United States

Department of

Agriculture

Forest Service

Pacific Northwest

Research Station

General Technical Report

PNW-GTR-402

November 1998

Recurrent Outbreak of the

Douglas-Fir Tussock Moth in

the Malheur National Forest:

A Case History

R.R. Mason, D.W. Scott, M.D. Loewen, and H.G. Paul
Authors

This publication reports research involving pesticides. It does not contain recommendations for their use, nor does it imply that the uses discussed here have been

registered. All uses of pesticides must be registered by appropriate State and

Federal agencies before they can be recommended.

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable

plants, and fish or other wildlife—if they are not handled or applied properly. Use all

pesticides selectively and carefully. Follow recommended practices for the disposal

of surplus pesticides and pesticide containers.

R.R. MASON is principal insect ecologist and H.G. PAUL is forestry technician,

Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, Oregon

97850-3399; D.W. SCOTT is forest entomologist, Wallowa-Whitman National

Forest, 1401 Gekeler Lane, La Grande, Oregon 97850-3399; and M.D. LOEWEN

is silviculturist, Burns Ranger District, Malheur National Forest, Star Route 4-12870

Hwy. 20, Hines, Oregon 97738.
Mason, R.R.; Scott, D.W.; Loewen, M.D.; Paul, H.G. 1998. Recurrent outbreak of

the Douglas-fir tussock moth in the Malheur National Forest: a case history.

Gen. Tech. Rep. PNW-GTR-402. Portland, OR: U.S. Department of Agriculture,

Forest Service, Pacific Northwest Research Station. 14 p.

Characteristics of an outbreak of the Douglas-fir tussock moth (Orgyia pseudotsugata

(McDunnough)) in 1991-95 on the Burns Ranger District of the Malheur National

Forest (eastern Oregon) are given and compared with an earlier infestation in the

same area in 1963-65. Results of monitoring with pheromone traps, evaluating

populations by sampling larvae, and predicting trends and defoliation are reported in

detail for the latest outbreak. Temporal patterns of each outbreak also are described

and compared with the classic behavior of other tussock moth outbreaks. Findings of

this analysis, and the recurrent behavior of tussock moth outbreaks in general,

reinforce the importance of maintaining a system for detection, evaluation, and

prediction in managing Douglas-fir tussock moth populations in the future.

Keywords: Douglas-fir tussock moth, Orgyia pseudotsugata, insect outbreaks,

sampling insects, pheromone traps, population monitoring.

Abstract

Introduction The Douglas-fir tussock moth (Orgyia pseudotsugata (McDunnough)) is a major

insect defoliator of true firs (Abies spp.) and Douglas-fir (Pseudotsuga menziesii var.

glauca (Beissn.) Franco) in the interior Northwest. Its populations are characterized by

regular fluctuations and sometimes explosive growth, often leading to abrupt and

unexpected defoliation of trees. An ever increasing abundance of fir trees in mixedconifer forests during the late 20th century has created stands with a chronic risk for

outbreaks of this insect.

Early history of tussock moth outbreaks in the Malheur National Forest is briefly

documented in old survey reports. The first record of an outbreak in the Malheur was

a minor infestation in 1928-29 near Seneca, Oregon (Buckhorn 1948). From 1937 to

1939, a large and apparently severe outbreak also occurred in the Rudio Mountain

area in the northwestern corner of the Forest (USDA 1946); and in 1947, two small

infestation areas were reported in the southwestern part of the Forest (Buckhorn 1948).

Although these early outbreaks developed in the Forest at fairly regular intervals, they

did not reoccur at the same locations.

The first major tussock moth outbreak in the Malheur National Forest to be completely

described occurred in 1963-65, north of the city of Burns, Oregon. In this infestation,

known as the “Burns Outbreak,” defoliation was concentrated in the Antelope Mountain, Gold Hill, and King Mountain areas in the south half of the Forest. Unusual insect

activity was first noticed in 1963 when minor defoliation occurred in the tops of trees

on 15 acres on Antelope Mountain. By summer 1964, defoliation of true fir and Douglasfir suddenly became visible over 38,960 acres (Orr and others 1965). In 1965, the

portion of the outbreak in the Malheur National Forest was terminated by aerially

treating 62,650 acres with an application of DDT (Crouch and Perkins 1968, Perkins

and Dolph 1967). For many years, this infestation was considered to be one of the

more significant tussock moth outbreaks on record (Wickman and others 1973).

