- Email: [email protected]
A nucleopolyhedrosis virus in populations of the Douglas-fir tussock moth, Hemerocampa pseudotsugata, in California
JOURh‘AL
OF INVERTEBRATE
PATHOLOGY
13, 264-271
( 1969)
A Nucleopolyhedrosis Virus in Populations of the Douglas-Fir Tussock Moth, Hemerocampa pseudotsugutu, in California D.
Department
L.
of Entomology
DAHLSTEN
AND
G.
M.
THOMAS
and Parasitology, University of California,
Berkeley 94720
Received September 19, 1968 A nuclear-polyhedrosis virus of the Douglas-fir tussock moth, Hemerocampa pseudotsugatu, was found in the white fir forests of California for the first time. Tussockmoth populations declined in 1965, and this virus may be one of the important mortality factors. Egg masses were collected in selected study areas and the larvae reared in petri dishes on artificial diet. Larvae were also collected in several instars during the course of the summer and reared individually in petri dishes. An attempt was made to design a survey technique that would indicate the rate of decline, due to virus, of tussock-moth populations in the field. Further refinement will be necessary before egg-mass collections can be used as a survey tool but these studies indicate a good potential for such use. Evidence from this study shows the rate of dying in sample dishes in the laboratory to be related to the rate of larval mortality in the field. The primary use of such a survey would be an indicator to those involved in pest control decisions.
one of the most important mortality factors (Dodge and Trostle, 1964; Keen, 1952). In The Dougals-fir tussock moth, Hemero- British Columbia, the outbreaks of 1939, campa pseudotsugata, is considered to be 1949, and 1956 were believed to have colan important defoliator of the true firs lapsed because of a nucleopolyhedrosis ( Abies spp. ) and Douglas fir (Pseudotsuga virus ( Sugden, 1957; Morris, 1963). EvenmenziesiC) in Western North America. In den and Jost (1947) reported that a nuCalifornia, the primary host is white fir, clear-polyhedrosis virus completely halted Abies concolor. Periodically populations the tussock-moth outbreak in areas that have increased to epidemic levels in British were not sprayed with DDT in the 1947 Columbia (Sugden, 1957) and in the west- Idaho-Washington outbreak. ern United States (Dodge and Trestle, The Douglas-fir tussock-moth nucleopoly1964). Although the infestation may occur hedrosis virus has been reported from Oreover a large area, as in the recent 1962-65 gon, Idaho, Washington, and Montana outbreak in California, the tussock moth ( Steinhaus, 1951; Steinhaus and Marsh, occurs locally in discrete patches, occasion- 1962) and British Columbia (Morris, 1963). ally as large as 3000 acres (Mathers, 1949). There is no record of the virus in California Although the reasons for periodic outbreaks although outbreaks of tussock moth ocare unknown, climatic causesare suggested. curred in 1906, 193138, and 1954-56 Populations characteristically collapse (Eaton and Struble, 1957). The reasons for quite dramatically, and such was the case in the decline of these infestations are unCalifornia in 1965. The tussock moth is known. The most recent outbreak of the inknown to have a number of natural enemies sect in California included approximately but a nucleopolyhedrosis virus is probably 78,000 acres (Pierce, 1964). In June, 1965, INTRODUCTION
264
TUSSOCK-MOTH
about 58,000 acres were aerially sprayed with $-lb DDT in 1 gal of diesel oil per acre (U.S.D.A., Forest Service, 1966). MATERIALS
265
NUCIJ3OPOLYHJ3DROSIS
AND METHODS
Description of Study Areas
A project was initiated in 1964 to investigate the influence of biotic agents on the Douglas-fir tussock moth. All of the study areas were located in northeastern California in Modoc, Lassen, and Plumas Counties
(Fig. 1). The forest is a mixed conifer type with white fir the predominant species. The study regions were as follows: I. Corral Creek, Goose Lake, not treated and not discovered until the fall of 1965, but populations collapsed in 1966. II. Stowe Reservoir, Cedar Pass, treated in early fall, 1964, with Malathion and in 1965 with DDT. III. There were six areas sampled within the large Knox Mountain area.
r
i
hlodoc
Co.
1 //
FIG. 1.
Douglas-fir
tussock-moth
study areas in northeastern
California.
