Monitoring Western Spruce 

Budworm With PheromoneBaited Sticky Traps to Predict 

Subsequent Defoliation

Christine G. Niwa and David L. Overhulser1

DEPAR TMENT OF AGRICULT URE

United States Department of Agriculture

Forest 

Service

Pacific Northwest 

Research Station

Research Note

PNW-RN-571

June

2015

1 Christine G. Niwa is a research entomologist (retired), U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Research Station, Forestry Sciences 

Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331-8550, and David L. 

Overhulser is an entomologist (retired), Oregon Department of Forestry, 2600 

State Street, Salem, OR 97310. 

Abstract

A detailed procedure is described for monitoring western spruce budworm with 

pheromone-baited sticky traps and interpreting the results to predict defoliation 

the following year. Information provided includes timing of the survey, how to 

obtain traps and baits, how many traps are needed, trap assembly, field placement of traps, and how to evaluate the catches.
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RESEARCH NOTE PNW-RN-571

Introduction

Because outbreaks of the western spruce budworm (Choristoneura freemani 

Razowski (= occidentalis Freeman)) can last a decade or more, there are 

many occasions when knowing the future course of stand defoliation will aid 

in management decisions. This is particularly true whenever an insecticide 

treatment is under consideration.

Forest Service scientists with the U.S. Department of Agriculture 

(USDA) have developed a technique for predicting spruce budworm 

defoliation.2 This technique relates the number of moths caught in 

pheromone-baited traps to stand defoliation the following year. Described 

herein are the methods for monitoring spruce budworm and procedures for 

evaluating trap catch to predict future impacts from this pest. 

Timing of the Survey

The timing of trap placement is critical to the accuracy of the survey. 

Throughout most of its range, the western spruce budworm (WSBW) 

completes its life cycle in 12 months. Depending on local conditions, moths 

usually emerge from pupal cases in July or early August; in the southern 

Rockies, adults often begin emerging in early July. 

Ideally, traps should go out at the time budworm larvae start to pupate. 

Budworm larvae near pupation are 1 to 1-1⁄4 in long with tan heads and white 

spots on an olive to reddish brown body (fig. 1). Budworm pupae are 1⁄2 to 3⁄4 in 

long (fig. 2) and are yellow- to reddish brown. Both mature larvae and pupae 

are found near the tips of branches.

If information on budworm pupation is not available, traps should be put 

out during the second week in July for most areas in eastern Oregon. When 

2 The trap technique for predicting budworm defoliation was developed by 

Charles Sartwell, Christine Niwa, and Gary Daterman, USDA Forest Service, 

Pacific Northwest Research Station, Corvallis, Oregon. Forestry Sciences 

Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331. 

http://www.fs.fed.us/pnw/contact/corvallis.shtml.
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Monitoring Western Spruce Budworm With Pheromone-Baited 

Sticky Traps to Predict Subsequent Defoliation

trap locations differ in elevation by more than 800 ft, the deployment of traps 

should be delayed approximately 1 week for each 800 ft of increased elevation. 

All traps should be retrieved and evaluated in late August. The WSBW pheromone bait uses a very low dosage, 0.0001 percent, and is less stable over time 

than the Douglas-fir tussock moth (Orgyia pseudotsugata McDunnough) bait; 

therefore, it is recommended to put the traps out after the larvae have pupated, 

but before they emerge. Putting traps out while the larvae are still feeding will 

not ensure accurate results.

Figure 1—Mature budworm larva.

Figure 2—Budworm pupa.

William M. Ciesla Christine Niwa
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RESEARCH NOTE PNW-RN-571

Obtaining Traps and Baits

The traps used for WSBW monitoring are white triangle sticky traps, the same 

as those used for Douglas-fir tussock moth. Traps may be ordered through 

regional Forest Service Forest Health Protection coordinators or local state 

forestry offices in January. The WSBW baits are purchased directly from 

semiochemical supply companies. Contact your regional or state entomologist 

for the current accepted manufacturer. Use only those baits and traps that have 

been recommended and calibrated. Different bait formulations could have a 

different chemical composition or release rate that could affect trap catch, 

and the correlation with defoliation could be different from what is currently 

being used. 

How Many Traps Are Needed?

Budworm traps provide a method of predicting defoliation levels on an area 

basis. To make an accurate defoliation estimate, traps should be distributed 

to cover as much of the entire survey area as posible. The following tabulation 

summarizes the approximate number of traps needed to survey a range 

of acreage.

Number of 

Acres surveyed traps required

500 or less 10

500 to 10,000 10 to 20 

10,000 to 25,000 20 to 30

25,000 to 50,000 30 to 40

50,000 to 100,000 40 to 50

When surveying large areas, traps should be labeled with an exact legal 

description. When many traps are deployed, location information is very 

important as adjacent traps with high moth catches may help delineate a hotspot where severe defoliation and tree damage are likely. 
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Monitoring Western Spruce Budworm With Pheromone-Baited 

Sticky Traps to Predict Subsequent Defoliation

Assembly of Traps

Equipment and supplies needed:

• 0.0001-percent WSBW baits from supplier—baits should be stored 

in a freezer prior to use and in a cooler in the field

• USDA Forest Service Douglas-fir tussock moth traps

• 1⁄4-in thick foam cut into 1⁄2- to 1-in squares (provided with traps)

• Wire ties (provided with traps)

• 1-1⁄4-in straight pins

• Stapler and staples

• Needle-nosed pliers or long tweezers

• Black waterproof marker

• Flagging

Instructions for trap assembly

Traps and baits may be assembled indoors prior to taking them to the field. 