Almost 30 years later in 1991-95, a second major outbreak of tussock moth erupted

in two of the same areas of the Malheur: Gold Hill and King Mountain. By this time,

however, detection and evaluation procedures had improved, thereby making it

possible to quantify insect populations and forecast trends that could not have been

done 30 years earlier. Because considerable effort went into following the course of

this second outbreak, we believe the results should be permanently documented, both

for scientific and historical reasons and for use in managing future tussock moth

outbreaks. Accordingly, in this paper we have chronicled basic features of the outbreak, including how resurgence of the population was detected first by trapping

moths and how subsequent trends in the population and defoliation were predicted by

monitoring larval densities. As our activities were not part of a designed study, results

are presented as a case history comparing explicit findings from the second outbreak

with general descriptions of the first outbreak. We hope our experience will benefit

others facing similar problems.

1
The 1991-95 outbreak occurred in the southern end of the Malheur National Forest,

15 to 30 miles north of Burns, Oregon. The total infestation included about 112,640

acres. For management purposes, it was divided into three parts: the Rattlesnake

(37,120 acres), Thompson Springs (40,960 acres) and Gold Hill (34,560 acres)

Analysis Units. Geographical location of the first two units was analogous, although

larger in size, to the former 1965 King Mountain Spray Unit (27,334 acres). Location

of the Gold Hill Analysis Unit was similar to the 1965 Gold Hill Spray Unit (11,655

acres) but was larger (fig. 1).

Methods

Description of Outbreak

Area

Figure 1–Geographic locations of 1991-95 and 1963-65 outbreaks of Douglas-fir tussock

moth, Malheur National Forest.

Burns

Moth trap locations

1992-95 analysis units

1965 spray units

N

W E

S

0 5 Miles

Highway

395

Thompson

Analysis Unit

Springs

(40,960 acres)

Thompson

Analysis Unit

Springs

(40,960 acres)

Gold Hill

Analysis Unit

(34,560 acres)

Gold Hill

Analysis Unit

(34,560 acres)

Highway

395

Rattlesnake Rattlesnake

Unit Unit

Analysis Analysis

(37,120

acres) acres)

2
 Adult Monitoring

 Larval Sampling

Analysis units were comprised largely of ponderosa pine (Pinus ponderosa Dougl. ex

Laws.) and mixed-conifer types of ponderosa pine, Douglas-fir, and white fir (Abies

concolor (Gord. and Glend.) Lindl. ex Hildebr., Abies grandis (Dougl. ex D. Don)

Lindl., or a hybrid of the two). White fir, the most common host species of Douglas-fir

tussock moth in the area, usually occurred either as understory beneath old-growth

ponderosa pine or as released stands after the harvesting of pine. Since 1937, the

proportion of such mixed-conifer types, often dominated by late-successional species

like white fir and Douglas-fir, had nearly tripled because of natural succession accelerated by fire exclusion and the removal of pine (Agee 1993, Powell 1994). Elevations in

the units range from 5,000 to 6,800 feet, and precipitation averages about 20 inches

annually, mostly as snowfall in winter. Between 1982 and 1986, this portion of the

Malheur National Forest also was part of a regionwide outbreak of the western spruce

budworm (Choristoneura occidentalis Freeman) that caused considerable defoliation

of fir trees during that period (Powell 1994, Sheehan 1996a). In summer 1983, part of

the area was treated with the chemical insecticide carbaryl to suppress budworm

populations (Sheehan 1996b).

Presence of male adults lured to pheromone-baited traps is widely used today as an

“early warning” of potential outbreaks. On the Burns Ranger District, moths have been

trapped annually since 1981 to detect such population increases. Standard procedures of monitoring adult flight were followed each year at permanent trap sites

(Daterman and others 1979). Preceding moth flight in early August, five sticky traps,

baited with synthetic attractant of the female tussock moth, were hung at 1-chain

intervals along a line at each site. Traps then were retrieved after flight was complete in

October. Results were summarized annually by the mean number of moths caught

per trap at each loca- tion. Data on moth catch used for this analysis were limited to

the six trap sites on the District that fell within the 1991-95 outbreak area (fig. 1).

Means of these six trap sites were used to summarize flight activity in the outbreak

area each year.

