Adult Allium Leafminer, photo by Sven Spichiger, Pennsylvania Department of Agriculture
In December 2015, the Pennsylvania Department of
Agriculture, in cooperation with the Penn State Extension, confirmed the
presence the Allium leafminer (Phytomyza gymnostoma (LOEW)) in Lancaster
County, Pennsylvania. This is the first detection of this non-native species,
also known as the onion leafminer, in the United States. Additional infestations
were found in Chester, Lehigh, Dauphin, and Delaware counties in the spring of 2016,
expanding the known range of this pest. Surveys are ongoing to determine other
areas in Pennsylvania have been impacted.
The Allium leafminer is an Agromizyid, or leafmining fly,
found throughout Europe and parts of Turkey, where it has become a major pest
of Allium crops in much of its range. The Allium leafminer infests a wide
variety of crops in the genus Allium.
In areas of high infestation, there have been reports of 100 percent damage and
loss, with examples of complete crop loss in Pennsylvania.
Leeks and onions are reported to
be the most heavily damaged crops, both of which have been documented in Pennsylvania.
Damage to other Allium crops such as garlic, chives, shallots and green onions has
been reported in Europe. Ornamental and wild Alliums are also affected, but it
is unknown to what extent. The damage caused by the feeding and mining of the
insect results in softened tissues in the plant, increasing risk of fungal and
bacterial infections as well as decreased marketability. Densities of up to 20 flies per leek stem
have been recorded in heavily infested areas. The Allium leafminer is
considered one of the most impactful pests of Allium production in Europe.
Early detection is vital for effective control and the limiting
spread of this pest, and the protection of Pennsylvania agriculture.
Who is at Risk from Allium Leafminer?
The threat is most problematic for organic
growers and homeowner/backyard gardens. Commercial growers with spray controls
programs in place have shown that their crops can be resistant to the Allium
leafminer already in Europe as part of system to control leafminers, cutworms,
thrips, onion maggot and other diseases. Organic growers and backyard growers
who do not have access or use commercial chemical controls will have to rely
upon other methods to safeguard their crops from Allium leafminer, such as
cultural controls. Control recommendations for both commercial growers and
organic/backyard growers can be found on the Penn State Extension factsheet, Penn State Extension Fact Sheet.
The Allium leafminer has shown a preference for attacking
garlic and onion in the spring. In the
fall leeks become the primary target, where they provide an overwintering
refuge for the fly. Other crops such as garlic, shallots and chives can be
affected in both the spring and fall. Wild plants may provide a refuge to Allium
leafminer year-round, making control difficult.
There is no human health risked associated with this
pest. All damage is associated with the
quality of the Allium crops and plants.
Pest Identification and Symptoms
Eggs: The eggs small, 0.5 mm, white and slightly curved.
Eggs would be laid on leaf material.
Larvae: A white to yellowish maggot that can reach up to 8
mm in length. Larvae can be found mining in the leaf tissue, with trails
widening as they travel downwards.
Pupa: The pupa is 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.
Allium leaf minear pupa extract from leek, photo by L.Donovall, USDA APHIS
A gray to black colored fly, with a distinctively colored head. The fly’s head
is yellow on the face lightening towards the top, with black behind the eyes.
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.
leafminers on onion, photo by Sven
Spichiger, Pennsylvania Department of Agriculture
Depending on the life stage, the Allium leafminer can cause
a variety of damage and be found on various portions of the plant. Adult
feeding damage can be seen on the end of leaves and typically appears as row of
white spots. Adults may also be found feeding on damaged leaves. Larval
galleries in the leaves can be seen closer to the base of the plant, widening
as the move deeper towards the bulb of the plant. These galleries, or mines,
are more apparent in alliums with thinner leaves. Pupa can be found in the base
of the plant, or down into the bulb itself. You may need to pull away several
layers of leaves to find older larva or pupa.
Damage from Allium leafminer can also cause the plant to
exhibit curly, wavy or distorted leaves. In heavy infestations, the plants
become susceptible to fungal or bacterial diseases, which can cause further
damage, resulting in unhealthy looking plants.
Feeding punctures from adult flies feeding, photo by Sven Spichiger, Pennsylvania
Department of Agriculture
Distressed plants with curly, wavy, distorted leaves, photo by Sven Spichiger, Pennsylvania
Department of Agriculture
As the Allium leafminer was just recently discovered in the
United States, little information is available on the life history of this pest
in Pennsylvania. Survey work toto determine life history as well distribution
of the pest is currently underway by the Department of Agriculture. The timings
for different emergences and life stages presented are based on European
countries with a similiar climate to Pennsylvania.
The overwintering will emerge as an adult in the spring,
late march to early April, with the first adults collected in mid-April in
Pennsylvania this year. Adults will lay eggs on Allium leaves where they will
hatch. The larva will begin feeding on the leaves and mining further into the
plant. The larva will then pupate, either in the plant or surrounding soil, but
will not emerge until the fall, likely September or October, when the second
generation will emerge. The second generation will lay eggs on Allium again,
with this generation overwintering as a pupa.
What can you do?
Limit the spread of the Allium leafminer: You can take steps
to limit the chance you may spread the Allium leafminer. Inspect your Allium
crops for signs of damage, larva, pupa and eggs. Finding any one of the life
stages will alert you to the possibility of infected material. If you believe
your crop may be infected, transporting the material will greatly increase the
risk of spread of the pest.
Collect a Specimen: If you are from Pennsylvania
you can turn adult specimens into the Pennsylvania Department of Agriculture's
entomology lab or to your local Penn State Extension Office for identification.
Adult specimen can be placed into alcohol or hand sanitizer in a leak proof
container. A sample submission form can be found in the publications section.
more information or to report possible populations of Allium Leafminer: