Larvicidal Efficacy of Aqueous Extract of Moringa Oleifera Seeds on Malaria Vector (Anopeheles gambiae) and Its Toxicity Effects on Mosquito Fish (Poecilia reticulata)

ABSTRACT

Malaria is one of the major public health problems in developing countries and its prevalence in Nigeria is dependent on the vector, Anopheles gambiae. The control of Anopheles is a major component of vector management but effectiveness has been limited by factors like insecticide resistance, cost, toxicity on non-target organisms and environmental pollution concerns. There is an increasing interest in developing plant-based insecticides as sustainable alternatives to chemical insecticides in mosquito control. This study was designed to determine the larvicidal efficacy of aqueous extract of Moringa oleifera seeds on the larvae of Anopheles gambiae and its acute toxicity effects on mosquito fish (Poecilia reticulata).

The study involved extraction of toxic components in Moringa oleifera seeds using aqueous extraction technique. Five aliquots of concentrations (1160, 1450, 2900, 5800 and 8700 µg/mL) were prepared by serial dilutions from the extract. Three independent experiments were run in quadruplicates on 1440 laboratory reared third instar larvae of Anopheles gambiae; twenty larvae per treatment were used and control group was exposed to distilled water. Larvicidal parameters, mortality and pupation were recorded 24-hourly for 5 days; larvae were considered dead if they were immobile and unable to reach water surface. Two independent toxicity experiments were run in triplicates on 480 male mosquito fishes exposed to three graded treatments (10, 20 and 30 mg/mL) of the extract. Behavioural responses, increased respiration, loss of orientation, discoloration, motility and mortality were observed hourly for 24 hours. Data were analysed using descriptive statistics, regression and probit analyses at p≤0.05. Probit was used to calculate the LC₅₀ and LC₉₀ values of the extract on Anopheles larvae and mosquito fish.

Larvicidal effects across the concentrations ranged from 59.0%-99.3%; at 2900 µg/ml, 59.0% mortality was observed within 24 hours and this increased with exposure duration across the different concentrations. There was a high linear relationship, (r=0.87) between larval mortality and concentration of the extract. The larval bioassay showed that the extract presented a 24hourLC₅₀ and LC₉₀ values of 2505.8 and 6293.4µg/ml respectively. At lower concentrations (1160 and 1450 µg/ml), the larvae lived as long as 7 days before pupating while in the control pupation was not delayed. Acute toxicity evaluation on the mosquito fishes gave a 96 hour- LC₅₀ and LC₉₀ of 24.0 and 82.0 mg/ml respectively and also showed that the fishes exhibited varying degrees of changes such as concentration and time-dependent progressive declines in fish 7 motility, discoloration, with a corresponding increase in respiration and in the proportion of dead fishes as concentration increased unlike in the control.

Moringa extract was highly toxic to Anopheles larvae, inhibited pupae development and had low toxicity on Poecilia reticulata. This extract should be used to improve vector control with minimal toxicity effects on non-target organisms.