From 1992 to 1995, density of early larvae (instars I, II) was estimated annually for

each analysis unit. The estimates for each year were based on a sample of 22 to 64

plots systematically located over the unit. Each plot included five host trees (white fir or

Douglas-fir) on which three 18-inch branch tips in the lower crown were sampled for

larvae. The branch tips were beaten in place to remove larvae, which were caught

on a drop cloth held underneath the branch (Paul 1979). Numbers of larvae sampled

per tree were averaged for the plot and converted to a standard expression of density

per square meter (1,550 square inches) in the midcrown (Mason and Paul 1994). A

density of about 30 early larvae per square meter is usually the threshold at which

defoliation in the crowns of trees first becomes visible (Mason and Wickman 1988).

Larval density for an analysis unit was calculated as the mean of the included plot

densities.

In 1995, after unexpectedly high moth counts had reoccurred in fall 1994, additional

larval sampling was conducted near moth trap sites to confirm the previously established downward trend of populations. For this sample, a larger plot of 50 trees was

sampled at each site to obtain a more precise estimate of larval density for the specific

3
Defoliation

Mortality Factors

Analyses

location (Mason and Paul 1994).

Visible defoliation in the outbreak was mapped in 1992-95 by annual aerial surveys

conducted cooperatively by the USDA Forest Service and the Oregon Department of

Forestry. Number of defoliated acres each year was obtained from the digitization of

mapped defoliation polygons (McConnell 1987).

Populations were not systematically sampled to determine causes of mortality. Field

observations of the condition of mature larvae and pupae, however, were made at

plot locations in early fall 1993. In addition, 30 egg masses were collected over the

outbreak and later bioassayed in the laboratory for disease incidence (Stelzer 1979).

In midsummer 1994 and 1995, larvae also were collected at random at several sample

points in each analysis unit. These larvae then were reared on artificial media in the

laboratory where they were observed for the presence of disease or parasitism.

To facilitate analyses, mean densities of larvae per square meter were multiplied by 100

to express all densities in terms of numbers per 100 square meters of branch area.

A value of 1.0 also was added to all mean densities to eliminate zeros in the data

set before transformation.

Population trends were examined visually by graphing mean trap data and mean

larval densities over time. Differences in means between years or analysis units were

compared by inspecting standard error of the mean (SEM) bars. Finite rates of

change, R, in larval density were calculated annually for each analysis unit by,

Rt

 = Nt+1/Nt , (1)

where N is mean density of larvae per 100 square meters, and t is the generation.

After sample estimates of larval density for 1992 and 1993 became available, R was

predicted for subsequent years by a second-order autoregressive model,

ln Rt = 1.54 - 0.065 ln Nt - 0.482 ln Nt-1 ± 2.114 , (2)

where lnN is the natural log of mean density of larvae per 100 square meters and

2.114 is the standard error of estimate (SEE). This model was developed previously

from time series of Douglas-fir tussock moth populations in the Umatilla and Wallowa-Whitman National Forests (Mason 1996). One-year predictions of log larval

density, ln Nt+1, then were calculated for 1994-96 by adding the estimate of ln Rt to ln

Nt so that,

ln Nt+1 = ln Nt + ln Rt ± 2.114 . (3)

Predicted and observed trends in density were graphed and compared visually.

Means of larval density usually include a wide range of plot densities. This explains

the patchy nature of defoliation in tussock moth outbreaks where visible defoliation

occurs only in stands where densities exceed the threshold of 3,000 early larvae per

100 square meters. Size of the defoliated portion of the outbreak was predicted

each year from the distribution of plot densities after normalization by logarithmic

transformation. A standard normal deviate, Z, was calculated by

Z = (ln 3000 - x)/s , (4)

where X is the mean of log-transformed plot densities and s is the standard deviation. Probability of a value greater than Z, which represents the portion under a

4
Outbreak Phases

Trap Counts and Larval

Density

normal curve that exceeds a density of 3,000 larvae, was read from a standard statistical

table (Steel and Torrie 1960). Predicted acres of defoliation were then calculated by

multiplying this probability by total area of the outbreak (112,640 acres). Accuracy of

the predictions was judged by comparison with mapped acres of defoliation.

