Production and quality of conidia by Metarhizium anisopliae var. lepidiotum: critical oxygen level and period of mycelium competence.

Mycoinsecticides application within Integral Pest Management requires high quantities of conidia, with the proper quality and resistance against environmental conditions. Metarhizium anisopliae var. lepidiotum conidia were produced in normal atmospheric conditions (21 % O2) and different concentrations of oxygen pulses (16, 26, 30, and 40 %); conidia obtained under hypoxic conditions showed significantly lower viability, hydrophobicity, and virulence against Tenebrio molitor larvae or mealworm, compared with those obtained under normal atmospheric conditions. Higher concentrations of oxygen (26 and 30 %) improved conidial production. However, when a 30 % oxygen concentration was applied, maximal conidial yields were obtained at earlier times (132 h) relative to 26 % oxygen pulses (156 h); additionally, with 30 % oxygen pulses, conidia thermotolerance was improved, maintaining viability, hydrophobicity, and virulence. Although conidial production was not affected when 40 % oxygen pulses were applied, viability and virulence were diminished in those conidia. In order to find a critical time for mycelia competence to respond to these oxidant conditions, oxygen pulses were first applied either at 36, 48, 60, and 72 h. A critical time of 60 h was determined to be the best time for the M. anisopliae var. lepidiotum mycelia to respond to oxygen pulses in order to increase conidial production and also to maintain the quality features. Therefore, oxygen-enriched (30 %) pulses starting at 60 h are recommended for a high production without the impairment of quality of M. anisopliae var. lepidiotum conidia.