Development of Beauveria bassiana for control of grasshoppers and locusts

Recognition of the potential of Beauveria bassiana (Balsamo) Vuillemin as a control agent of grasshoppers and locusts occurred as early as 1936, in South Africa. Field testing of B. bassiana as an inundative control agent of grasshoppers and locusts has been facilitated by development of a solid substrate method for mass-production of the fungus and has resulted in the registration of a strain against grasshoppers in the United States. In some, but not all field trials, application has resulted in substantial reductions in grasshopper populations. Numerous environmental constraints, including temperature and ultraviolet (UV) radiation, may limit field efficacy of the fungus. Laboratory studies suggest that low humidity does not limit the ability of the fungus to initiate disease. Sunlight is the major cause of mortality of conidia on leaf surfaces. The incorporation of UVB protectants in formulations can increase conidial survival; however, these have not yet been evaluated for their effects on field efficacy of B. bassiana against insects. Thermoregulation by grasshoppers has been implicated in resistance to mycosis. Results of laboratory studies indicate that grasshoppers infected with B. bassiana preferentially seek temperatures between 40 and 42 degrees C and these temperatures are inhibitory to disease development. In field-cage trials, a higher prevalence and more rapid development of disease were observed in grasshoppers placed in shaded cages than in grasshoppers placed in cages exposed to full sunlight. In laboratory experiments simulating grasshopper thermoregulation during daylight periods, application of both Metarhizium flavoviride Gams and Rozsypal and B. bassiana simultaneously resulted in a final prevalence of disease that was greater than M. flavoviride alone in the hot temperature environment, and equal to B. bassiana alone in the cool temperature environment. Incorporation of sublethal levels of Dimilin with conidia of B. bassiana increased efficacy of the fungus against grasshoppers in laboratory and field trials. Once environmental constraints are better quantified, it may be possible to overcome them through improved formulation, strain selection, genetic or phenotypic manipulation, and inoculum targeting. Ultimately, success of B. bassiana as a microbial control agent will depend on our ability to overcome environmental and other constraints and/or to predict its efficacy under various environmental conditions.