Allium Leafminer Identification, Biology and Control

Distribution

Allium leafminer (ALM), Phytomyza gymnostoma (Loew), is a specialist pest that infests wild and cultivated plants in the Allium genus. ALM is native to Europe and was first detected in North America in 2015 near Lancaster, Pennsylvania (USA). Since then, it has spread to Connecticut, Delaware, Maryland, Massachusetts, New Jersey, New York, Virginia, and Washington DC. Generally, ALM attacks allium crops such as chives (A. schoenopanasum), garlic (A. sativum), leek (A. porrum), onion (A. cepa), ramps (A. tricoccum), and scallion (A. fistulosum). In some instances, ALM damage can cause total crop loss.

Biology and Life History

Allium leafminer (ALM) has two generations per year (bivoltine). ALM overwinters in the pupal stage either within its host or in soil adjacent to its host. Adults emerge in the spring between late March and late April and the first generation is completed in early June.

ALM undergoes aestivation (a summer inactive period) in the pupal stage, and the fall generation of adults emerge in early September. These adults continue to be active through December, but the second generation is typically completed in November and December (Figure 2).

Female ALMs puncture the leaf surface with an ovipositor (egg laying structure) multiple times, which creates a row of white dots (Figure 3). Male and female ALMs then feed on the sap that exudes from these punctures. (Figure 4). A small percentage of these leaf punctures have eggs laid inside the leaf tissues. After eggs hatch, larvae will mine (feed between the upper and lower leaf surface) through the leaf moving downward to the base of the leaf or bulb to pupate.

Allium crops grown in the fall tend to become more severely infested than those in the spring, presumably because populations are higher in the fall than in the spring. Allium crops grown under organic production practices and those in home gardens also tend to become more frequently damaged than those grown following conventional production regimes that may be treated more frequently with broad-spectrum and highly effective insecticides.

Damage by Allium Leafminer

The series of egg laying punctures created by female ALM (Figure 3) and mining by larvae in the foliage can cause significant cosmetic injury to chives and scallions (Figure 5). Feeding by larvae in crops such as leeks and scallions can be extensive, and in some cases can cause plant deformity (Figure 6).

Feeding damage also creates openings for pathogenic fungi and bacteria to establish and cause rot (Figure 7). Finally, these crops can be contaminated with ALM pupae during harvest, which can make the crop unmarketable (Figures 7.2 and 8).

Although larvae have been recovered from onion leaves before bulbing, larvae and pupae have rarely been encountered in onion bulbs. One exception was found near the outer portion of the bulb (Figure 9). Therefore, the least amount of economic risk associated with ALM damage occurs in bulb onions.

Management of Allium Leafminer 

Cultural control

Escape in time: Transplanting Allium plants in late spring and summer (after peak adult flight period and when pupae undergo summer diapause) and harvesting in early September (before fall generation emerges) may help escape the infestation of ALM from both spring and fall generations.

Crop rotation:  Avoid planting susceptible crops in the same location multiple years. If the susceptible plant in Allium genus is planted in the same field, pupae may overwinter in crop residues or soil, which will serve as an ALM source for the next season’s crop.

Escape in space: Growing allium crops as far away as possible from previous plantings of allium crops could be helpful to reduce the probability of infestation as ALM does not seem to be a long-distance flier.

Good Sanitation practices: Removal of plant debris after each season will help remove overwintering pupae that may serve as a source of ALM for the next season.

Solarization of planting bed is yet another tactic where clear plastic cover can be spread over the planting bed in the spring and left covered for about a month to kill ALM pupae. Additionally, this tactic will help to kill soil pathogens and build up beneficial microbes.

Physical Control

Predicting the adult flight period and egg-laying period is useful for timing the use of barriers for protecting plants from infestation. An example would be to use exclusion netting or a light weight floating row cover before the flight period starts (spring: March/April and fall: September/October) until 8 weeks after the tentative start of the flight. The netting or row cover should completely surround the plants with edges buried in the soil.

Reflective mulch can reduce infestation levels by ALM, but is unlikely to provide enough protection to avoid economic damage (Figure 10). Therefore, other tactics will be needed in addition to reflective mulch.
 

Chemical control

Insecticide use is the most effective tool for managing ALM. Two or three well-timed applications of an insecticide should provide an acceptable level of ALM control as long as the first application is 2- 3 weeks after initial detection of ALM flies and subsequent sprays are timed at 1–2-week intervals. Examples of highly effective insecticides applied as a foliar spray include dinotefuran, spinetoram and cyantraniliprole. One OMRI- labeled option that works well is spinosad. Insecticides such as abamectin, cyromazine, imidacloprid, and lambda-cyhalothrin can also help reduce ALM densities.

Biological control

Parasitoids such as Halticoptera circulus (often associated with ALM) and Chrysocharis oscinidis have been reported to parasitize ALM pupae (Figure 11).