Several important vegetable crops grown outdoors in temperate climates can be damaged by the root-feeding larvae of Diptera; the larvae of Delia radicum (L.) (cabbage root fly), D. platura (Meigen) (bean seed fly), D. florilega (Zetterstedt) (root fly), D. antiqua (Meigen) (onion fly), D. floralis (Fallén) (turnip fly) and Chamaepsila rosae (Fabricius) (carrot fly). All species damage the root systems of host plants, and some occasionally attack above-ground plant parts as well. This can lead to considerable losses in crop quality and overall yield. Root-feeding fly larvae themselves are difficult targets for insecticidal control because they are in the soil or inside the plant tissue, and non-systemic insecticides applied as sprays do not readily penetrate to reach the target. Adult flies are also difficult targets because there needs to be opportunities for them to acquire a sufficient dose of insecticide, either through direct contact or through contact with treated surfaces. Due to environmental and human health considerations, some formerly effective insecticidal active ingredients have been banned in several European countries, making crop protection more challenging for the growers. Overall, there is a European ambition to reduce pesticide use, articulated in the new EU Biodiversity and Farm to Fork Strategies.
The project has focused on the tools and approaches that might be part of an IPM package to manage root-feeding fly larvae on vegetables and this is particularly 1) biocontrol with entomopathogens and nematodes, 2) physical barriers, 3) combinations of attractants and repellents and 4) the development of a push-pull strategy using trap crops. Underlying all of this is a need to know when pests are present, through crop monitoring and weather-based forecasts. With the exception of monitoring and forecasting, very few of the tools are used by growers currently. The research focus was particularly on the cabbage root fly (Delia radicum).
The consortium considered the phenology of the target pests and methods of monitoring and forecasting them. A collaborative study led from Norway showed that most populations of D. radicum tested were of the early-emerging biotype. Methods of monitoring all species are available and similar monitoring approaches are used in all countries. Day-degree and more complicated forecasting models are available or being developed for all species. The more complicated tools require updating and this is underway for the German SWAT models, but not for the UK models, which need a software upgrade. The likely impacts of climate change have been explored for both C. rosae and D. radicum. The effects, or possible effects, of climate change on C. rosae are already apparent in Switzerland and the UK, where the life-cycle appears to be getting closer to the situation in south-western France.
Research in France identified the best varieties of Chinese cabbage to be used as a trap crop in a push-pull design against the cabbage root fly, D. radicum, together with a spatial design that could optimize the effect of this trap crop on the colonisation of broccoli plants by this pest. Volatiles that could be used to manipulate this colonisation behaviour in such a design were unreliable in their effects in the field, but other non-volatile compounds were identified that show good potential. The most effective compound was tested in a large-scale push-pull trial. In this experiment the compound was sprayed onto young broccoli plants (i.e. target crop) surrounded by Chinese cabbage strips (i.e. trap crop). This product displayed a very promising potential to reduce D. radicum oviposition but further experiments on product formulation, quantity used, persistence etc. need to be conducted before use in commercial fields.
The activities in WP3 focused on interactions between the plants, biological control agents such as entomopathogenic fungi (EPF) (Denmark) and entomopathogenic nematodes (EPN) (Ireland), and the target pest D. radicum. Research produced new insights about practical application methods for EPF and EPNs, and the effects on control of D. radicum larvae under realistic cultivation regimes were evaluated. New knowledge was obtained concerning the stimuli that influence host-plant finding by D. radicum when plants are, or are not, inoculated with EPF, and the application methods showed promise as a “kill” component in combination with a trap crop strategy. Research in the UK showed that the broad range of temperature regimes under which EPF are active demonstrates their versatility independent of origin. Temperature conditioning of EPN did not seem to improve mortality and crop damage, but adjustments of application rates and EPN concentrations may enhance control efficacy against D. radium larvae in the field.
Other control methods were investigated, both as individual practices and in combination. The tactics investigated included a large field experiment in Norway with cauliflower as the main crop and the following methods to reduce attack by D. radicum: trap cropping (Chinese cabbage as trap), repellent+trap crop, exclusion fences, trap crop+exclusion fences. There was a clear reduction in attack rate in the treatments with exclusion fences with or without trap crop, compared to all other treatments. Greenhouse and field experiments in Switzerland investigated the repellent effects of sage extracts in different formulations and in combination with other IPM tools. Sage extracts were tested in different combinations with an attractant (rutabaga juice), fungal biocontrol agents and insect netting. The sage extracts greatly reduced the number of D. radicum eggs laid and the number of larvae that developed on the plants in the greenhouse experiments. The fungal biocontrol agent also reduced the number of larvae and pupae in the greenhouse experiments, as well as the amount of damage in the field experiments. Insect netting was the most beneficial control method in the Swiss field experiments. In an experiment in the UK several biological insecticides were compared either as pre-planting or at planting applications for control of D. radicum. Root and foliage weight were greater and root and stem damage was reduced by pre-planting treatment with spinosad (produced from metabolites of soil bacteria) or azadirachtin (from extracts of the neem tree). The combination of a reduced dose of spinosad and a fungal control agent gave slightly less effect and the fungal treatment alone showed little difference from the control. Field trials in Germany in 2018 and 2019 investigated fungi and nematodes as biological control agents for D. radicum. The 2019 trials added a trap crop and a repellent to combine the elements of push-pull. The timing of the trials was according to the SWAT forecasting model, in order to test the benefits of using this model in an IPM strategy. The 2018 trial showed a significant reduction in D. radicum survival in the plots treated with the fungal biocontrol. Overall, the applied biocontrol agents were not very effective in controlling D. radicum. The 2019 trial showed that the different methods or "IPM tools" could easily be combined to develop a push-pull strategy. However, more research is needed to optimise conditions to favour the biological control agents.
Of the tools that have been evaluated, there seems to be real potential for improved forecasting systems for several species. In addition to the current use of crop covers to exclude certain pests, vertical barriers seem to be a good option for management of D. radicum in certain situations. Whilst the push-pull technique requires further work before it is commercially-viable, there is real potential to use it as part of an IPM strategy for D. radicum in future. Interestingly, although this research has confirmed that the mortality of D. radicum larvae achieved by the fungal pathogens tested is ‘too late’ to protect the current crop, it could be applied to a trap crop to improve its function as a ‘dead end’.
The consortium members have engaged with stakeholders in Europe through publications and events and papers have been prepared for a wider audience, some of which are published, some under review or in press, and some in preparation. Engagement with stakeholders and knowledge exchange will continue after the end of the project. A number of Masters students have been involved in the research. In conclusion, whilst few of the approaches considered will be taken immediately into commercial practice the project has identified routes that individual countries might pursue to develop effective strategies and increase the uptake of IPM.
