This is the leek and onion miner fly, which I am battling with on the allotment. According to the Central Science Laboratory the first record of Phytomyza gymnostoma in England was in 2003 when the fly was formally identified on allotments around Wolverhampton. I reckon that it was on my allotment in 1999 at least. I had decided to give up half of my allotment in 1998 and then used the top bed on the allotment to grow onions. I remember someone saying my onions look a little worse for wear. The onions were slowly twisting up.
As I am an organic gardener and there is no effective chemical method of combating this little fly anyway, the way to protect the onions Allium cepa; the leeks Allium porrium and the garlic Allium sativum from Phytomyza gymnostoma is to cover them with a barrier.
The adult flies are about 3mm long so the mesh of the barrier needs to be finer than this. There can be no gaps in the barrier and the best way to construct it is with hoops covered with enviromesh netting.
The dates that I am going to cover the onions and leeks are March 15th 2012 till May 20th. 2012. This should cover the time that the adults are flying. They are probably flying for less time than this but it will ensure that the Alliums are covered for the correct period. The nets will go back on the leeks on the 1st. of September 2011 and come off on the 1st of November 2011 when the adults are no longer flying.
There is some evidence that the Larvae, which is 6mm long, can travel through the soil when the Allium plant it is infecting rots away. Hopefully, burying the enviromesh edges will prevent the Phytomyza gymnostoma larvae from reaching the protected area.
Regardless of how weedy the onions, leeks and garlic get during this period, I will not remove the mesh until the dates listed. I have seen some people's onions when they have removed the mesh for weeding and it is obvious that the fly has reached the plants. I will be watering through the mesh.
That's how I am going to deal with Phytomyza gymnostoma.