Abstract
The objective of this study was to examine the effect of interacting conditions of water stress (0.995-0.96 water activity; aw), 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 aw 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 aw 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 aw) 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 aw 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.
Highlights
There has been significant interest in the impact that abiotic change scenarios may have on economically important crops and the associated pests and diseases. most climate change models suggest that thereBorisade and Magan will be a marked decrease in summer precipitation and increases in temperature, which will result in related drought stress episodes interspersed with periods of unusually high precipitation depending on the part of the world (European Commission, 2007; Solomon et al., 2009; Chalcraft, 2009)
The objective of the present study was to examine the effect of interactions between aw, temperature and elevated CO2 on (a) growth and (b) sporulation of three strains each of B. bassiana, M. anisopliae and I. farinosa
For M. anisopliae there was a marked difference in growth between one of the strains (Ma29) and the others in both normal atmospheric air and elevated CO2, regardless of aw and temperature treatment
Summary
There has been significant interest in the impact that abiotic change scenarios may have on economically important crops and the associated pests and diseases. A recent study has predicted that, on a global scale, pests and diseases are moving to the poles at the rate of 3-4 km/year (Bebber et al, 2013). The possible implications this may have for the development of strategies to minimize pest and fungal pathogens of staple crops, especially the use of biological control agents has not been addressed. The objective of the present study was to examine the effect of interactions between aw, temperature and elevated CO2 on (a) growth and (b) sporulation of three strains each of B. bassiana, M. anisopliae and I. farinosa. The cricket was chosen for this bioassay because it survived the degree of abiotic stress factors being examined in the absence of fungal inoculum and is a useful bioassay system to examine the impact of the abiotic change scenarios on efficacy of such entomopathogenic biocontrol agents
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