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Allium Leafminer Adults Emerging Now in Southeastern Pennsylvania

Allium Leafminer Adults Emerging Now in Southeastern Pennsylvania
By Timothy Elkner and Shelby Fleischer
In past years, allium leafminer (ALM) emergence started around April 18, and this year we used a model that predicted emergence would start in southeastern PA by April 6, based on temperatures that had occurred until March 6, plus climatological averages projected beyond that date. We were updating this model with real-time temperatures and lower baseline thresholds, which was moving the date earlier (as early as March 29). However, we confirmed adult activity in Lancaster, York, and Perry counties on March 17—a month prior to results from recent years, and 12 days earlier than our current models.
The first signs of ALM are the linear series of round white dots on allium leaves. You can also distinguish the adult fly by the orange patch on the head, and the wings folded horizontally over its back. The flies tend to be found at the tips of the leaves in the morning.
Note the linear white dots caused by the female allium leafminer. Photo: Brandon Lingbeek, Penn State
ALM attacks plants in the Allium genus including high-value crops such as onion, garlic, leek, scallions, shallots, and chives. ALM 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.
The white dots are made by the female with her ovipositor. Both males and females feed on leaf sap, and egg-laying occurs at this same time. Larval development progresses to the pupal stage but is then delayed as the pupa undergoes summer aestivation (a resting period with little to no development), and they do not emerge again until late September for another 5-7-week flight.
Control measures are only needed during the adult flight period to target adults and developing larvae, and very shortly after to target any remaining developing larvae. Control can be achieved with row covers during the adult flight, or insecticides. We’ve obtained good control with both conventional and organic options.
Among the organic options, using approximately weekly applications, Entrust has worked well. Aza Direct has also given some control, but not as consistently as Entrust. Avoid Pyganic – we’ve seen it result in higher infestations than untreated controls.
Among conventional options, Scorpion, Exirel, and Radiant have all given excellent control, with fewer applications. Alliums have a very waxy leaf, so including a spreader-sticker, or a soap product for organic production, is a good idea.
However, you may be able to escape damage in the spring, depending on the crop and timing of both the crop and pest. We’ve not seen much damage into the bulbs with sweet onions if the egg-laying is limited to the outer leaves that will become the scale leaves at harvest. Scallions seem to be preferred, and since you are marketing the leaf tissue, scallions are at risk of economic damage. We’ve seen egg-laying scars and leaf-mining to garlic and have had grower reports of significant crop loss to garlic.
A good place to look for signs of ALM is on wild garlic, and on actively growing scallions if you have them on your farm.
Allium leafminer on scallion. Note the yellow patch on the head and the white dots indicating egg-laying activity. Photo: Brandon Lingbeek, Penn State
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