266
DAHLSTEN
AND
1964 and the spring of 1965 and larval collections were made in untreated areas during the summer of 1965. In the Corral Creek infestation (Fig. 1, I), egg masses and cocoons were collected in the fall of 1965 and the spring of 1966 and larval collections were made during the summer of 1966. Larvae from these collections were reared individually either on white-fir foliage from uninfested areas or on artificial media (Lyon and Flake, 1966). All dead larvae and unemerged pupae were examined for pohyhedra. In addition, a systematic egg-mass survey was made in the spring of 1965. Egg masses were only ,collected in the areas listed in Table 1. Sterile plastic petri dishes (15 X 100 mm) were taken in the field in plastic bags, and forceps were carried in vials of 70% ethyl alcohol. Samples of approximately 100 egg masses were collected from each area, in previously established study plots, with sterile forceps. One egg mass was placed in each petri dish for transport to the laboratory where they were reared at room temperature (22°C). The petri dishes were checked regularly and artificial media was added when larvae began emerg-
(a) Canyon Creek, treated with DDT in 1965. (b) Likely Mill, not treated. (c) Sweagert Flat, treated with Malathion for white-fir sawfly control in July, 1964 and with DDT in 1965 for tussock moth. (d) Tom’s Creek near Hermit Butte, treated with DDT in 1965. (e) Middle Ridge, treated with DDT in 1965. (f) Ambrose Canyon, within the 1965 spray project boundary but inadvertently missed. IV. Lower Fredonyer, Eagle Lake, treated with DDT in 1965. V. Fredonyer Peak, Eagle Lake, and Fredonyer Peak Lookout, not treated. VI. Diamond Mountain, Skyline Boulevard, treated in 1965 with DDT. VII. Roney Flat, near Adin, not treated. Larval
Rearing
Egg masses and larvae were collected periodically from each of the areas listed above in connection with life history and natural enemy investigations. Egg masses and cocoons were collected in the fall of DETERMINATION
OF VIRUS
THOMAS
TABLE 1 BY EGG MASS REARINGS
INCIDENCE
OF EGG MASSES
COLLECTED Egg
Study
,
area
Stowe Reservoir Roney Flatb Diamond Mtn. Fredonyer Pk.b Tom’s Creek Canyon Creek Likely Millb Sweagert Flat Totals 0 b 0 d
Egg masses collected
97 98 118 133 58 57 59 60
Dishes diseased
with larvae %
72 59 9 40 33 50 52 44
74.2 60.2 7.6 30.1 56.9 87.7 88.1 73.3 52.8
No. 18 23 64 68 20 3 4 0
Numbers and percentages are recorded on a per egg mass or per petri Areas not treated with DDT; the other five areas were treated. Aspergillus j7uuu.s~oyzae group found in two of the dishes. Penicillium sp. found in two dishes and Aspergillus found in one dish.
dish basis.
Fatea
% 18.6 23.5 54.2 51.1 34.5 5.3 6.8 -
7c 16 45d 25 5 4 3 16
29.4
200
1965
Dishes with unknown mortality
Dishes prod. adults
No.
359
mass
IN MAY,
121
TUSSOCK-MOTH
NUCLEOPOLYHEDROSIS
ing from the eggs. This media was added as needed until all larvae were dead or until the larvae had spun cocoons. Observations were made in the field until the study areas were treated or until the populations collapsed in the untreated areas. RESULTS
AND
DISCUSSION
The first diseased larva and the first record of a nuclear polyhedrosis in the tussock moth in California was collected at the Sweagert Flat study area (Fig. 1, III) on 6 October, YL964,after most of the larvae had spun cocoons. The diseased larva was hanging from a branch in the typical posture of the terminal ,stages of a nucleopolyhedrosis. The results from rearing all the larvae from a single egg mass in one petri dish on artificial media show that mortality in the laboratory may give an indication of field mortality. However, further correlation with incidence of virus in the field must be made before such a method can be proved valid for field estimation of virus incidence. During 1965, population collapse occurred sooner in those areas where a high incidence of virus was observed in the laboratory (Table 1). However, all except three of the areas listed in Table 1 were treated shortly after tussock-moth larval eclosion. Field observations for early-instar diseased larvae were made in all of the areas prior to spraying and dead larvae were obvious in those areas in which 70% or more of the dishes produced diseased larvae in the laboratory (Table 1, Stowe Reservoir, Canyon Creek, Likely Mill, and Sweagert Flat). The percentage of the dishes containing living larvae in the laboratory is depicted graphically (Figs. 2 and 3). Thirty days after larval eclosion, the eight areas appear to be divided roughly into four groups on the basis of ‘the number of dishes with
