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Cooperative Forestry and Pest Management conducted a pilot control
project in central Montana to determine the effectiveness of two commercial
formulations of Bacillus thuringiensis Berliner applied at 8
billion international units (BIU) per acre (19.8 BIU per hectare) when
used under operational conditions against the western spruce budworm,
Choristoneura occidentalis Freeman. Nine blocks 2,000-4,200 acres
(810-1,701 hectares) were selected in areas of budworm infestation on
State, private, BLM, and Forest Service lands near Butte, Montana.
Three blocks were randomly assigned Dipel 4L treatment, three blocks
were assigned Thuricide 16B treatment, and the remaining three served
as control blocks.
Time and sequence of treatment was determined by larval development
sampling in each block. Blocks were released for treatment when 50
percent of the larvae had developed to or beyond the fourth instar. A
prespray population sample was collected within 24 hours of spraying
from 25 three-tree cluster plots established in each block. Treating
began on June 29 and ended on July 5. Spray was applied with two Ayers
Turbo Thrush aircraft equipped with fulljet one-eighth 66A8W nozzles
calibrated at 1 gallon per acre (1.5 liters per hectare). Dipel was
applied to 8,600 acres (3,493 hectares), and Thuricide to 8,150 acres
(3,301 hectares). Eight thousand acres (3,240 hectares) were used as
control areas. Postspray population samples were collected at 7, 14,
and 21 days following treatment. Population densities are expressed
in number of larvae per 100 shoots. Following egg deposition, percent
defoliation and egg mass densities were collected from each cluster
tree.
Unadjusted budworm mortality at 21 days was 48 and 62 percent for Dipel
and Thuricide respectively. Budworm mortality in control blocks averaged
26 percent. Percent defoliation was 33, 36, and 47 for Dipel,
Thuricide, and control. Adjusted mortality for all larvae (combined)
prior to the egg stage was 73 and 91 percent for Dipel and Thuricide.
In 1981, percent parasitism for all defoliators in the Dipel, Thuricide,
and control blocks was 21.5, 19.8, and 26.0, respectively. Egg mass
densities per square meter (per square foot) in 1981 were 18.3 (5.6),
17.6 (5.4), and 38.5 (11.7) for Dipel, Thuricide, and control.
Mean budworm larvael densities in 1982 were 4.4, 3.5, and 12.7 larvae
per 100 shoots for the Dipel, Thuricide, and control areas. Rearing
samples showed a greater proportion of females emerging in sprayed
than in control blocks (61.4 and 62.0 percent for Dipel and Thuricide
compared to 52.4 percent in the controls); however, adult female weight
was higher in the control blocks (126.3 mg compared to 121.3 and 113.9
mg for the Dipel and Thuricide blocks, respectively). Average percent
parasitism of budworm larvae and pupae in 1982 was 5.1, 8.6, and 7.3
for the Dipel, Thuricide, and control blocks. We were unable to distinguish
any effects of B.t. on individual budworm parasite species
during the study. Egg mass densities in 1982 were 44.5 (13.6), 39.7
(12.1), and 78.6 (24.0) per square meter (per square foot) for Dipel,
Thuricide, and control. |
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