Difference in the susceptibility of Culex larvae against methoprene and its slow-release formulation A by various stages of the development

Lethal effects of a juvenile hormone analogue, methoprene (ZR-515○!R), and its slow-release formulation A (SR-10○!R), were tested on larvae and pupae of Culex pipiens pallens and Culex tritaeniorhynchus at different developmental stages. Methoprene, which contains more than 90% of the active component, was emulsified with 0. 03% (w/w) Tween 80 and 5% alcohol solution. Certain amount of the diluted emulsion or slowrelease formulation A, was stirred into glass jars containing 200ml of distilled water and 30 mosquito larvae or pupae at the particular developmental stage. Minute amount of brewered yeast was added as a food. The temperature was maintained at 25℃. The individuals which failed to moult into normal pupae or adults were counted every day until all survived larvae became adults. EC_ values of methoprene against Culex pipiens pallens were 0. 03ppm at late 3rd instar, 0. 02ppm at late 4th instar, 0. 0006ppm at pharate pupal stage and 1. 0ppm at one-day-old pupae, respectively. The lethal effect of slow-release formulation A to C. pipiens pallens larvae proved to be more potent than methoprene perse, but not to pupae. Almost the same figure was found in the experiments on Culex tritaeniorhynchus. The phenomenon that the susceptibility to the compound increases gradually toward the pupation and decreases drastically after the pupation was in accord with the theory on the secretion and action of juvenile hormone in the physiological and endocrinological studies.