Metarhizium robertsii protease and conidia production, response to heat stress and virulence against Aedes aegypti larvae

Abstract

Nutritional factors exert signifcant infuence on the growth of entomopathogenic fungi, one of the main agents employed commercially in the biological control of arthropods. Thus, the objective of this work is to optimize the culture medium and solid fermentation time for production of proteases and conidia of Metarhizium robertsii ARSEF 2575 and to evaluate the interference of ribofavin and salts on virulence and resistance to abiotic stress factors. In the frst step, nine groups were separated: negative control, positive control, and seven supplementation groups: ammo nium nitrate, ammonium chloride, potassium nitrate, sodium nitrate, ammonium sulfate, ammonium phosphate, urea. Sodium nitrate showed signifcant diference in protease production at the time of 20 days of solid fermenta tion. Then, diferent concentrations of sodium nitrate and ribofavin as supplement were evaluated. Response surface methodology demonstrated that ribofavin and sodium nitrate infuence proteolytic activity and conidia produc tion, but without synergism. Supplementation of the medium with the optimal concentration of sodium nitrate and ribofavin did not interfere with the germination of conidia without exposure to abiotic stress, but did increase the thermotolerance of conidia. The presence of ribofavin and sodium nitrate at optimal concentrations in the culture medium did not alter fungal virulence with and without exposure to heat stress, varying according to the presence or absence of the supernatant during exposure, evidencing that resistance to heat exposure is multifactorial and dependent on intra- and extracellular factors. Moreover, the supplementation increased the larvicidal activity of the supernatant against Aedes aegypti.