Effect of differences in leaf litter from organic and conventionally managed apple orchards on the growth and reproduction of Metaphire hilgendorfi (Haplotaxida: Megascolecidae): Verification by a breeding experiment

Fertilization and pesticide use in conventional farming greatly affects the soil invertebrate community structure. A recent study reported that megascolecid earthworms were more abundant in an organic apple orchard with no fertilization or agrochemicals compared with an adjacent, conventionally managed apple orchard in Japan. Understory plant and leaf litter compositions also largely differed between these orchards. Under conventional farming practices, monocotyledonous species were dominant, with only a few dicotyledonous species, whereas both monocotyledonous and dicotyledonous species were dominant in the organically farmed orchard. Here, we examined the effect that differences in leaf litter had on the growth and reproduction of megascolecid earthworms to elucidate why megascolecid earthworms dominate in organic farms. Breeding experiments were conducted for 12 weeks using the important megascolecid earthworm Metaphire hilgendorfi (Michaelsen, 1892). We used four leaf-litter treatments: 1) monocotyledonous leaf litter from conventional farming, 2) monocotyledonous leaf litter from organic farming, 3) dicotyledonous leaf litter from organic farming, and 4) no leaf litter. We found that earthworms in treatments containing monocotyledonous leaf litter from conventional farming or dicotyledonous leaf litter from organic farming were heavier after 9-12 weeks than those in the other treatments. Cocoon viability was lower in the conventional farming treatment than in treatments with leaf litter from organic farming. Survival rates and the number of cocoons did not differ among the leaf-litter treatments. Our results indicate that dicotyledonous leaf litter from organic farming is the most suitable food resource for M. hilgendorfi, suggesting that organic apple orchards are dominated by megascolecid earthworms because they provide adequate dicotyledonous leaf litter.