NEW HOLLAND, Pa. — Researchers are starting to understand the ways of the allium leafminer after their first growing season studying the pest.

The leafminer, native to northern Europe, feeds on onions, leeks, garlic, chives, even the ornamental flowering giant allium.

“Anything allium seems to be fair game,” said Tim Elkner, the Penn State Extension educator who in December 2015 documented the leafminer’s appearance in the Western Hemisphere.

The leafminer, first found by a Lancaster County produce grower, has now been found in 17 counties in southeastern Pennsylvania and in Hunterdon County, New Jersey. Elkner has even found it in his home garden.

This wide distribution suggests that the leafminer was in the region for a while before it was reported, Elkner said Monday during New Holland Vegetable Day at Yoder’s Restaurant.

The leafminer scrapes openings in the allium leaves and drinks the sap. The tidy line of dots from the scraping is easy to recognize, Elkner said.

The grower who initially asked Elkner to come to his farm was finding leeks rotting in the field and getting complaints that there was something in the leeks.

Leafminers can damage allium bulbs, reducing onions and leeks to culls or No. 2 product, Elkner said.

In Austria, researchers have found that the allium leafminer has flights in April and September. Elkner and his colleagues found a similar pattern in Pennsylvania last year, with the flies’ activity peaking in mid-April and mid-October.

Those two flights, which bookend the leafminer’s summerlong resting stage, are when farmers should be watching and ready to control the pest, Elkner said.

The leafminer can be distinguished from other flies by the yellow patch on it head. There is a native fly with similar markings, but Elkner did not see it this year.

The leafminer also has yellow on its abdomen. “They’re easy enough to recognize,” he said.

The leafminer is not flighty and will let people get close enough to make an identification. “They are a cooperative fly” in that sense, he said.

Elkner often saw leafminers sitting on the tops of allium leaves.

He has found that the leafminer seems to prefer feeding on larger plants, not tiny ones. That is bad news for spring onions, which are planted early in the season and have some size when the leafminer gets active.

Sweet onions might be planted late enough to escape the leafminer, and garlic growers could push back their planting dates to avoid the fly, Elkner said.

Leeks may face the biggest challenge, as their growing season coincides with the fall leafminer flight, he said.

To avoid the leafminer, one wholesale grower planted and harvested his leeks early. Community-supported agriculture operators may be less willing to move up their leek crop because leeks are a good item to have for the fall-winter transition period, Elkner said.

Farmers could try to fight wild alliums, but wild onions are common enough that control might be difficult. Elkner found leafminer damage on wild garlic on a farm where no allium crops were growing.

Row covers can be used to exclude the leafminer from the crops.

Covers are unrealistic for large plantings, so they might work best for crops planted in small quantities, like chives, he said.

Elkner tested different colored sticky traps in Berks, Lancaster and Lebanon counties to see if the cards attracted the flies.

The yellow trap with a grid pattern seemed to do best, though clear, yellow and black traps were also decent at catching leafminers, he said.

Still, the traps did not work well enough for Elkner to recommend their use for monitoring leafminers. He sometimes found damage or adult flies before he found any leafminers on the traps.

Farmers could use a small planting of spring onions as an indicator crop to track when the leafminers are active in their fields. They could also target sprays to the spring onions, Elkner said.

Insecticides available for the leafminer include pyrethroids, Radiant and, for organic growers, Entrust. Scorpion is also labeled, but Elkner does not have experience with using that chemical against leafminers.

In 2016, the allium leafminer emerged simultaneously at the various monitoring sites around southeastern Pennsylvania.

The flies even emerged at the same time in a half-forgotten jar Elkner had left in the research farm barn.

“When they came out, they came out in force,” he said.

If that patterns holds true, only a few monitoring sites might be needed in the future to track when the leafminers are active, he said.

Elkner is planning more research to determine what environmental trigger, such as day length or temperature, causes the leafminer to change from a pupa to an adult.

The research could eventually lead to a model, like those for other pests based on growing degree days, to predict leafminer activity, he said.

The leafminer seems well suited to the Mid-Atlantic and is likely here to stay.

“Will the cold weather kill them? They’re native to Poland,” Elkner said.

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