Larvicidal Potency of Ashes of Two Insecticidal Plants against the Activities of Anopheles coluzzii and Culex quinquefasciatus Mosquitoes

Insecticidal plants are widely studied materials that have intense applications in various fields of vector, pest, and disease control. They are among the recommended strategies to tackle the already established resistance in mosquitoes causing prevailing diseases in the world, especially in Africa.

The study aimed to assess the biological potency of ashes of scent leave (Ocimum gratissimum) and lemon grass (Cymbopogon citratus) against the larvae of Anopheles and Culex mosquitoes.

Larvae of mosquitoes were sourced, and plant ashes were prepared and exposed to 20 larvae of both mosquitoes in 1g, 2.5g, 5g, 10g, and 15g concentrations. Treatment concentrations were formulated by mixing ashes in 100ml of water in triplicate. Mortality, acute toxicity, and sub-chronic toxicity data were obtained.

Mosquito larval mortality increased with time at all concentrations of test plants, and sub-chronic toxicity showed complete mortality in all treatments. Acute toxicity of Culex larvae was highest in 15g of lemon grass and complete mortality was recorded after 30 minutes of exposure. There was no acute toxicity recorded with scent leave exposure. The Lethal Dose (LD₅₀) for Anopheles mosquitoes recorded with scent leave ashes was 0.319g (y=1.928x+0.96; R²=0.221, p= 0.407), and for Culex mosquitoes, it was recorded to be 0.424g with lemon grass exposure (y=1.86x+0.69; R²=0.221, p= 0.240). Remarkably, lemon grass at a concentration of 1.250g and 3.247g caused 95% toxicity in Anopheles (y=15.85x-0.25; R²=0.633, p> 0.05) and Culex (y=2.918x-0.18; R²=0.388, p= 0.254) mosquitoes, respectively. LT₅₀ of mosquitoes was between 21.3 minutes and 1451.4 minutes, whereas LT₉₅ was between 37.1 minutes and 1740.4 minutes, respectively. No adult mosquito emergence was recorded.

Ashes of both plants, especially lemon grass, could be considered better materials for local treatment of the mosquito breeding sites.