Allium Leaf Miner (ALM)
Allium leaf miner feeding and egg laying injury was reported on chives in southern Cape May County on Thursday. The West Cape May weather station (closest station) has recorded 295 growing degree days (GDD) base 39˚F as of Thursday. 39˚F is close to the lower developmental temperature (38.3˚ F) which our colleagues at Penn State recommend for predicting the first emergence of ALM adults. 295 GDD at this temperature is in excess of the 250 GDD base 38.3˚ F that is the threshold for adult emergence and activity. In recent years, initial feeding/egg laying scars have been detected within a few days of local weather stations recording the 250 GDD threshold. Growers who feel they may be impacted by this pest should check the accumulated GDD from local weather stations. Growers in southern counties may reasonably assume there will be ALM activity in their area soon, if not already happening. To determine GDD from local weather stations, the NEWA website https://newa.cornell.edu/ is very helpful. 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 (1/01/23) 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.
Photo: Sabrina Tirpak. Allium leafminer oviposition scars on onion
Growers should consider initiating the control method of their choice at this time. Affected crops include chives, scallions, garlic, onions and leeks. Look for neat rows of white spots descending from the upper tips of allium leaves (see photo at left). Initial injury often occurs on the tallest leaves. Under warmer, less breezy conditions, adults may be seen near the tips of leaves (see photo of adult at lower right).
Floating row covers, kept on until this flight ends will help minimize access to plants. Insecticide applications targeting adults may be helpful as well, although frequency of applications is uncertain. Spinosyn materials (Radiant, Entrust (OMRI approved)), pyrethroids (Mustang Maxx, 
Warrior), neonicotinoids (Scorpion, Venom), the diamide Exirel (section 2ee recommendation) and the insect growth regulator Trigard are labeled for miner control.
Adult activity and observations of feeding will be reported on in the IPM Update as they occur.
