Birch tar oil is an effective mollusc repellent: field and laboratory experimentsusing Arianta arbustorum (Gastropoda: Helicidae) and Arion lusitanicus(Gastropoda: Arionidae)

Abstract: Populations of two molluscs, the land snail Arianta arbustorum and the Iberian slugArion lusitanicus, have increased substantially in many places in the northern Fennoscandia inrecent years. In this study birch tar oil (BTO), a new biological plant protection product, wastested against these molluscs. We examined whether 2 types of BTO, used either alone, mixedtogether, or mixed with Vaseline®, could be applied as a repellent against 1) snails when paintedon a Perspex® fence, and 2) slugs when smeared on pots containing Brassica pekinensisseedlings. Both the fences and the pots with seedlings were placed in each field with a highpopulation of the target organism. The BTO – barriers were effective in repelling both snails andslugs. However, the repellent effect of BTO alone against the molluscs was short-term. Repeatedtreatments were required to keep the slugs away from the plants and we found that the intervalbetween treatments should not exceed two weeks. Most noticeably, the BTO+Vaseline® mixtureprevented the land snails from passing over the treated fences for up to several months. Theresults of these experiments provide evidence that BTO, especially when mixed with Vaseline®,serves as an excellent long-term repellent against molluscs.