Greetings from the Veg IPM team, and happy first day of Spring! As we are slowly transitioning into warmer weather, now is a great time to be thinking about Allium Leafminer (ALM), a pest of many bulbous crops such as onions, scallions, chives, and garlic.
Allium Leafminer (Phytomyza gymnostoma) is a small grey-bodied fly with a yellow head (A). In warm, still weather, you may see adult flies settling on foliage. The females lay eggs in neat rows, leaving behind a line of white dots on the leaves (B). As the larvae hatch and begin to feed, they will tunnel downwards, causing damage to leaves and bulbs. ALM have two generations per year; the first in early spring, and the second occurring in the fall. Crops such as chives, scallions, garlic, onions and leeks are attacked by ALM, but leeks, and scallions seem to be the most affected.
An Allium Leafminer adult (A) and the characteristic oviposition scars left behind after females lay eggs (B). Photos by Maria Cramer and Amanda Quadrel
Be on the lookout for oviposition scars on both crop alliums and wild alliums such as wild onion/garlic and ramps. You can also use yellow sticky cards to monitor for adult presence in the field. However, another way to the predict the first generation’s activity is by using growing degree days (GDDs). The first emergence of ALM adults occurs around 250 GDDs (Base temperature: 39˚F). Below is a table that shows the number of GDDs that have accumulated from January 1st to March 19th (the date this post was written), as well as the number of GDDs predicted to accumulate by March 24th.
| Location | GDDS for 3/19 | GDDs for 3/24 (forecast) |
| Northern NJ (Chester) | 102 | 129 |
| Central NJ (Cream Ridge) | 162 | 202 |
| Southern NJ (Upper Deerfield) | 187 | 231 |
While ALM risk is currently low for northern and central NJ, areas in southern NJ could reach 250 GDDs by the end of next week, thus being at a higher risk for ALM activity. To find out how many GDDs have accumulated in your specific area, the NEWA website: https://newa.cornell.edu/ has a helpful GDD calculator. From the NEWA home screen, select your nearest weather station from the drop down menu or map at the center of the page. Next, scroll down to the “Weather Tools” section on the lower right side of the page, and select “DD Calculator”. Select your start date (1/01/26) and end date (typically the day you check), as well as Degree Day Type (Base 39 F) from the menu at the left of the page. The site will automatically generate a table of the number of accumulated GDDs for the specified date range and a forecast of predicted GDD accumulations over the next five days.
If you are concerned about crop injury due to ALM, you may want to prepare for control efforts. Floating row covers, kept on until the first flight ends (around the end of May) can restrict ALM’s access to vulnerable plants. Reflective mulches may also provide some additional crop protection. Adults and larvae can be targeted through well-timed insecticide sprays. Materials labeled for ALM control include spinosyns (IRAC 5- Radiant or Entrust (OMRI approved)), pyrethroids (IRAC 3A- Mustang Maxx, Proaxis, and Warrior II), neonicotinoids (IRAC 4A- Scorpion, Venom), the insect growth regulator Trigard (IRAC 17), and diamide products (IRAC 28- Exirel, Minecto Pro). The newly registered product Vertento (IRAC 30) may also be used for ALM control. However, growers who have trouble managing onion thrips may want to save this active ingredient for thrips sprays, as there is a limit of two applications per season. The first spray application of a material should be completed 2- 3 weeks after initial detection of ALM (either oviposition scars or the fly itself). Subsequent sprays should be completed in 1–2-week intervals. Two or three sprays should provide adequate control. As always, be sure to follow label rates and application instructions for any pesticide that you plan to use.
For more pictures and detailed information on the biology and management of ALM, Cornell has an excellent fact sheet that can be found here: https://cals.cornell.edu/integrated-pest-management/outreach-education/fact-sheets/allium-leafminer. In the meantime, we will keep our eyes open for any ALM activity in our respective areas and provide updates in the next coming weeks. We are looking forward to warmer weather and a productive season!
~Amanda Quadrel and Maria Cramer, Senior Program Coordinators- Vegetable IPM
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