Bipalium kewense ~ Flatworm, Planaria
The flatworm Bipalium kewense is increasingly common throughout the US. Related species are a serious problem in the London area. Bipalium kewense is native to Malaysia and the Orient. They likely came to this country in the soil of imported plants. These terrestrial Planaria are predators of earthworms. They can destroy entire earthworm populations, resulting in great harm to the soil structure. Some of the Japanese species are reported to kill chickens if the chickens eat them. The flatworms will reproduce by division if they are cut or broken. The bodies are very soft and easily damaged, resulting in multiple viable pieces. They are about the diameter of an earthworm but can be 18" long. The flatworms can be found in moist conditions under rocks. They come to the surface if the soil becomes saturated. They seem most likely to be found in damp soil conditions. They in fact encourage these conditions by eliminating the organisms that would aerate the soil. Thus far there are no biological remedies on the market, though many nematodes are controlled by the entangling mycelae of fungi. It is thought by many that the soil fungi are harmed by urea based fertilizers. This line of thinking would match with the conditions at the places that I have found these pests.
High resolution photos of Bipalium kewense are part of our garden image collection