TOXICITY OF 4 COMMON INSECTICIDES TO FIELD-COLLECTED BEET ARMYWORM LARVAE

Relative toxicity of chlorpyrifos, cypermethrin, diflubenzuron, and th iodicarb to seven consecutive generations of field collected beet army worm, Spodoptera exigua (Hubner), from Alabama (Macon Co.), Georgia [B artow Co., Ben Hill Co., Decatur Co., Tift Co. (Gibbs and Rigdon Farms )], and Mississippi (Yazoo Co.) was determined. Insecticide tolerance varied among locations, insecticides and generations tested. No single collection-site colony (F-1) was significantly more tolerant to all i nsecticides than the susceptible laboratory colony. Macon Co., AL, ins ects were significantly more tolerant to chlorpyrifos, cypermethrin, a nd diflubenzuron than the laboratory colony; and Bartow Co. and Yazoo Co. exhibited significant tolerance to more than one evaluated insecti cide. In most instances, tolerance to insecticides was inversely relat ed to time in culture with a majority of colonies losing 50% of their tolerance to tested insecticides within two generations following remo val of insecticide pressure. However, some fluctuations in tolerance e xisted from generation to generation, indicating that there was some n aturally occurring genetic variation in colony susceptibility.