Effects of culture media on larvicidal property of secondary metabolites of mosquito pathogenic fungus Chrysosporium lobatum (Moniliales: Moniliaceae)

In order to determine the potential role of secondary metabolites of Chrysosporium lobatum as a biological control agent for mosquitoes, effects of culture media on the larvicidal property of secondary metabolites was evaluated. The secondary metabolites of C. lobatum released in the Sabouraud's dextrose broth (SDB) and chitin broth (CB) were collected by filtering through Whatman No. 3chr filter after 7 days of growth. First, second and third instars of Anopheles stephensi and Culex quinquefasciatus were exposed to six different concentrations of secondary metabolites for the calculation of LC50 and LC90 values. C. quinquefasciatus were more susceptible than A. stephensi to secondary metabolites in SDB and CB. The LC50 and LC90 values for first instars of both the mosquitoes were significantly lower than second and third instars. A significantly higher mortality was recorded in both the mosquitoes to the secondary metabolites in CB than SDB. In the field trial, secondary metabolites in CB reduced 56.3% of A. stephensi and 66.42% of C. quinquefasciatus populations after 5 days of application. The LT50 of first instars of both the mosquitoes were lower than the second and third instars. The precipitates of the secondary metabolites were more effective to both the mosquitoes than the secondary metabolite and heat-shocked secondary metabolites. In addition, the insecticidal effect of the secondary metabolites was reduced after exposure for 5 min at 120 °C, suggesting that the bioactive compounds were proteinaceous. The secondary metabolites in CB contained proteases which may cause more mortality than the secondary metabolites in SDB.