CHAMBERSBURG, Pa. — Keep a lookout for Allium leafminer on any garlic or early-planted alliums you may have this season. The unusually mild weather we’ve had throughout this winter is leading to earlier emergence than we have seen in recent years. Evidence of leafminer activity has been detected in western Maryland for a few weeks so we expect they’ve likely begun to emerge in south central and southeast PA, as well.
Allium leafminer (ALM), is an invasive fly from Poland first detected in Lancaster Co., Pennsylvania in December 2015 (Fig. 1). ALM attacks plants in the Allium genus including onion, garlic, leek, scallions, shallots, and chives. It overwinters as a pupa in leaf tissue or adjacent soil, emerges in the spring, and adult flight occurs over a 5-7-week period. Females puncture leaves with their ovipositor and both males and females feed on leaf sap. Oviposition results in a characteristic linear series of round wounds (Fig. 2). Larval development progresses to the pupal stage but is then delayed as the pupa undergoes summer aestivation, and they do not emerge again until late September for another 5-7-week flight.
Knowing when adults start flying helps with management. You can protect crops by applying netting prior to flight and removing it after a flight-period ends. In most of PA, now would be the time to get the netting put on. If you’re in the warmer portions of southeast PA, intial flight may have started, but netting could still protect the plants during the time most of the flight occurs. In past years, spring flight occurred over about 5-7 weeks. The flight period may be extended this year if we keep having intermixing of warm and cool (or near normal) weather.
Another option is to apply insecticides during the flight period, targeting adults and developing larvae. Systemics tend to work best because the larvae are mining inside the leaf tissue. Since Allium leaves are very waxy, a nonionic surfactant is recommended whenever applying insecticides to allium crops to help the product spread across and stick to the leaf surface. In trials conducted by researchers at Cornell University and Penn State, the highest and most consistent control of ALM occurred using foliar applications of dinotefuran (Scorpion), cyantraniliprole (Exirel) and spinetoram (Radiant). For OMRI-listed options, spinosad (Entrust) was the best performer. Those trials used weekly applications starting as soon oviposition / feeding marks are detected. More recent trials suggest that very good control can be achieved by waiting about 2 weeks after first detection.
That said, knowing when to start is an important question! Through a growing degree day model we’ve developed, we estimate that ALM first emerge in the spring after 350° Celsius degree days above a threshold of 1°C, have been accumulated starting from January 1. This tends to coincide with when daffodils and forsythia had been blooming for a week and ornamental pear is in bloom in urban areas. If you have internet access, you can use the Northeast Weather Associate to estimate when this occurs for select weather stations. Go to https://newa.cornell.edu/, click ‘Weather Tools’, click ‘Degree Day Calculator’, choose a site, set the start date to Jan 1 and end date to today, and select base of 1° C as the degree day type.
If you don’t have internet access, you can use the bloom period of forsythia, daffodils, and ornamental fruit trees as a rough estimate or call your local extension office for assistance predicting exact GDD accumulation. In Hagerstown, MD which is the closest NEWA site to the southern portion of the Cumberland Valley, we reached 350°C around March 17th. This is slightly ahead of when first emergence occurred in the past (Table 1) which has ranged from the 17th of March to the 27th of April in PA. In years with warm winters, we have seen some exceptionally early initial emergence, about 10 days before our prediction, but most of the population emerged much closer to our prediction. In Halifax, PA, which is the closest monitoring site to the northern half of the Cumberland Valley, we have only accumulated 277°C so far and are likely not experiencing emergence yet. On the other side of the mountains, we are forecasted to hit 350°C in Biglerville, PA on March 25th
In warmer regions of PA, and within a few weeks in cooler areas, now is the time to start scouting your Allium crops and prepare for management. Finding adults is easiest in the cool temperatures of early morning and by looking at the tops of the leaves. Finding the feeding scars on leaves is often easier than finding adults, especially on onions and when scouting during windy conditions. Most of our first detections were in wild garlic, or other weedy alliums, along fence lines or forested borders of farms. Among Allium species, we tend to find more ALM in early spring scallions and green onions so that is also a great place to search.
For help predicting ALM emergence or for questions about management in the south central portion of PA, feel free to reach out to Karly Regan at the Franklin Extension Office at 717-263-9226. For other regions, reach out to your local extension office and they’ll be able to find someone to help you.
–Karly Regan, Tim Elkner, and Shelby Fleischer, Penn State Extension