Mar 11, 2020

Allium Leafminer’s first emergence of 2020 tracked

Allium leafminer attacks plants in the Allium genus including high-value crops such as onion, garlic, leek, scallions, shallots, and chives.

Allium leafminer (ALM), Phytomyza gymnostoma (Diptera: Agromyzidae), is an invasive leafmining fly from Poland first detected in Lancaster County, Pennsylvania in December 2015. ALM (Figure 1, above) has two generations per year. It overwinters as a pupa in leaf tissue or adjacent soil, emerges in the spring, and adult flight occurs over 4-5 weeks. Females puncture leaves with their ovipositor, and both males and females feed on leaf exudates. 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.

In order to more effectively manage ALM, with the support of the Pennsylvania Vegetable Growers Association (PVGA), we scouted throughout southeast and central Pennsylvania for this pest in the Spring of 2019 and combined that with lab work, to develop a degree-day model for predicting when first emergence will occur.

We estimate that ALM first emergence occurs after a degree-day accumulation of 250°D with a minimum temperature threshold of 3.5°C. In Centre County, this was when daffodils and forsythia had been blooming for about a week, and ornamental pear was in bloom in urban areas. In 2019, ALM emerged in the Southeast (Landisville and Pottstown) on April 16 and on April 25 (Penn’s Valley) in the center of the State, and the degree-day accumulation with a 3.5°C minimum temperature threshold on March 6, 2019, was 20.8°D in Landisville and 8.50°D in State College.

The months of January, February, and March in 2020 have been much warmer than last year. As of March 6, 2020, the degree-day accumulation with a minimum temperature threshold of 3.5°C is 89.8°D in Landisville and 57.0°D in State College. Based on current weather forecasts and 30-year temperature averages, we predict allium leafminer will emerge in southeastern Pennsylvania on April 7, 2020, and on April 21, 2020, in central Pennsylvania.

However, we may continue to experience warmer temperatures than historical (30-year) averages, in which case emergence will occur earlier than today’s predictions. We will continue to update this projection as future weather forecasts become more accurate, and we draw closer to the expected date of ALM first emergence.

Above, Figure 2. Oviposition and feeding scars on a leek leaf. Photo: Brandon Lingbeek, Penn State

– Brandon Lingbeek and Shelby Fleischer, PH.D.