Time to Prepare for Protecting Allium Crops From Allium Leafminer

Allium leafminer (ALM) attacks plants in the Allium genus including onion, garlic, leek, scallions, shallots, and chives (Figure 1.) It overwinters as a pupa in leaf tissue or adjacent soil, emerges in the spring, and adult flight occurs over a 4-to-5-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) which is unlike any other insect damage in allium crops in our area. 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. Left: Allium leafminer fly and feeding damage on leek leaf showing adult size. Right: Allium leafminer feeding damage on onion leaves. Photos: Tim Elkner, Penn State

Knowing when adults start flying and laying eggs helps with management. You can protect crops by applying netting prior to flight and removing it after flight-period ends. Flight periods have been about 5-7 weeks (to be safe, remove netting about 8 weeks after flight starts). Another control 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 surfactant is recommended whenever applying insecticides to these crops. Based on several years of research trials, the highest and most consistent control of ALM occurred using foliar applications of dinotefuran (Scorpion), cyantraniliprole (Exirel), and spinetoram (Radiant), and spinosad (Entrust) among OMRI-labeled options. Those trials used weekly applications. Preliminary data suggest fewer applications are as effective, but we don’t yet have good data about what schedules will work best.

But when to start—with nets or insecticides—is a question we are often asked by growers. With the support of the Pennsylvania Vegetable Growers Association (PVGA), we scouted throughout the southeast and central Pennsylvania in early spring and combined that with lab work and scouting data from New York to develop a degree-day model for predicting when the first emergence will occur. We estimate that spring ALM first emergence occurs after a degree-day accumulation of 350°D above a threshold of 1°C, starting from January 1. In the State College area, this was when daffodils and forsythia had been blooming for about a week, and ornamental pear was in bloom in urban areas.

Using FAA data from Lancaster, PA, as of March 25, 2021, the degree-day accumulation above 1°C since January 1 is 208, which is 142 degree-days short of predicted emergence at 350 degree-days. During the last week, there has been an average of 8.2 degree-days per day. Assuming this will increase to about 10 per day, we will reach 350 in about 14 more days, or two weeks. So, a rough estimate for predicted emergence in the Lancaster area is April 8, or sooner. This is in the ballpark of when first emergence occurred in the last few years (Table 1) which has ranged from the 17th of March to the 27th of April. Of course, this is just the best guess. In years with a very 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. Also, our current model uses a statistical method that gives a range of what might be the best lower threshold and degree-day accumulation requirement. Further work is needed to refine these parameters.

Regardless of our modeling caveats, now is the time to start scouting your Allium crops and prepare for management, if needed. Finding adults is easiest in the cool temperatures of early morning and 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.

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