Allium Leafminer | Phytomyza gymnostoma Loew

Allium Leafminer
California Pest Rating for
Phytomyza gymnostoma Loew: Allium Leafminer
Diptera: Agromyzidae
Pest Rating: A

Initiating Event:

On July 12, 2016 USDA inquired if states were interested in surveying for Phytomyza gymnostoma (Allium leafminer).  A pest rating proposal is required to determine a permanent rating for this pest.

History & Status:

Background:  Phytomyza gymnostoma is a leafmining fly that feeds on plants in the genus Allium including leeks (Allium porrum), onion (Allium cepa), garlic (Allium sativum), chive (Allium schoenoprasum), shallot (Allium cepa), and green onion (Allium fistulosum)1. The flies overwinter as pupae in plant leaves or bulbs or soil1.  In Pennsylvania adult flies emerge from March through May and females lay eggs at the base of plant stems1.  Larvae mine leaves, moving into the base of leaves or bulbs to pupate1.  This feeding causes leaves to become distorted1.  A second generation of adults emerges in September and October1.  Adult females also puncture tissue to feed on the plants, opening them up to invasion by secondary decay organisms1.  Allium leafminer may be transported long distances when infested plants, bulbs, or soil is moved.

Worldwide Distribution: Phytomyza gymnostoma is native to Poland and Germany1.  It recently started to expand its range and has spread throughout Europe, to Turkey, Russia, and Turkmenistan1.  The flies were first detected in the Western Hemisphere in Pennsylvania in December 2015.

Official Control: Phytomyza gymnostoma is listed as a harmful organism by Chile, Costa Rica, and Peru3.

California Distribution:  Phytomyza gymnostoma has never been found in the environment of California.

California Interceptions Phytomyza gymnostoma has never been intercepted by California.

The risk Phytomyza gymnostoma (Allium leafminer) would pose to California is evaluated below.

Consequences of Introduction: 

1) Climate/Host Interaction: Phytomyza gymnostoma is expected to be able to establish a widespread distribution in California wherever Allium plants grow. It receives a High (3) in this category.

Evaluate if the pest would have suitable hosts and climate to establish in California.  Score:

Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Phytomyza gymnostoma is only known to feed on plants in the genus Allium.  It receives a Low (1) in this category.

Evaluate the host range of the pest. Score:

Low (1) has a very limited host range.

Medium (2) has a moderate host range.

High (3) has a wide host range.

3) Pest Dispersal Potential: Agromyzid flies have high reproductive potential and Phytomyza gymnostoma can rapidly spread long distances when infested bulbs are moved.  It receives a High (3) in this category.

Evaluate the natural and artificial dispersal potential of the pest. Score:

Low (1) does not have high reproductive or dispersal potential.

Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

4) Economic Impact: California is the largest producing state in the U.S. of garlic, onions, and green onions.  The state produces 90%+ of the commercial garlic, is the largest producer of processing onions, and is one of the top fresh market onion producers2.  Both garlic and onion crops are valued at $150-$300 million each annually2.  California also leads the nation in the production of green onions with a 2009 crop value of $28 million in Monterey and Riverside county alone4.  If Phytomyza gymnostoma were to establish in California it is expected to lower crop yields and increase production costs of these crops, especially on organic farms.  Infestation rates of 20 to 100 pupae per plant and 100% of plants in a field have been reported1.  Its presence in the state would likely affect markets for fresh garlic and onions.  Growers in other places infested with Allium leafminer have changed cultural practices including delaying planting until after the leafminer flight, covering fall plantings, and separating crops by large distances1.  The leafminer is not expected to vector other organisms, injure animals, or interfere with water supplies.  Phytomyza gymnostoma receives a High (3) in this category.

Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Low (1) causes 0 or 1 of these impacts.

Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

5) Environmental Impact: If Phytomyza gymnostoma were to establish in California it is not expected to lower biodiversity, disrupt natural communities, or change ecosystem processes.  It is likely to affect threatened and endangered species such as Munz’s onion (Allium munzii) and Yosemite onion (Allium yosemitense).  Allium leafminer would not be expected to disrupt critical habitats.  It is likely to trigger additional treatment programs in agriculture and in residential gardens.  Species of Allium are grown in home/urban gardens and would be significantly affected by this pest.  Phytomyza gymnostoma receives a High (3) in this category.

Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Phytomyza gymnostoma (Allium leafminer):  High (13)

Add up the total score and include it here.

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Phytomyza gymnostoma has never been found in California and receives a Not established (0) in this category.

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.

Not established (0) Pest never detected in California, or known only from incursions.

Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: High (13)


Phytomyza gymnostoma is a new arrival to the Western Hemisphere and is a known pest.  There is low uncertainty.

Conclusion and Rating Justification:

Phytomyza gymnostoma has never been found in California and is expected have significant economic and environmental impacts if it establishes in the state.  An “A” rating is justified.


1 Fleisher, Shelby and Tim Elkner. 2016. Pest Alert – Allium Leafminer.  Penn State Department of Entomology.

2 California Garlic & Onion Research Advisory Board.

3 USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD).

4 Smith, Richard, Michael Cahn, Marita Cantwell, Steven Koike, Eric Natwick, and Etaferahu Takele. 2011.  Green Onion Production in California.  UC Vegetable Research & Information Center Vegetable Production Series.

Responsible Party:

Jason Leathers, 1220 N Street, Sacramento, CA, 95814, (916) 654-1211,[@]

Pest Rating: A

Posted by ls