Control of grasshoppers and locusts has traditionally relied on synthetic insecticides, and for emergency situations this is unlikely to change. However, a growing awareness of the environmental issues associated with acridid control as well as the high costs of emergency control are expanding the demand for biological control. In particular, preventive, integrated control strategies with early interventions will reduce the financial and environmental costs associated with large-scale plague treatments. The recent development of effective oil formulations of Metarhizium anisopliae spores in Africa, Australia, and Brazil opens new possibilities for environmentally safe control operations. Metarhizium biopesticide kills 70%-90% of treated locusts within 14-20 days, with no measurable impact on nontarget organisms. An integrated pest management strategy, with an emphasis on the use of Metarhizium, that incorporates rational use of chemical pesticides with biological options such as the microsporidian Nosema locustae and the hymenopteran egg parasitoids Scelio spp., has become a realistic option.

1. Thermoregulatory behaviour of the Senegalese grasshopper, Oedaleus senegalensis (Krauss), was investigated in the field following a spray application of an oil‐based formulation of Metarhizium flavoviride Gams and Rozsypal in Niger, West Africa. 2. Measurements of environmental temperature, wind speed and solar radiation were made in conjunction with measurements of internal body temperatures of grasshoppers from a control (unsprayed) and treated plot using microthermocouples and hand‐held thermometers. Grasshoppers were monitored for 4 days from the third day after application. 3. Oedaleus senegalensis utilized a range of thermoregulatory behaviours to maximize body temperatures during periods of low insolation and ambient temperature, and to minimize excessive heat loading during the hottest periods. Preferred body temperature of uninfected grasshoppers was 39 °C, with a range from 24 °C in the early morning to a high of 46 °C during periods of high insolation and ambient temperature. 4. Infected grasshoppers altered their thermoregulatory behaviour and showed a behavioural fever response to the pathogen. Preferred body temperatures of infected individuals were raised to a new set point of ≈ 42 °C. This is believed to be the first evidence for a behavioural fever in response to a microbial infection for any natural population. In the present study, its effects appeared to provide little therapeutic advantage to hosts infected following application. Preliminary evidence from other studies, however, indicates that modifications to host thermoregulation could be a significant constraint to the pathogen and may limit its impact under certain conditions.