Duration and intensity of the outbreak were clearly illustrated by the synchronous

behavior of larval densities in the three analysis units (fig. 2). In 1992, the first year

that larvae were sampled, mean densities were similar in all units and still increasing.

Populations peaked, however, in 1993 at mean densities above the defoliation threshold

in each analysis unit. Mean densities then declined steadily over the next 2 years. This

trend is summarized well by the finite rates of change for each analysis unit (table 1).

Between 1992 and 1993, mean densities increased in all units but at a faster rate in

the Rattlesnake Analysis Unit (10.81X) than in the Thompson Springs (6.64X) or Gold

Hill Analysis Units (5.73X). Consequently, population mean of the Rattlesnake Analysis Unit peaked at a significantly higher density than in other analysis units. Decline of

mean density was fastest in the Gold Hill Analysis Unit.

Patterns in the overall trend of small larvae conformed reasonably well with the sequence of phases observed in classical tussock moth outbreaks (Mason and

Wickman 1988, 1991) (fig. 2). Such outbreaks typically begin with at least 2 years of

rapid change when the population increases quickly from a low density to a high or

peak density. Although larval sampling did not begin until 1992, 1991 most likely was

a year of sharp population increase or “release” and the beginning of the outbreak

(phase 1) (table 1). Other phases followed in sequence, with 1994 apparently being

the last year of significant activity and, consequently, the end of the outbreak (phase

4). This conclusion was supported by the low densities estimated in the final year of

larval sampling in 1995.

Pheromone traps clearly signaled a significant increase in tussock moth abundance

beginning in 1990 (fig. 3). After the outbreak was underway, however, some anomalies

emerged. The high trap count in fall 1990 (42.6 moths per trap) warned of a probable

larger number of larvae than usual. The forecast was repeated in 1991 when counts

were even higher (57.5 moths per trap). These predictions ultimately were confirmed

by results of larval sampling in 1992 that showed mean densities in analysis units to

be approaching outbreak numbers, with many plots already at larval densities above

the defoliation threshold (fig. 2). Trap counts continued to rise in fall 1992, and larval

densities multiplied an average of eight times by early summer 1993 (table 1). Surprisingly, the moth count then dropped precipitously in fall 1993 even though larval densities continued at a lower but still relatively high density in 1994 (fig. 3). More surprisingly, however, the count rebounded again in 1994, bringing into question the downward trend established by larval sampling earlier in the year. This anomaly was shortlived, however, when extensive sampling in 1995 confirmed the continuing decline (fig.

2). Additional intensive sampling of larvae at trap sites in the same year also produced

nearly identical results as the extensive sample (138 vs. 111 larvae per 100 square

meters, respectively) and supported the virtual population collapse.

Results

Trends of Larval

Density

5
Figure 2—Trends of density (mean ± SEM) of Douglas-fir tussock moth larvae in three

analysis units, Malheur National Forest, 1992-95.

Analysis unit

Thompson Whole

Year Phase Rattlesnake Springs Gold Hill outbreak

1991 1 — — — —

1992 2 10.81 6.64 5.73 8.00

1993 3 .15 .38 .05 .17

1994 4 .08 .01 .08 .05

1995 4+ — — — —

Table 1—Finite rates of change (R)a of Douglas-fir tussock moth

larvae by year, outbreak phase, and analysis unit, Malheur National

Forest, 1991-95

Year

Density of larvae per 100 m2 (log scale)

10

100

1,000

10,000

100,000

1992 1993 1994 1995

Defoliation threshold

1991

Rattlesnake Analysis Unit

Gold Hill Analysis Unit

Thompson Springs Analysis Unit

Probable trend

Phase 1 Phase 2 Phase 3 Phase 4

a Rt

 = Nt+1 N⁄ t

6
Figure 3—Number (mean ± SEM) of Douglas-fir tussock moth adult males caught in pheromone traps in outbreak area, Malheur National Forest, 1989-95.

Mortality Factors

The above results indicate that artificially baited traps may have been less efficient at

high population densities because of increasing competition from natural pheromone

released by female moths. Once the outbreak developed, trap counts likely dropped

because of competition from natural sources of pheromone, and then increased again

as the population declined and competition from natural sources eased. Pheromone

traps, obviously, are most efficient at monitoring low-density populations, where they

are highly effective as an early warning of initial increases in abundance.