267
living larvae: (1) Diamond Mountain, 99%; (2) Fredonyer Peak, 79% and Tom’s Creek, 65%; (3) Roney Flat, 49%, Stowe Reservoir, 44oJ0, and Sweagert Flat, 38%; (4) Likely Mill, 26% and Canyon Creek, 22%. Incidence of virus was also determined by individual rearing of the larvae because of concern over crowding and stress in the petri dishes. Two egg-mass samples taken late in June showed a high incidence of virus. One sample of 58 larvae from Middle Ridge gave 89.6% mortality and another of 30 larvae from Stowe Reservoir gave 80.0% mortality. The Stowe Reservoir sample compared favorably to the earlier sample from that area in which all larvae from a single egg mass were reared in the same petri dish (74.2% mortality, Table 1). The other areas for which group rearings as well as individual rearings were available showed variable results. Likely Mill showed 88.1% mortality when groupreared but only 55.0% when larvae were reared separately. The populations in this area, however, were among the first to collapse (Table 2). Also, it should be remembered that the incidence of virus by egg mass, which includes many larvae, is being compared with virus incidence in individual larvae. Morris (1963) found that the incidence of virus increased as the season progressed. There was an indication of this in this study from samples taken in the unsprayed areas (Table 2). Note that Roney Flat is an exception to this. There is no explanation for this occurrence. Larvae were extremely difficult to find during the first week in September. During this week at Fredonyer Peak, larvae were found in small pockets but were otherwise absent. It is also interesting that this is the only ,study area where tussock moth has been collected since the population decline (one egg mass and one larva in 1967). In 1966, the only area that could be found with a tussock-moth infestation was
Mill
Likely
Peak
area
a No larvae
Total
Fredonyer
Flat
Roney
Study
or cocoons
could
Late Late
be found
1st & 2nd Late -a
1st Late Late
Larval instar
on this date.
VIII-19 IX-4
VII-13 VIII-17 IX-6
VII-5 VIII-18 IX-6
Date
143 49
80 86 0
44 158 26
Number collected
3 23 4
15 21 82 21 17
-i-f
81 26
369
-
127
Pupae No.
Mortality
456
102 43
-
18 44 86
141 22
No.
Total
LARVAE 1965
due to virus
TUSSOCK-MOTH STIJDY AREAS,
129 15
Larvae No.
TABLE 2 IWWENCE OF Vmus IN INDIVIDUALLY REARED ON VARIOUS DATES IN THREE UNTREATED
77.8
71.3 87.8
69.2 55.0 100.0
89.2 50.0
%
COLLECTED
6 1
-59
21 3
-
-
17 11
Other mortality
71
-
20 3
-
2 35 0
110
Adults reared _____ No.
%
12.1
14.0 6.1
-
7.7 43.8 -
25.0 -
E cn
2
5
3
E
u P+
TUSSOCK-MOTH
Q Canyon Creek l Swcagert Flat A Stowe Res. A Diamond Mt.
10.
269
NUCLEOPOLYHEDROSIS
k
20
30
40
50
60
Days
in
FIG. 2. Survival curves northeastern California.
for
individually
reared
tussock-moth
at Corral Creek near Goose Lake (Fig. 1, I). The incidence of virus in this area decreased as the season progressed (Table 3). Since the population completely collapsed TABLE
INCIDENCEOF VIRUS ~LARVAE Collection Date 8 25 31 Totals
Number collected 1580 47 25 230
COLLECTEDAT~ORRAL
Mortality caused by virus
No. reaching
Pupal
No.
%
pupal stage
120 33 14
75.9 70.2 56.0
38 14 11
19 1
63
ZTT-
167
-
72.6
-
a Larvae were reared five per dish. b Determined by macroscopic examination
Virusb
only.
egg masses
from
four
infestation
areas
an increase in virus incidence as observed by Morris (1963) would have been expetted. Miscellaneous samples were taken and 3 CREEK,MODW
COUNTY,CALIFORNIA,JULY,
mortality
Total % virus
Parasites
Other
mortality
3 3
88.0 70.2 60.0
2 4
-
81.3
No. adults --
reared
6
0
Total
11 7 6
3 4
14 11 6
7
31
?ii
--
1966
$I 8.9 23.4 24.0 13.5
270
DAHLSTEN
AND
the incidence of virus in these areas was also variable (Table 4). The nuclear-polyhedrosis virus of the Douglas-fir tussock
THOMAS
moth was widespread, as indicated by these studies. Virus was found in every area sampled. It is interesting that the population
L g 40 2 c : 30 % 2 ; ct. 20
10
tol
.. 0 A A
Likely Mill Roney Flat Fredonyer Peak Tom’s Creek
0
IO
20
30
40
50
66
Days
in
FIG. 3. Survival curves northeastern California.
