Abstract

The objective of this study was to examine the effect of interacting conditions of water stress (0.995-0.96 water activity; a_w), elevated temperature (25-37°C) and CO₂ (350, 1000 ppm) on growth and sporulation of strains of three entomogenous fungi, Beauveria bassiana, Metarhizium anisopliae and Isaria farinosa. Subsequently, using bioassay systems with locust (Schistocerca gregaria), we examined the effect of elevated CO₂ (control, 350; 650; 1000 ppm CO₂) on efficacy of strains of all three species and used crickets (Acheta domesticus) to examine interacting conditions of elevated temperature and CO₂ at two relative humidities (25-35°C; 350, 1000 ppm CO₂; 96, 98 and >99% RH) on efficacy of a strain of B. bassiana for the first time. The 3-way interacting factors had a significant effect on growth of the strains of all three species, especially at 35-37°C and 0.96-0.98 a_w and 1000 ppm CO₂. Under these conditions, only one strain of B. bassiana and M. anisopliae was able to grow at a reduced rate as compared to the controls. No strain of I. farinosa was able to grow at 35-37°C either in normal air or in elevated CO₂ at 0.995-0.96 a_w showing a high level of sensitivity to these interacting factors. Sporulation of the three strains of each species was also significantly affected by these three-way environmental interactions. There were some intra-strain differences and in most cases for the three species, water stress (0.98-0.96 a_w) at 35-37°C and 1000 ppm CO₂ resulting in either no sporulation or no growth. One strain of M. anisopliae (Ma 29) was particularly tolerant at 0.96 a_w at 37°C and 1000 ppm CO₂. Bioassays with the S. gregaria showed when CO₂ was elevated from 350 to 650 and to 1000 ppm, the relative virulence of two strains of each species was reduced over a 6-day temporal study. Further studies with B. bassiana in a detailed bioassay using crickets under three way abiotic interactions (25-35°C, 99-96% RH and  350 or 1000 ppm CO₂) showed that virulence was decreased with no efficacy occurring at 30-35°C and 1000 ppm CO₂ at 96% RH. This study suggests that climate change factors could have a profound impact on the efficacy of such biocontrol agents and thus have major implications for pest control using such approaches.

Key words: Water stress, temperature, elevated CO₂, growth, sporulation, entomopathogenic fungi, pest control.