^\-.l>J^^\ United States

^' * ft) Department of

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Agriculture

Combined Forest Pest

Research and

Development Program

Agriculture Handbook

No. 536

Douglas-Fir

Tussock Moth

Handbook

5

Larval Instars of

the Douglas-fir

Tussock Moth

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Larval Insíars

of the Douglas-fir

Tussock Meíh

by R.C. Beckwithi

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 Dou;glas-fir tussock moth.

Douglas-fir tussock moth larvae go

through five, six, and occasionally

seven developmental stages, or instars, before pupating. Entomologists

often need to identify the larval

stages in field and laboratory situations. For example, the instars need

to be accurately identified for lifetable studies in the field. Accurate

identification is also important when

rearing tussock moth larvae for

behavioral studies, virus incidence

determination, predator and parasite

studies, and bioassay of microbial

and chemical spray formulations.

Ability to identify instars in the field

is also important in planning and carrying out control operations.

This handbook provides a means

to identify larval instars of the

Douglas-fir tussock moth by visible

characteristics. It is not designed to

function as a taxonomic description

(McDunnough 1921.) All characteristics used for identification—

except the head capsule measurements—can be seen with the naked

eye or a lOX hand lens. Microscopic

examination and measurement of the

head capsule will help verify the

identification.

The dichotomous key (table 1)

separates the first through fifth

instars; additional instars are not

included in the key because the differences are minor.

^Principal insect ecologist, Forest Service,

Pacific Northwest Forest and Range Experiment Station, Forestry Sciences Laboratory,

Corvallis, Oreg.
Table 1.—Key to larval instars

0

Thoracic horns and abdominal tufts absent;

abdominal glands absent or visible as yellow

areas below the surface of the integument on

abdominal segments 6 and 7.

First instar

Thoracic horns and abdominal tufts present,

although they may be difficult to see with the

naked eye; abdominal glands protrude above

the integument on abdominal segments 6 and 7.

See 0

0

Thoracic horns short, usually shorter than the

length of the head capsule, three dorsal

abdominal tufts (located on abdominal

segments 1, 2, and 8); dorsal abdominal glands

light yellow to yellow.

Thoracic horns longer than the head capsule;

more than three dorsal abdominal tufts (located

on abdominal segments 1, 2, 3, usually 4, and

8); dorsal abdominal glands orange to red.

Second instar

See 0

©

Abdominal tufts 1 and 2 predominately black. See 0

Abdominal tufts 1 and 2 buff, brown, or brown

with a darker brown to black fringe. See

Auxiliary tuft on abdominal segment 8 absent

or very short and erect; abdominal tufts 3 and

4 short and difficult to see, number 4 may be

missing.

Auxiliary tuft on abdominal segment 8 present

and usually leaning against the anal tuft;

abdominal tufts 3 and 4 shorter than 1 and 2,

white, and conspicuous.

Third instar

Fourth instar

AuxiUary tuft black; abdominal tufts 1-4

variable in height and color; tufts 1 and 2

larger and fuller than tufts 3 and 4; tufts 3 and

4 white.

Auxiliary tuft brown.

Fourth instar

See

Abdominal tufts variable in height, tufts 3 and

4 shorter and less dense than tufts 1 and 2,

tufts 3 and 4 white or rarely brown.

Abdominal tufts more uniform in height and

quite full; tufts 3 and 4 brown or brown with a

dark fringe.

Fourth instar

Fifth and

sixth instars
' \\/í ' I '. " . 1 '

Abdominal Tufts Auxiliary

Anal Tuft

The key uses the characteristics and

terminology shown in figure 1. Each

instar is also described in more detail

to aid proper identification. Finally,

schematic drawings (fig. 2) and color

photographs of each instar provide

additional help. The photographs

show representative larvae and do

not span the variances in size or

color patterns. Table 2 lists various

tuft color patterns.

The information was developed

from £md is designed for identifying

living larvae. Accurate identifications

can also be made of cast skins or

dead dried larvae if they are handled

carefully. This makes it possible to

determine the instar killed by

parasites or other factors.

