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

Abstract

BackgroundEntomopathogenic 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.MethodsLaboratory 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.ResultsBeauveria 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.ConclusionsThis 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.

Adulticidal and larvicidal activity of Beauveria bassiana and Metarhizium anisopliae against housefly, Musca domestica (Diptera: Muscidae), in laboratory and simulated field bioassays

“…They also obtained a positive correlation between the fungal concentration and its lethality for the treated larvae, as has been observed in this study. Similarly, Bukhari et al (2010) used B. bassiana and M. anisopliae against larval stages of Anopheles gambiae and An. stephensi and found M. anisopliae to be more effective than the B. bassiana.…”

Pathogenicity of Fusarium oxysporum against the larvae of Culex quinquefasciatus (Say) and Anopheles stephensi (Liston) in laboratory

“…Metarhizium anisopliae and Beauveria bassiana are highly effective in reducing larval survival and adult emergence. When insecticide resistance is increasing, it is important to have a wider option of new biological and environmentally friendly control agents (Bukhari et al 2010). Similarly, F. oxysporum has shown potential in the control of larvae of C. quinquefasciatus and A. stephensi.…”

“…Fungal species have the potential to kill mosquitoes in the larval stage, and the mortality rate depends on the fungus species itself, larval stage targeted, larval density, and amount of nutrients available to the larvae. Increasing the concentration of fungal spore or reducing the exposure time to the spore did not show a proportional increase and decrease in mortality rate, respectively, because the spores clumped together (Bukhari et al 2010). We, therefore, are interested in the laboratory bioassays of F. oxysporum metabolites against the larvae of C. quinquefasciatus and A. stephensi.…”

Survival of anopheline eggs and their susceptibility to infection with Metarhizium anisopliae and Beauveria bassiana under laboratory conditions

“…Recent research has demonstrated that the entomopathogenic fungi Beauveria bassiana (Ascomycota: Cordycipitaceae) and Metarhizium anisopliae (Ascomycota: Clavicipitaceae) are able to shorten the lifespan of adult anopheline mosquitoes (Scholte et al 2005;Kanzok and Jacobs-Lorena 2006;Achonduh and Tondje 2008;Stevenson 2008), thus sparking interest in the use of such fungi as biological control agents. Furthermore, both B. bassiana and M. anisopliae have been reported to cause mortality of the larval stages of anopheline (Scholte et al 2004;Bukhari et al 2010) and aedine species (Silva et al 2004(Silva et al , 2005.…”