Evaluation Of Thermal Adaptation, Carbendazim Resistance And Ovicidal Activity Of Six Paecilomyces Fumosoroseus Strains Against Spider Mites

The entomopathogenic fungus, Paecilomyces fumosoroseus, is one of the most promising fungal species that takes important part in microbial control of insect pests, particularly with sucking mouthparts. To search for potential fungal candidates against spider mites, this study was performed to evaluate colony growth, conidiation capacity and conidal viability of six P. fumosoroseus wild strains grown on the plates of Sabouraud dextrose agar plus yeast (SDAY) at 15-35℃, to assay their sensitivities to gradient concentrations of carbendazim, a common fungicide widely applied to plant disease control, and ovicidal activity against the carmine spider mite, Tetranychus cinnabarinus. The results are summarized as follows.Thermal adaptation. Six P. fumosoroseus strains derived from whiteflies or psyllids were grown on SDAY plates at 15, 20, 25, 30, and 35℃, respectively, for 9-day daily diameter measurements of their colony growths. Conidiation capacity was measured by harvesting conidia on day 9 or on day 14 at the lowest temperature and conidial viability examined using 24-h germination rates in a liquid medium for gemination. The three indices were always best at 25 ℃ for all the fungal strains, but differed significantly at different temperatures for a given strain or among the strains tested at a given temperature. The range of 20-30℃ was optimal for the colony growth, conidiation and viability of the strains Pfr4205, Pfr116, Pfr6206 and Pfr2175. Of those, Pfr6206 and Pfr2175 tended to adapt to the high temperature of 35℃ than others. Generally, the low temperature of 15℃ affected the three indices of the tested strains more than the high temperature of 35℃. The two strains Pfr116 and Pfr6206 adapted to the wide range of 15-35℃ better or more stably than the other strains.Resistance or tolerance of the fungal strains to cabendazim. The sensitivities of the six P. fumosoroseus strains to gradient concentrations of the fungicide carbendazim were assayed at the optimal temperature based on the diameters of their colony growths and the counts of colony forming units (CFU) on SDAY plates (colony growth: 0-16 μg/mL;CFU counts: 0-256 μg/mL). As a consequence, the sensitivities of the strans to the fungicide differed significantly. Pfr6206 was least susceptiable to carbendazim and had a colony diameter of 19.2 mm at 1.0 ug/mL, which was 41.0% smaller than that of the blank control. In contrast, Pfr116 was most susceptible to the fungicide with a colony diameter of only 8.2 mm, being inhibited by 90.2%. The inhibitory effects of the fungicide on the colony growths of the other strains were intermediate between Pfr6206 and Pfr116. The CFU counts for the conidia of the fungal strains on carbendazim-inclusive SDAY plates fit wellto a logistic equation (i?>0.90), generating the estimates of minimal inhibitory concentration (MIC;unit: jj.g/mL) of the fungicide against them. Low resistance was found in the strains Pfr4205 and Pfrl 16 due to MIC<20 whereas the other four strains fell in medium, resistance due to 20100 and significantly greater than the MIC values of the rest strains.Ovicidal activity. The three strains Pfr6206, Pfr2175 and Pfr4205 were chosen to assay their ovicidal activity against the mite pest based on their thermal adaptation and carbendazim resistance.The mite eggs on fava bean leaves were exposed to sprays of conidial suspensions of each strain, generating four concentrations ranging from 11-1677 conidia/mm2. The sprayed eggs started hatching five or six days after exposure. The hatch rates of the eggs sprayed at different conidial concentrations were lower or significantly lower than those in blank controls and became stable 10 days after exposure. The egg mortalities based on the counts of the unhatched eggs were corrected using the background mortalities in blanck controls and then subjected to probit analysis, yielding a linear concentration-mortality relationship for each fungal strain against the mite eggs. The LC50 with 95% confidence intervals was estimated as 247 (187-340) conidia/mm2 for P&6206 and 361 (263-537) conidia/mm2 for Pfr2175, respectively. Both represent highly ovicidal activities. However, Pfr4205 was poor in ovicidal activity due to its LC50 as high as 4791 conidia/mm2.Conclusive remarks. Based on the thermal adaptation, carbendazim resistance and ovicidal activity of the six P. Jumosoroseus strains against the carmine spide mite, Pfr6206 was the most potential strain as a fungal candidate for the mite control.