|Wraight, Stephen - Steve|
Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/29/2009
Publication Date: 7/2/2009
Citation: Faria, M., Hajek, A.E., Wraight, S.P. 2009. Imbibitional damage in conidia of the entomopathogenic fungi Beauveria bassiana, Metarhizium anisopliae, and Metarhizium acridum. Biological Control. 51:346-354. Interpretive Summary: The common insect pathogenic fungi Metarhizium and Beauveria comprise more than 70 percent of the fungus-based biopesticides developed worldwide. The active ingredients in most of these products are dry spores (conidia). A 1997 study by British and African researchers showed that dried conidia of M. acridum lost substantial viability following immersion in water, but retained viability if hydrated slowly by humidification prior to immersion. It is well known that when dried organisms are immersed in water, rapid uptake of water (imbibition) can cause lethal damage to cell membranes. Susceptibility of fungi to this type of damage has important implications with regard to their use as pest control agents, yet this problem has received little attention since 1997. In this study, we investigated effects of conidial powder moisture content and water temperature on susceptibility of Metarhizium and Beauveria to imbibitional damage. We also examined the potential of oil formulation to slow imbibition and reduce damage. In general, the drier the conidia and the colder the water, the greater the imbibitional damage. Dry Metarhizium conidia were highly susceptible to damage, losing substantial viability after immersion at temperatures as high as 25 deg C. In contrast, dry conidia of Beauveria bassiana were highly resistant to damage, losing substantial viability only after mixing in ice-cold water. Damage was largely preventable in all cases by using warm (33-34 deg C) water for hydration or by hydrating the conidia slowly in a humid environment prior to mixing. Damage was also reduced when conidia were formulated in oil. This study underscores a need to develop standardized methods for handling these fungi during experimental and commercial pest control applications.
Technical Abstract: When dried organisms are immersed in water, rapid imbibition may cause severe damage to plasma membranes; in unicellular organisms, such damage is usually lethal. This study investigated effects of pre-immersion moisture levels and immersion temperature on imbibitional damage in three insect pathogenic fungi. Conidial powders of Beauveria bassiana (Bb), Metarhizium anisopliae (Ma) and M. acridum (Mac) were dried/hydrated to water activities of 0.02-0.96 prior to immersion in water at 0.5-33 deg C. Imbibitional damage increased with decreasing water activity of the conidia and decreasing temperature of the immersion water. Germination of dry Metarhizium conidia was reduced to less than or equal to 5 percent after immersion at 0.5 deg C and 60 percent following immersion at 15 deg C. Ma was most susceptible to damage, with viability of the driest conidia reduced to 66 percent after treatment at 25 deg C. In contrast, Bb was highly resistant to damage, with significant reductions in viability (to levels as low as 43-51 percent) occurring only when dry conidia were immersed at 0.5 deg C. Damage was prevented when conidia were hydrated slowly by humidification prior to immersion and increasing immersion temperature to 33-34 deg C; germination of all fungi was greater than or equal to 94 percent under optimal conditions. However, immersion of the driest Bb, Ma, and Mac powders in water at 33 deg C also resulted in high viabilities (95, 89, & 94 percent, respectively), and slow-hydrated conidia also retained high viability following immersion at 0.5 deg C (87, 92, & 83 percent, respectively). Imbibitional damage was also reduced by formulation of conidia in pure (non-emulsifiable) paraffinic oil. This study underscores a need for establishing standard protocols for preparing aqueous suspensions of sensitive fungi for research and commercial applications.