INCIDENCE
Study
area
Lower Fredonyer Fredonyer L.O. Ambrose
Canyon
for
individually
reared
tussock-moth
egg masses
from
TABLE 4 OF VIRUS IN LATE-INSTAR LARVAE FROM AREAS NOT TREATED IN NORTHEASTERN CALIFORNIA, 1965 Mortality caused by virus
Collection date
Number collected
No.
“/o
pupal stage
Virus
IX-4 IX-5 IX-28 IX-6
16 85 40= 23
1 12 1 5
6.2 14.1 2.5 21.7
15 73 39 18
3 56 20 2
ii
11.6
Totals 0 All but one of these
Is4 was collected
No. reaching
in the pupal
145 stage.
Pupal
5iziT
mortality
infestation
2 2
areaa
WITH DDT
Total % virus mortality
Parasitesother 12 10 13 14
four
25.0 80.0 52.5 30.4 61.0
Adults reared No.
%
-
0 5.9 15.0 0
5 6 -11
6.7
TTJSSOCK-MOTH
NUCLEOPOLYHJZDROSIS
crash of the tussock moth took place in all areas in northeastern California in 1965 except for one small area that crashed in 1966. Preliminary evidence from this study indicates that the virus was an important natural control agent in this spectacular decline. This study indicates that it may be possible to develop an egg-mass survey for incidence of virus that relates to actual mortality in the field. If laboratory mortality can be correlated with mortality in the field on this basis, then the forester will have a valuable tool available for an efficient and extensive biological evaluation of tussock-moth nucleopolyhedrosis before control decisions are reached. ACKNOWLEDGMENTS
The authors thank the California Division of Forestry, Sacramento, and the U.S. Forest Service, Regional Office, San Francisco for their assistance in this study. Gratitude is also expressed to the Regional Office for permission to cite unpublished reports. Finally, we thank Dr. R.W. Stark, Dr. G.O. Poinar, Jr., Dr. L.A. Falcon, and Dr. D.L. Wood for reviewing the manuscript, and Mr. W.A. Copper for valuable assistance in the laboratory and field.
REFERENCES
H. R., AND Douglas-fir tussock Forest Sew. Forest EATON, C. B., AND DODGE,
TROSTLE,
G.
moth. U.S. Pest Leaflet STFWBLE,
G.
C.
1964. Agr.
Dept. 86, 7
pp.
R.
1957.
271
Douglas-fir tussock moth in California. PanPacific Entomologist, 33, 105-108. EVENDEN, J. C., AND JOST, E. J. 1947. Tussock moth control, north Idaho. U.S. Dept. Agr., Forest Service & Bur. Entomol. & Plant Quarantine. ( Mimeo. ) 28 pp. KEEN, F. P. 1952. Insect enemies of western forests. U.S. Dept. Agr. Misc. Publ. 273, 280 PP. LYON,
R. L., AND FLAKE, H. W., JR. 1966. Rearing Douglas-fir tussock moth larvae on synthetic media. J. Econ. Entomol. 59, 696 698. MATHERS, W. G. 1949. Douglas-fir tussock moth. Can. Dept. Agr., Forest insect Invest., Bi-monthly Progr. Rept. 5( 1 ), 4. MORRIS, 0. N. 1963. The natural and artificial control of the Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough, by a nuclear-polyhedrosis virus. .I. Insect Pathol., 5, 401-414. PIERCE, J. R. 1964. Douglas-fir tussock moth infestations in northern California - 1964. U.S. Forest Service, Region 5, Division of Timber Management, San Francisco, Cahf., Dec. 14, 1964. Processed, 14 pp. STEINHAUS, E. A. 1951. Report on diagnoses of diseased insects 1944-1956. Hilgardia, 20, 629-678. STEINHAUS, E. A., AND MARSH, G. A. 1962. Report of diagnoses of diseased insects 1951-1961. Hilgardia, 33, 349-490. SUGDEN, B. A. 1957. Brief history of outbreaks of the Douglas-fir tussock moth, Hemerocampa pseudotsugata McD., in British Columbia. Proc. Entomol. Sot. British Columbia, 54, 37-39. U.S. Department of Agriculture, Forest Service. 1966. The Douglas-fir tussock moth in California-a cooperative control project. U.S. Forest Service, Division of Timber Management, San Francisco, Cahf., March, 1966. Processed, 34 pp.
