1469 MATTHEW B. THOMAS 1 AND NINA E. JENKINS 2 *  Leverhulme Unit for Population Biology and Biological Control, NERC Centre for Population Biology, Imperial College, Silwood Park, Ascot, Berks. SL5 7PY, U.K.  International Institute of Biological Control, Silwood Park, Buckhurst Road, Ascot, Berks. SL5 7TA, U.K. The temperature optima for germination, radial hyphal extension rate and sporulation were determined for two isolates of Metarhizium ﬂavoviride from acridoid hosts. I91-609 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 I91-609 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. A number of research projects around the world are trying to develop biological pesticides based on entomopathogenic hyphomycete fungi, for locust and grasshopper control (e.g. Johnson et al., 1992 ; Lobo-Lima, Brito & Henry, 1992 ; Prior et al., 1992 ; Zimmermann et al., 1994). Several studies have demonstrated signiﬁcant population reductions of grasshoppers following microbial applications (e.g. Johnson & Goettel, 1993 ; Lomer et al., 1993 ; Douro-Kpindou et al., 1995 ; Lomer et al., in press), suggesting considerable promise for these agents. However, results of studies in North America, using applications of Beauveria bassiana (Bals-Criv.) Vuill., have proven to be very variable with environmental factors, particularly high temperatures, appearing to severely limit the performance of the pathogen at certain times (Goettel, Johnson & Inglis, 1995). Results of other studies using oil formulations of Metarhizium ﬂavoviride Gams & Rozsypal against a range of locust and grasshopper species in Africa have been more consistent but have suggested, although not conﬁrmed, that temperature eﬀects on disease incubation may substantially slow down mortality rates in the ﬁeld (e.g. Lomer et al., 1993 ; Thomas, Langewald & Wood, 1996). These observations are consistent with work by Carruthers, Larkin & Firstencel (1992) which demonstrated that the rangeland grasshopper, Camnula pellucida (Scudder), can slow the development and cause mortality of the fungal pathogen Entomophaga grylli (Fresen.) Batko (pathotype I) by raising its * Current address : IITA Plant Health Management Division Biological Control Centre for Africa, B.P. 08-0932, Cotonou, Benin. body temperature during active thermoregulation in the ﬁeld. Boorstein & Ewald (1987) report a similar ﬁnding for another rangeland grasshopper, Melanoplus sanguinipes (F.), infected with the microsporidian pathogen Nosema acridophagus Henry. In this case, infected individuals were shown to actually prefer higher temperatures than uninfected controls. These higher temperatures were found to be detrimental to pathogen development and survival. It is clear that environmental temperature and host thermoregulation have the potential to alter the eﬃcacy of a mycoinsecticide. The consequences of this will depend on the system. For example, speed of kill is considered one of the major factors limiting the utility of mycoinsecticides for control of many pests, including locusts and grasshoppers (Bateman & Thomas, 1996 ; Prior & Streett, in press). However, not all control scenarios demand instant ‘ knock-down ’ and much of the treatment of locusts and grasshoppers can be done in non-crop habitats where control is preventive rather than curative (Prior & Streett, in press). Under these circumstances, slow speed of kill need not be a limitation, especially as feeding may be reduced some time before death (Moore et al., 1992). Therefore, the role of temperature on disease development and overall disease dynamics needs to be considered if the consequences of microbial applications are to be fully understood and their eﬀectiveness maximized. Determining the eﬀects of temperature on growth and sporulation are also fundamental to optimizing mass pro- duction of microbial agents. This paper presents a study to examine the eﬀects of Mycol. Res. 101 (12) : 1469–1474 (1997) Printed in the United Kingdom Eﬀects of temperature on growth of Metarhizium ﬂavoviride and virulence to the variegated grasshopper, Zonocerus variegatus