Enhancing the Thermotolerance of Entomopathogenic Isaria fumosorosea SFP-198 Conidial Powder by Controlling the Moisture Content Using Drying and Adjuvants

Abstract

Entomopathogenic fungi are promising pest-control agents but their industrial applicability is limited by their thermosusceptibility. With an aim to increase the thermotolerance of Isaria fumosorosea SFP-198, moisture absorbents were added to dried conidial powder, and the relationship between its water potential and thermotolerance was investigated. Mycotized rice grains were dried at 10℃, 20℃, 30℃, and 40℃ and the drying effect of each temperature for 24, 48, 96, and 140 hr was determined. Drying for 48 hr at 10℃ and 20℃ reduced the moisture content to < 5% without any significant loss of conidial thermotolerance, but drying at 30℃ and 40℃ reduced both moisture content and conidial thermotolerance. To maintain thermotolerance during storage, moisture absorbents, such as calcium chloride, silica gel, magnesium sulfate, white carbon, and sodium sulfate were individually added to previously dried-conidial powder at 10% (w/w). These mixtures was then stored at room temperature for 30 days and subjected to 50℃ for 2 hr. The white carbon mixture had the highest conidial thermotolerance, followed by silica gel, magnesium sulfate, and then the other absorbents. A significant correlation between the water potential and conidial thermotolerance was observed in all conidia-absorbent mixtures tested in this study (r = -0.945). Conidial thermotolerance in wet conditions was evaluated by adding moisturized white carbon (0~20% H2O) to conidia to mimic wet conditions. Notably, the conidia still maintained their thermotolerance under these conditions. Thus, it is evident that conidial thermotolerance can be maintained by drying mycotized rice grains at low temperatures and adding a moisture absorbent, such as white carbon.

VOLCANIC MATERIALS AS CARRIERS FOR THE FORMULATION OF MYCOINSECTICIDES USING THE FUNGUS Beauveria bassiana

“…Conidia stored with these materials and exposed to 50 ºC for two hours showed no significant decrease in viability, while the germination was reduced by 10, 17 and 23% for unformulated conidia and conidia stored with silica gel and or with zeolite, respectively. It is surprising that those materials with higher hydric retention capacity, i.e., silica gel and zeolite, did not enhance the thermotolerance, because other studies have shown that this property may protect spores when exposed to high temperatures (McClatchie et al, 1994;Moore et al, 1996;Kim et al, 2014a). It is possible that the higher proportion of material used in this study has influenced conidial viability.…”

“…Some authors have observed that the addition of silica gel to oil formulations of M. anisopliae favors conidial viability due to the capacity of this material to adsorb humidity (McClatchie et al, 1994;Moore et al, 1996). Other studies have shown that the use of different desiccant materials enhances the conidial thermotolerance of the entomopathogenic fungi Isaria fumorosea and correlate this with water potential (Kim et al, 2014a). Thus, it is important for the ingredients in the formulation to have properties that favor the high temperature resistance of infective units and that can maintain suitable humidity conditions.…”

“…These materials were selected based on their stability, abundance and low cost, as well as for their hydric retention capacity, which may enhance conidial thermotolerance. In addition, non-indicating silica gel (SiO 2 ) was used because of its known desiccant properties and reported capacity to enhance conidia viability (Moore et al, 1996;Kim et al, 2014a). Silica gel has an amorphous microporous structure with a distribution of pore opening sizes of approximately 3-60 angstroms.…”

Mixtures between Beauveria bassiana and potassium silicate to manage thrips in tomato plants for industrial processing

“…Bell & Hamal (1974) reported that B. bassiana, Metarrhizium anisopliae and Metarhizium rileyi fungi spores maintained their virulence to insects after three years stored at -20 o C in tubes containing silica gel crystals (a synthetic product made by the silicate reaction of sodium and sulfuric acid) compared to those without silica gel. Kim et al (2014) have also confirmed higher thermotolerance mediated by Si compounds, through moisture retention, for the insect-parasitic fungus Cordyceps fumosorosea. The higher resistance to fungus applied under field conditions may have potentiated the degree of synergism between Bb+KSil observed in the present work in terms of CE%.…”

“…This synergism has been demonstrated through different interactions, including the breakdown of lipid barriers in the insect cuticle by the K 2 SiO 3 , facilitating the fungi penetration and colonization of the insect body (Storm et al, 2016). The synergism between the fungus B. bassiana and K 2 SiO 3 increased the mortality of the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae), in mono and dicotyledonous plants (Gatarayiha et al, 2010), and thermotolerance and pathogenicity of fungus spores (Kim et al, 2014).…”