Plant Garlic Late to Avoid Allium Leafminer

Allium leafminer (ALM) is an invasive pest that attacks plants in the Allium genus, including onions, leeks, chives, and garlic. The adult fly punctures and deposits eggs in the leaves of onion-related crops. As the larvae develop, they eat leaf material and burrow into the bulb, stunting the plant's growth and making the leaves and bulb unpalatable.

Allium leafminer's first detection in Pennsylvania was in Lancaster County in 2015. The insect spread to nearby states and is probably throughout Pennsylvania. One way to monitor whether ALM is in your area is to inspect local onions—especially wild onions. Look for a series of telltale holes on the leaves, which reveal where the fly has punctured them to lay eggs.

If you enjoy growing garlic, you should take steps to avoid having Allium leafminer destroy your crop. One simple step is to plant garlic late in the season to avoid ALM’s fall flight as it seeks to deposit eggs. Extension Horticultural Educator Tim Elkner advises planting garlic as late in the fall as possible so that garlic will only grow a few inches before winter. That way, there will not be enough growth in which ALM can lay eggs. In southeastern Pennsylvania, this planting date would be around November 1. If you live in colder parts of Pennsylvania, consider planting garlic about two weeks later than usual. For example, if you usually plant garlic around October 1, delay until October 15.

Unfortunately, planting garlic late in the autumn will not protect it from Allium leafminer through harvest. In spring, another generation of ALM will seek a host for eggs. In spring, the best protection is to cover your garlic crop with a floating row cover. (Planting garlic late in the fall saves you the work of doing this twice.) To know when to apply the cover in spring, search the Penn State Extension website for up-to-date information on ALM activity. For more information, see Preventing Allium Leafminer in Garlic Plantings.