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

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 2-5 days old adults were used for adulticidal assays. After 24 h, 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 48 h. Ni. tabacum (0.50 mg/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 0.25 mg/ml of Ni. tabacum (20.3 min), and the lowest KdT95 was observed with 0.10 mg/ml of Ag. conyzoides (35.97 min) 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.

Multiple insecticide resistance mechanisms in urban population of Anopheles coluzzii (Diptera: culicidae) from Lagos, South-West Nigeria.

Malaria is a major public health challenge in Africa with Nigeria accounting for the highest burden of the disease in the world. Vector control has proved to be a highly effective component of malaria control, however, the development and spread of insecticide resistance in major vectors of malaria have been a major challenge. This study assessed resistance mechanisms in Anopheles coluzzii populations from Kosofe, Lagos mainland and Ojo Local Government Areas in Lagos, Nigeria where An. gambiae s.l is resistant to DDT and Permethrin. WHO susceptibility bioassay test was used in determining resistance status of An. coluzzii to discriminating doses of DDT and Permethrin while synergist assay was used to assess the involvement of monooxygenases in resistance development. Sub-species of An. gambiae s.l (An. gambiae and An. coluzzii) were identified using polymerase chain reaction (PCR) and Restriction Fragment Length Polymorphism (PCR-RFLP) while Allele-Specific Polymerase Chain Reaction (AS-PCR) assay was used to detect knockdown mutation (kdr-West; L1014F). Biochemical assays were used in determining the activities of metabolic enzymes. High DDT resistance was recorded in An. coluzzii populations from the three sites. Mortality rate of mosquitoes exposed confirmed Permethrin resistance in Kosofe (50%) and Lagos mainland (48%) but resistance was suspected in Ojo (96%). All specimens tested were confirmed as An. coluzzii with low kdr frequency; 11.6%, 16.4% and 6.7% in Kosofe, Lagos mainland and Ojo respectively. Pre-exposure to synergist (PBO) before exposure to Permethrin led to increased mortality in all populations. Esterase activity was insignificantly overexpressed in Kosofe (p = 0.849) and Lagos mainland (p = 0.229) populations. In contrast, GST activity was significantly lower in populations from Lagos mainland (63.650 ± 9.861; p = 0.007) and Ojo (91.765 ± 4.959; p = 0.042) than Kisumu susceptible strains (120.250 ± 13.972). Monooxygenase activity was higher in Lagos mainland (2.371 ± 0.261) and Ojo (1.361 ± 0.067) populations, albeit significantly in Lagos mainland (p = 0.007) only. Presence of target-site mutation in all populations, increased mortality with pre-exposure to PBO and elevated monooxygenase in Lagos mainland population were confirmed. Multiple resistance mechanisms in some urban populations of An. coluzzii from Lagos, Nigeria calls for appropriate resistance management strategies.