Effect of age on phenotypic insecticide resistance in anopheles gambiae sensu lato

An. gambiae s.l. are the major malaria vectors in Africa and Tanzania in particular, and are highly resistant to insecticides. Proper monitoring of insecticide resistance is of paramount importance in malaria vector control programs. The present study assessed the effect of age on the status of insecticide resistance in An. gambiae s.l. during the dry and the rainy seasons. Female An. gambiae s.l. of unknown age and of 2, 3, 5, and 10 days old, were exposed to six insecticides (deltamethrin, permethrin, lambdacyhalothrin, bendiocarb, DDT and pirimiphos-methyl). Knock down records were taken within one hour of exposure, while mortalities were recorded 24 hours holding time. Results showed a mortality rate that ranged from 65% to 90% for unknown age adults exposed to all pyrethroids and bendiocarb, DDT and pirimiphos-metyhl caused 100% mortality in both seasons. Resistance by An. gambiae s.l. to pyrethroids was attributed to frequent exposure to these insecticides used in agricultural activities and in public health purposes. Season did not influence resistance against pyrethroids in wild An. gambiae s.l (DF = 1, P > 0.05); resistance to bendiocarb was higher in the dry than rainy season (P < 0.05). Resistance to insecticides decreased with the age of the mosquitoes (F = 24.3941, DF = 3, P < 0.05) which is possibly due to decrease in mosquito vitality with age. Resistance to pyrethroids in larvae reared mosquitoes (aged 2, 3, 5, 10 days old) was higher during the dry season than the rainy season (F = 51.836, DF = 1, P < 0.05). The susceptibility to the pyrethroid by An. gambiae s.l. was influence by age, whereby younger mosquitoes (2 to 5 days old) were more resistant than older ones.