Assessment of the virulence and proteolytic activity of three native entomopathogenic fungi against the larvae of Oryctes agamemnon (Burmeister) (Coleoptera: Scarabaeidae)

Abstract

Susceptibility of the third larval instar of the rhinoceros beetle, Oryctes agamemnon (Burmeister) (Coleoptera: Scarabaeidae), to the infection with three native entomopathogenic fungi (EPFs), Beauveria bassiana, Metarhizium anisopliae, and Lecanicillium lecanii, was determined under laboratory conditions. Three concentrations (104, 106, and 108 conidia/ml) of each EPF isolate were tested. Respective mortality rates of the larvae reached 34.2, 48.1, and 55.5% for B. bassiana, 39.5, 57.4, and 60.8% for M. anisopliae, and 17.4, 26.4, and 29.2% for L. lecanii. Mycosis test was performed to confirm the fungal infection of dead larvae. The LT50 values for O. agamemnon larvae were 9.9 and 11.3 days, for the concentration which achieved 50% mortality for B. bassiana and M. anisopliae, respectively. The proteolytic activity of the three EPFs was investigated under different factors (incubation periods, temperature, and pH of fungal cultures). The results showed that the maximum activity of protease, produced by the three EPFs, was recorded after 6 days of incubation periods at 28 °C, in a medium initially adjusted to pH 6.