During feeding, the female will lay an egg in one of the shot holes near the base between the epidermis layers. The larvae that hatch mines toward the leaf edge and then turns inward causing the characteristic comma-shape blotch. When mature, the larvae break the epidermis and drop to the ground where they pupate. Most damage is caused by the larvae mining through the leaflet, but some is caused by the leaf loss during harvesting and feeding by the adult. Most insecticides are ineffective and yield loss is generally minor, so treatment is not recommended.
Early harvest is recommended to expose young larvae to high soil temperatures.
Last year for the first time treatment for alfalfa blotch leafminer with Warrior resulted in significantly higher yields. If you had problems last year and see extensive shot holing, I recommend you experiment with Warrior. Maybe try treating half of a field to see whether it is effective for you. I say this based on severe infestations in the fourth harvest of 1998 and first harvest of 1999, but minor to no infestations in the second, third, and fourth harvest of 1999 at Fargo. The alfalfa blotch leafminer should be just a problem for only a few years and then disappear.