Effect of temperature on local and Philippine isolates of Metarhizium anisopliae and their virulence on larvae of Oryctes rhinoceros, a pest of coconut in Sri Lanka

The effect of temperature on mycelial growth and spore production in Sri Lankan (local isolate) and Philippine isolates (DRC) of Metarhizium anisopliae (Metsch.) Sorokin and their virulence to larvae of the black beetle, Oryctes rhinoceros L. were compared in the laboratory. Mycelial growth and spore production in each isolate were evaluated at 25°C, 27.5°C, 30°C and 32.5°C. Healthy third instar larvae of black beetle were inoculated with spore suspensions of 10⁶spores/ml of each isolate to compare the virulence. There was no considerable difference between the two isolates with respect to mycelial growth at the end of the experimental period of 40 days. However, the mycelial growth of the local isolate significantly differed among the four temperatures, with the highest growth at 27.5°C and lowest at 32.5°C. The growth of DRC isolate was similar at all temperatures. The two isolates differed significantly in spore production. Spore production was extremely low at 32.5°C in both isolates. The local isolate produced a significantly higher number of spores at other three temperatures than that of the DRC isolate. In the local isolate spore production was higher at 25°C and 27.5°C and in the DRC isolate it was highest at 25°C. Both isolates caused death in black beetle larvae, the local isolate recorded a significantly higher mortality (67%) than the DRC isolate (42%), 17 days after inoculation. The LT₅₀of the local and DRC isolates were 14 and 18 days respectively. The study reveals that the local isolate has more desirable characteristics than the DRC isolate for use in the management of black beetle larvae in Sri Lanka.

COCOS Vol. 20: 49-58 (2013)