Sep 29, 2016
Growers urged to be on lookout for invasive onion pest

An insect that attacks onions, leeks, garlic and related crops may be emerging now in parts of Pennsylvania, and growers should be prepared to take measures to manage the pest, according to a Penn State entomologist.

The allium leafminer, which never had been seen in the Western Hemisphere until its discovery last winter in Pennsylvania, produces two generations per year, and the second generation could emerge in September or October, according to Shelby Fleischer, professor of entomology in Penn State’s College of Agricultural Sciences.

“The adults that emerged in the spring laid eggs, and the resulting larvae fed during the spring, in some cases causing serious crop damage,” Fleischer said. “Then the larvae or pupae entered a long resting stage that carried them through the summer. After pupating, they will emerge as adults in the fall, when they will lay another round of eggs. So it’s important to manage any infestations now before they can start another generation.”

As the eggs laid in the fall hatch into larvae, they will mine leaves before moving downward into the base of leaves or into bulbs, where they will pupate and overwinter, emerging as adults in the spring, likely between March and May.

The allium leafminer — also known as the onion leafminer — is a threat to several species of crop plants in the Allium genus, such as onion, leek, garlic, chive, shallot and green onion. Fleischer noted that the insect’s full range of host plants is unknown.

The invasive pest’s first confirmed U.S. appearance was in Lancaster County, where it was found infesting leeks and onions. The Pennsylvania Department of Agriculture since has reported confirmed infestations in at least 12 additional southeastern Pennsylvania counties.

Native to Germany and Poland, the allium leafminer’s geographic range has been expanding rapidly, most likely transported with commercial cargo, in shipments of affected crop plants or in passenger baggage, according to state agriculture officials.

“More research is needed to assess the potential impact of allium leafminer under Pennsylvania conditions, but literature from other countries suggests that organic and market-garden production systems and home gardens tend to experience more damage than conventional production systems,” Fleischer said.

“Conventional growers may have fewer problems due to the insecticidal controls they are likely to use and to shorter time windows in which host plants are available,” he said. “However, wild Allium species that exist as weeds in our agroecosystems may alter this.”

The allium leafminer never had been found in the Western Hemisphere until it was discovered recently in Lancaster County, Pennsylvania. Photos: Sven Spichiger
The allium leafminer never had been found in the Western Hemisphere until it was discovered recently in Lancaster County, Pennsylvania. Photos: Sven Spichiger

Plant symptoms occur when female leafminers make repeated punctures in leaf tissue with their ovipositors, and both females and males feed on the plant fluids. Plant damage results from the larvae mining into leaf tissue and at the base of plants into leaf sheaths or bulbs. Both the leaf punctures and mines serve as entry routes for bacterial and fungal pathogens.

“Leaf punctures arranged in a linear pattern toward the distal end of leaves may be the first sign of damage,” Fleischer said. “Leaves can be wavy, curled and distorted.”

He said high rates of infestation were reported in early spring 2016. “There can be from 20 to 100 pupae per plant, and 100 percent of plants in a field may be infested.”

Allium leafminer adults are small grey or matblack-colored flies with a distinctive yellow or orange patch on the top and front of the head and yellow on the side of the abdomen. They hold their wings horizontally over their abdomen when at rest. Their legs have distinctive yellow “knees.”

The larvae are headless white, cream or yellowish maggots, measuring up to 8 millimeters long at their final instar. The insect’s pupa stage is dark brown and 3.5 millimeters long.

Although more research is needed to determine how to monitor effectively for the active adult stage, growers can use yellow sticky cards or yellow plastic bowls containing soapy water to capture adults. Excluding the pest by covering plants starting in February and continuing through spring emergence of adults may help to protect crops.

In addition, Fleischer suggested that infestation rates can be reduced by delaying planting until late spring to avoid the adult egg-laying period. Covering fall plantings during the second generation flight also can be effective.

“Systemic and contact insecticides can be effective, but EPA registrations vary among Allium crops,” he said. “Check labels to ensure the crop is listed and for rates and days-to-harvest intervals.”

More information about the allium leafminer is available on the Penn State Department of Entomology website.

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