Refer to figure 3, and follow these steps when assembling traps and preparing 

the baits:

1. Cut 1⁄2- to 1-in square pieces of foam.

2. Push the pin through the center of the foam square with the head of 

the pin on the bottom of the square; impale a bait pellet on the end of 

the pin and slide part way down the pin. Place the foam squares with 

the pinned baits in a waterproof container.

3. The entire inside surface of the sticky trap should be evenly coated 

with adhesive. If it is not, rub the inside surfaces of two traps 

together in order to obtain an even distribution of adhesive. 

4. Staple the flaps together to form the triangular shape of the trap. 

5. Attach the wire hanger through the center hole of the trap and twist 

it several times.
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RESEARCH NOTE PNW-RN-571

Figure 3—Bait placement in traps. Place the foam square on the 

bottom of the trap, as close to the center of the trap as possible. 

Note that the pin and bait sit upright and do not come in contact 

with the sides of the trap. (Photos 1 through 5 by Mark Hitchcox; 

photo 6 by Karen Ripley)

1

2

3
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Monitoring Western Spruce Budworm With Pheromone-Baited 

Sticky Traps to Predict Subsequent Defoliation

4

6

5

Figure 3—Continued.
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RESEARCH NOTE PNW-RN-571

6. When ready to place a trap in the field, use a pliers or tweezers to 

grasp the pin and place the foam square on the bottom side of the 

trap, as far to the center of the trap as possible. It is important that 

the bait never touches the adhesive. Keep the baits in a cooler when 

in the field.

Field Placement of Traps

Distribute traps singly throughout the trapping area; however, traps should 

be at least 20 m (21.8 yd) apart. Individual traps should be hung in the lower 

crown of open-grown Douglas-firs (Pseudotsuga menziesii (Mirb.) Franco) 

that are at least 30 to 60 ft in height (fig. 4). The outer branches on clumps of 

trees are also acceptable sites for traps. Do not hang traps in dense foliage 

or in thickets. Trees selected for hanging traps should be at least 75 ft off the 

road. Use a black waterproof marker to label the trap with location information. Bright flagging visible from the road should be used to mark the trap 

location. Trap location records should include geographic coordinates and be 

marked on a map to aid in retrieval. 

Evaluating Trap Catches

In late August, traps should be collected and the number of moths in each 

trap counted and recorded. It is not important that you can distinguish spruce 

budworm from other moths in the trap. Virtually all moths in the traps during 

a budworm outbreak will be WSBW (fig. 5). The total number of moths in all 

traps is calculated and divided by the number of traps, to give an average trap 

catch. Using the average trap catch, it is possible to estimate the approximate 

level of defoliation (current foliage loss) expected next year (table 1).
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Monitoring Western Spruce Budworm With Pheromone-Baited 

Sticky Traps to Predict Subsequent Defoliation

Figure 4—Trap placement in trees.

Karen Ripley

Jeff Jones

Douglas-fir

Place trap at head

height Trees should

be 75 feet from

any road Cut branches if

blocking trap

opening
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RESEARCH NOTE PNW-RN-571

Figure 5—(A) Budworm moth, and (B) moths in traps. 

Chris Niwa Jeff Jones

A

B
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Monitoring Western Spruce Budworm With Pheromone-Baited 

Sticky Traps to Predict Subsequent Defoliation

In table 1 predictions are for the entire area surveyed, not for individual 

or small groups of traps. There will be stands within the survey area with 

higher and lower levels of defoliation.

An average trap catch of 40 or more moths indicates heavy tree defoliation the next year. In instances where trees in the survey area are already 

moderately defoliated, a trap catch of greater than 40 moths indicates the 

potential for serious tree damage such as top-kill and tree mortality. 

Acknowledgments 

Special thanks to Mark Hitchcox for ensuring that the manuscript was in 

final form.

Metric Equivalents

When you know: Multiply by: To find:

Inches (in) 2.54 Centimeters (cm)

Feet (ft) 0.305 Meters (m)

Acres 0.405 Hectares

Table 1—Predicted percentage of defoliation of new 

growth and defoliation over the entire area based on 

average number of moths per trap

Predicted percentage Predicted

Average number of defoliation defoliation

of moths per trap of new growth over entire area

Percent

15 20 Next year light

25 40 Moderate

40 60 to 80 Heavy

60 or more 100 Heavy
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RESEARCH NOTE PNW-RN-571

Other Resources

Fellin, D.G.; Dewey, J.E. 1982. Western spruce budworm. Forest Insect and 

Disease Leaflet 53. USDA Forest Service. http://www.na.fs.fed.us/spfo/

pubs/fidls/westbw/fidl-wbw.htm. 

Sartwell, C.; Daterman, G.E.; Twardus, D.B. 1985. Moth captures in 

pheromone-baited traps relative to subsequent defoliation of Douglasfir by western spruce budworm. In: Sanders, C.J.; Stark, R.W.; Mullins, 

E.J.; Murphy, J., eds. Recent advances in spruce budworms research. 

Proceedings of the CANUSA spruce budworms research symposium. 

Ottawa, ON: Canadian Forest Service. 240 p.


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