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Date
2025
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Abstract
ABSTRACT
Emerging adult Anopheles and Culex mosquitoes migrate to human dwellings,
at great risks of insecticide exposure, in search of bloodmeal. Countries like Nigeria,
targeting this joint mosquito host-seeking behaviour have launched Malaria-Lymphatic
Filariasis (Malaria-LF) co-intervention plans using synergistic insecticide-based control
tools. However, insecticide resistance currently threatens mosquito vector control. This
study investigated separate and combined pyrethroid insecticide exposure response of
Anopheles gambiae sensu lato (s.l.) and Culex pipiens complex mosquitoes collected from
two mixed breeding sites in Nigeria. Adult Anopheles and Culex mosquitoes reared from
larval collections were exposed in separate and joint insecticide susceptibility tests to
permethrin 0.75%, deltamethrin 0.05% and alphacypermethrin (0.05%) using World Health
Organization (WHO) kits. The mosquitoes were identified morphologically. Time to
mosquito knockdown, percentage knockdown, and mortality were monitored. The results
for the different mosquito and exposure types were compared. Both mosquitoes were
resistant to all pyrethroid insecticides tested. Significantly higher Anopheles knockdown was
recorded compared to Culex for the joint (Anopheles 79-89% vs Culex 44-50%, P ≤ 0.011)
and separate (Anopheles 67-88% vs Culex 46-47%, P < 0.048) exposures respectively. Time
to 95% Anopheles and Culex mosquito knock down were shorter during separate (2200.9
mins) than in joint (4513.9 mins) exposures. Faster Anopheles knockdown (kdt50 18.7 - 68.8
mins) were recorded compared to Culex (kdt50 22.1 - 98.8 mins). Higher but not significantly
different (P ≥ 0.083) mosquito mortalities were recorded from separate than in joint
Anopheles and Culex exposures. There were no significant (P ≥ 0.072) differences between
Anopheles 43-77% and Culex 41-72% mortalities for separate and joint exposures. The speed
of action and morbidity effects of the pyrethroid insecticides tested were lower during joint
than in the separate mosquito exposures. Taken together, reduced pyrethroid speed of
action and morbidity along with multiple mosquito resistance noted could threaten
insecticide user-acceptability, performance perception and Malaria-LF co-intervention in
the field.