Temperature-dependent growth and virulence, and mass production potential of two candidate isolates of Metarhizium anisopliae (Metschnikoff) Sorokin for managing Maruca vitrata Fabricius (Lepidoptera: Crambidae) on cowpea

Entomopathogenic fungi are increasingly gaining recognition as a viable alternative to synthetic insecticides for pest management; their efficacy is influenced by several biotic and abiotic factors. In this study, the effect of temperature on germination, growth and virulence of two isolates of Metarhizium anisopliae, ICIPE 18 and ICIPE 69, against first-instar larvae of Maruca vitrata at temperatures ranging from 15-35 degrees C, and mass-production potential of the isolates were assessed under laboratory conditions. Minimum, optimum and maximum threshold temperatures for efficacy of the best performing isolate, ICIPE 69, were estimated by the quadratic equation which emerged as the best modelamongthe nonlinear mathematical expressions tested. Neither isolate germinated at 15 degrees C while germination was low at 35 degrees C for both. The optimum temperature for germination, radial growth and virulence of ICIPE 69 and ICIPE 18 ranged between 25-30 degrees C and 25-33 degrees C, respectively. Mortality of M. vitrata by both isolates ranged between 56.0 and 91.6% across the different temperatures, with the highest mortality occurring at 25 and 30 degrees C for ICIPE 69 and ICIPE 18, respectively. The shortest LT50 values were 2.5 days for ICIPE 18 and 2.2 days for ICIPE 69 at 33 degrees C and 25 degrees C, respectively. The quadratic model predicted 25-30 degrees C as the optimum temperature range for maximum efficacy of M. anisopliae isolate ICIPE 69 against M. vitrata. Upper and lower threshold temperatures were predicted as 40 degrees C and 10 degrees C, respectively. Comparison of aerial conidia production on rice showed that ICIPE 69 yielded higher amounts of conidia (18.4 g conidia/kg rice) than ICIPE 18 (12.4 g conidia/kg rice). The results suggest that ICIPE 69 is a promising candidate for biocontrol of M. vitrata.