Carbaryl bioassays were conducted in the field with natural populations of three species of grassbugs (Hemiptera: Miridae). Insecticide-coated vials and a portable incubator were used. Grassbug populations differed in carbaryl LC50with respect. to species, sex, date, and location. Pretreatment of the insects with piperonyl butoxide inhibited the monooxygenase (mixed-function oxidase) system, depriving the insects of a key defense against carbaryl. The differences in synergized and unsynergized LC50values were used to calculate an estimated “percent dependency upon monooxygenase detoxification” for the grassbugs, Plotting these values against carbaryl LC50illustrated hypothetical population differences in tolerance and detoxification potential. These were tested in the laboratory with two species, Males were found to be more susceptible to carbaryl than were the females, with population LC50values falling as the season progressed.Labops hesperiusUhler was the most tolerant species, with some Over-lapping values withLabops hirtusKnight.Labops utahensisSlater was the most susceptible and contained the lowest level of monooxygenase activity.