Jun 1, 2017

Allium leafminer damage seen in Pennsylvania onions and garlic

Penn State University Extension has issued a spring update on the allium leafminer.

The flight is over, but the war may have just begun! The larvae have taken hold and will remain steadfast as pupa throughout the summer.

The allium leafminer adult spring flight is done for the season!  However, the larva and pupa are the now the main concern.

Eggs will hatch within a week of being laid and the larvae begin to mine. As the larvae grow they will begin to mine down the leaves toward the stem of the plant – even mining through leaf layers deeper into the stem for protection and food.

Once the larvae are large enough, about 2-3 weeks of development, they will pupate within plant tissue. We estimate that the majority of the ALM larval population will have completed pupation by the 2nd week of June. At this point the pupa “hibernate” (called aestivation) through the summer months and will hatch as adults at the end of September.

In the late larval and pupal stages the majority of damage from ALM has already occurred to the plant. We do not know if this pest transmits any diseases but we currently do not believe that ALM does. What ALM does do is open up entryways for pathogens to infect the plant, which can be a major issue for yields.

So what can you do? Well, if you mostly have larva, then targeting the larva using some form of control that can penetrate the plant is best method.

If you have more pupa than larva, then treatment for the insect may not be effective due to the decreased metabolism of the pupa.  The next best thing to do would be to try and protect your crop from disease and harvest before the adult flight in the fall, destroying all larva and pupa you find.

For chemical control recommendations, Shelby Fleischer created an allium leafminer amendment for the Mid-Atlantic Commercial Vegetable Production Guide 2016-2017.

For more information about the allium leafminer, including descriptions of the life stages and the updated confirmed counties, the Pennsylvania Department of Agriculture and Penn State posted reports and a pest alert to these websites:

• Allium Leafminer, PA Department of Agriculture
• Allium Leafminer, Penn State Extension

Source: Penn State University Extension