Allium Leafminer Identification, Biology and Control

Key Points

  • Two generations of allium leafminer (ALM) are expected per year in the Northeast. Spring generation emerges between late March and late April, and fall generation emerges in early September.

  • Summer diapause and winter hibernation occur in the pupal stage

  • Females lay eggs on the tip of Allium leaves making a row of white dots.

  • Egg laying marks cause cosmetic damage to some allium crops and larval tunnelling and feeding can cause extensive damage to leaves and bulbs.

  • Risk of ALM infestation varies by season. Generally, chives, garlic, ramps, scallions, and ornamental alliums are at greatest risk in the spring, while chives, leeks, scallions, and ornamental alliums are at higher risk during the fall.

  • Early monitoring of egg laying marks and adults can be helpful to determine when to take management actions. Prediction of spring generation adult emergence can be done using a degree-day model.

  • Protect against ALM damage by applying suitable registered insecticides at an optimal rate along with other Integrated Pest Management tactics.  

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.

Key Features of Allium Leafminer

There are several key features that distinguish ALM from other vegetable leafminers in North America. ALM adults are larger than other leafminers; approximately 0.11 inch long (Figure 1). The area on the top and front of the head (frons) is yellow; halteres are white, and the distal end of the femur (the “knee area”) is yellow. ALM larvae are about 0.31 inch long and yellowish white. Pupae are 0.11- 0.15 inch long and reddish brown. The underside of the end of female’s abdomen is solid black, whereas male has a yellow V-shaped mark.

pale oblong egg

Allium leafminer egg. Photo by Pin-Chu Lai.

yellowish white larvae

ALM larvae are about 0.31 inch long and yellowish white. Photo by Pin-Chu Lai.

reddish brown pupa

Pupae are 0.11- 0.15 inch long and reddish brown. Photo by Pin-Chu Lai.

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.

Figure 2. Life History of Allium Leafminer

Illustration prepared by Rekha Bhandari

Spring adult female lays eggs by probing on leaves using her ovipositor.

Spring larvae tunnel through leaves and leaf sheaths of Allium plants.

Spring pupae undergo summer aestivation on leaf sheaths or on the plant surrounding

Fall adult females lay eggs by probing on leaves using their ovipositor

Fall larvae tunnel through the leaf and leaf sheaths of Allium plants

Fall pupae overwinter on leaf sheaths and in the soil surrounding plants.

Host Range and Plant Preference

ALM will infest many different Allium species, but their risk for attack varies during the season. For example, chives, garlic, ramps, scallions and ornamental alliums are more likely to be infested in the spring compared with onions that are transplanted. Onions that are established by seeding are typically not at risk. The primary reason for these differing levels of risk is the availability of leaf tissue for egg-laying females. Very few onions that are established by seeding will emerge in time to produce foliage large enough for females to lay their eggs.

In the fall, chives, leeks, scallions and ornamental alliums are at high risk for ALM attack because these crops have sufficient canopy to be attractive to females. In contrast, garlic is not planted until later in the fall and likely escapes females that are searching for a host. Additionally, most onion fields are harvested by the time ALM adults emerge from aestivation and are at very low to no risk for infestation.

Risk

of Infestation

Potential

Spring Generation

  • High Risk: Chives, ramps, scallions (green onion) and ornamental alliums
  • Medium Risk: Onion established by transplanting and garlic
  • Low RiskOnion established by seeding

Fall Generation

  • High Risk: Chives, leeks, scallions (green onion), and ornamental alliums
  • Medium Risk: N/A
  • Low RiskOnion and garlic

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.

a green stalk with white dots on it

Figure 3: White dots are oviposition punctures

a droplet of sap on a plant

Figure 4: Leaf sap exudates from the puncture

trails on allium plant

Figure 5: Larval mining

damaged leek

Figure 6: Damage in leek (P.C. Pin-Chu Lai)

damaged leeks

Figure 7.1: Damage in leek (P.C. B. Nault)

damaged leek

Figure 7.2: Damage in leek (P.C. B. Nault)

damaged garlic bulb

Figure 8: Damage in garlic (P.C. T. Rusinek)

damaged onion

Figure 9: Damage in onion (P.C. T. Rusinek)

Monitoring allium leafminer activity

Monitoring the emergence of ALM adults and their subsequent egg laying is important for making control decisions. There are various ways to detect the tentative timing of adult flight and the egg-laying period.

Predicting the timing of spring ALM emergence or initial feeding/egg laying using a Growing Degree Day model

According to lab and field-based research at Penn State University, the best prediction for the emergence of adult ALM is based on the accumulation of 350 growing degree days (GDD), at GDD base temperature of 1°C starting January 1. For predicting site-specific adult emergence, weather data from NEWA (Network for Environment Weather Applications) can be extracted and calculated for the site.


Site-specific ALM adult emergence prediction instructions:

  • Go to NEWA website https://newa.cornell.edu/
  • Select the weather tools tab.
  • Select the degree day calculator link.
  • Select the station of interest.
  • Select the start date as January 1 and end date as the current date.
  • Select degree day type as 1°C (as a baseline temperature)
  • The spreadsheet displaying degree days and accumulated degree days will appear on the right side of the screen.

The ALM emergence date can be predicted by subtracting the current degree day accumulation from 350 and dividing by the average daily degree days from the previous week. For example, if the current degree day accumulation is 310 and the average degree day accumulation for the past week is 3 per day, emergence is approximately 13 days away (350-310=40, 40/3=13.3) if temperatures are similar to the previous week. It is better to check the approximate time of emergence more frequently as GDD accumulations approach 350, especially if temperatures are warm.

Monitoring infestation (egg laying marks and adults)

Initial egg laying marks can be prominent towards the tip of the leaf, mainly in a tall leaf. Sometimes, egg laying scars are not prominent, but adults can be spotted near the tip of allium leaves when the weather conditions are warm and less breezy. Sometimes leaves may have to be peeled back to observe ALM larvae and pupae later in the growing season.
 

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).
 

An Extension and Outreach Assistantship from Cornell University, Entomology Department supported graduate student Rekha Bhandari’s position as an Extension and Outreach Assistant, Spring 2023. We thank Abby J. Seaman, Vegetable IPM Coordinator at NYSIPM Program for reviewing the document and providing us with constructive suggestions. 

Brian Nault portrait
Brian Nault

Professor

Department of Entomology

Cornell AgriTech

Brian Nault