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.
Les ceufs de la pyrale du mais, Ostrinia nubilalis (Hfibner), sont tr~s sensi-"bles h Metarhizium anisopliae (Metsch.)/Sorokin et Paecilomyces fumosoroseus (Wize) (Brown-Smith). Toutes conditions 6gales par ailleurs, les premiers stades larvaires meurent plus rite mais dans une proportion plus faible que les stades plus ~g6s. Beauveria bassiana (Bals.) (Vuillemin) n ~ 147 et M. anisopliae n ~ 139 sont particuli~rement pathog~nes pour ce stade tandis que P. fumoso-roseus attaque facilement les chrysalides. Par adjonction d'une dose faible de chlorpyriphos on sensibilise les larves h la mycose.Les chenilles diapausantes sont tr~s sensibles ~ la mycose puisque m6me des doses tr~s faibles (40 sp/cm 2) tuent plus de 50 % de la population.
A simple experimental device has been described for controlling relative humidity (RH) using saturated salt solutions. This device was suitable for studies requiring a 14–82% RH (± 3%) at 27 ± 0.7°C. The RH and temperature set points were not affected by the presence of grasshoppers. Perturbations of 1.5 min due to daily insect provisioning introduced a 9- to 60-min disturbance of the RH and temperature equilibria in the chambers containing NaCl and LiCl, and a 7.5-h disturbance in the MgCl2 chamber.
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