Effects of temperature and relative humidity on sporulation of Metarhizium anisopliae var. acridum in mycosed cadavers of Schistocerca gregaria

The effects of relative humidity (RH) and temperature on the sporulation of Metarhizium anisopliae var. acridum on mycosed cadavers of desert locust, Schistocerca gregaria, were assessed in the laboratory. Quantitative assessments of conidial production over 10 days under constant conditions showed that sporulation was optimized at RH >96% and at temperatures between 20 and 30°C. Under both these conditions > 10⁹ conidia/cadaver were produced. At 25°C, conidial yield was maximized under conditions in which cadavers remained in contact with damp substrate. Relatively little sporulation occurred at 15°C (<3 × 10⁷ conidia/cadaver) and 40°C (<4 × 10⁶ conidia/cadaver) and no sporulation occurred at 10 or 45°C. Following incubation, conidial yield was closely related to the water content of locust cadavers. In separate tests, locust cadavers were incubated for 10 days under diurnally fluctuating temperature and RH that comprised favorable (25°C/100% RH) alternating with unfavorable (40°C/80% RH) conditions for sporulation. In this case, fewer conidia were produced compared with cadavers that were incubated under the favorable conditions for an equal period cumulatively but were not periodically exposed to unfavorable conditions. However, this reduced sporulation observed with the fluctuating condition was not observed when cadavers were similarly incubated under favorable/unfavorable conditions of temperature but were not periodically exposed to the low RH condition. This result implies that sporulation is a dynamic process, dependent not only on periodic exposure to favorable RH but also on the interrelation of this with low RH. Associated tests and the monitoring of changes in cadaver weights imply that the mechanism driving the reduced sporulation under fluctuating RH is the net water balance of cadavers, i.e. the cumulative ability of the fungus/cadaver to adsorb water necessary for sporulation at high RH is restricted by water loss associated with intermittent exposure to a low RH. The duration of daily exposure to high humidity appears to be a crucial constraint to the recycling ability of M. anisopliae var. acridum.