Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
The allium leafminer (ALM) or onion leafminer is an invasive species that was detected in Lancaster, PA in 2015. This was the first confirmed detection in the Western Hemisphere. Since this detection, it has spread to New Jersey, New York, and Maryland. It has not been detected in Delaware but is something you should be aware of.
The allium leafminer is known to infest species in the genus Allium. This includes leeks, onions, garlic, chive, shallot, and green onion. The adults, small grey or black flies with a yellow or orange marking on the top and front of head, emerge in late winter through spring (March-May) and begin laying eggs at the base of plant stems. The larvae mine leaves, moving downward to the base of leaves or into bulbs where they pupate. Pupae may move into soil. During the summer months, ALM undergoes diapause in the pupal stage before developing into adults which emerge in the fall (September/October). This generation also attacks Allium spp. before overwintering as pupae.
Here are a couple links from Penn State University and University of Maryland for more information on allium leafminer including pictures of adults, pupae, monitoring, damage, and management:
http://ento.psu.edu/extension/vegetables/pest-alert-allium-leafminer
https://extension.umd.edu/learn/allium-onion-leafminer
If you suspect you have damage or a life stage of the allium leafminer, please contact Steve Hauss at the Delaware Department of Agriculture (302-698-4500 or Stephen.hauss@state.de.us) for official confirmation since this pest has not been detected in Delaware.