Systematic studies of natural mortality were not conducted in the infestation. In field

observations and laboratory rearings of larvae, however, the same mortality factors

commonly associated with tussock moth outbreaks were identified. Nucleopolyhedrosis virus (NPV), especially, was widely observed in larvae and pupae as early as

late summer 1993. Virus also was found later to be present in 43 percent of the egg

masses collected at the same time. In 1994 and 1995, 37.3 and 28.8 percent, respectively, of early to mid larvae sampled in all analysis units were infected with NPV (table 2).

Although these percentages indicate a significant presence of virus disease in the

population, they do not necessarily reflect a between-year difference in disease

incidence because of the inadequate sample and different ages of larvae collected.

Parasitization of larvae was lower, but rates were typical for this stage of an outbreak

(Mason 1976, Mason and Wickman 1988).

Year

Mean number of moths per trap

0

20

100

80

60

40

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1989 1990 1991 1992 1993 1994 1995

7
1992 1993 1994 1995 1996

0.1

1

10

100

1,000

10,000

100,000

Defoliation threshold

Observed

Predicted

Probable trend

Figure 4—Observed trend of density (mean ± SEM) of Douglas-fir tussock moth larvae

compared with 1-year predictions (± SEE) of autoregressive model, Malheur National

Forest, 1989-95.

Table 2—Mortality in laboratory-reared field collections of Douglas-fir

tussock moth larvae, Malheur National Forest, 1994 and 1995

Date of No. of larvae Number Percent

collection and (instars) Mortality factor died mortality

July 11-12, 1994 59 (III, IV) Virus diseasea 22 37.3

Insect parasitoidsb 3 5.1

Unknown 1 1.7

 Total 26 44.1

July 11-13, 1995 52 (I, II) Virus disease 15 28.8

Insect parasitoidsb 10 19.2

Unknown 11 21.2

 Total 36 69.2

a 43 percent of 30 egg masses collected in the previous fall (1993) also contained virus.

b Phobocampe pallipes (Provancher) (Hymenoptera: Ichneumonidae).

Year

Density of larvae per 100 m2 (log scale)

Observed

Predicted

Probable trend

8
Predictions of

Population Trend

Mean densities of larvae calculated from equation (3) produced a downward trend

generally analogous to the actual trend observed in the last 3 years of the outbreak

(fig. 4). Systematic sampling was not conducted in 1996, but all observations indicated that larvae were rare and probably had declined to low densities similar to those

predicted. Although the error bars are large for predicted densities, there is little doubt of the

overall downward trend forecast for the outbreak. Predictions based on the model for

any year are strongly dependent on delayed feedback of larval densities in the previous generation so that high densities, such as those in 1993, always will foretell a

decline 2 years later.

Over large areas, sample plot densities of Douglas-fir tussock moth larvae usually

have a log-normal distribution (Mason and Paul 1994). Transformation of arithmetic

plot densities to natural logarithms, therefore, adjusted density frequencies each year

toward a more normal distribution (fig. 5). The small size of plots unfortunately resulted

in a large number of zero values that skewed distributions to the left, especially for years

with relatively low mean densities. Data sets, however, were close enough to normality

for predicted frequencies to represent observed values. Considering the many possible

errors associated with sampling larvae and mapping the infestation, the number of

defoliated acres predicted for each year was surprisingly similar to the number actually

observed (table 3). The largest discrepancies occurred late in the outbreak (1994 and

1995) when considerably more acres were observed with defoliation than predicted.

These could have been caused partially by the cumulative effects of damage in tree

crowns, thereby making current defoliation viewed from the air difficult to separate from

year-old defoliation. The thresholds at which defoliation becomes visible also may be

lower in crowns where foliage has been depleted by previous feeding, thereby causing

new defoliation to show up at lower overall larval densities. Either of these possibilities

could have inflated the observed number of currently defoliated acres. Regardless of

acreage discrepancies, the pattern of amount of defoliation predicted each year was

similar to that observed and illustrates how total area impacted in an outbreak can be

estimated from the mean and variance of larval density.

Predictions of

Defoliation

9
Figure 5—Frequency distributions of arithmetic and log-transformed densities of Douglasfir tussock moth larvae in all sample plots by year, Malheur National Forest, 1992-95.

Broken line in each year is the normal curve for given mean and standard deviation of

transformed densities.

