Biodegradable Insecticides in Forestry
173
extrapolating toxicity response from one species to another should be avoided.
Similar results have been noted for two different populations of western blackheaded budworm (Robertson et at., 1973). Stock and Robertson (1979) report
that in Douglas fir tussock moth, variation in sibling group responses of different
populations to acephate and carbaryl were related quantitatively to esterase isozymes.
7.6.1.7. Toxicity Tests on Different Ins tars
The relative susceptibility of selected developmental stages-instars-to
insecticide treatments provides data for gauging the correct time of application
for maximum mortality under field conditions. Timing the application is critical
if the insecticide is short-lived. If protecting the foliage is the purpose of insect
population reduction, an early application strategy will be required. The toxicity
of mexacarbate to various ins tars of western spruce budworm has been determined
(Robertson et at., 1976). Third to fifth instars and immature sixth instars were
the most susceptible. Under laboratory conditions first instars were the least
susceptible. Early instars of the Douglas fir tussock moth and western spruce
budworm were more susceptible to diflubenzuron than the last or large instars
(Gillette et at., 1978). Acephate, carbaryl, and permethrin were most toxic to
first instars (Robertson and Boelter, 1979a,b). Spray chamber treatments yielded
similar results: acephate, carbaryl, and permethrin were most toxic to first instars,
while diflubenzuron was most toxic to second instars. Sixth instars were least
susceptible to acephate and permethrin. Yet, in California oakworm (Phryganidia
californica Packard), the first ins tars were most susceptible to mexacarbate
(Robertson, 1972).
7.6.1.8. Evaluation of Sublethal Dose Effects
When sublethal doses of methomyl were applied to western spruce budworm
larvae, the toxicological response was that of a juvenomimetic (Smith, 1971).
Sublethal doses of mexacarbate produced a similar loss in larvae weight and
premature development due to modified diuresis. Reversal of cholinesterase
inhibition from mexacarbate-treated mice has been demonstrated to be rapid
(Lang and Miskus, 1967). The metabolism to nontoxic metabolites is also rapid
(Roberts et al., 1969). These results suggest that the rapid disappearance of
mexacarbate, especially in doses common to sublethal studies, must contribute
significantly to the diuresis.
7.6.1.9. Resistance Studies
Mexacarbate treatment in 14 generations of nondiapausing western spruce
budworm larvae showed no trend toward increased tolerance (Robertson and
173
extrapolating toxicity response from one species to another should be avoided.
Similar results have been noted for two different populations of western blackheaded budworm (Robertson et at., 1973). Stock and Robertson (1979) report
that in Douglas fir tussock moth, variation in sibling group responses of different
populations to acephate and carbaryl were related quantitatively to esterase isozymes.
7.6.1.7. Toxicity Tests on Different Ins tars
The relative susceptibility of selected developmental stages-instars-to
insecticide treatments provides data for gauging the correct time of application
for maximum mortality under field conditions. Timing the application is critical
if the insecticide is short-lived. If protecting the foliage is the purpose of insect
population reduction, an early application strategy will be required. The toxicity
of mexacarbate to various ins tars of western spruce budworm has been determined
(Robertson et at., 1976). Third to fifth instars and immature sixth instars were
the most susceptible. Under laboratory conditions first instars were the least
susceptible. Early instars of the Douglas fir tussock moth and western spruce
budworm were more susceptible to diflubenzuron than the last or large instars
(Gillette et at., 1978). Acephate, carbaryl, and permethrin were most toxic to
first instars (Robertson and Boelter, 1979a,b). Spray chamber treatments yielded
similar results: acephate, carbaryl, and permethrin were most toxic to first instars,
while diflubenzuron was most toxic to second instars. Sixth instars were least
susceptible to acephate and permethrin. Yet, in California oakworm (Phryganidia
californica Packard), the first ins tars were most susceptible to mexacarbate
(Robertson, 1972).
7.6.1.8. Evaluation of Sublethal Dose Effects
When sublethal doses of methomyl were applied to western spruce budworm
larvae, the toxicological response was that of a juvenomimetic (Smith, 1971).
Sublethal doses of mexacarbate produced a similar loss in larvae weight and
premature development due to modified diuresis. Reversal of cholinesterase
inhibition from mexacarbate-treated mice has been demonstrated to be rapid
(Lang and Miskus, 1967). The metabolism to nontoxic metabolites is also rapid
(Roberts et al., 1969). These results suggest that the rapid disappearance of
mexacarbate, especially in doses common to sublethal studies, must contribute
significantly to the diuresis.
7.6.1.9. Resistance Studies
Mexacarbate treatment in 14 generations of nondiapausing western spruce
budworm larvae showed no trend toward increased tolerance (Robertson and