Influence of oscillating temperatures on the competitive infection and colonization of the migratory grasshopper by Beauveria bassiana and Metarhizium flavoviride

A study was conducted to determine the effects of temperature on the competitive infection and colonization of grasshoppers (Melanoplus sanguinipes) by Beauveria bassiana and Metarhizium flavoviride applied alone and in combination, Nymphal mortality and proliferation of the fungi in the hemocoel were measured in four environments with the same mean daily temperature (25 degrees C) but differing in the degree to which they oscillated daily (constant 25 degrees C, 20 to 30 degrees C, 15 to 35 degrees C, or 10 to 40 degrees C). For nymphs inoculated with B. bassiana and to a lesser extent M. flavoviride, mortality decreased as the amplitude of temperature increased. Fungal populations in the hemocoel of nymphs increased over time in all environments, but fewer colony-forming units (CFU) were recovered from nymphs as the degree of temperature oscillation increased. Densities of CFU were significantly correlated with mortality, Both B. bassiana and M. flavoviride were isolated from the hemocoel of nymphs that were coinoculated, but coapplication of the two fungi did not significantly affect the prevalence of mortality. In some instances total fungal populations in the hemocoel of nymphs coinoculated with B. bassiana and M. flavoviride were smaller than those in nymphs treated with each fungus alone, indicating a degree of interspecies antagonism, Furthermore, more B. bassiana than M. flavoviride CFU were recovered from coinoculated nymphs at constant 25 degrees C, but as the amplitude of temperature increased, populations of M. flavoviride increased relative to B. bassiana, suggesting that temperature influenced their competitiveness. The results of this study indicate that M. flavoviride is superior to B. bassiana against grasshoppers under conditions of oscillating temperature and that the advantage of applying B. bassiana in combination with M. flavoviride will primarily be when daytime conditions are overcast and/or cool. (C) 1999 Academic Press.