Risk screening and management of alien terrestrial planarians in The Netherlands

Worldwide over 910 terrestrial planarian species have been described. They mainly occur in tropical and subtropical regions. In Europe, 22 alien terrestrial planarian species have been recorded over the last decades. In The Netherlands, 9 alien species have been found so far, mostly in greenhouses. Three of these species have established populations in gardens (i.e., Marionfyfea adventor, Caenoplana variegata and Parakontikia ventrolineata). Alien terrestrial planarians that consume earthworms and are established outdoors can have a negative impact on biodiversity and soil quality by reducing earthworm populations. Their impact on earthworm populations can be high, but is difficult to assess due to limited knowledge of the feeding patterns and ferocity of most terrestrial planarian species. Risk assessments for The Netherlands carried out with the Harmonia+ scheme shows that only the New Zeeland land planarian Arthurdendyus triangulatus scores high for potentially risks due to its ability to significantly reduce earthworm densities. This species has not yet been found in The Netherlands, but already occurs in the United Kingdom, Ireland, and Iceland. Obama nungara obtained a medium risk score and all other species a low risk score. Due to the limited information about terrestrial planarians and their potential impact, the certainty of most risk scores is low to moderate. Therefore, it is recommended to update their risk assessments periodically based on new information about their invasion biology. Phytosanitary measures can limit the unintentional import of alien planarian species.