Backyard Gardening

By Katelyn Smith

While we’re familiar with garden pests like the spongy moth and spotted lantern fly, a relatively new threat to Pennsylvania’s backyard gardeners is crucial to share. This pest, known as the Allium leafminer (Phytomyza gymnostoma), or ALM, was first identified in Lancaster County, Pennsylvania, in 2015. If you’re a fan of growing Allium crops like onions, garlic, leek, scallions, chives, and ramps, it’s essential to familiarize yourself with the ALM.

Originally from Europe, this pest has rapidly spread across several states, including Pennsylvania, Connecticut, Delaware, Maryland, Massachusetts, New Jersey, New York, Virginia, and Washington, DC (Cornell College of Agriculture and Life Sciences).

The first step is to learn to identify them. Adults are gray or black flies, measuring about 3 mm long. They have a yellow-orange spot on the top of their head and yellow spots on their legs near the joints. Larvae are white to yellow headless maggots up to 8 mm long. Pupas are small and brown and measure 3.5 mm long. Eggs are white and 0.5 mm long. As the adults feed on the leaves, they leave straight lines of white spots. Females will lay their eggs in some feeding scars (University of Massachusetts Amherst).

The life cycle of the ALM begins with the pupae overwintering in the plant tissue of allium crops or the surrounding soil. As spring arrives, the pupae transform into adult flies. According to Penn State Extension, the emergence of this pest coincides with the blooming of forsythia bushes and daffodil flowers (University of Massachusetts Amherst).

Knowing when adult ALM starts their journey is crucial in protecting your plants. You can begin by applying netting or row covers before their journey starts in the spring and remove them at the end. A second option is to use an insecticide. When applying, follow the instructions and wear appropriate gloves, glasses, and long sleeves.  According to results from a Penn State study, participants who used insecticides achieved reasonable control. You should wait about two weeks after detecting the feeding scars to begin treatment (Penn State Extension).

Check your allium crops for puncture holes around September-October. These indicate that the leafminer has begun feeding on them. Some gardeners have found yellow sticky cards helpful in trapping the adult flies (Maine Organic Farmers and Gardeners).

ALM will infest many Allium crops, but its risk for attack varies during the season. Crops like scallions, ramps, garlic, chives, and ornamental allium are more likely to be infested in the spring. Onions have a better chance of surviving this pest. Those crops are at a high risk in the fall because they have now developed a large leaf canopy, which will attract the females. 

The feeding scars created by the female ALM and mining by larvae can cause significant cosmetic injury or deformity to chives and scallions. Once feeding scars are established, pathogenic fungi and bacteria have an open invitation to cause your crop to rot. By monitoring the emergence of ALM in your area, you can plan and decide on a control method. There are various ways to detect the tentative timing of adult ALMs and their egg-laying period.

According to field and lab-based research at Penn State University, the best prediction method for the ALM is based on the accumulation of 350 growing degree days or GDD. To do this, you’ll want to familiarize yourself with weather data from NEWA (Network for Environment Weather Applications). The website is: https://newa.cornell.edu/. Select the weather tools tab, select the degree day calculator link, select the station of interest, select the start date as January 1 and end date as the current date, and select the degree day type as 1℃. The GDD calculator tool plots growing degree units which measure heat accumulation in order to predict plant and insect development (Climate Smart Farming: Cornell University). 

Katelyn Smith has been a certified Penn State Extension Master Gardener since 2023. She can be reached at clintonmg@psu.edu or the Clinton County Master Gardener Hotline at (570)-858-0198.