Evaluating a Mustard Extraction Technique for Sampling Earthworms

We examined efficacy of a mustard extraction technique for quantifying earthworm occupancy and density using a mesocosm experiment. In May 2017, we placed 12 earthworms from 2 ecological groups (6 surface dwelling [epigeic] and 6 deep dwelling [anecic]) into mesocosms (0.144 m² surface area) constructed from soils typically found in northern hardwood forests of Michigan, USA. We evaluated 2 soil depths (15 and 30 cm) and a soil moisture gradient (dry to wet). We found occupancy of epigeic earthworms was accurately portrayed 92% of the time through leaf searches, and subsequent mustard phase did not improve accuracy. For anecic earthworms, we found that occupancy was accurately portrayed 50% of the time through leaf searches and mustard phase increased accuracy to 92%. We found that mustard extraction produced 0–83% (median = 50%) of epigeic earthworms available for capture, with most recovered through leaf searches (median = 42%). We found that mustard extraction produced 0–67% (median = 33%) of anecic earthworms available for capture, with most (median = 20%) recovered during the mustard phase. Our results lend support for using mustard extraction as a technique for quantifying occupancy of epigeic and anecic earthworms, but caution that density estimates are likely low.