Figure 1.—Mature larva, showing

characteristics discussed in text.
INSTAR

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Figure 2.—Tuft color patterns of

instars 1 through 6.
Table 2. — Tuft color patterns of

Douglas-fir tussock moth larval

instars! ^

Instar

Thoracic

horns 1&2

Abdominal tufts

3 4

1st none none none none

2nd Short

Black

Black none none

3rd Black Black Short

White

Short

White

Black

Black

Black

Black

Short

White

none

none

none

4th Black

Black

Black

Black

Black

Black

Black

Black

Black with

brown tip

Brown with

black tip

Black with

brown tip

Brown

Brown

Brown

White

White

White

White with

brown tip

White

White

Brown

White

White

White

White

White

White

Brown

5th Black

Black

Black

Black

Buff with

brown tip

Buff with

black tip

Brown

Brown

Buff w,ith

brown tip

Buff with

black tip

Brown

Brown

Buff with

brown tip

Buff with

black tip

Brown

Brown

^/ From numerous rearings and various

geographical areas during 1973-77.
Glands

Auxiliary

tuft

Anal

tuft Remarks

Yellow areas beneath

integument

none none Larva appears fuzzy because

of long hairs especially at

ends of the larva.

Yellow none Black Tufts and horns difficult

to see without a hand lens.

Orange Short

Black

Upright

Black

Orange none Black

Orange none Black Occasional specimen.

Red/orange-red Black Black Tufts 3, 4 shorter than

tufts 1, 2.

Red/orange-red Black Black

Red/orange-red Black Black

Red/orange-red Black with

some brown

Black Occasional specimen.

Red/orange-red Black Black

Red/orange-red Brown Black Occasional specimen.

Red/orange-red Brown Black Rare specimen.

Red Brown Black Tufts 1-4 rather uniform

in height.

Red Brown Black

Red

Red

Brown

Brown

Black

Brown with

black hairs

Rare specimen.
General Characteristics

The size and color of Douglas-fir

tussock moth larvae depend on a

number of factors, including instar,

sex, and genetic makeup. The characteristics of the larvae are further influenced by external conditions such

as temperature, food supply, and

population density. Stress brought on

by some unfavorable condition can

force development of additional

instars that may not look normal.

These odd-appearing individuals are

not included in the key or

descriptions.

The general colors of tussock moth

larvae vary from light cream

(blondes) to dark gray or black

(brunettes), a difference that is

especially noticeable in later instars

(fig 3). In addition to the lighter

body color, blondes also tend to have

lighter colored tufts than brunettes.

Some larvae appear blonde in the

early instars but darken by the last

instar.

Figure 3.—Two color variations

of Douglas-fir tussock moth larvae: light cream, or "blonde,"

and dark brown or black, or

"brunette".
The characteristic tufts or tussocks

on all instars except the first arise

from specific areas (plates) on the

larval integument (skin). Tufts are

composed of one or two different

types of hairs (fig. 4 A and B) which

must be present before they are considered to be true tufts. In addition,

long body hairs (fig. 4C) may be

found in the tuft position, especially

in the early instars; these are not

tufts unless at least one of the other

two hair types is present.

Figure 4.—Types of larval hairs:

"a" and "b" are tuft hairs; "c"

is body hair.

An eversible abdominal gland occurs

dorsally on abdominal segments 6

and 7 in all instars except the first

(fig. 1). In the first instar, the gland

can often be seen through the integument as a yellow circular area. As

the larva matures, the gland changes

from light yellow to deep red. This

color change can be used as a

secondary characteristic to help

determine the instar.
Head-capsule width can be used to

help determine instar. But remember

that overlap can occur, especially in

the fourth, fifth, and sixth instars.

Also, head-capsule size depends on

many factors such as population

source, larval food, and temperature

during the larval stage. Mean headcapsule widths of larvae from many

rearings on an artificial diet and

natural food imder laboratory conditions are shown (Table 3).