OF INVERTEBRATE
PATHOLOGY
13, 264-271
( 1969)
A Nucleopolyhedrosis Virus in Populations of the Douglas-Fir Tussock Moth, Hemerocampa pseudotsugutu, in California D.
Department
L.
of Entomology
DAHLSTEN
AND
G.
M.
THOMAS
and Parasitology, University of California,
Berkeley 94720
Received September 19, 1968 A nuclear-polyhedrosis virus of the Douglas-fir tussock moth, Hemerocampa pseudotsugatu, was found in the white fir forests of California for the first time. Tussockmoth populations declined in 1965, and this virus may be one of the important mortality factors. Egg masses were collected in selected study areas and the larvae reared in petri dishes on artificial diet. Larvae were also collected in several instars during the course of the summer and reared individually in petri dishes. An attempt was made to design a survey technique that would indicate the rate of decline, due to virus, of tussock-moth populations in the field. Further refinement will be necessary before egg-mass collections can be used as a survey tool but these studies indicate a good potential for such use. Evidence from this study shows the rate of dying in sample dishes in the laboratory to be related to the rate of larval mortality in the field. The primary use of such a survey would be an indicator to those involved in pest control decisions.
one of the most important mortality factors (Dodge and Trostle, 1964; Keen, 1952). In The Dougals-fir tussock moth, Hemero- British Columbia, the outbreaks of 1939, campa pseudotsugata, is considered to be 1949, and 1956 were believed to have colan important defoliator of the true firs lapsed because of a nucleopolyhedrosis ( Abies spp. ) and Douglas fir (Pseudotsuga virus ( Sugden, 1957; Morris, 1963). EvenmenziesiC) in Western North America. In den and Jost (1947) reported that a nuCalifornia, the primary host is white fir, clear-polyhedrosis virus completely halted Abies concolor. Periodically populations the tussock-moth outbreak in areas that have increased to epidemic levels in British were not sprayed with DDT in the 1947 Columbia (Sugden, 1957) and in the west- Idaho-Washington outbreak. ern United States (Dodge and Trestle, The Douglas-fir tussock-moth nucleopoly1964). Although the infestation may occur hedrosis virus has been reported from Oreover a large area, as in the recent 1962-65 gon, Idaho, Washington, and Montana outbreak in California, the tussock moth ( Steinhaus, 1951; Steinhaus and Marsh, occurs locally in discrete patches, occasion- 1962) and British Columbia (Morris, 1963). ally as large as 3000 acres (Mathers, 1949). There is no record of the virus in California Although the reasons for periodic outbreaks although outbreaks of tussock moth ocare unknown, climatic causesare suggested. curred in 1906, 193138, and 1954-56 Populations characteristically collapse (Eaton and Struble, 1957). The reasons for quite dramatically, and such was the case in the decline of these infestations are unCalifornia in 1965. The tussock moth is known. The most recent outbreak of the inknown to have a number of natural enemies sect in California included approximately but a nucleopolyhedrosis virus is probably 78,000 acres (Pierce, 1964). In June, 1965, INTRODUCTION
264
TUSSOCK-MOTH
about 58,000 acres were aerially sprayed with $-lb DDT in 1 gal of diesel oil per acre (U.S.D.A., Forest Service, 1966). MATERIALS
265
NUCIJ3OPOLYHJ3DROSIS
AND METHODS
Description of Study Areas
A project was initiated in 1964 to investigate the influence of biotic agents on the Douglas-fir tussock moth. All of the study areas were located in northeastern California in Modoc, Lassen, and Plumas Counties
(Fig. 1). The forest is a mixed conifer type with white fir the predominant species. The study regions were as follows: I. Corral Creek, Goose Lake, not treated and not discovered until the fall of 1965, but populations collapsed in 1966. II. Stowe Reservoir, Cedar Pass, treated in early fall, 1964, with Malathion and in 1965 with DDT. III. There were six areas sampled within the large Knox Mountain area.
r
i
hlodoc
Co.
1 //
FIG. 1.
Douglas-fir
tussock-moth
study areas in northeastern
California.
