Intraspecific tolerance of Metarhizium anisopliae conidia to the upper thermal limits of summer with a description of a quantitative assay system

Conidial tolerance to the upper thermal limits of summer is important for fungal biocontrol agents, whose conidia are formulated into mycoinsecticides for field application. To develop an efficient assay system, aerial conidia of eight Metarhizium anisopliae, four M. anisopliae var. anisopliae, and six M. anisopliae var. acridum isolates with different host and geographic origins were wet-stressed for ≤180 min at 48 °C or incubated for 14 d colony growths at 10–35 °C. The survival ratios (relative to unstressed conidia) of each isolate, examined at 15-min intervals, fit a logistic equation (r2 ≥ 0.975), yielding median lethal times (LT50s) of 14.3–150.3 min for the 18 isolates stressed at 48 °C. Seven grasshopper isolates from Africa had a mean LT50 of 110 (73–150) min, but could not grow at 10 or 15 °C. The mean LT50 of five non-grasshopper isolates capable of growing at 10–35 °C was 16 (10–26) min only. Three isolates with typically low (type I), medium (type II), and high (type III) levels of tolerance to 48 °C were further assayed for ≤4-d tolerance of their conidia to the wet stress at 38, 40, 42, or 45 °C. The resultant LT50s decreased to 20, 53 and 167 min at 48 °C from 507, 1612, and 8256 min at 38 °C for types I, II and III, respectively. For the distinguished types, the logarithms of the LT50s were significantly correlated to the temperatures of 38–48 °C with an inverse linearity (r2 ≥ 0.88). The method developed to assay quantitatively fungal thermotolerance would be useful for screening of fungal candidates for improved pest control in summer.