Earthworm mediated dispersal of baculovirus occlusion bodies: experimental evidence from a model system.
The soil is the most important reservoir of baculovirus occlusion bodies (OBs) in the environment. The ability of the earthworm Eisenia fetida to transport OBs of Spodoptera frugiperda multiple nucleopolyhedrovirus was examined in laboratory terraria filled with an artificial soil. OBs were detected in soil samples using a soil-diet incorporation bioassay, for which the 50% lethal concentration was estimated at 2.7×106 OBs/g soil in S. frugiperda second instars. Incubation of earthworms in soil containing 109 OBs for 7 days did not result in a significant loss of OB virulence compared to soil without earthworms. The earthworm intestine was found to be slightly acidic, with acid-base indicators applied to lengths of dissected intestine suggesting a pH of 6.0-6.3. Despite their epigean habits, E. fetida individuals were observed to form burrows up to 22.5 cm deep in laboratory terraria. Soil-diet bioassays indicated the presence of OBs at depths of 6-8, 14-16 and 22-24 cm in samples taken at 1, 7 and 14 days following the application of 109 OBs to the surface of terraria containing earthworms. In contrast, OBs were only detected in samples from the soil surface in terraria without earthworms. We conclude that earthworms likely affect the distribution and dynamics of OB populations in soil habitats.