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;^^^ United States

Department of

Agriculture

Combined Forest Pest

Research and

Development Program

Agriculture Handbook

No. 532

Douglas-Fir

Tujssock Moth

Handbook

How to Time

the Sampling of

Tussock Moth

Larvae

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How to Time the Sampling of

Tussock Moth Larvae

by B. E. Wickman»

In 1974 the U.S. Department of

Agriculture initiated the Combined

Forest Pest Research and Development Program, an interagency

effort that concentrated on the

Douglas-fir tussock moth in the

West, on the southern pine beetle

in the South, and on the gypsy moth

in the Northeast. The work reported

in this publication was funded in

whole or in part by the program.

This manual is one in a series on

the Douglas-fir tussock moth.

Forest managers and entomologists

concerned about outbreaks of the

Douglas-fir tussock moth in the West

need to keep close track of increasing

insect populations. This is best done

in the spring, soon after the larvae

emerge from the egg masses and

move to the newly flushed foliage to

feed.

In the past, early instar larval

sampling times were based on

estimates of the date of egg hatch.

But predicting exactly when egg

hatch will occur has been difficult.

For one thing, the time of egg hatch

varies considerably from year to year

and from place to place. In addition,

it has been difficult to keep track of

the development of the egg masses to

know when the larvae begin to

emerge. During non-outbreak years,

many egg masses are located in the

tops of trees where they are hard to

see. During an outbreak, egg masses

are easy to find in the lower crown

— but during later stages of the

outbreak many of the eggs in a mass

may fail to hatch. This hatching

failure can give a false impression of

the stage of egg mass development.

'Forest Service, Pacific Northwest Forest and

Range Experiment Station.
A solution to the problem of timing

larval sampling is to keep track of

tree development rather than insect

development. These are closely linked

since the same environmental factors

that affect tussock moth egg

development also affect the timing of

bud burst and shoot elongation in

Douglas-fir and true fir. Since the

newly hatched larvae feed only on

new foliage, they must begin feeding

shortly after the new foliage begins to

grow or they will starve. It is this

synchronization of insect and host

development that indicates when to

begin sampling early instar larvae.

The timing is roughly as follows

(± 1 to 5 days):

Stage I

Day 1—first buds burst

Stage II

Day 10—most buds burst, egg hatch

begins

Stage III

Day 17—egg hatch peaks

Stage IV

Day 22—larvae dispersing off egg

masses, shoots 50 percent or more

elongated.

Research has shown that larvae begin

to hatch from the eggs at about Day

10, when approximately 90 percent

of the tree buds have burst. Due to

local environmental conditions, egg

hatch can last anywhere from about

6 to 14 days. Thus, while bud burst

may be a good indicator of egg

hatch, it does not indicate when most

of the larvae will have moved off the

egg mass and into the foliage. Within

a few days, however, insect and tree

development become even more

closely linked. By the time shoots

are about half grown (2 to 3.5 cm

long), most of the larvae will have

moved off the egg masses and

begun feeding (Stage IV). This is

the best time to sample for firstand second-instar larvae.
Stage I

Stage II

Stage IV

-I cm

Figure 1.—Tussock moth

development is linked with tree

development :

Stage 1 New buds burst, egg hatch

imminent

Stage 11 Most buds burst, some

elongation occurs, eggs hatching

Stage IV New shoots about 50 percent elongated, larvae have left egg

masses and started feeding.
If possible, tree development should

be monitored in the field from before

the time of bud burst. This can be

done by establishing a series of plots

(containing about 10 trees each) at

different elevations and then

periodically examining the lower and

mid-crown foliage. Sometimes it may

be impossible to be in the forest this

early or for this long. In this case, a

usable estimate can be made by

measuring shoot growth on several

dozen trees in a given location.

To be accurate, insect sampling

should be done when the larvae are

well dispersed in the foliage. By this

time, early hatching larvae may have

caused some defoliation. While this

damage may be unsightly, it will

cause little or no effect on tree

growth.
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* Washington, D.C. 20402

^ G.P.O. 1978-796-823