Methoprene tolerance in Aedes nigromaculis in Fresno County, California

Methoprene, a juvenile hormone analogue, has been used for at least 20 year s as the primary insecticide to control the pasture mosquito Aedes nigromac ulis in Fresno County, California. First reports of apparent methoprene con trol failures were noted in a pasture west of Fresno in September 1998. Ins ufficient control was noted in 12 different pastures the following season f rom April to September 1999. In September of 1999, field trials were conduc ted to better ascertain the level of control. Results based on pupal counts from different methoprene formulations and rates of application indicated that in some pastures low levels of control were achieved with Altosid(R) ( Liquid Larvicide) and Altosid XR-G. Control with Altosid Pellets was report ed at 52-99%.