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Background  Entomopathogenic fungi have shown great potential for the control of adult malaria vectors. However, their ability to control aquatic stages of anopheline vectors remains largely unexplored. Therefore, how larval characteristics ( Anopheles  species, age and larval density), fungus (species and concentration) and environmental effects (exposure duration and food availability) influence larval mortality caused by fungus, was studied.    Methods  Laboratory bioassays were performed on the larval stages of  Anopheles gambiae  and  Anopheles stephensi  with spores of two fungus species,  Metarhizium anisopliae  and  Beauveria bassiana . For various larval and fungal characteristics and environmental effects the time to death was determined and survival curves established. These curves were compared by Kaplan Meier and Cox regression analyses.    Results   Beauveria bassiana  and  Metarhizium anisopliae  caused high mortality of  An. gambiae  and  An. stephensi  larvae. However,  Beauveria bassiana  was less effective (Hazard ratio (HR) <1) compared to  Metarhizium anisopliae. Anopheles stephensi  and  An. gambiae  were equally susceptible to each fungus. Older larvae were less likely to die than young larvae (HR < 1). The effect of increase in fungus concentration on larval mortality was influenced by spore clumping. One day exposure to fungal spores was found to be equally effective as seven days exposure. In different exposure time treatments 0 - 4.9% of the total larvae, exposed to fungus, showed infection at either the pupal or adult stage. Mortality rate increased with increasing larval density and amount of available food.    Conclusions  This study shows that both fungus species have potential to kill mosquitoes in the larval stage, and that mortality rate depends on fungus species itself, larval stage targeted, larval density and amount of nutrients available to the larvae. Increasing the concentration of fungal spores or reducing the exposure time to spores did not show a proportional increase and decrease in mortality rate, respectively, because the spores clumped together. As a result spores did not provide uniform coverage over space and time. It is, therefore, necessary to develop a formulation that allows the spores to spread over the water surface. Apart from formulation appropriate delivery methods are also necessary to avoid exposing non-target organisms to fungus.

Factors affecting fungus-induced larval mortality in Anopheles gambiae and Anopheles stephensi