Toxicity potentials of selected insecticidal plant oils against Anopheles gambiae (Diptera: Culicidae) under laboratory conditions

Abstract Despite increasing reports and concerns about resistance development to public-health insecticides in malaria-vectors, significant steps have been put into the quest for novel strategies to disrupt the disease transmission cycle by targeting insect-vectors hence sustaining vector management. This study evaluates the toxicity potential of oils of insecticidal plants shortlisted in an ethnobotanical survey on the larvae and adult stages of Anopheles gambiae. Oils from leaves of Hyptis suaveolens, Ocimum gratissimum, Nicotiana tabacum, Ageratum conyzoides and fruit-peel of Citrus sinensis were extracted by steam-distillation using a Clevenger apparatus. Larvae and female adults of deltamethrin susceptible Anopheles gambiae were gotten from an already established colony in the Entomological Research Laboratory, University of Ilorin. Twenty-five third instar stage larvae were used for larvicidal assays while twenty 2-5 days old adults were used for the adulticidal assays in five replicates. After which, coupled gas chromatography-mass spectrometry analysis (GC-MS) was performed to determine the major chemical-constituents of plant oils. A. gambiae exposed to H. suaveolens and C. sinensis demonstrated significantly higher larval toxicity (94.7-100%) after 24 hours. At 48 hours, the mortality induced by the oils of the four plants peaked at 100%. N. tabacum (0.50 mg/ml) induced the highest percentage adult mortality (100%) on A. gambiae which compared favourably with the positive control Deltamethrin (0.05%). The lowest KdT 50 was observed with 0.25 mg/ml of N. tabacum (20.3 minutes) while the lowest KdT 95 was observed with 0.10mg/ml of A. conyzoides (35.97 mins) against adult A. gambiae. D-limonene is the key chemical-constituent in oils from C. sinensis and A. conyzoides. The significant larval and adult mortality rates, lower lethal concentration and knockdown times demonstrated by the evaluated plant oils showed promising outcomes that can be further developed for vector control management. Graphical Abstract