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

The viability of Anopheles gambiae sensu stricto and Anopheles arabiensis (Diptera: Culicidae) eggs over time and the ovicidal activity of Beauveria bassiana (Ascomycota: Cordycipitaceae) and Metarhizium anisopliae (Ascomycota: Clavicipitaceae) were investigated. Eggs were incubated in soil or leaf litter for up to 12 weeks at 26°C and 75%, 86% or >98% relative humidity (RH). Eggs were treated topically with M. anisopliae ICIPE-30 or B. bassiana I93-825 conidia in either water or oil-in-water formulations. Survival of eggs whether treated or not with fungus was similar, and untreated eggs generally did not survive longer than 2 weeks regardless of the substrate or humidity tested. After a minimal 5-day exposure, M. anisopliae at 5×10 conidia/cm clearly reduced the number of larvae. The efficacy of the fungus increased when it was oil-in-water formulated, and eclosion was completely prevented regardless of the conidial concentration (10 -10 conidia/cm ) after a 10-day exposure in soils at >98% RH. Treatment of eggs with B. bassiana, however, failed to reduce the number of eclosing larvae. This is the first demonstration of the ovicidal activity by M. anisopliae against either A. gambiae s. s. or A. arabiensis and the results underline the potential of this fungus against anopheline mosquitoes