Two probable infestations of allium leafminer have been discovered. One of these was on the Hunterdon/Warren County border involving overwintered garlic and onions planted this spring. The second, a very minor infestation, was found near Princeton in Mercer County on overwintered chives. While we await word from USDA as to where specimens should be sent for final confirmation, the signs of infestation are very characteristic of allium leafminer, and larvae and eggs were found in association with the egg laying scars (oviposition scars) on the leaves of onions and garlic. Both of these infestations appear to be very recent, with larvae in very early growth stages.
It is likely that this pest is more widespread in the state than was assumed, given that we only became aware of it in PA several weeks ago. The most serious damage from this pest is likely to be from rot-causing organisms that enter plant tissue via wounds caused as the miners burrow down the foliage and pupate near the bulbs. The easiest way to determine whether this pest is present is to observe upper leaf tissue for characteristic signs of oviposition (see photos by Sabrina Tirpak). This will consist of pale circular-to-oval scars in straight lines near the edges of foliage on flat leafed hosts like garlic and leeks. On onion leaves, the scars might be anywhere on the leaf surface, but will usually be closer to the tip.
It is not known the extent to which this pest could be an economic threat this season. The current known infestations appear light, but reports from Europe, where the pest is endemic, indicate that it can be quite serious. Therefore, we would advocate regular scouting of onion and related crop fields. Look for the appearance of oviposition scars. If these are found, it may be prudent to make an insecticide application. Adult (fly) activity should decline after 2-3 more weeks, according to available information on this pest. After this point, larvae will pupate and remain dormant for much of the summer. Flies will emerge in late summer and early fall to infest the fall crop.
A list of useful insecticides was posted in the previous allium leafminer alert on May 2nd. These include Entrust (spinosad), Radiant (spinetoram), Scorpion (dinotefuran) and Triguard (cryomazine). All are labeled for leafminer on onion. Be sure to check labels for specific crops.