• Markin, George P.
  USDA Forest Service, Pacific Southwest Station
  
• Johnson, David R.
  USDA Forest Service, Pacific Southwest Station
  

Since carbaryl was first tested in 1975, all subsequent field tests and eventual operational programs have been at the same dosage rate of 1.12kg a.i. per hectare (1lb/acre), the rate at which this insecticide is now registered. To determine the effectiveness of lower dosage rates, an aerial field test was conducted in July 1979, north of the community of White Sulphur Springs in western Montana. Rates of 0.28 and 0.56 kg a.i. per hectare (0.25 and 0.5 lb a.i./acre) were compared with the registered rate of 1.12 kg a.i. per hectare. Each dosage was applied to five selected 20-hectare plots by helicopter with Beecomist nozzles while the larvae were in the fifth and early sixth instars. Branch samples were collected from 15 randomly selected Douglas-fir trees before spraying and at intervals of 5, 10, and 15 days after spraying to determine larval populations. An additional sample was collected when 95% pupation has occurred to determine loss of current year's foliage due to larval feeding. All tree treatments significantly reduced larval populations and the amounts of foliage destroyed when compared with untreated controls. It appears, however, that the rates of 1.12 and 0.56 kg/ha provided a comparable degree of population control while the rate of 0.28 kg/ha was possibly 10% less effective.