Living with the Enemy: How to Minimise the Negative Effects of the Invasive New Zealand Flatworm on Small-Scale Food Production

Pest species management is key to sustainable crop production. Many pests directly suppress crops, while others have an indirect influence through modification of the soil environment. The New Zealand flatworm (Arthurdendyus triangulatus) is one such organism as it preys predominantly on earthworms, with the latter essential for soil health. During the 1990s the species expanded rapidly, reaching areas away from known source populations including allotments, crofting and other agricultural land. It is the flatworm-induced decline in earthworm populations with associated hydrological (water logging) and soil productivity (reduced aeration and bioturbation) impacts which are deemed of greatest societal and economic consequence. Ways to mitigate the impact of NZ flatworms on earthworms and their soil environment are now called for.

In 2015 we launched a UK-wide NZ flatworm survey www.opalexplorenature.org/nzflatworm. This led to the most up-to-date distribution of the species, demonstrating further expansion, but also many detailed accounts of its history in allotments – a habitat where NZ flatworms flourish but where biologists typically do not work – and numerous views on how to control flatworms and promote earthworms. A subsequent pilot study revealed strikingly large differences in earthworm density within two large allotments despite long-term and extensive presence of NZ flatworms in both places. The ways individual plots were managed (level of soil disturbance) and how this influenced the presence of refugia for both earthworms (grassed-over area) and flatworms (plastic matting, woodchips) appeared to be key to differences in earthworm density.

A targeted survey of allotments across the UK is proposed to determine the spread of NZ flatworms and scale of the problem. Time and pathway of introduction will be analysed and models developed to explain spread and severity of impacts on earthworms, providing a richer data source on the predator-prey dynamics of NZ flatworm and earthworms than could possibly be obtained by long-term experiments (PhD Paper 1). Through semi-structured interviews with allotment holders, the student will gain systematic and well-evidenced insights into the impacts NZ flatworms have and the response of allotment holders in terms of gardening practices. This will elucidate differences as to why some allotment holders have maintained earthworm populations in the face of NZ flatworm presence, whereas others have not (PhD Paper 2). Building on the insights gained from working with allotment holders, replicated field trials will be conducted to manage the land in the presence of NZ flatworms by assessing the removal of refugia, no-till approaches, liming, and earthworm-promoting measures (PhD Paper 3). Knowledge from the allotment studies will then be applied to crofting land in remote rural communities, where the existing prevalence of the NZ flatworm and spread to nearby pasture land is a significant socio-economic concern. Ways to mitigate the impact of NZ flatworms on earthworms and their soil environment will be studied. This will become increasingly relevant to farmers, who will encounter NZ flatworms (and absence of earthworms) more frequently, yet at present may have little awareness of this pest species and how to mitigate through specific land-use measures (PhD Paper 4).