Midpoint of classes

Frequency of sample plots

Arithmetic Logarithmic

16000

18000

20000

22000

24000

8000

10000

12000

14000

2000

4000

6000

X =

s =

2.80

3.75

X =

s =

7.71

2.71

25

5

0

20

15

10

01 23 4 5 6 7 8 9 10 12 11

01 23 45 6 7 8 9 10

X = 2.06

s = 2.66

40

0

50

20

30

10

70

60

01234 567

X =

s =

4.44

3.55

1992

1993

1994

1995

1629

s = 3841

X =

X = 12,730

s 22,729

X =

s =

1933

4376

X =

s =

111

220

50

60

15

20

5

10

0

100

20

10

0

90

40

30

10

0

40

30

20

012 3 4 5 678 9 10

70

0

60

20

10

80

0

60

40

20

120

0

100

40

30

10

20

=

0

110000

120000

130000

60000

70000

80000

90000

100000

10000

20000

30000

40000

50000

0

16000

18000

20000

22000

8000

10000

12000

14000

2000

4000

6000

0

1000

1100

1200

1300

1400

300

400

500

600

700

800

900

0

100

200

X = 1629

s = 3841

X = 4.44

s = 3.55

X = 2.06

s = 2.66

X = 1,933

s = 4,376

X = 12,730

s = 22,279

X = 7.71

s = 2.71

X = 1,629

s = 3,841

X = 111

s = 220

X = 2.80

s = 3.75

10
Table 3—Comparison of acres of current defoliation

predicted by sampling Douglas-fir tussock moth larvae

and observed by aerial survey, Malheur National Forest,

1992-95

Acres Acres

Year predicteda observed

1992 9,315 6,004

1993 51,420 45,351

1994 17,718 26,530

1995 1,430 2,931

Discussion and

Conclusions

The 1991-95 tussock moth outbreak appears to be simply another episode in what

likely has been a long series of population eruptions in this part of the Malheur National Forest. It is now common knowledge that numbers of tussock moths fluctuate

between low and high densities with some regularity and that population peaks occasionally reach outbreak densities in favored habitats. The extent to which densities

fluctuate seems to be more limited (stable) in some environments than in others

(unstable) for reasons not yet fully understood. The variability with which densities in

an area have fluctuated before, therefore, is a good measure of the likelihood of an

outbreak occurring in the future (Mason 1996).

Although reporting between outbreaks has been inconsistent, there are enough

records of tussock moth larvae being found in the Malheur National Forest at other

times in the past to indicate that populations probably are always present at lower densities. Trap records, for example, clearly showed increased tussock moth activity in the

early 1980s even though no outbreaks were reported. For these reasons, as well as

because of the increasing abundance of susceptible host type, much of the Forest

should be regarded as a high risk for future outbreaks.

Comparing phases of the two infestations is difficult without having estimates of larval

density from which population trends can be positively established. However, the fact

that populations erupted in 1964 to cause extensive defoliation, after almost no

defoliation was reported in 1963, clearly identifies 1963 as the year of population

release in the first outbreak (Orr and others 1965, Wickman and others 1973). New

versus old egg mass comparisons in fall 1964 showed that populations may have

peaked that year at Gold Hill but still were increasing at King Mountain (Perkins and

Dolph 1967). Virus disease also was detected in dead larvae throughout the outbreak

in 1964, with rates being especially high at Gold Hill (51 percent) (Perkins and Dolph 1967).

Such observations indicate that tussock moth populations in 1963 and 1991 were of a

similar dynamic state, with numbers still relatively low but poised to increase dramatically by the next year (phase 1). Conditions also were comparable in 1964 and 1992

when defoliation was observed over a relatively large area for the first time in both outa Number of infested acres (112,640) X probability of density greater

than 3,000 larvae per 100 square meters.

11
breaks (phase 2). The 1965 suppression project, most likely, was conducted

against a population in which densities had just peaked and, thus, were analogous

to the peak densities of 1993 (phase 3). The difference was that the first outbreak

was terminated in the third phase by DDT treatment, whereas the second outbreak

continued on to cause more defoliation in that phase and for another year during

population decline (phase 4).

