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 compet itive infection and colonization of grasshoppers (Melanoplus sanguinipes) b y Beauveria bassiana and Metarhizium flavoviride applied alone and in combi nation, Nymphal mortality and proliferation of the fungi in the hemocoel we re measured in four environments with the same mean daily temperature (25 d egrees C) but differing in the degree to which they oscillated daily (const ant 25 degrees C, 20 to 30 degrees C, 15 to 35 degrees C, or 10 to 40 degre es C). For nymphs inoculated with B. bassiana and to a lesser extent M. fla voviride, mortality decreased as the amplitude of temperature increased. Fu ngal populations in the hemocoel of nymphs increased over time in all envir onments, but fewer colony-forming units (CFU) were recovered from nymphs as the degree of temperature oscillation increased. Densities of CFU were sig nificantly correlated with mortality, Both B. bassiana and M. flavoviride w ere isolated from the hemocoel of nymphs that were coinoculated, but coappl ication of the two fungi did not significantly affect the prevalence of mor tality. In some instances total fungal populations in the hemocoel of nymph s coinoculated with B. bassiana and M. flavoviride were smaller than those in nymphs treated with each fungus alone, indicating a degree of interspeci es antagonism, Furthermore, more B. bassiana than M. flavoviride CFU were r ecovered from coinoculated nymphs at constant 25 degrees C, but as the ampl itude of temperature increased, populations of M. flavoviride increased rel ative to B. bassiana, suggesting that temperature influenced their competit iveness. The results of this study indicate that M. flavoviride is superior to B. bassiana against grasshoppers under conditions of oscillating temper ature 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.