Allium Leaf Miner, Phytomyza gymnostoma is a pest of onions and their relatives such as chives, garlics and leeks. This fly has spread across Europe and invaded Great Britain in 2002. It has spread to Asia. This year, it has been reported in Pennsylvania.
Adult flies lay their eggs at the base of the onion stem. The larvae feed on the leaves making tunnels or “mines” beneath the out layer of the leaf. As larvae grow, they move down the leaf and can burrow into the onion. The burrowing exposes the interior of the onion to disease that promote rot.
The flies emerge in spring and again in September. Commercial growers will probably be able to control the damage with insecticides, but home graders may find control more difficult. Only gardeners in Pennsylvania are likely to see them this year, but in the next decade gardeners in all of North America are likely to get these invasive pests.