Slug

Terrestrial gastropods are pervasive pests, especially of agriculture in temperate regions. While current management options exist, they primarily consist of baited pellets with active ingredients such as methiocarb or metaldehyde. These pesticides are highly effective at controlling slug populations and feeding damage incurred on crops, but they are also now widely accepted to have devastating non-target effects on other macroinvertebrates, mammals, birds, and invertebrate communities in nearby soil and waterways. With the use of methiocarb currently being suspended by the European Union and metaldehyde recently disallowed in the United Kingdom, and other available methods (e.g., ferric phosphate pellets, inundative biological control using the parasitic nematode Phasmarhabditis hermaphrodita) demonstrating inconsistent efficacy, there is a clear and pressing need for the development of additional, sustainable control strategies for pestiferous slugs. Knowledge of marsh flies (Diptera: Sciomyzidae) being natural enemies of molluscs has garnered growing interest since the first early observations of the family in the 1950s. This diverse, globally distributed family is composed of a multitude of species which have evolved to be associated with one or a small suite of mollusc species, either as predators or parasitoids. Many of these are associated with semi-aquatic snails and have been the focus of biological control aimed at reducing the intermediate vectors of filarial diseases. A small subset of the Sciomyzidae, however, have evolved to target terrestrial slugs. Tetanocera elata is a species possessing a wide Holarctic distribution (including Ireland), and is an obligate parasitoid and predator of Deroceras reticulatum (Stylommatomorpha: Agriolimacidae) in the larval stage; both of which make this species a prime candidate for use in slug control. Previous research has addressed essential questions of T. elata biology and behaviour, such as the determination of optimal rearing conditions of laboratory cultures and describing predating behaviour of larvae on D. reticulatum. However, before T. elata can be realistically considered as a biological control agent, the existing knowledge base requires expansion in several key areas. The research presented in this thesis addresses many of these areas. Of primary importance is an understanding of the biological requirements of T. elata at each life stage. While previous investigations are predominantly focused on larval requirements (as this is the life stage which affects pest slugs), almost nothing to date on adult requirements has been investigated. Such requisites were examined to determine dietary components utilised by T. elata (and other marsh fly species) in nature. Studies presented here discovered that adult marsh flies feed on hemipteran honeydew. Another important factor for the evaluation of T. elata as a biological control agent is a determination of its prey range and preference, in order to avoid potential non-target effects. Choice and no-choice behavioural assays were conducted to determine the realised prey range of predaceous larvae, with additional investigation into the physiological consequences of the utilisation of alternative prey species on larval survivorship. Larvae showed no clear preference for prey species, however attacks were more efficacious on D. reticulatum. Finally, in an effort to optimise the establishment of T. elata populations within agroecosystems, an evaluation of habitat requirements was undertaken. This study addressed potential ecological associations of T. elata with plant community as well as structural features such as plant growth forms and field boundaries, finding that T. elata presence were correlated with hedgerow proximity and height and percentage cover of dead vegetation. These results will form the basis for any future ecological management recommendations. Overall, the findings presented here contribute to the progression of realising the full ecological potential of T. elata for pest control and propose how this research may be applied in an ecological management and conservation biological control context.2023-09-2