Remember, a large fourth instar

could easily be confused with a small

fifth or a large fifth instar with a

small sixth if identification were

based on head-capsule size alone.

Relative head-capsule size and larval

lengths are shown for six instars

(figs. 5 and 6).

#9 A m

1 mm

/

Figure 5.—Relative head capsule

sizes of instars 1 through 6.

Figure 6.—Relative body lengths

of instars 1 through 6.

Table 3.—Head capsule

measurements of laboratory-reared

Douglas-fir tussock moth larvae

Instar Number Mean width Standard error

measured (mm)

1 358 0.53 0.001

2 350 0.82 0.002

3 336 1.26 0.005

4 314 1.82 0.010

5 115 2.34 0.017

61/ 20 3.34 0.143

'^ Data supplied by Milt Stelzer, Corvallis,

Oreg.

10
Larval Instars

First Instar

Larvae in the first instar (fig. 7) are

distinctive because they are very small

and lack tufts (fig. 2). Long, fine

body hairs occur where tufts eventually appear. The newly hatched larvae appear fuzzy, but the fuzziness

diminishes when they lengthen out

after feeding. The general body color

is light cream with black markings.

Larvae become quiescent and darken

1 or 2 days before molting. The

abdominal glands have not protruded

through the skin but can often be

seen as yellow circular spots beneath

the skin on abdominal segments 6

and 7. Mean head-capsule width of a

large sample was 0.53 mm (range

0.48-0.56).

Figure 7.—First instar larva.

First instars can be difficult to see on

foliage, especially when the larvae lie

along a twig or on the underside of a

needle and do not move. Larvae are

usually found in trees by beating a

branch over a white drop cloth. The

first instar is the primary stage of

dispersion between trees. The young

larvae spin long silken strands that

carry them on wind currents. In

dense populations, these silk strands

frequently cause "tenting" at the

tops of heavily infested trees.

11
Figure 8.—Second instar larva.

Second Instar

The beginnings of true tufts are

found on larvae in the second instar

(fig. 8). The tufts, regardless of their

location, have groups of hairs with

feathery tips. Thoracic horns arising

on either side of the head are short,

usually shorter than the head capsule.

Abdominal tufts 1 and 2 are

present, erect, short, and black; the

black anal tuft occurs on the rear

part of segment 8. The light-yellow

to yellow abdominal glands protrude

through the skin on abdominal

segments 6 and 7. Mean head-capsule

width is 0.82 mm (range 0.72-0.92).

The short thoracic horns and the lack

of abdominal tufts 3 and 4 are key

characteristics used to distinguish

second instars. The larvae are difficult to see on foliage unless insect

populations are dense. Second instars

also spin silken threads, but not as

readily as larvae in the first instar.

Also, second instars do not usually

move from one tree to another on

these strands.

12
Figure 9.—Third instar larva. Figure 10.—Fourth instar larva.

Third Instar

Larvae in the third instar (fig. 9)

have tufts that can be seen easily.

The thoracic horns are longer than

the head capsule. Abdominal tufts 1

and 2 are black and of uniform

height. Tuft 3 and sometimes tuft 4

are present; however, they are often

short, sparse, and white. A hand lens

may be required to see them. The abdominal glands are yellow orange to

orange. The auxiliary tuft is

sometimes absent; if present, it is

very short and erect, rising from the

middle of the eighth segment. The

mean head-capsule width is 1.26 mm

(range 0.92-1.34).

Third instars are readily seen in the

trees except at low population levels.

The larvae will drop on spun threads

when disturbed, but not as readily as

earlier instars.

Fourth Instar

The prominent, white abdominal

tufts 3 and 4, which are shorter than

tufts 1 and 2, are characteristic of

this instar (fig. 10). Tufts 1 and 2 can

be black, black with a brown fringe,

brown with a black fringe, or solid

brown. The auxiliary tuft is usually

black and appears to lean against the

anal tuft. Variation in tuft color can

sometimes cause confusion with the

fifth instar. In those cases, abdominal tufts 1 - 4 and the auxiliary

tuft are brown. These larvae can be

distinguished as fourth instar by the

noticeably short tufts 3 and 4, compared with tufts 1 and 2. The abdominal glands are red-orange to red.

