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 1 936, in South Africa. Field testing of B. bassiana as an inundative co ntrol agent of grasshoppers and locusts has been facilitated by develo pment of a solid substrate method for mass-production of the fungus an d 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. Numero us environmental constraints, including temperature and ultraviolet (U V) radiation, may limit field efficacy of the fungus. Laboratory studi es suggest that low humidity does not limit the ability of the fungus to initiate disease. Sunlight is the major cause of mortality of conid ia on leaf surfaces. The incorporation of UVB protectants in formulati ons can increase conidial survival; however, these have not yet been e valuated for their effects on field efficacy of B. bassiana against in sects. Thermoregulation by grasshoppers has been implicated in resista nce to mycosis. Results of laboratory studies indicate that grasshoppe rs infected with B. bassiana preferentially seek temperatures between 40 and 42 degrees C and these temperatures are inhibitory to disease d evelopment. In field-cage trials, a higher prevalence and more rapid d evelopment of disease were observed in grasshoppers placed in shaded c ages 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 an d Rozsypal and B. bassiana simultaneously resulted in a final prevalen ce of disease that was greater than M. flavoviride alone in the hot te mperature environment, and equal to B. bassiana alone in the cool temp erature environment. Incorporation of sublethal levels of Dimilin with conidia of B. bassiana increased efficacy of the fungus against grass hoppers in laboratory and field trials. Once environmental constraints are better quantified, it may be possible to overcome them through im proved formulation, strain selection, genetic or phenotypic manipulati on, and inoculum targeting. Ultimately, success of B. bassiana as a mi crobial control agent will depend on our ability to overcome environme ntal and other constraints and/or to predict its efficacy under variou s environmental conditions.