Identification of Metarhizium strains highly efficacious against Aedes, Anopheles and Culex larvae

Entomopathogenic fungi, such as Metarhizium anisopliae and Beauveria bassiana, have been shown to be efficacious in killing mosquito larvae of different mosquito species. The current study compared the pathogenicity and efficacy of two formulations of three fungal strains against different instars of three mosquito species with the aim of identifying the most virulent strain for use under field conditions. Three strains of Metarhizium, ARESF 4556, ARSEF 3297 and V275, were assayed against early (L2-3) and late (L3-4) instar larvae of Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus. Two formulations of the fungi were tested, dry conidia and aqueous suspensions (i.e. 'wet' conidia). Effects of all combinations of conidia, mosquito species, instar, fungal strain and concentration on mosquito mortality were analysed using Cox regression and Kaplan-Meier analyses. Strain ARSEF 4556 was more virulent than ARSEF 3297 and V275, with LT50 values ranging from 0.3 to 1.1 days, with Anopheles and Culex being more susceptible than Aedes. Early and late instars were equally susceptible independent of species. Although the formulation did influence mortality rates, both 'wet' and 'dry' conidia applications were highly effective in killing mosquito larvae. Viable spores were more efficacious than heat killed spores. The latter did cause mortality but only at high concentrations. Metarhizium sp. has proved to be effective in reducing survivability of all larval stages of Aedes, Anopheles and Culex under laboratory conditions. Aedes larvae were generally more tolerant than Anopheles and Culex irrespective of fungal strain.