Mean head-capsule width is 1.82 mm

(1.40- 1.92); head capsules of larger

larvae may be wider than those of

small fifth instars.

Larvae in the fourth instar will occasionally spin a silken thread when

disturbed, but only rarely. Usually

they just fall from the branch.

13
Figure 11.—Fifth instar larva.

Fifth Instar

In the fifth instar (fig. 11) abdominal

tufts 1 - 4 are nearly uniform in

height and color. Tuft color varies

from buff with dark brown tip, and

buff with black tip to solid brown.

The auxiliary tuft is brown, full, and

conspicuous. The anal tuft is usually

black but may rarely have several

brown hairs scattered through it. The

abdominal glands are red to deep

red. The mean head-capsule width is

2.34 mm (1.82-2.68). The low end

of this scale overlaps with the fourth

instar and the high end with the sixth

instar.

Fifth instars usually do not spin

threads when disturbed. In the field,

most males and some females pupate

after this stage.

Figure 12.—Sixth instar larva.

Sixth and Seventh Instars

Sixth and seventh instars (fig. 12) are

similar to the fifth in tuft pattern and

color. Tufts are fuller and body size

is larger. General body color becomes

quite dark in the brunette strain. The

larvae in these instars are usually

females; however, male larvae under

stress sometimes have a sixth instar.

Seventh-instar females are also usually the product of stress. Additional

instars are probably more common in

larvae reared on artificial diets under

laboratory conditions than in nature.

The mean head-capsule width of

sixth instar females is 3.34 mm

(3.10-3.65).

14
41p

Abdominal Tufts Auxiliary

Thoracic Horn 1,2 34 Abdominal Tuft

^^ ^^^^ 'Glands'

Anal Tuft
Table l.—Key to larval instars

0

Thoracic horns and abdominal tufts absent;

abdominal glands absent or visible as yellow

areas below the surface of the integument on

abdominal segments 6 and 7.

Thoracic horns and abdominal tufts present,

although they may be difficult to see with the

naked eye; abdominal glands protrude above

the integument on abdominal segments 6 and 7. ©

©

Thoracic horns short, usually shorter than the

length of the head capsule, three dorsal

abdominal tufts (located on abdominal

segments 1, 2, and 8); dorsal abdominal glands

light yellow to yellow.

Thoracic horns longer than the head capsule;

more than three dorsal abdominal tufts (located

on abdominal segments 1, 2, 3, usually 4, and

8); dorsal abdominal glands orange to red.

Second instar

0

0

Abdominal tufts 1 and 2 predominately black. See ©

Abdominal tufts 1 and 2 buff, brown, or brown

with a darker brown to black fringe. ©

©

Auxihary tuft on abdominal segment 8 absent

or very short and erect; abdominal tufts 3 and

4 short and difficult to see, number 4 may be

missing.

Auxiliary tuft on abdominal segment 8 present

and usually leaning against the anal tuft;

abdominal tufts 3 and 4 shorter than 1 and 2,

white, and conspicuous.

©

Auxiliary tuft black; abdominal tufts 1-4

variable in height and color; tufts 1 and 2

larger and fuller than tufts 3 and 4; tufts 3 and

4 white.

Auxiliary tuft brown. ©

©

Abdominal tufts variable in height, tufts 3 and

4 shorter and less dense than tufts 1 and 2,

tufts 3 and 4 white or'rarely brown.

Abdominal tufts more uniform in height and

quite full; tufts 3 and 4 brown or brown with a

dark fringe.

Fifth and

sixth instars
References

McDunnough, J. 1921. New British

Columbia tussock moth,

Hemerocampa pseudotsugata. Can.

Ent. 53: 53-56.

15