Breeding Site Distribution of <i>Anopheles gambiae</i> s.l. Mosquitoes and Their Susceptibility to Some Classes of Recommended Insecticides in Amansea, Nigeria

Abstract

The widespread emergence of insecticide resistance in malaria vectors poses a serious threat to the effectiveness of current control strategies. This study assessed the abundance, breeding habitats, and insecticide susceptibility status of Anopheles gambiae complex in Amansea, Awka, Anambra State, Nigeria. Mosquito larvae were collected from five villages between May and September 2023, reared to adults under insectary conditions, and identified morphologically. Susceptibility bioassays were conducted on 3–5-day-old, non-blood-fed female An. gambiae (s.l.) using World Health Organization (WHO) standard procedures against six insecticides representing four chemical classes: pirimiphos-methyl (organophosphate), dichloro-diphenyl-trichloroethane (DDT) (organochlorine), bendiocarb and propoxur (carbamates), and deltamethrin and permethrin (pyrethroids). Knockdown rates, knockdown times (KDT₅₀ and KDT₉₅), and 24-hour mortality were determined. A total of 1,433 larvae were collected, of which 1,301 emerged as adults (90.79% emergence). Potholes constituted the most productive breeding habitat, accounting for over 60% of larval abundance. Marked variation in susceptibility was observed across insecticides (P < 0.05). Bendiocarb showed the highest efficacy with rapid knockdown (KDT₅₀ = 47.77 min) and the highest mortality (94.0 ± 3.46%), followed by propoxur (84.0 ± 2.82%). Pyrethroids induced moderate to low mortality (19–38%), while pirimiphos-methyl and DDT produced negligible knockdown and mortality (≤ 5%), indicating strong resistance. Widespread multi-class insecticide resistance was observed in Anopheles gambiae complex in Amansea with carbamates retaining comparatively higher efficacy but falling below WHO susceptibility thresholds. This resistance threatens long-lasting insecticidal net (LLIN) and indoor residual spraying (IRS) effectiveness, necessitating strengthened surveillance, resistance management, and integrated vector control strategies.