Cocoon Heat Tolerance of Pheretimoid Earthworms Amynthas tokioensis and Amynthas agrestis

In the United States earthworms of two Asian species (Amynthas tokioensis and A. agrestis) are leading a northward invasion into temperate forests and horticultural landscapes. Some studies have shown temperature sensitivity in earthworms, but none has explicitly tested the range of heat tolerance for A. tokioensis and A. agrestis cocoons. This study tested the hypothesis A. tokioensis and A. agrestis cocoons would become nonviable when exposed to 55 C in a laboratory setting. Clitellate earthworms of A. tokioensis and A. agrestis were established in replicate incubation cultures to quantify cocoon production rate and to obtain cocoons of known source. Cocoons were then exposed to heat treatments (3 or 15 d at 20, 30, 40, 45, 50, 55, or 60 C) to determine viability after heat exposure. Twelve A. tokioensis and A. agrestis earthworms produced as many as 74 and 82 cocoons in 20 d, respectively (overall mean = 0.14 +/- 0.10 cocoons earthworm(-1) d(-1)). All heat treatments >= 40 C resulted in zero viability (P < 0.001), but the 30 C treatment was no different than the control at 20 C (P = 1.000). These findings were consistent regardless of species, treatment length, or period of incubation during which the cocoons were collected. The threshold of tolerance was between 27.1 C (maximum of the 30 C treatment) and 38.1 C (minimum of the 40 C treatment) for A. tokioensis and A. agrestis cocoons. These data may guide management decisions concerning the spread of A. tokioensis and A. agrestis to new locations.