The fly infests plants in the Allium genus, including leeks (A. porrum) onion (A. cepa), garlic (A. sativum), chive (A. schoenoprasum), shallot (A. cepa var. aggregatum), and green onion (A. fistulosum). We've also found feeding signs on wild garlic (A. vinealae), garlic chives (A. tuberosum), and an ornamental- Allium 'Globemaster' (A. christophii x A. macleanii). The Pennsylvania Department of Agriculture and Penn State posted reports and a pest alert to these websites:
Adult females puncture leaves in a linear pattern with their ovipositor for feeding and egg laying (Figure 1). Leaves from infested plants can be wavy, curled and distorted (Figure 2). Larvae mine leaves moving towards and into bulbs and leaf sheathes (Figure 3) where they pupate (Figure 4). It is often necessary to peel back the leaves to find the insect. Both the leaf punctures and mines serve as entry routes for bacterial and fungal pathogens.
Allium leafminer overwinters as a pupa. Reports suggest that the fly has a distinct spring flight, a summer aestivation (dormancy) period, and a fall flight. One useful report from Fuchsenbigl, Austria (1Kahrer, 1999) recorded the adult flight based on emergence of adults within cages that held infested plants. Kahrer noted that adults were active during the spring from mid-April to mid-May, and in the fall from early September to early October. Last year we used sticky traps to estimate timing of fall flight from 5 farms in Lancaster County and 3 farms in Berks County. Adults were active during the last week of September until we stopped trapping in the second week of November. We also recorded data about how the fly responds to different visual stimuli, as part of ongoing studies to optimize traps.
The adult fly (Figures 1, 5, 6, 7) has a charismatic orange face, yellow "knees" (end of femurs), and matte black body. At 3mm-4mm, the adult is small (larger than a fruit fly but much smaller than a house fly). Eggs are laid in or on plant tissue, and larvae are well embedded into the plant tissue.
The timing of planting and harvest affect risks of damage. We seem to be getting most damage to allium crops that are showing strong vegetative growth at the time of adult flight activity. For example, last year, bulb onions planted after the spring flight, and harvested before the fall flight, mostly escaped damage. But settings with alliums present during the flight periods were most at risk. Last year, spring onions were damaged during the spring flight, and leeks and other alliums were damaged during the fall flight. Row covers during the flight period should prevent damage.
Insecticides labelled for leafminers in various allium crops (the list is on page 1 and 2, right after the cover page).
We anticipate emergence of the adults in the beginning of April. We are deploying traps and will report when we see flight activity using the 1-800-PENN-IPM phone line, and postings to the Vegetable Extension Team's news site.
1Kahrer, A. (1999). Biology and control of the leek mining fly, Napomyza gymnostoma. Integrated Control in Field Vegetable Crops IOBC Bulletin, 22 (5), pp. 205-211.
Figure 1. Female puncturing leaf with ovipositor. Note linear puncture marks, which is a sign of adult activity. Photo: T. Elkner
Figure 2. Twisted leaves of infested crop. Photo: L. Donoval
Figure 3. Larvae feeding in onion. Photo: S. Spichiger
Figure 4. Pupae in base of allium plants. Photo: L. Donoval
Figure 5. Adult. Top view. Note yellow patch on head, and yellow 'knees'. Photo: A. Megroz
Figure 6. Adult, preserved specimen. Note yellow head patch and yellow knees. Photo: N. Sloff
Figure 7. Adult. Side view. Photo: A. Megroz