Photo credit: All photos from the Pennsylvania Department of Agriculture

The Allium leaf miner (Phytomyza 

gymnostoma) (also known as the 

onion leaf miner) infests crops 

and flowering plants in the Allium 

family such as leeks, onions, 

garlic, chives, shallots, and green 

onions. Leeks appear to be a 

preferred host plant. The insect’s 

feeding softens plant tissues, 

increasing the risk of fungal 

and bacterial infections and 

impacting marketability. Up to 20 

insects per leek stem have been 

recorded on heavily infested 

plants. 

The Allium leaf miner is native to 

Poland and Germany. Recently, 

the geographic range has rapidly 

expanded. It is now present 

throughout Europe, reaching 

the United Kingdom in 2004. It 

was recently reported in Asia, 

Turkey, and parts of Russia and 

Turkmenistan. The status of leaf 

miners in areas where they have 

established can rapidly change 

from insignificant to serious. In 

areas of high infestation, there 

are reports of 100 percent 

damage and crop loss, and 

the insect appears to be the 

most damaging in organic or 

noncommercial backyard 

production. However, there are 

also reports of severe damage in 

traditional Allium production.

Pest Alert Plant Protection and Quarantine

September 2016

Allium (Onion) Leaf Miner 

(Phytomyza gymnostoma)

Early detection is vital for 

effectively controlling and 

limiting this pest’s ability to 

spread and for protecting 

agricultural resources. A 

potential pathway for this insect 

is through commercial cargo or 

in passenger baggage.

Recently, the Allium leaf miner 

was collected as pupae from 

a leek and onion farm in 

Pennsylvania, marking the first 

detection in the United States. 

Identification was confirmed by 

USDA in February 2016. The 

pest was also confirmed in New 

Jersey in June 2016.

Pest Identification and 

Symptoms

The Allium or onion leaf miner 

has four life stages: egg, larva, 

pupa, and adult.

Eggs—small, 0.5 mm, white and 

slightly curved; eggs are laid on 

leaves 

Larva—white to yellowish 

maggots that can reach up to 8 

mm in length; larva can be found 

chewing into the leaf tissue with 

trails widening as they travel 

downward (the chewing activity 

gives “leaf miner” its name)

Pupa—reddish to dark brown, 

3.5 mm long oval; pupa can be 

found at the end of feeding trails, 

located near or inside the bulb of 

the plant. Multiple pupae may be 

found in a single leaf and multiple 

leaves can be infested.

Adult—small greyish flies 3 mm 

long, with a largely yellow head 

with black behind the eyes; wing 

length varies from 2.9 mm in 

males to 4.0 mm in females. Legs 

are dark with yellowish knees. 

The yellow color may also be 

seen on the abdomen sides and 

the tips of the leg segments. The 

wings are clear with dark veins. 

Positive identification must be 

confirmed in laboratory through 

the dissection of an adult fly. 

Adults can fly, but more studies 

are needed on flight periods and 

distances. 

Damage

Depending on the life stage, the 

Allium leaf miner can cause a 

variety of damage. Adult females 

make repeated punctures in leaf 

tissue with their ovipositor, and 

both females and males feed 

on the plant exudates. These 

punctures are the first sign the 

flies are active. Adult feeding 
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Program Aid No. 2204

damage can be seen on the end 

of leaves and typically appears 

as a row of white spots. Adults 

may also be found feeding on 

damaged leaves. Larval galleries 

in the leaves are closer to the 

base of the plant, widening as 

they move deeper toward the 

bulb of the plant. These galleries, 

or mines, are more apparent in 

Allium with thinner leaves. Pupa 

can be found in the base of the 

plant, or down into the bulb itself. 

Pulling away several layers of 

leaves can reveal older larva or 

pupa.

Damage from Allium leaf miner 

can cause plant leaves to curl, 

appear wavy, or distorted. 

Leaf mines are most evident in 

species with thin leaves (chives). 

In species with larger leaves, it is 

often necessary to peel back the 

leaves to find the insect. In heavy 

infestations, plants can develop 

fungal or bacterial diseases, 

which cause further damage and 

result in unhealthy-looking plants.

Leeks and onions are reported 

to be the most heavily damaged 

crops. Damage to other Allium 

crops such as garlic, chives, 

shallots, and green onions has 

been reported in Europe. 

The Allium leaf miner overwinters 

as pupae attached to plant 

tissues. At the beginning of 

spring, adults emerge. Eggs are 

laid within plant tissues, usually 

at the leaf base. Larvae mine the 

leaves (moving downward into 

the stalk and eventually to the 

bulb), and pupate at the end of 

their galleries. 

During summer, the pest 

develops within the plants. 

Another generation of adults 

emerges toward the end of 

summer and beginning of fall. 

In spring, damage is observed 

after the first adults take flight. 

On leeks that are small plants 

at that time of the year, just a 

few larvae can kill a plant, so an 

infested field can rapidly show a 

large number of missing plants. 

In autumn, plants are larger 

and tolerate higher levels of fly 

populations. 

What You Can Do 

Early detection is vital for 

protecting agriculture and for 

effectively controlling and limiting 

the spread of this pest. The 

following steps will help limit the 

spread of the onion leaf miner: 

• Check your transplants for 

leaf mining damage before 

planting them in your vegetable 

garden;

• If you are located in USDA 

Plant Hardiness Zones 5, 6, or 

7, cover plants in February, prior 

to the emergence of adults, 

and keep plants covered during 

spring emergence;

• Pick off and destroy infested 

leaves via shredding, deep burial, 

or incineration;

• Rotate crops that are not 

hosts for the insect; and

• Cut forage crops, removing 

remaining old plants and tilling 

the ground after harvesting. 

Inspect Allium crops for signs 

of damage, larva, pupa, and 

eggs. Finding any of the onion 

leaf miner life stages should alert 

you to the possibility of infected 

material. If you believe your crop 

may be infected, do not move 

material out of the area, as this 

will greatly increase the risk of 

pest spread.

If you find signs of damage or 

an insect that you suspect is the 

Allium leaf miner, please contact 

your local Extension office or 

State Plant Regulatory Official 

as soon as possible. To locate 

an Extension specialist near 

you, visit the USDA-National 

Institute of Food and Agriculture 

Extension Web site at www.

nifa.usda.gov/. A directory 

of State Plant Regulatory 

Officials is available on the 

National Plant Board Web site 

at www.nationalplantboard.org/

membership. A list of USDA 

State Plant Health Directors can 

be found at http://www.aphis.

usda.gov/planthealth/sphd .