The residual effect of lambda-cyhalothrin, deltamethrin and dichlorodiphenyltrichloroethane in Zhombe, Kwekwe district, Zimbabwe

Indoor residual house spraying using lambda-cyhalothrin, deltamethrin and dichlorodiphenyltrichloroethane (DDT) was conducted in Zhombe Resettlement area, Zimbabwe. A total of 204/219 (93.1%), 224/260 (86.2%) and 257/325 (79.1%) rooms were sprayed with lambda-cyhalothrin, deltamethrin and DDT wettable powders respectively. Bioassays were conducted on sprayed walls and roofs using 3-5 day old laboratory reared susceptible Anopheles gambiae sensu lato mosquitoes placed in World Health Organization cones.
Bioassays conducted on sprayed walls (1 month), showed that efficacy of lambda-cyhalothrin was the same with DDT but different with deltamethrin and this trend continued in the 2^nd month. During the 3rd month, lambda-cyhalothrin killed more mosquitoes than deltamethrin (P=1.931x10^-14), DDT killed more mosquitoes than deltamethrin (P=0.0001) and lambda-cyhalothrin killed more mosquitoes than DDT (walls). Efficacy of lambda-cyhalothrin and DDT was the same 4 months post spray (P=0.487), notable differences were seen in lambdacyhalothrin and deltamethrin (P=2.57x10^-6), DDT and deltamethrin (P=2.17-10^-8). Efficacy of lambda-cyhalothrin and DDT was the same 5 months post spray (P=0.244), major differences were found in lambdacyhalothrin and deltamethrin (P=0.000), DDT and deltamethrin (P=5.18x10^-5) and this trend continued in the 6^th month. One month after spraying roofs, mortality of mosquitoes due to lambdacyhalothrin/ deltamethrin (P=2.56x10^-5), lambda-cyhalothrin/DDT (P=1.2x10^-7) and deltamethrin/DDT (P=0.013) were significantly different and this continued in the 2^nd month. However, 3 months after spraying, mortality due to lambda-cyhalothrin/deltamethrin (P=1.46x10^-6), lambda-cyhalothrin/DDT (P=0.048), and deltamethrin/DDT (P=0.004) were significantly different and this continued in the 4^th month. Five months after spraying roofs, mortality due to lambda-cyhalothrin/ deltamethrin (P=0.000) and deltamethrin/DDT (P=6.6x10^-7) were significantly different. Six months after spraying, lambdacyhalothrin/ deltamethrin (P=0.34), lambda-cyhalothrin/DDT (P=0.982), and deltamethrin/DDT (P=0.64) were not significantly different. When using exit window traps, no mosquitoes were collected from rooms sprayed with each of the insecticides over a 6-month period. However, 17, 6, 14, 7, 2 and 3 fed An. gambiae sl mosquitoes were collected in the 1^st, 2^nd, 3^rd, 4^th, 5^th and 6^th month respectively from unsprayed rooms and none of them died after 24 h.