Infection of insects and persistence of Metarhizium (Hypocreales: Clavicipitaceae) species on apple bark

Entomopathogenic fungi (EPF) are cosmopolitan soil borne pathogens that cause epizootics in various insect orders. EPF isolates of Metarhizium brunneum and M. pinghaense have shown the potential for use as biological agents of important agricultural insect pests. The aim of the current study was to test for the persistence of M. brunneum and M. pinghaense on apple bark over a period of three weeks, under laboratory conditions. Apple bark was sprayed with conidial suspensions of both fungi, at a standard infective conidial concentration of 107 conidia/ml. The persistence, or survival, of the conidia on apple bark was measured using codling moth larvae (CM) (Cydia pomonella) and females of woolly apple aphid (WAA) (Eriosoma lanigerum) as indicator species. The results showed that conidia of M. pinghaense can induce mortality of insect pests through contact with an EPF-treated substrate, with mortality of 39% to 82% for WAA over a period of 10 days post application, and with mortality of 3% to 68% for CM over a period of 7 days, after application to apple bark. Further evaluation showed that the conidia of M. pinghaense persisted longer on apple bark, up to 63%, than did M. brunneum, up to 11%, three weeks post application of the conidial suspensions. The study provides insights into the potential persistence of fungal isolates on apple bark over time post application. Further evaluation of the persistence of the isolates on apple bark under both glasshouse and field conditions should be conducted.