2022 Pest Alert: Allium Leafminer Emerging Soon

Allium leafminer (ALM) (Figure 1) 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 (Figure 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.

Figure 2. Oviposition and feeding scars. Photo: B. Lingbeek, Penn State

Knowing when adults start flying helps with ALM management. You can protect crops by applying netting prior to flight and removing it after the flight-period ends. Another option is to apply insecticides during the flight period, targeting adults and developing larvae. Systemics with an added surfactant tend to work best because the larvae are mining inside the leaf tissue. Since Allium leaves are very waxy, a surfactant is recommended whenever applying insecticides to allium crops. Results from our research trials found that the highest and most consistent control of ALM occurred using foliar applications of dinotefuran (Scorpion), cyantraniliprole (Exirel), spinetoram (Radiant), and spinosad (Entrust) among OMRI-labeled options. Those trials used weekly applications starting as soon oviposition/feeding marks were detected. More recent trials suggest that very good control can be achieved by waiting about two weeks after the first detection.

But when to start? With the support of the Pennsylvania Vegetable Growers Association (PVGA) and USDA, we developed a degree-day model. We estimate that spring ALM first emerge after 350°Celsius degree-days above a threshold of 1°C, starting from January 1. It's a statistical estimate from field data, a bit rougher than most phenology models, which use controlled lab work, but it's the best we've got because of knowledge gaps for rearing ALM (see Lingbeek et al 2021). In State College, this was when daffodils and forsythia had been blooming for a week, and ornamental pear was in bloom in urban areas.

You can use the Northeast Weather Association (NEWA) to estimate when this occurs for select weather stations.

• Go to NEWA, click 'Weather Tools', click 'Degree Day Calculator', pick a site, insert the start date of Jan 1 and end date of today, and insert the degree day type as 1°C. This is important because you need to accumulate in °C instead of °F for the 350° threshold. If you use °F, the conversion will mean we need ~650 degree-days (dd).
• You get a chart and below that a cumulative graph. Figure 3 is the graph for Biglerville on March 29, 2022. We are predicted to reach 305 dd by April 3—about 45 dd below 350 and accumulating at 10–15 dd per day. So, on April 3, we are about three days shy of the first emergence using the 15/day rate.
• Thus, emergence in Biglerville will potentially start around April 6-7. This is in the ballpark of when the first emergence occurred in past (Table 1) which has ranged from March 17 to April 27. Of course, this is just the best guess. In years with a warm winter, we saw some exceptionally early initial emergence, about 10 days before our prediction, but most of the population emerged much closer to our prediction.

Starting now in southeast PA, and within a few weeks in cooler areas, it is the time to start scouting your Allium crops and prepare for management. Finding adults is easiest in the cool temperatures of early morning—look 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, which is also a great place to search.

Figure 3: Accumulated Celsius degree days above a base threshold of 1°C, at Biglerville, PA. ALM emergence is predicted to start at 350°C degree days.

Table 1: Date of the first detection of ALM adults or feeding/oviposition wounds in 2019 and 2020.

Citation for Phenology

Lingbeek, B., D. Roberts, T. Elkner, M. Gates, and S. J. Fleischer. 2021. Phenology, development, and parasitism of Allium Leafminer (Diptera: Agromyzidae), a recent invasive species in the US. Environmental Entomology 50(4): 878-887.

Citations for Insecticide Management

Nault, B. A., L. E. Iglesias, R. S. Harding, E. A. Grundberg, T. Rusinek, T. Elkner, B. Lingbeek and S. J. Fleischer. 2020. Managing Allium leafminer (Diptera: Agromyzidae): an emerging pest of allium crops in North America. J. Econ. Entomol. 113 (5): 2300-2309.

Nault, B. A., K. R. Sandhi, R. S. Harding, E. A. Grundberg, and T. Rusinek. 2022. Optimizing Spinosyn Insecticide Applications for Allium Leafminer (Diptera: Agromyzidae) Management in Allium Crops. J. Econ. Entomol. doi.org/10.1093/jee/toac016