Effects of temperature on growth of Metarhizium flavoviride and virulence to the variegated grasshopper, Zonocerus variegatus

The temperature optima for germination, radial hyphal extension rate and sporulation were determined for two isolates of Metarhizium flavoviride from acridoid hosts. 191-609 exhibited lower temperature optima for hyphal extension (25.5 °C) and sporulation (24°) than IMI 330189 (27°and 25°, respectively), although temperature optima for germination were similar (ca 30°). The temperature optimum for sporulation was lower than that for hyphal extension. Further studies were carried out on 191-609 to investigate the effect of temperature and dose on the infection mortality rate of the grasshopper pest, Zonocerus variegatus. Increasing dose resulted in a decrease in mean survival time regardless of incubation temperature. Dose and temperature were shown to interact, with the effects of dose increasing at temperatures either side of the optimum. Furthermore, for each dose, the optimum temperature for infection was estimated to be ca 5°higher than the optimum for growth, and no significant relationship was identified between hyphal extension rate in vitro and host survival time. The implications of these results are discussed with regard to the selection of isolates for use in biological control.