Susceptibility of laboratory colonies of members of the Anopheles gambiae complex to entomopathogenic fungi Beauveria bassiana

Abstract

Control of the major African malaria vectors of the Anopheles gambiae complex continues to rely heavily on the application of insecticides either by indoor residual house spraying (IRS) or bednet impregnation (ITNs). However, growing concerns about their negative impact on human health, their potential impact on the environment, and the ever-increasing spread of insecticide resistance across sub- Saharan Africa has diverted attention to alternative strategies that are not based on chemicals. Therefore, alternatives such as vector control with biological agents may be effective. Levels of susceptibility to the entomopathogenic fungus Beauveria bassiana were assessed against insecticide-resistant and insecticide-susceptible laboratory colonies as well as a wild-caught sample of the An. gambiae complex of which most members are major or localised malaria vectors. Insecticide susceptibility tests against all four classes of insecticides recommended by WHO for vector control was performed according to the standard WHO bioassay protocol. Fungal susceptibility tests using suspensors dusted with dry conidia of B. bassiana isolate IMI 391510 were performed on laboratory-reared colonies of An. gambiae complex members as well as on a sample of wild caught, insecticide susceptible An. arabiensis from Malawi. Our data showed that there is no interaction between insecticide susceptibility and fungal susceptibility. Survival of both insecticide susceptible and insecticide resistant colonies as well as wild mosquitoes was significantly affected (P< 0.001) by fungal infection. All infected mosquitoes succumbed to fungal infection within 7-20 days post-exposure compared to control groups for which 70-85% of mosquitoes were still alive at that time. The evidence of fungal infection was confirmed with more than 90% of the mosquito cadavers sporulating 3-5 days after incubation. These results provide hope for alternative control strategies for adult anopheline mosquitoes other than those that rely entirely on insecticides. There was evidence that insecticide susceptible and resistant mosquitoes can be killed by the fungus B. bassiana within the 14 day period before they become infective with the malaria Plasmodium parasite. However, conidia formulations and concentrations as well as a suitable delivery method for efficient coverage remain the areas of focus for field implementation.