Breaking news from Jerry Brust, UME IPM Vegetable Specialist:
"Hello Everyone...another new pest--the Allium (or onion) leafminer has been found in Pennsylvania. It seems to be worse in organic systems and in home gardens, but it is something we should watch for over the summer and fall. It seems to like leeks the best, but will infest any allium spp. Its population appears to build through the spring and summer and can become quite high on some farms that do not spray any insecticides."
Link to a full IPM report with photos from Penn State Extension: http://ento.psu.edu/extension/vegetables/pest-alert-allium-leafminer.
The report is for commercial growers (organic and conventional). Home gardeners in MD should focus on scouting for this pest and reporting any finds to HGIC. Covering early spring alliums with floating row covers will help exclude the allium leafminer and other pests, such as onion maggot.