Toxicity of insecticidal plant oils on the larval and adult stages of a major malaria vector (Anopheles gambiae Giles 1920)

Abstract

Despite increasing reports and concerns about the development of resistance to public-health insecticides in malaria vectors, significant progress has been made in the search for alternative strategies to disrupt the disease transmission cycle by targeting insect vectors and thus sustaining vector management. The use of insecticidal plants is a strategy that can be employed and this study investigates the toxicity potential of insecticidal plant oils shortlisted in an ethnobotanical survey on Anopheles gambiae larvae and adult stages. The shortlisted plants parts, the leaves of Hyptis suaveolens, Ocimum gratissimum, Nicotiana tabacum, Ageratum conyzoides, and Citrus sinensis fruit-peel were collected and extracted using a Clevenger apparatus. Larvae and female adults of deltamethrin-susceptible Anopheles gambiae were obtained from an already-established colony at the University of Ilorin's Entomological Research Laboratory. In five replicates, twenty-five third instar stage larvae were used for larvicidal assays and twenty 2-5days old adults were used for adulticidal assays. After 24h, An. gambiae exposed to Hy. suaveolens and Ci. sinensis exhibited significantly higher larval toxicity (94.7-100%). The mortality induced by the oils of the four plants peaked at 100% after 48h. Ni. tabacum (0.50mg/ml) induced the highest percentage of adult mortality on An. gambiae (100%) when compared to the positive control Deltamethrin (0.05%). The lowest KdT50 was observed with 0.25mg/ml of Ni. tabacum (20.3min), and the lowest KdT95 was observed with 0.10mg/ml of Ag. conyzoides (35.97min) against adult An. gambiae. The evaluated plant oils demonstrated significant larval and adult mortality rates, lower lethal concentrations, and knockdown times, indicating promising results that can be further developed for malaria vector management.