Growth Characteristics and Bio-Efficacy of Different Isolates of Metarhizium anisopliae (Metschnikoff) Sorokin against Certain Key Insect Pests

An attempt was made to know the variation in field collected isolates of Metarhizium anisopliae (Metschnikoff) Sorokin under laboratory conditions. Isolates of M. anisopliae collected from different geographical regions were studied for various parameters like growth and sporulation on potato dextrose agar (PDA) medium and rice based medium, and exhibited considerable variation. Ma3 and Ma4 isolates took 6 days to initiate sporulation while Ma1 and Ma2 isolates took 2 to 3 days on PDA. Mycelial growth in Ma1 and Ma2 isolates, being on par, was significantly higher (38.60 and 42.30 mm, respectively) than the other two isolates (Ma3-32.30 mm and Ma4- 33.19 mm). Ma1 and Ma2 isolates grew faster on rice based medium and yielded conidial yield of 8.93 and 9.32 g per 100 g of medium respectively. Isolate Ma2 was the most virulent against Helicoverpa armigera (Hübner) and Plutella xylostella (Linnaeus) with less LC₅₀ values of 1.77 x 10⁶ and 1.98 x 106 conidia per ml followed by Ma1, which was more virulent against Oryctes rhinoceros (Linnaeus). The lethal time (LT₅₀) was low in Ma2 for H. armigera and P. xylostella (126.15 h and 69.74 h, respectively) in contrast to Ma1 that recorded lower LT₅₀ value against O. rhinoceros (294.47h). The results indicate that Ma2 was the best isolate against lepidopteran insects and Ma1 was the best against coleopterans.