Susceptibility of Culex pipiens (Diptera: Culicidae) Field Populations in Cyprus to Conventional Organic Insecticides, Bacillus thuringiensis subsp israelensis, and Methoprene

Cidex pipiens pipiens L. populations on Cyprus were Sampled over a 6-yr period front 2002 to 2008 to evaluate the status of insecticide resistance toward the insecticides temephos, chlorpyrifos, and permethrin and to study susceptibility levels toward the recently introduced bacterial insecticide Bacillus thuringiensis subsp. israelensis De Bariac and the Juvenile hormone analog, methoprene. Susceptibility to the three conventional chemical insecticides varied between different collections, with most collections showing moderate or low resistance. The 2004 Akrotiri collection had the highest temephos resistance ratio, 167-fold at the LC(95) although later sampling showed that the populations returned to susceptibility after treatments stopped. Chlorpyrifos resistance was generally higher than temephos resistance. Four collections showed high resistance, and the resistance ratios of two collections were notably high with resistance ratios of 110- and 248-fold at the LC(95). Three collections showed high permethrin resistance (22.5-, 23.9-, and 86.3-fold). The frequency of elevated esterase activity in populations was estimated using a filter paper test, and frequencies varied from 0.9 to 65% among collections. The levels of temephos resistance and the frequency of elevated esterases in this survey were generally lower than in earlier reports, suggesting a decline in temephos resistance. Dose-response values for B. thuringiensis subsp. israelensis is covered an approximate eight-fold range, but no resistance was detected. Methoprene values showed a 4.7-fold and 16-fold range at the LC(50) and LC(95), respectively. Two populations showed significant resistance ratios at the LC(95). These data are discussed in relation to the changes in larval control practices underway in Cyprus.