Measurement of internal Beauveria bassiana to ascertain non-target impacts on arthropods in field environments

Abstract

ABSTRACT Effects of Beauveria bassiana on non-target arthropods were assessed following field application on rangeland and alfalfa agroecosystems in Canada. Densities and prevalence of B. bassiana within non-target arthropods were measured by two methods, surface sanitation of non-target cadavers followed by homogenisation and plating on selective medium, or incubation in a high humidity environment. From rangeland, numbers of B. bassiana colony-forming units (cfu) in spiders, and carabid and tenebrionid beetles were low (≤2.0 × 103 cfu/ individual) and none of the 2500 arthropods incubated in high humidity were colonised. In alfalfa, increases in numbers of B. bassiana cfu in homogenised specimens were observed only in the first four days in coccinellid beetles (Coccinellidae) while high cfu numbers persisted in some harvestmen (Phalangidae). Laboratory assays with coccinellids indicated that recovered B. bassiana emanated from ingested conidia within the alimentary tracts rather than from hyphal bodies within hemocoels. A relatively low prevalence of non-target arthropods from which B. bassiana was isolated in rangeland and alfalfa coupled with the decrease in numbers of internal cfu as a function of time indicated that the fungus imparted a minor and short-lived impact on non-target arthropods. Although B. bassiana did not affect leafcutting bee larvae, prepupae or adult emergence following overwintering diapause, a small number of foraging bees were infected under field conditions. Measuring internal fungal populations on selective medium or incubation of cadavers in a high humidity environment were found to be convenient and efficient methods for safety assessment to non-target arthropods of a mycopesticide applied under field conditions.