well as being relatively small targets for 

rust spores. Therefore, it is not unusual to find 

infection increasing with tree size. It is also generally 

suspected that ribes plants in high elevations may be 

somewhat rare, or unlikely to become infected due to 

very narrow windows of proper weather conditions 

for infection. Therefore, infections may be due to 

spores traveling from distant sources (Schwandt and 

Guyon 1999, Goheen et al. 2000). If this is the case, 

it is also more likely that most infections would occur 

on the larger mature trees, simply because they are a 

bigger target and reach higher into the wind currents. 

Although a few dead regeneration were tallied, it is 

also possible that very small trees may have an even 

higher rate of mortality, but were not tallied because 

they were less than four feet tall. 

MPB activity will continue to be monitored in these 

high-elevation stands to provide mortality trends and 

additional information regarding possible 

relationships to BR infection. Additional trees should 

be felled to better determine the actual amount of BR 

in mature trees. 

ACKNOWLEDGEMENTS 

Gary Kempton and Ray Hoff assisted with the field 

surveys. Michael Marsden conducted the statistical 

analysis. 

LITERATURE CITED 

Goheen, E.M., Danchok, R.S., Goheen, D.J., 

Marshall, K., Petrick, J.A., White, D.E. 2000. The 

Status of Whitebark Pine along the Pacific Crest 

National Scenic Trail on the Umpqua National 

Forest. USDA Forest Service, Pacific Northwest 

Region, Forest Insect and Disease Service Center 

Rpt. 00-1, 13p. 

Hadfield, J., Flanagan, P.1996. Whitebark pine 

mortality survey in the eastern Washington 

Cascade Range. Nutcracker Notes, number 7, 

December 2, 1996. 

Kegley, S., Schwandt, J., Gibson, K. 2001. Forest 

Health Assessment of whitebark pine on Pyramid 

Pass, Russell Mountain, and Burton Ridge in the 

Selkirk Mountains on the Idaho Panhandle 

National Forests. USDA Forest Service, Northern 

Region, Forest Health Protection Rpt. 01-08, 11p. 

Kegley, S., Gibson, K. Schwandt, J., Marsden, M. 

2003. A test of verbenone to protect individual 

whitebark pine from mountain pine beetle attack. 

USDA Forest Service, Northern Region, Forest 

Health Protection Rpt. 03-9, 6p. 

Kegley, S., Schwandt, J., Gibson, K. 2004. Forest 

Health Assessment in Selected Stands in the 

Selkirk Mountains of Northern Idaho. USDA 

Forest Service, Northern Region, Forest Health 

Protection Rpt. 04-5, 8p. 

Schwandt, J., Guyon, J. 1999. Forest Health Service 

Trip report to Babbitt Mountain and Dog Valley, 

Humbolt-Toiyabe National Forests. USDA Forest 

Service, Intermountain Region, Forest Health 

Protection Trip Report Oct. 27, 1999 3p.