The infectivity of the entomopathogenic fungus Beauveria bassiana to insecticide-resistant and susceptible Anopheles arabiensis mosquitoes at two different temperatures

Abstract

Background: Control of the major African malaria vector species continues to rely extensively on the application of residual insecticides through indoor house spraying or bed net impregnation. Insecticide resistance is undermining the sustainability of these control strategies. Alternatives to the currently available conventional chemical insecticides are, therefore, urgently needed. Use of fungal pathogens as biopesticides is one such possibility. However, one of the challenges to the approach is the potential influence of varied environmental conditions and target species that could affect the efficacy of a biological 'active ingredient'. An initial investigation into this was carried out to assess the susceptibility of insecticide-susceptible and resistant laboratory strains and wild-collected Anopheles arabiensis mosquitoes to infection with the fungus Beauveria bassiana under two different laboratory temperature regimes.

Methods: Insecticide susceptibility to all four classes of insecticides recommended by WHO for vector control was tested on laboratory and wild-caught An. arabiensis, using standard WHO bioassay protocols. Mosquito susceptibility to fungus infection was tested using dry spores of B. bassiana under two temperature regimes (21 +/- 1 degrees C or 25 +/- 2 degrees C) representative of indoor conditions observed in western Kenya. Cox regression analysis was used to assess the effect of fungal infection on mosquito survival and the effect of insecticide resistance status and temperature on mortality rates following fungus infection.

Results: Survival data showed no relationship between insecticide susceptibility and susceptibility to B. bassiana. All tested colonies showed complete susceptibility to fungal infection despite some showing high resistance levels to chemical insecticides. There was, however, a difference in fungus-induced mortality rates between temperature treatments with virulence significantly higher at 25 degrees C than 21 degrees C. Even so, because malaria parasite development is also known to slow as temperatures fall, expected reductions in malaria transmission potential due to fungal infection under the cooler conditions would still be high.

Conclusions: These results provide evidence that the entomopathogenic fungus B. bassiana has potential for use as an alternative vector control tool against insecticide-resistant mosquitoes under conditions typical of indoor resting environments. Nonetheless, the observed variation in effective virulence reveals the need for further study to optimize selection of isolates, dose and use strategy in different eco-epidemiological settings.

Figure 1

Insecticide susceptibility status of four laboratory strains and one field strain of Anopheles arabiensis. Data show the average mosquito mortality (%) after a 1 hour exposure to insecticide treated papers, recorded 24 hours post-exposure, of two replicates of 25 mosquitoes for the susceptible groups and three replicates for the resistant groups.

Figure 2

Daily cumulative proportional survival of F1 offspring of An. arabiensis collected in Karonga, Malawi. Data show the average ± SE survival of nine replicate groups of 25-35 mosquitoes infected with B. bassiana (open symbols) and uninfected control groups (closed symbols) kept at 21 ± 1°C (red) or 25 ± 2°C (blue).

Figure 3

Daily cumulative proportional mosquito survival of insecticide-susceptible (blue) and insecticide-resistant (red) An. arabiensis laboratory colonies originating from Sudan (SENN) or Kwazulu/Natal (MBN). Data show the average ± SE survival of nine replicate groups of 25-35 mosquitoes infected with B. bassiana (open symbols) and uninfected control groups (closed symbols) kept at 21 ± 1°C (top) or 25 ± 2°C (bottom).