It is noteworthy that virus disease was conspicuously present in phase 3 of the

second outbreak but did not necessarily bring about a quick overall collapse of the

population. This supports other observations where, even though virus epizootics

have contributed significantly to population decline, they have not always terminated

outbreaks with the speed that sometimes is expected (Mason 1981). Direct suppression of outbreaks before NPV develops naturally, as occurred in 1965, prevents

production of virus in older larvae and has been suspected of reducing the accumulation of virus in forest soil where it is available for infecting future outbreak populations (Thompson and Scott 1979). On the other hand, there also is speculation that

NPV has a destabilizing effect on tussock moth populations in general so that,

ultimately, it may be primarily responsible for the rapid changes in density that lead

to outbreaks in the first place (Berryman and others 1990). The overall influence of

virus disease in tussock moth dynamics obviously is complex and remains somewhat

of an enigma.

Besides documenting details of the 1991-95 tussock moth outbreak, much can be

learned from the above analysis in the way of evaluation techniques. When used in

low-density situations, monitoring adults with baited traps is an efficient technique

for early detection of a population buildup. After an outbreak has developed, however,

moth traps no longer are reliable for predicting changes in population density. High

trap counts indicate only the potential for an outbreak and signal the need for more

intensive evaluation (Daterman and others 1979). Followup sampling of larvae then

is the most efficient method for estimating basic population parameters to establish

population trend, predict larval densities, and forecast defoliation. All these procedures produce important information needed for decisionmaking in dealing with

outbreaks and should be a permanent part of any tussock moth management

system.

We thank G.E. Daterman, Pacific Northwest Research Station, Corvallis, OR; J.E.

Dewey, Northern Region, Missoula, MT; D.L. Overhulser, Oregon Department of

Forestry, Salem, OR; and J.C. Weatherby, Intermountain Region, Boise, ID, for

their technical reviews of an early draft, and J.W. Hazard, Statistical Consulting

Service, Bend, OR, for his statistical review.

Agee, James. 1993. Fire ecology of Pacific Northwest forests. Washington, DC:

Island Press. 493 p.

Berryman, A.A.; Millstein, J.A.; Mason, R.R. 1990. Modelling Douglas-fir tussock

moth dynamics: the case for simple theoretical models. In: Watt, A.D.; Leather,

S.R.; Hunter, M.D.; Kidd, N.A.C., eds. Population dynamics of forest insects.

Andover, UK: Intercept: 369-380.

Acknowledgments

Literature Cited

12
Buckhorn, W.J. 1948. Defoliator situation in the fir stands of eastern Oregon and

Washington, season of 1947. Unnumbered rep. Portland, OR: U.S. Department of

Agriculture, Bureau of Entomology and Plant Quarantine, Forest Insect

Laboratory. 11 p.

Crouch, Glenn L.; Perkins, Randall F. 1968. Surveillance report, 1965 Burns

Project, Douglas-fir tussock moth control. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Region. 20 p.

Daterman, G.E.; Livingston, R.L.; Wenz, J.M.; Sower, L.L. 1979. How to use

pheromone traps to determine outbreak potential. Agric. Handb. 546. Washington,

DC: U.S. Department of Agriculture. 11 p.

Mason, R.R. 1981. Numerical analysis of the causes of population collapse in a

severe outbreak of the Douglas-fir tussock moth. Annals of the Entomological

Society of America. 74: 51-57.

Mason, R.R. 1996. Dynamic behavior of Douglas-fir tussock moth populations in the

Pacific Northwest. Forest Science. 42: 182-191.

Mason, R.R.; Paul, H.G. 1994. Monitoring larval populations of the Douglas-fir

tussock moth and western spruce budworm on permanent plots: sampling

methods and statistical properties of data. Gen. Tech. Rep. PNW-GTR-333.

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Research Station. 23 p.

Mason, R.R.; Wickman, B.E. 1988. The Douglas-fir tussock moth in the interior

Pacific Northwest. In: Berryman, Alan A., ed. Dynamics of forest insect populations,

patterns, causes, implications. New York: Plenum Press: 180-209.

Mason, R.R.; Wickman, B.E. 1991. Integrated pest management of the Douglas-fir

tussock moth. Forest Ecology and Management. 39: 119-130.

Mason, Richard R. 1976. Life tables for a declining population of the Douglas-fir

tussock moth. Annals of the Entomological Society of America. 69: 948-958.

McConnell, Tim. 1987. Region six forest pest management annual aerial insect

detection survey map processing handbook. Unnumbered rep. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Region, Forest Pest

Management. 76 p.

