SUSCEPTIBILITY OF THE MIGRATORY GRASSHOPPER, MELANOPLUS SANGUINIPES (FAB.) (ORTHOPTERA: ACRIDIDAE), TO BEAUVERIA BASSIANA (BALS.) VUILLEMIN (HYPHOMYCETE): INFLUENCE OF RELATIVE HUMIDITY

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Abstract
In laboratory tests, Melanoplus sanguinipes (Fab.) was susceptible to infection by Beauveria bassiana (Bals.) Vuillemin conidia. Infection occurred independently of the relative humidity (RH) (12, 33, 76, 100% RH). No significant difference was observed in the final percentage mortality of the treated grasshoppers and in the lethal times (LT50)(P > 0.05) under the RH conditions studied. However, the daily rate of mortality after treatment was higher at 76% RH than at 33% RH. High natural mortality occurred at 100% RH.At 100% RH, treatment with B. bassiana was associated with an outgrowth of microorganism that created septicemic conditions in the absence of the characteristic symptoms of mycosis. Below 100% RH, mycelial growth on cadavers was never observed, even though partial or total insect mummification occurred, and a pink body coloration on approximately 80% of the insects indicated Beauveria as the killing agent. Once transferred to 100% RH, no more than 5% of the treated grasshoppers exhibited external mycelial growth.The microenvironment at the cuticular level of the grasshopper allows expression of conidial pathogenicity regardless of ambient RH. Hence, together, these observations suggest the feasibility of field testing with B. bassiana as a bioinsecticide against M. sanguinipes in semi-arid climatic areas.