266
DAHLSTEN
AND
1964 and the spring of 1965 and larval collections were made in untreated areas during the summer of 1965. In the Corral Creek infestation (Fig. 1, I), egg masses and cocoons were collected in the fall of 1965 and the spring of 1966 and larval collections were made during the summer of 1966. Larvae from these collections were reared individually either on white-fir foliage from uninfested areas or on artificial media (Lyon and Flake, 1966). All dead larvae and unemerged pupae were examined for pohyhedra. In addition, a systematic egg-mass survey was made in the spring of 1965. Egg masses were only ,collected in the areas listed in Table 1. Sterile plastic petri dishes (15 X 100 mm) were taken in the field in plastic bags, and forceps were carried in vials of 70% ethyl alcohol. Samples of approximately 100 egg masses were collected from each area, in previously established study plots, with sterile forceps. One egg mass was placed in each petri dish for transport to the laboratory where they were reared at room temperature (22°C). The petri dishes were checked regularly and artificial media was added when larvae began emerg-
(a) Canyon Creek, treated with DDT in 1965. (b) Likely Mill, not treated. (c) Sweagert Flat, treated with Malathion for white-fir sawfly control in July, 1964 and with DDT in 1965 for tussock moth. (d) Tom’s Creek near Hermit Butte, treated with DDT in 1965. (e) Middle Ridge, treated with DDT in 1965. (f) Ambrose Canyon, within the 1965 spray project boundary but inadvertently missed. IV. Lower Fredonyer, Eagle Lake, treated with DDT in 1965. V. Fredonyer Peak, Eagle Lake, and Fredonyer Peak Lookout, not treated. VI. Diamond Mountain, Skyline Boulevard, treated in 1965 with DDT. VII. Roney Flat, near Adin, not treated. Larval
Rearing
Egg masses and larvae were collected periodically from each of the areas listed above in connection with life history and natural enemy investigations. Egg masses and cocoons were collected in the fall of DETERMINATION
OF VIRUS
THOMAS
TABLE 1 BY EGG MASS REARINGS
INCIDENCE
OF EGG MASSES
COLLECTED Egg
Study
,
area
Stowe Reservoir Roney Flatb Diamond Mtn. Fredonyer Pk.b Tom’s Creek Canyon Creek Likely Millb Sweagert Flat Totals 0 b 0 d
Egg masses collected
97 98 118 133 58 57 59 60
Dishes diseased
with larvae %
72 59 9 40 33 50 52 44
74.2 60.2 7.6 30.1 56.9 87.7 88.1 73.3 52.8
No. 18 23 64 68 20 3 4 0
Numbers and percentages are recorded on a per egg mass or per petri Areas not treated with DDT; the other five areas were treated. Aspergillus j7uuu.s~oyzae group found in two of the dishes. Penicillium sp. found in two dishes and Aspergillus found in one dish.
dish basis.
Fatea
% 18.6 23.5 54.2 51.1 34.5 5.3 6.8 -
7c 16 45d 25 5 4 3 16
29.4
200
1965
Dishes with unknown mortality
Dishes prod. adults
No.
359
mass
IN MAY,
121
TUSSOCK-MOTH
NUCLEOPOLYHEDROSIS
ing from the eggs. This media was added as needed until all larvae were dead or until the larvae had spun cocoons. Observations were made in the field until the study areas were treated or until the populations collapsed in the untreated areas. RESULTS
AND
DISCUSSION
The first diseased larva and the first record of a nuclear polyhedrosis in the tussock moth in California was collected at the Sweagert Flat study area (Fig. 1, III) on 6 October, YL964,after most of the larvae had spun cocoons. The diseased larva was hanging from a branch in the typical posture of the terminal ,stages of a nucleopolyhedrosis. The results from rearing all the larvae from a single egg mass in one petri dish on artificial media show that mortality in the laboratory may give an indication of field mortality. However, further correlation with incidence of virus in the field must be made before such a method can be proved valid for field estimation of virus incidence. During 1965, population collapse occurred sooner in those areas where a high incidence of virus was observed in the laboratory (Table 1). However, all except three of the areas listed in Table 1 were treated shortly after tussock-moth larval eclosion. Field observations for early-instar diseased larvae were made in all of the areas prior to spraying and dead larvae were obvious in those areas in which 70% or more of the dishes produced diseased larvae in the laboratory (Table 1, Stowe Reservoir, Canyon Creek, Likely Mill, and Sweagert Flat). The percentage of the dishes containing living larvae in the laboratory is depicted graphically (Figs. 2 and 3). Thirty days after larval eclosion, the eight areas appear to be divided roughly into four groups on the basis of ‘the number of dishes with
