Variation In Conidial Thermotolerance And Virulence Of Originally Different Beauveria Bassiana Strains Against Myzus Persicae

As a classic fungal biocontrol agent, Beauveria bassiana has been developed into formulations for application to control of numerous crop or forest insect pests using aerial conidia as active ingredients. Environmental temperature is known to affect conidia] germination and infection and thus becomes a factor influential on either ambient shelf-life of the formulations or field performance after application. To search: for more potential candidates for formulations adaptable to pest control in southern China during hot summer, this study was performed to evaluate conidial thermotolerance of 165. bassiana strains derived from different host and geographic origins and compare colony growth rates of thermotolernace-different strains at 25-35 °C and their virulence toward green peach aphid Myzus persicae at 16-30 °C, generating results as follows.Screening of thermotolerable strains based on conidial viability decline after exposure to thermal stress. Aerial conidia of each of the 16 B. bassiana strains were uniformly suspended in 1-mL liquid medium (2% sucrose plus 0.5% peptone) in 10-ml glass tubes, exposed to thermal stress of 48 °C in water bath for 45 mir., and then allowed to germinate at 25 °C by shaking the tubes for 24 h. The resultant residue viability of the conidia was used in an index for the tolerance of the conidia of each strain to the thermal stress. As a result, the residue viabilities differed significantly among the strains tested (P <0.01), ranging from 4.6 to 87.1%. Three of the 16 strains had residue viabilities exceeding 40% after exposure, showing strong thermotolerance whereas four strains with the residue viabilities lower than 10% fell within a weak thermotolerance group. Ths rest strains were intermediate in conidial thermotolerance due to a range of their residue viabilities from 11.0 to 23.6%. A representative strain taken from each of the three groups was further exposed to 30-min thermal stresses at 45, 50, 55, 60, and 65 °C, respectively. The data fit well to a survival model, yielding estimates of the half-lethal temperatures of 50.9, 48.3 and 45.5 °C for the strains with strong, intermediate and weak thermotolerance, respectively.Colony growths of thermotolerance-different strains at variable temperatures. The three highly thermotolerable strains grown on SDAY plates for 7 days had colonydiameters of 21-33.3 mm at 25 °C, 15-31.7 mm at 30 °C, and 8.3-15.3 mm at 35 °C, respectively. These observations differed significantly among the strains at each of the temperatures (PO.01). Moreover, three representative strains with the residue viabilities of 40.8, 15.8 and 4.6% after exposure to the 45-min stress of 48 °C weie also compared for their colony growths at different temperatures. Their 7-day-old colonies were in a range of 28-30 mm at 25 °C but became significantly larger at 30 °C if their conidial thermotolerance was greater. At 35 °C, however, only the most thermotolerable strain, B. bassiana 124, formed visible colonies in mean diameter of 12.5 mm. This strain was rare for B. bassiana due to its ability to grow and sporulate at 35 °C.Virulence of thermotolerance-different strains to aphids at variable temperatures. The three strains, B. bassiana 734, 1829 and 3153 with strong, intermediate and weak thermotolerance were separately bioassayed for their virulence to vigorous apterous adults of M. persicae at 16, 25 and 30 °C, respectively. Each bioassay consisted of three gradient concentrations (no. conidia/mm2) plus a blank control, being replicated three times (30-40 aphids per replicate on a detached cabbage leaf) using a spray tower method for inoculation. Based on the fitted time-dose-mortality relationships, the LC50S and associated 95% confidence intervals for the three strains against the aphid species on day 7 after fungal spray were estimated as 40.6 (21.6-76.1), 132.5 (86.8-202.1) and 405.2 (315.3-520.8) conidia/mm2 at 30 V, 215.0 (162.1- 285.2), 493.9 (332.0-734.6) and 204.4 (148.3- 281.6) conidia/mm2 at 25 °C, and 3684.7 (2037.3-6664.2), 1202.3 (800.9-1804.7) and 1192.3 (657.2-2163.3) at 16 °C, respectively. For the most thermotolerable strain, the virulence was best at 25-30 °C but poor at 16 °C. The strain with intermediate thermotolerance showed acceptable virulence at the full range of 16-30 °C. However, the least thermotolerable strain had poor virulence at 30°C but was most virulent at the lower temperatures.Based on the results presented above, variation in conidial thermotolerance was large among B. bassiana strains. Colony growths and virulence of the strains at high temperatures were generally in accordance with their conidial thermotolerance but more thermotolerable strains did not necessarily have growth and sporulaton traits desired for candidates in microbial control. This suggests necessity and possibility to search for morepotential fungal candidates with all desired traits that warrant not only better adaptation to stressed weather but higher efficacy for insect control in the field. The strain B. bassiana 734 found in this study would suit to more tropical climate for insect control in southern China while another strain, B. bassiana 1829, could be more appropriate for formulations against insect pests along the Changjiang River.