Abstract

Background: The current method of controlling malaria vectors using synthetic chemicals has caused serious problems for human health and the environment. Hence, a need for alternative, cheap, readily available, and acceptable mosquito control methods.

Objective: To evaluate the bioinsecticidal efficacy of Ocimum gratissimum and Mesosphaerum suaveolens against Anopheles gambiae using the bioassay method.

Methods: Oil extracts in 5 ml, 25 ml, and 75 ml serially diluted twice in 100 ml of olive oil formed 0.05%, 0.25%, and 0.75%, while 1 m in 100 ml formed 1%. Olive oil only served as a control. Twenty-five female Anopheles mosquitoes, aged 2-4 days, were blown into tubes containing filter paper-coated essential oils of various concentrations in quadruplicates.

Results: Over 50% mortality was recorded in mosquitoes exposed to 0.75% and 1% O. gratissimum compared to others (p <0.05). Knockdown time (KDT) was higher in mosquitoes exposed to 1% O. gratissimum compared to others (p <0.05). Generalized linear regressions showed that the essential oils of O. gratissimum and M. suaveolens contributed 4% (f2 = 0.042) and 14% (f2 = 0.167) to the variability of mortality with time. KDT50 (9.34 - 131.5 minutes) and KDT95 (77.6 - 275 minutes) of O. gratissimum as well as KDT50 (162.8 - 233.6 minutes) and KDT95 (302.8 - 415 minutes) of M. suaveolens were recorded. Stigmastane; 4,22-Stigmastadiene-3-one (Area: 45.88%) in O. gratissimum and gamma-Sitosterol (16.02%) in M. suaveolens were identified by Gas Chromatogram as the highest dose of chemical compounds.

Conclusion: The low bioinsecticidal activities of these plant extracts could be attributed to the low occurrence of terpenoids and alkaloids.

Keywords: Anopheles, bioinsecticide, Mesosphaerum suaveolens, Ocimum gratissimum, insecticide resistance, susceptibility.