267
living larvae: (1) Diamond Mountain, 99%; (2) Fredonyer Peak, 79% and Tom’s Creek, 65%; (3) Roney Flat, 49%, Stowe Reservoir, 44oJ0, and Sweagert Flat, 38%; (4) Likely Mill, 26% and Canyon Creek, 22%. Incidence of virus was also determined by individual rearing of the larvae because of concern over crowding and stress in the petri dishes. Two egg-mass samples taken late in June showed a high incidence of virus. One sample of 58 larvae from Middle Ridge gave 89.6% mortality and another of 30 larvae from Stowe Reservoir gave 80.0% mortality. The Stowe Reservoir sample compared favorably to the earlier sample from that area in which all larvae from a single egg mass were reared in the same petri dish (74.2% mortality, Table 1). The other areas for which group rearings as well as individual rearings were available showed variable results. Likely Mill showed 88.1% mortality when groupreared but only 55.0% when larvae were reared separately. The populations in this area, however, were among the first to collapse (Table 2). Also, it should be remembered that the incidence of virus by egg mass, which includes many larvae, is being compared with virus incidence in individual larvae. Morris (1963) found that the incidence of virus increased as the season progressed. There was an indication of this in this study from samples taken in the unsprayed areas (Table 2). Note that Roney Flat is an exception to this. There is no explanation for this occurrence. Larvae were extremely difficult to find during the first week in September. During this week at Fredonyer Peak, larvae were found in small pockets but were otherwise absent. It is also interesting that this is the only ,study area where tussock moth has been collected since the population decline (one egg mass and one larva in 1967). In 1966, the only area that could be found with a tussock-moth infestation was
Mill
Likely
Peak
area
a No larvae
Total
Fredonyer
Flat
Roney
Study
or cocoons
could
Late Late
be found
1st & 2nd Late -a
1st Late Late
Larval instar
on this date.
VIII-19 IX-4
VII-13 VIII-17 IX-6
VII-5 VIII-18 IX-6
Date
143 49
80 86 0
44 158 26
Number collected
3 23 4
15 21 82 21 17
-i-f
81 26
369
-
127
Pupae No.
Mortality
456
102 43
-
18 44 86
141 22
No.
Total
LARVAE 1965
due to virus
TUSSOCK-MOTH STIJDY AREAS,
129 15
Larvae No.
TABLE 2 IWWENCE OF Vmus IN INDIVIDUALLY REARED ON VARIOUS DATES IN THREE UNTREATED
77.8
71.3 87.8
69.2 55.0 100.0
89.2 50.0
%
COLLECTED
6 1
-59
21 3
-
-
17 11
Other mortality
71
-
20 3
-
2 35 0
110
Adults reared _____ No.
%
12.1
14.0 6.1
-
7.7 43.8 -
25.0 -
E cn
2
5
3
E
u P+
TUSSOCK-MOTH
Q Canyon Creek l Swcagert Flat A Stowe Res. A Diamond Mt.
10.
269
NUCLEOPOLYHEDROSIS
k
20
30
40
50
60
Days
in
FIG. 2. Survival curves northeastern California.
for
individually
reared
tussock-moth
at Corral Creek near Goose Lake (Fig. 1, I). The incidence of virus in this area decreased as the season progressed (Table 3). Since the population completely collapsed TABLE
INCIDENCEOF VIRUS ~LARVAE Collection Date 8 25 31 Totals
Number collected 1580 47 25 230
COLLECTEDAT~ORRAL
Mortality caused by virus
No. reaching
Pupal
No.
%
pupal stage
120 33 14
75.9 70.2 56.0
38 14 11
19 1
63
ZTT-
167
-
72.6
-
a Larvae were reared five per dish. b Determined by macroscopic examination
Virusb
only.
egg masses
from
four
infestation
areas
an increase in virus incidence as observed by Morris (1963) would have been expetted. Miscellaneous samples were taken and 3 CREEK,MODW
COUNTY,CALIFORNIA,JULY,
mortality
Total % virus
Parasites
Other
mortality
3 3
88.0 70.2 60.0
2 4
-
81.3
No. adults --
reared
6
0
Total
11 7 6
3 4
14 11 6
7
31
?ii
--
1966
$I 8.9 23.4 24.0 13.5
270
DAHLSTEN
AND
the incidence of virus in these areas was also variable (Table 4). The nuclear-polyhedrosis virus of the Douglas-fir tussock
THOMAS
moth was widespread, as indicated by these studies. Virus was found in every area sampled. It is interesting that the population
L g 40 2 c : 30 % 2 ; ct. 20
10
tol
.. 0 A A
Likely Mill Roney Flat Fredonyer Peak Tom’s Creek
0
IO
20
30
40
50
66
Days
in
FIG. 3. Survival curves northeastern California.
