Allium Leafminer Emerging Now in Southeastern PA
Leafminer fly with oviposition and feeding scars on scallion. Photo: Tim Elkner, Penn State Extension.
The first signs are the linear series of round white dots on allium leaves. 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. 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. A good place to look for signs of ALM is on green onions and leeks if you have them on your farm.
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. Eggs are laid in allium leaf tissue. 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 the fall.
Control measures are only needed during the adult flight period to target adults, and very shortly after to target developing larvae. Control can be achieved with row covers during adult flight, or insecticides. Foliar applications of Scorpion or Venom (dinotefuran), Exirel (cyantraniliprole), and Radiant (spinetoram) are the most effective at controlling both adults and larvae in the leaves. Chemigation treatments are not effective. Alliums have a very waxy leaf, so including a spreader-sticker in any insecticide application is recommended. For organic control, two consecutive sprays of Entrust(spinosad) mixed with a 1-5% v/v solution of a soap, applied 2-4 weeks after the first detected ALM emergence is recommended. An additional application of pyrethrins plus azadirachtin with an adjuvant (choose a product that does not raise the spray solution above a pH of 6.5) is also needed.
If you plan to use row covers for control, first scout your fields closely to be sure that ALM are not already active in the field, so you do not trap adults under the cover. Row covers need to stay in place until the end of adult emergence which generally lasts 4-6 weeks or until the first hard freeze.
