Helminths associated with terrestrial slugs in Swedish agricultural fields

Slugs are important agricultural pests causing yearly yield losses. However, parasitizing helminths potentially could affect the size of the slug population. Here, a survey of terrestrial slug-parasitic helminths (nematodes and trematodes) was conducted for the first time in Sweden. In total, 268 terrestrial slugs were collected from 27 agricultural field edges in three seasons over 2020 and 2021 and dissected for presence of helminth parasites. Slugs belonging to the genus Arion were molecularly identified by mitochondrial DNA cytochrome c oxidase subunit I (COI) while parasites were identified using ribosomal RNA (18S). Overall, 13% of the collected slugs had helminth parasites and the likelihood of a slug being parasitized was highest in autumn. Slugs identified as Arion vulgaris were more likely to be parasitized than native slug species. The prevalence of nematodes and trematodes were similar; the dominant species found were Alloionema appendiculatum and Brachylaima thompsoni, respectively. This is the first record of the presence of these two species in Sweden.