Comparative studies on effects of three chitin synthesis inhibitors on common malaria mosquito (Diptera: Culicidae)

Toxicities of three chitin synthesis inhibitors (diflubenzuron, nikkomycin Z and polyoxin D) were evaluated using second instars of the common malaria mosquito, Anopheles quadrimaculatus Say (Diptera: Culicidae). Neither nikkomycin Z nor polyoxin D at 50 microg/liter caused significant larval mortality, although they reduced the body weight of the survivors by 20.5 and 33.8%, respectively, in 48 h. In contrast, exposures of the larvae to diflubenzuron at 12.5 microg/liter for 48 h resulted in 86.7% larval mortality and reduced the body weight of the survivors by 29.1%. Exposure of the pupae (<12 h old) to diflubenzuron at 100 microg/liter for 48 h caused 18.9% pupal mortality and consequently reduced the adult emergence by 24.7% from the surviving pupae. Furthermore, exposure of third instars to diflubenzuron at 4, 20, 100, and 500 microg/liter for 24 h resulted in the reduction of larval chitin contents by 4.25, 33.2, 35.2, and 57.7%, respectively. Such an effect seemed to be associated with only cuticular chitin synthesis because the same exposures did not significantly affect chitin contents in the guts. Our results indicated that diflubenzuron was highly toxic to second instars by not only causing high larval mortality but also by affecting their growth. Diflubenzuron was also fairly toxic to pupae by not only causing pupal mortality but also affecting the adult emergence. Our results suggest that diflubenzuron might affect only chitin synthesis in the cuticle but not in the peritrophic matrix, which is probably due to diflubenzuron's direct contact to mosquito larvae in water, slow distribution in insect body, rapid degradation in the insect gut, or a combination.