Effectiveness of electroshocking and subsurface barriers when manipulating earthworm populations

Background/Question/Methods

Invasive earthworms dominate assemblages in many North American landscapes.  Feeding and burrowing activities of earthworms influence physical and biological soil processes, which can lead to far-ranging effects on the structure of microbial, plant, and animal communities.  Researchers often wish to investigate earthworm effects on soil properties or other organisms by creating conditions in which earthworms are absent.  Such studies frequently are relegated to microcosm experiments because of logistic constraints inherent in removing and excluding earthworms in a field setting without disturbing physical and chemical soil properties.  Our goal was to test the efficacy of removing earthworms from experimental plots lined with fiberglass screen using an electroshocking extraction method.  In the spring of 2008, we established 66 one m² experimental quadrats within four forest sites at the Smithsonian Environmental Research Center (SERC), Maryland, USA.  Earthworm treatments included removal, addition, and unmanipulated control quadrats.  A fiberglass screen barrier was installed around the perimeter of the removal and addition quadrats from 80 cm belowground to 10 cm aboveground.  Aluminum rods were installed to a depth of 50 cm in all treatments.  During shocking, alternating current was run from a portable generator through the rods, causing earthworms to come to the surface. 

Results/Conclusions

Earthworms were removed from removal quadrats nine times between the spring of 2008 and spring of 2010.  On four occasions, all 66 quadrats were shocked to measure density and biomass.  Averaging across sites and occasions, density within removal quadrats (116 ind m^-2) was 33% lower than the density in addition quadrats (174 ind m^-2); whereas biomass was 41% lower within removal quadrats (30.7 g m^-2 versus 51.8 g m^-2).   Density and biomass in control quadrats was similar to addition quadrats.  The treatments did not affect the smallest earthworms (29% density reduction, 35% biomass reduction) to the degree they affected the largest earthworms (56% reduction in density and biomass).  Upon conclusion of the study, we observed significant root penetration through the fiberglass screens through which earthworms likely could pass.  Thus, this method may be useful to researchers who wish to significantly reduce earthworm density in the field.  A stronger barrier may help increase the effectiveness of the removal treatments.