Effect of temperature and humidity on the virulence of beetle-derived Beauveria bassiana (Balsamo) Vuillemin (Deuteromycetes: Moniliales) against the daikon leaf beetle, Phaedon brassicae Baly (Coleoptera: Chrysomelidae)

In this study, the effect of temperature and humidity on the virulence of a Beauveria bassiana isolate, SCAU-BB01D, to Phaedon brassicae, a serious insect pest of cruciferous vegetables in South China, was studied in the laboratory. The mortality of P. brassicae was significant lower at temperature of 17℃ and 29℃ than at other temperatures. At temperature of 23℃, the mortality of adult on the 14th day after treatment reached its highest, 95.60%. For 2nd instar larvae the highest mortality (96.00%) was obtained at both 23℃ and 26℃ on the 10th day after treatment. The pathogenicity of B. bassiana to P. brassicae was enhanced as relative humidity increased. When relative humidity (RH) was increased from 50% to 100%, adult mortality 14 days after treatment was enhanced from 55.70% to 88.80% and 2nd instar larvae mortality 10 days after treatment was enhanced from 62.00% to 96.00%. At 17, 20, 23, 26, and 29℃, the LT_ 50 values were 13.63, 10.27, 8.05, 8.87 and 12.41 days for adults, and 6.06, 5.72, 4.90, 4.86 and 6.73 days for 2nd instar larvae, respectively. LT_ 50 values at RH of 50%, 60%, 70%, 80%, 90% and 100% were 12.97, 10.97, 10.13, 9.79, 9.14, 8.29 for adults and 7.68, 6.92, 5.76, 5.57, 5.48, and 4.82 days for 2nd instar larvae, respectively. Therefore, temperatures between 23-26℃ and RH higher than 80% were the most suitable conditions for promoting infection of B. bassiana to P. brassicae.