Thermoregulatory behavior was studied in two key acridid pest species from west and south Africa. Locustana pardalina Walker (Orthoptera: Acrididae) from the arid Karoo region of South Africa was an active behavioral thermoregulator using postural adjustments and microhabitat selection to elevate and then maintain body temperatures at a preferred level between 38 and 41°C for much of the day. Both cool weather and time of season significantly affected the ability of these locusts to reach and maintain these preferred temperatures. Hieroglyphus daganensis Krauss (Orthoptera: Acrididae) from the humid tropical river valleys of west Africa was not an active behavioral thermoregulator and showed none of the postures or habitat selection associated with such behavior. Body temperatures varied little across the day being generally around 32°C. The humid, wet habitat where this species occurs and the uniformity of the thermal environment appeared to preclude the development of elaborate thermal behaviors seen in L. pardalina and other behaviorally thermoregulating ectotherms. The implications of host thermal behavior and the marked difference both within and between species under different environmental conditions are discussed in terms of biological control using a biopesticide based on an entomopathogenic fungus. It is concluded that the fundamental importance of host body temperature in relation to the fitness of the insect and its ability to cope with disease challenge, although generally overlooked in biocontrol programs, has significant implications for the successful development of microbial pest control.