INCIDENCE
Study
area
Lower Fredonyer Fredonyer L.O. Ambrose
Canyon
for
individually
reared
tussock-moth
egg masses
from
TABLE 4 OF VIRUS IN LATE-INSTAR LARVAE FROM AREAS NOT TREATED IN NORTHEASTERN CALIFORNIA, 1965 Mortality caused by virus
Collection date
Number collected
No.
“/o
pupal stage
Virus
IX-4 IX-5 IX-28 IX-6
16 85 40= 23
1 12 1 5
6.2 14.1 2.5 21.7
15 73 39 18
3 56 20 2
ii
11.6
Totals 0 All but one of these
Is4 was collected
No. reaching
in the pupal
145 stage.
Pupal
5iziT
mortality
infestation
2 2
areaa
WITH DDT
Total % virus mortality
Parasitesother 12 10 13 14
four
25.0 80.0 52.5 30.4 61.0
Adults reared No.
%
-
0 5.9 15.0 0
5 6 -11
6.7
TTJSSOCK-MOTH
NUCLEOPOLYHJZDROSIS
crash of the tussock moth took place in all areas in northeastern California in 1965 except for one small area that crashed in 1966. Preliminary evidence from this study indicates that the virus was an important natural control agent in this spectacular decline. This study indicates that it may be possible to develop an egg-mass survey for incidence of virus that relates to actual mortality in the field. If laboratory mortality can be correlated with mortality in the field on this basis, then the forester will have a valuable tool available for an efficient and extensive biological evaluation of tussock-moth nucleopolyhedrosis before control decisions are reached. ACKNOWLEDGMENTS
The authors thank the California Division of Forestry, Sacramento, and the U.S. Forest Service, Regional Office, San Francisco for their assistance in this study. Gratitude is also expressed to the Regional Office for permission to cite unpublished reports. Finally, we thank Dr. R.W. Stark, Dr. G.O. Poinar, Jr., Dr. L.A. Falcon, and Dr. D.L. Wood for reviewing the manuscript, and Mr. W.A. Copper for valuable assistance in the laboratory and field.
REFERENCES
H. R., AND Douglas-fir tussock Forest Sew. Forest EATON, C. B., AND DODGE,
TROSTLE,
G.
moth. U.S. Pest Leaflet STFWBLE,
G.
C.
1964. Agr.
Dept. 86, 7
pp.
R.
1957.
271
Douglas-fir tussock moth in California. PanPacific Entomologist, 33, 105-108. EVENDEN, J. C., AND JOST, E. J. 1947. Tussock moth control, north Idaho. U.S. Dept. Agr., Forest Service & Bur. Entomol. & Plant Quarantine. ( Mimeo. ) 28 pp. KEEN, F. P. 1952. Insect enemies of western forests. U.S. Dept. Agr. Misc. Publ. 273, 280 PP. LYON,
R. L., AND FLAKE, H. W., JR. 1966. Rearing Douglas-fir tussock moth larvae on synthetic media. J. Econ. Entomol. 59, 696 698. MATHERS, W. G. 1949. Douglas-fir tussock moth. Can. Dept. Agr., Forest insect Invest., Bi-monthly Progr. Rept. 5( 1 ), 4. MORRIS, 0. N. 1963. The natural and artificial control of the Douglas-fir tussock moth, Orgyia pseudotsugata McDunnough, by a nuclear-polyhedrosis virus. .I. Insect Pathol., 5, 401-414. PIERCE, J. R. 1964. Douglas-fir tussock moth infestations in northern California - 1964. U.S. Forest Service, Region 5, Division of Timber Management, San Francisco, Cahf., Dec. 14, 1964. Processed, 14 pp. STEINHAUS, E. A. 1951. Report on diagnoses of diseased insects 1944-1956. Hilgardia, 20, 629-678. STEINHAUS, E. A., AND MARSH, G. A. 1962. Report of diagnoses of diseased insects 1951-1961. Hilgardia, 33, 349-490. SUGDEN, B. A. 1957. Brief history of outbreaks of the Douglas-fir tussock moth, Hemerocampa pseudotsugata McD., in British Columbia. Proc. Entomol. Sot. British Columbia, 54, 37-39. U.S. Department of Agriculture, Forest Service. 1966. The Douglas-fir tussock moth in California-a cooperative control project. U.S. Forest Service, Division of Timber Management, San Francisco, Cahf., March, 1966. Processed, 34 pp.