Earthworms as phoretic hosts for Steinernema carpocapsae and Beauveria bassiana: Implications for enhanced biological control

Abstract

Prior research indicated that earthworms may serve as phoretic hosts to entomopathogenic nematodes. Therefore, we hypothesized that biocontrol efficacy of entomopathogenic nematodes could be enhanced in the presence of earthworms based on increased nematode dispersal through the soil. We also hypothesized that earthworms may serve as phoretic hosts to other entomopathogens, such as entomopathogenic fungi. We tested the first hypothesis by comparing the ability of two strains of Steinernema carpocapsae to suppress the pecan weevil, Curculio caryae, in the presence or absence of the earthworm Lumbricus terrestris. Under greenhouse conditions, S. carpocapsae was applied to the soil surface in pots containing C. caryae larvae. We tested the second hypothesis by determining the effects of L. terrestris on dispersal of the entomopathogenic fungus Beauveria bassiana in soil columns. Our results indicated that earthworm presence improved the ability of S. carpocapsae (Sal strain) to control C. caryae below the soil surface. Additionally, S. carpocapsae (All strain) only caused significant C. caryae suppression relative to the controls in the presence of earthworms. B. bassiana dispersal was enhanced by earthworm presence. The fungus was carried through soil by the earthworm; conidia recovered from the earthworm casts remained viable and was pathogenic to Galleria mellonella. Earthworms and other phoretic hosts may assist in dispersal of entomopathogenic nematode and fungal populations in various ecosystems and thereby assist in regulation of insect pests. Additionally, it may be conceivable to combine the application of earthworms with entomopathogens to achieve enhanced levels of inundative or inoculative biocontrol.