Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu
The new pest of onion, leek and garlic, the Allium leafminer, is active now in our area. This new pest was first found in Lancaster County Pennsylvania in December 2015. It has since been found in Maryland in only a few northeastern counties, but my guess is that the pest is probably in many northern/central areas of Maryland. New transplants or seedings of onions or leeks should be watched closely for the tell-tale signs of the fly’s damage which are several very small dots in a row along the leaf of an allium plant (Fig. 1). Figure 1 is an excellent picture by Sarah May of Penn State, that not only shows what and where the feeding is observed on a plant but also the relative size of the oviposition/feeding damage and what you should look for. Penn State has a great deal of good information about the new pest which can be found at: Penn State Allium Leafminer Pest Alert page. Figure 2 shows the adult female as she is making the incisions into the allium leaf causing the white spots. Growers should look for these tell-tale signs or the fly itself on any newly planted allium species. You can cover any new allium plantings with row cover to keep the flies off or treat with insecticides.
Figure 1. Oviposition/feeding spots (red circles) on onion transplants from Allium leafminer
Figure 2. Allium leafminer female adult on onion leaf