Studies on Artioposthia triangulata (Dendy) (Tricladida: Terricola), a predator of earthworms

Observations had linked the disappearance of earthworms from a grass field to the presence of Artioposthia triangulata. An experiment demonstrated that this land planarian could severely and quickly reduce numbers of four earthworm species. Two different approaches to sampling A. triangulata were investigated on a grassland site. The first used four different trap types (wood, ceramic tile, 'corriboard' plastic and 5 mm polystyrene beneath a ceramic tile) measuring 15 cm x 15 cm which were compared over a period of 18 wk. The second used one, or two, applications of dilute formalin solution to quadrats followed by counting the residual planarians by hand-sorting the soil beneath the quadrat to a depth of 30 cm. The polystyrene traps were the most effective for detecting the presence of A. triangulata. No planarians were ever found by hand-sorting after two formalin applications and it was concluded that formalin sampling provided a good estimate of the population density. Counts under traps were not related to absolute estimates of population densities so polystyrene type traps should only be used for detection of the planarians. A multiple regression relationship of A. triangulata weight over time in 5% formalin is presented and allows the original liveweight to be calculated from the weight in preservative. Planarian and earthworm population densities in eight fields were sampled and the planarians were found to be randomly distributed. Counts of planarians in 0.25 m² quadrats were not related to numbers of earthworms but overall field population densities were. It is concluded that A. triangulata is a severe threat to the earthworm populations of Northern Ireland.