The influence of age on insecticide susceptibility of Anopheles arabiensis during dry and rainy seasons in rice irrigation schemes of Northern Tanzania.

BACKGROUND: Insecticide resistance is the major emerging challenge facing the malaria vector control programmes in Tanzania. Proper monitoring and detection is of paramount importance guiding the vector control programmes. This paper presents the effect of mosquito aging on insecticide resistance status in Anopheles arabiensis populations in dry and rainy seasons in northern Tanzania.

METHODS: Anopheles gambiae s.l. larvae were sampled from rice fields in both dry and rainy seasons and reared in the insectary to adults. The emerged females in batches of 2, 3, 5, and 10 days old were exposed to six insecticides (deltamethrin, permethrin, lambda-cyhalothrin, DDT, bendiocarb and pirimiphos-methyl) to see the effects of age on insecticide resistance. Mosquitoes were exposed to insecticides using WHO standard susceptibility test kits. Knockdown was recorded during the 1-h exposure, while mortality and resistance ratio were recorded 24 h later. Mosquito specimens were identified to species level using the polymerase chain reaction (PCR) method.

RESULTS: Among the 326 specimens processed by PCR, 323 (99.1%) were identified as Anopheles arabiensis. There was reduced mortality (ranging from 61 to 97.7%) when adults reared from larvae were exposed to all pyrethroids and bendiocarb in both dry and rainy seasons, while they were fully susceptible to DDT and pirimiphos-methyl. There was a significant increase in mortality rate with increase in mosquito's age in both dry and rainy seasons following exposure to pyrethroids (DF = 1, P < 0.05). Mosquitoes showed significantly higher mortality rates in the rainy season than in the dry season after being exposed to pyrethroids (DF = 1, P < 0.05). Higher mortality rates (94.0-99.8%) were observed in all ages and seasons when mosquitoes were exposed to bendiocarb compared with pyrethroids. Pirimiphos-methyl was only tested in the rainy season so no comparison with dry season mosquitoes could be made.

CONCLUSIONS: Results showed that An. arabiensis were resistant to pyrethroids in both seasons and that the young age groups exhibited higher levels of resistance compared with the older age groups. Mosquitoes were full susceptible to DDT and pirimiphos-methyl irrespective of the season and age.