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.