Several important vegetable crops grown outdoors in temperate climates can be damaged by root-feeding fly larvae. All species damage the root systems of host plants, and some occasionally attack above-ground plant parts as well, leading to considerable losses in crop quality and overall yield. Root-feeding fly larvae are not easily amenable to insecticidal control, as their larvae are concealed in the soil or inside the plant tissue, and adults are only killed when targeted by a sufficient dose of insecticide, either through direct contact or through contact with treated surfaces. Due to environmental and human health concerns, some insecticidal active ingredients have been banned in several European countries, while consumers demand ambitious reductions of pesticide use, making crop protection more challenging for the growers.
The project has focused on the tools and approaches that might be part of an IPM “toolbox” to manage root-feeding fly larvae on vegetables and this is particularly 1) biocontrol with entomopathogenic fungi and nematodes, 2) physical barriers, 3) combinations of attractant and repellent olfactory stimuli 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).
Research on the potential of biological control agents produced new insights about practical application methods for entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN), and their effects on D. radicum larvae under realistic cultivation regimes. 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. The broad range of temperature regimes under which EPF are effective confirmed their versatility. Temperature conditioning of EPN did not improve fly control, but adjustments of application rates and concentrations may contribute to enhancing their efficacy against D. radicum larvae in the field.
Progress was made in the identification of the most suited varieties of Chinese cabbage to be used as a trap crop in a push-pull design against the cabbage root fly together with a spatial arrangement that could optimize the effect of this trap crop on the pest’s colonization of broccoli plants. Some non-volatile compounds showed good potential to reinforce the manipulation of colonization behaviour. The most effective compound was tested in a large-scale push-pull trial. 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, although further experiments on formulation, dosage and persistence will need to be conducted before use in commercial set-ups.
Other control methods were investigated, both as individual practices and in combination. For example, in a large field experiment in Norway, trap cropping (Chinese cabbage as trap), repellent and trap crop, exclusion fences, trap crop and exclusion fences were combined to reduce attack by D. radicum on cauliflower as the target crop. There was a clear reduction in attack rate in the treatments with exclusion fences with or without trap crop, compared to all other treatments. The repellent effects of sage extracts in different formulations and together with other IPM tools was tested in the field and in the greenhouse, in different combinations with an attractant (rutabaga juice), EPF 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. 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. 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 a weaker effect and the fungal treatment alone showed little difference from the control. Field trials investigated EPF and EPN as biological control agents for D. radicum. Further trials added a trap crop and a repellent to combine the elements of push-pull. While overall the applied biocontrol agents were little effective in controlling D. radicum, the approach showed that the different "IPM tools" could easily be combined to develop a push-pull strategy. However, more research is needed to optimize conditions to promote biological control.
In general, there is a recognized need for improved forecasting systems for several species, as the efficacy of many of the evaluated methods and tools greatly benefits from optimal timing of application. In addition to the current use of crop covers to exclude certain pests, vertical barriers are 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 induced by the fungal pathogens tested is ‘too late’ to protect the current crop, they may support a push-pull strategy when applied to a trap crop serving 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 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.