Orr, P.E.; Pettinger, L.F.; Dolph, R.E. 1965. Forest insect conditions in the Pacific

Northwest during 1964. Portland, OR: U.S. Department of Agriculture, Forest

Service, Pacific Northwest Region. 90 p.

Paul, H. Gene. 1979. How to construct larval sampling equipment. Agric. Handb.

545. Washington, DC: U.S. Department of Agriculture. 11 p.

Perkins, Randall F.; Dolph, R.E. 1967. Operational and entomological report, 1965

Burns Project, Douglas-fir tussock moth control. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Region. 28 p.

Powell, David C. 1994. Effects of the 1980s western spruce budworm outbreak on

the Malheur National Forest in northeastern Oregon. Tech. Publ. R6-FI&D-TP-12-94.

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Region. 176 p.

13
Sheehan, Katherine A. 1996a. Defoliation by western spruce budworm in Oregon and

Washington from 1980 through 1994. Tech. Publ. R6-NR-TP-04-96. Portland, OR: U.S.

Department of Agriculture, Forest Service, Pacific Northwest Region. 9 p. (plus

appendix)

Sheehan, Katherine A. 1996b. Effects of insecticide treatments on subsequent

defoliation by western spruce budworm in Oregon and Washington: 1982-92.

Gen. Tech. Rep. PNW-GTR-367. Portland, OR: U.S. Department of Agriculture,

Forest Service, Pacific Northwest Research Station. 54 p.

Steel, Robert G.D.; Torri, James H. 1960. Principles and procedures of statistics

with special reference to the biological sciences. New York: McGraw-Hill. 481 p.

Stelzer, M.J. 1979. How to determine the occurrence of virus in egg masses. Agric.

Handb. 548. Washington, DC: U.S. Department of Agriculture. 7 p.

Thompson, C.G.; Scott, D.W. 1979. Production and persistence of the nuclear

polyhedrosis virus of the Douglas-fir tussock moth, Orgyia pseudotsugata

(Lepidoptera: Lymantriidae), in the forest ecosystem. Journal of Invertebrate

Pathology. 33: 57-65.

U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine.

1946. Forest insect conditions in Oregon and Washington, 1938-1942.

Unnumbered rep. Portland, OR: Forest Insect Laboratory. 57 p.

Wickman, Boyd E.; Mason, Richard R.; Thompson, C.G. 1973. Major outbreaks

of the Douglas-fir tussock moth in Oregon and California. Gen. Tech. Rep. PNW-5.

Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest

Forest and Range Experiment Station. 18 p.

14
The Forest Service of the U.S. Department of Agriculture

is dedicated to the principle of multiple use management of

the Nation’s forest resources for sustained yields of wood,

water, forage, wildlife, and recreation. Through forestry

research, cooperation with the States and private forest

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provide increasingly greater service to a growing Nation.

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Pacific Northwest Research Station

333 S.W. First Avenue

P.O. Box 3890

Portland, Oregon 97208-3890

Mason, R.R.; Scott, D.W.; Loewen, M.D.; Paul, H.G. 1998. Recurrent outbreak of

the Douglas-fir tussock moth in the Malheur National Forest: a case

history. Gen. Tech. Rep. PNW-GTR-402. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station. 14 p.

Characteristics of an outbreak of the Douglas-fir tussock moth (Orgyia pseudotsugata

(McDunnough)) in 1991-95 on the Burns Ranger District of the Malheur National Forest

(eastern Oregon) are given and compared with an earlier infestation in the same area in

1963-65. Results of monitoring with pheromone traps, evaluating populations by

sampling larvae, and predicting trends and defoliation are reported in detail

for the latest outbreak. Temporal patterns of each outbreak also are described

and compared with the classic behavior of other tussock moth outbreaks. Findings

of this analysis, and the recurrent behavior of tussock moth outbreaks in general,

reinforce the importance of maintaining a system for detection, evaluation, and

prediction in managing Douglas-fir tussock moth populations in the future.

Keywords: Douglas-fir tussock moth, Orgyia pseudotsugata, insect outbreaks,

sampling insects, pheromone traps, population monitoring.
U.S. Department of Agriculture

Pacific Northwest Research Station

333 S.W. First Avenue

P.O. Box 3890

Portland, Oregon 97208-3890

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