Parasites of terrestrial gastropods : prevalence of nematodes and trematodes, and effects of a commercially available nematode
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- Master's theses (INA) 
The invasive slug Arion vulgaris was first recorded in Norway in 1988, and has since become an agricultural pest and a serious nuisance for many garden owners. It may cause substantial damage to certain crops, herbs and ornamental plants. Furthermore, it is reported to hybridize with the native species Arion ater. Many traits are likely contributing to A. vulgaris’ success, such as its ability to utilize a wide range of food sources, rapid reproduction, robustness when facing unfavourable conditions, and a lack of natural enemies. The slug parasitic nematode Phasmarhabditis hermaphrodita has lately become much used in efforts to control the invasive species. A slug control product containing this nematode, Nemaslug®, produced by BASF Agricultural Specialities Ltd., is commercially available and used in many Norwegian gardens. However, scientific studies have shown that this nematode mainly kills the youngest A. vulgaris, but is effective for controlling another pest, Deroceras reticulatum. Several non-target gastropod species have also been reported to be susceptible. The first objective of this study was to investigate if the success of the invasive slug is influenced by a lack of parasites. Its nematode and trematode prevalence and number was compared to that of three native gastropod species. A total of 61 A. vulgaris, 47 Arion fasciatus, 54 Arion circumscriptus, and 65 specimens of the snail Arianta arbustorum from two sites in south-eastern Norway were collected, dissected and searched for parasites. A. vulgaris turned out to have the highest prevalence of both parasite groups. With 44 % being nematode infected, its nematode prevalence was significantly higher than that of two of the native species. Its trematode prevalence of 74 % was significantly higher than all native species. It seems safe to conclude that parasite release is not a reason for the success of A. vulgaris in Norway. The second objective was to test the effects of P. hermaphrodita on the two non-target species A. fasciatus and A. arbustorum, compared to A. vulgaris, one of the main target species for the commercial product. In a 20-day bioassay, five specimens of each of the three species were exposed to soil containing a high dose of nematodes (128/cm2), five were exposed to a low dose (64/cm2), and five were unexposed controls. Gastropods were weighed before and after, fed with 4x4 cm cabbage leaf squares, and finally dissected. This was followed by another identical bioassay. Mortality, infection, weight change and amount cabbage eaten was noted and compared. The non-target species A. fasciatus was most affected, with 100 % infection, 60 % mortality and near complete feeding inhibition. A. vulgaris also experienced 100 % infection, but only 20 % mortality and not much feeding inhibition. Only 35 % of A. arbustorum were infected and none died. The most severe effect on this species was a complete feeding inhibition in the high dose treatment. It also seemed to be actively avoiding the nematode infested soil by going into hibernation. Application of P. hermaphrodita in fields and gardens could potentially harm local populations of A. fasciatus, while A. arbustorum populations are not likely to be much affected.
60-studiepoengs masteroppgave av Henrik Antzée-Hyllseth, ved NMBU. Forekomst av nematoder og ikter i fire sneglearter, samt effekt av en kommersiell nematode på tre av de samme artene sammenlignes.