THE EFFECTS OF TEMPERATURE ON THE PATHOGENICITY OF HEAT-SENSITIVE MUTANTS OF THE ENTOMOPATHOGENIC FUNGUS, BEAUVERIA-BASSIANA, TOWARD THE MIGRATORY GRASSHOPPER, MELANOPLUS-SANGUINIPES

Heat-sensitive (HS) mutants of Beauveria bassiana were shown to kill g rasshoppers at the growth-per missive temperature (PT) of 20 degrees C but not at 30 degrees C or more, the nonpermissive temperatures (NPT) . The migratory grasshopper, Melanoplus sanguinipes, exhibited near co mplete poikilothermy in the bioassay system employed for B. bassiana s trains. Temperature shift, from 20 to 30 degrees C, during the bioassa ys was used to halt the mutant cell development postinfection. The res ultant mortality data were analyzed using median effect plots, Timing of the infection was correlated with the required time at PT for grass hopper death. An incubation period of 4.6 to 6.4 days, postinfection, at PT was needed in order for the values for onset of infection, morta lity rates, and the LT(50) for the three HS mutants to reach those ins ects infected with control strain, GK2016, at the PT. The time of PT e xposure required to ultimately kill half of the test insect population , termed LE(50), ranged from 3.9 to 5.1 days. A minimum time for the i nsect infection at PT, defined by the LE(50) value, is required for in fection to lead to mortality, This time, the initial 4.4 days of infec tion, may reflect certain critical event(s) after which, whether or no t additional fungal growth at NPT occurs, grasshopper death is unavoid able, These observations should help the development of models for pre dicting the outcome of infection processes and elucidation of critical events. (C) 1996 Academic Press, Inc.