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."