In several countries of mainland Europe Phytomyza gymnostoma has become the major pest of Allium spp.. It can infest a high proportion (80-100%) of a susceptible crop (Darvas et al., 1988). Plants can be completely destroyed or reduced in market value. UK garden-grown leeks have been so badly infested that they were completely inedible (CSL unpublished data). 

There is a high likelihood that P. gymnostoma will continue to spread in the UK and will eventually reach commerical premises. Economic damage would be expected on Allium crops, particularly leek and onion crops from autumn sowings. Measures to prevent the spread of P. gymnostoma from the West Midlands, e.g. to the Vale of Evesham, a major leek growing region of England, relatively close by, are likely to be difficult to implement.