A glass vial technique was developed to monitor for resistance to organophosphate and carbamate insecticides in the tobacco budworm, Heliothis virescens (F.), and the boll weevil, Anthonomus grandis Boh. Laboratory data showed that toxicity of Lorsban and Curacron residues in glass vials disappeared within a week in vials held at room temperature, but not in vials held in a freezer. The loss of toxicity was shown to be due to hydrolysis of the insecticides rather than volatilization. Adding an organic acid such as benzoic or decanoic greatly increased residue life. Residues of Larvin did not lose toxicity as rapidly. The usefulness of the technique was confirmed by preliminary data obtained with tobacco budworm males collected from pheromone traps in Texas Arkansas and Louisiana. Similar tests for resistance to Guthion and Baythroid were conducted with laboratory and field-collected boll weevils.