Parasitism of Douglas-fir tussock moth eggs in Denver

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PARASITISM OF DGUGLAS-FIR TUSSOCK

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IN DENVER

Forest Insect and Disease Management
State and Private Forestry
Rocky Mountain Region
USDa, r orest Service
lli/7 Wp 8th Avenue
Lakewood, Colorado 8022b

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PARASITISM OF DOUGLAS-FIR TUSSOCK MOTH EGGS IN DENVER

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by

Robert D.

Averi 1 1 ,

Entomologist

Technical Report R2-6
November 1976

Forest Insect and Disease Management
State and Private Forestry
Rocky Mountain Region
USDA, Forest Service
11177 W. 8th Avenue
Lakewood, Colorado 80225

INTRODUCTION

Egg parasite surveys were conducted in May 1976 in the Denver area
to examine parasitism of Douglas-fir tussock moth (DFTM) eggs.

Orgy i a pseudotsugata McD. , on Federal and selected private lands.
These surveys were conducted by Bob Averill and Larry Yarger,
Entomologists, Forest Insect and Disease Management (FIDM) and
Ken Hostetler, Denver District Forester, Colorado State Forest
Service.

METHODS

Egg masses were removed from branches within the lower crowns of
spruce ( Picea spp.) trees defoliated by DFTM in 1975. Egg masses
from each of seven locations (Figure 1) were placed in large vials
and transported to the FIDM laboratory in Lakewood, Colorado.

Each egg mass was placed in a plastic petri dish maintained at
room temperature and given 8 hours of room light until emergence
was completed. One hundred forty-one new and 3 old egg masses
were collected.

Insects which emerged from the egg masses were tentatively iden¬
tified. Representative specimens were shipped for verification to
Dr. Torolaf Torgersen, Pacific Northwest, USFS Corvallis, Ore.
and to the United States National Museum. Additional specimens
have been retained in the FIDM insect collection in Lakewood.

A subsample of 12 egg masses were analyzed to determine the
average number of eggs per egg mass. Eggs were separated from
the egg mass by gently crumbling the egg mass, placing the broken
mass in a beaker containing a 2 percent Clorox solution and
agitating with a magnetic stirrer for 10 minutes. The solution
was then filtered. The eggs were spread out on filter paper and
were counted under a stereomicroscope.

RESULTS

Telenomus aalifornicus Ash. was the only species of egg parasite
recovered. Six males of the family Pseudococcidae, probably —
Phenaoocous minimus Tinsley, were recovered from one old egg mass.

The mean number of eggs per egg mass was 187 +_ 59.5. The mean
number of DFTM larvae hatching per egg mass was 16.2 +_ 20.4; the

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Determined by D. R. Miller, USNM

I

mean number of T. calif ovnicus was 18.6 +_ 20.4 per egg mass.
Statistics by sample site for larval and parasite emergence are
presented in Table 1.

TABLE 1 Larval, parasite emergence and percent parasitism
within emerging populations of DFTM in East Denver

DFTM

Site

Number of
egg masses

larvae

X SD

T. calif ovnicus %

X SD T.

Parasitism by

calif ovnicus

1

19

9.5

14.5

16.8

14.0

64.0

2

8

0.1

0.4

1.1

3.2

90.0

3

13

12.5

21.6

2.6

6.0

17.4

4

19

17.4

24.7

8.7

10.7

35.4

5

6

15.3

17.6

4.0

3.3

20.7

6

6

39.5

24.2

25.5

3.7

39.2

7

70

75.6

49.3

3.0

7.4

3.7

DISCUSSION

Thi s

is the first

known attempt

to examine

parasi ti sm

of DFTM

eggs in the Denver area. The current outbreak probably started
in 1972 or 1973. The insect was a problem in 1973 at Fort Carson,
south of Colorado Springs. In 1973 F I DM became aware of an
outbreak in the Denver area. Application of pesticides has been
the tactic employed to mitigate the DFTM population. However,
control has not been achieved in the Denver area and the effects
of the population continue to be a concern to landowners.

The data show a wide range o'f parasitism (3.7 - 90%) in the DFTM
population. Habitat differences, including presence of alternate
hosts, and improper timing of pesticide application may explain
the vast population range. Pesticides applied either before the
egg hatch or after adult emergence could be detrimental to the
egg parasite populations.

Less than 20 percent of the eggs in the subsample produced viable
DFTM larvae or parasites. Whether this low emergence is typical,
or an indicator of a collapsing population in the Denver area is
unknown. Egg masses were not checked for incidence of a virus,
which also could cause the low emergence.

Torgersen (personal communication) indicates that four genera of
parasites attack DFTM eggs: Trichogramma (Trichogrammatidae) ;
Tetrastichus (Eulophidae) ; Anastatus (Eupelmidae) and Telenomus
(Seel ionidae) . Trichogramma and Telenomus are common parasites
of DFTM egg masses while Anastatus is uncommon. Tetrastiahus has
been common in DFTM egg masses collected in New Mexico. We are
uncertain why we recovered only Telenomus. Although Trichogramma
is present in the Denver area it is absent from our collection.

The absence may indicate a failure to capture it in the laboratory.

RECOMMENDATIONS

The DFTM egg mass sampling should be extended to include an early
winter sample as well as one in early spring just prior to egg
eclosion. The survey should be expanded into areas of Denver
where DFTM is increasing. Also any virus activity should be
verified. An expanded survey would provide useful information
to determine where and when to spray, and in time, provide data
for predicting population trend of Douglas-fir tussock moth.

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