The ability of four antifungal agents to suppress growth of Aspergillus niger (USDA, ARS Robert T. Gast Rearing Laboratory, Mississippi State, MS isolate) in an artificial diet for Lygus hesperus Knight (Heteroptera: Miridae) was tested by inoculating a standard number of conidial spores into artificial diet with different antifungal agents added and determining the amount of ensuing microbial growth. The effects on biological fitness of L. hesperus also were measured. Measured characteristics of biological fitness included total number of surviving adults, mean biomass (dry weight) accumulated per cage over the total treatment period, egg production, time to adult emergence, and time to the beginning of egg laying. Benzoic acid, high concentrations of formalin, and a high concentration of sorbic acid suppressed the growth of A. niger. Propionic acid and low concentrations of sorbic acid and formalin were less effective at suppressing A. niger growth. Biological fitness was negatively affected in insects reared on diets containing high levels of A. niger growth and in diets with high concentrations of formalin. This study demonstrates how the antifungals that are added to the artificial diet of L. hesperus to control contamination by A. niger must be chosen carefully with consideration not only of their effectiveness at controlling contamination, but also to the tolerance level of the insect to the chemical compound.

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