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Amanda Quadrel – March 20, 2025 – Plant and Pest Advisory
Happy first day of Spring! Many parts of New Jersey are close to accumulating 250 growing degree
days (GDDs) base 39˚F, which is when the first emergence of Allium leafminer (ALM) adults is predicted
to occur. In some areas of New Jersey, 250 GDDs are forecasted to be surpassed as early as
this weekend. In Cape May, oviposition scars have already been spotted. If you would like to know
how many GDDs have accumulated in your area, the NEWA website https://newa.cornell.edu/ has a
helpful GDD calculator. From the NEWA home screen, select the nearest weather station from drop
down menu at the center of the page. Next, scroll down to “Weather Tools” on the right side of the
page, and select “DD Calculator”. Select your start date (1/01/25) and end date, as well as Degree
Day Type (39 F) from the menu at the left of the page. The site will automatically generate the
accumulated GDD base 39F to the last day of your requested sample, and then offer a forecast of
accumulated GDD for the next week.
ALM can cause injury to chives, scallions, garlic, onions and leeks, but leeks and scallions seem to
be the most effected. Look for neat rows of white spots descending from the upper tips of allium
leaves. Initial injury often occurs on the tallest leaves. Adults may be seen in warmer conditions at the
tips of the leaves- look for small flies with grey-black bodies and yellow heads.
If you are concerned about crop injury due to Allium leafminer, 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. Adults can be targeted through insecticide sprays. Appropriate
materials that are labeled for ALM control include spinosyns (Radiant, Entrust (OMRI approved)),
pyrethroids (Mustang Maxx, Warrior), neonicotinoids (Scorpion, Venom), and the insect growth
regulator Trigard. The first application should be applied 2- 3 weeks after initial detection of ALM flies
(if you see oviposition scars or the fly itself) and subsequent sprays should be completed in 1–2-week
intervals. Two or three sprays should provide adequate control.
If you have any questions/concerns, feel free to reach out to me at amanda.quadrel@rutgers.edu or
732-742-8441.