Apr 11, 2016
Allium leafminer detected in Pennsylvania

The allium leafminer (also known as the onion leafminer) has recently been detected and confirmed from infested leeks and onions in Lancaster County, Pennsylvania. This is the first confirmed infestation in the Western Hemisphere. Your assistance is needed for monitoring and controlling this new invasive species.
Host plants, damage symptoms

The allium leafminer has been reported to infest species in the genus Allium. Leeks (A. porrum) tend to be described as the most damaged host, which may be influenced by the timing of the second generation and the planting of leeks. Infestations have also been reported in onion (A. cepa), garlic (A. sativum), chive (A. schoenoprasum), shallot (A. cepa), and green onion (A. fistulosum). There are many ornamental species of Allium, some wild species are common weeds, and at least one species is endangered (A. munzii in California). Current literature varies in reporting damage in ornamental and wild Allium spp. The full host range is unknown.

Wavy, curled and distorted leaves

Wavy, curled and distorted leaves from the allium leafminer. Photo: L. Donovall.

Adult females make repeated punctures in leaf tissue with their ovipositor, and both females and males feed on the plant exudates. Leaf punctures arranged in a linear pattern towards the distal end of leaves may be the first sign of damage. Leaves can be wavy, curled and distorted. Larvae mine leaves, and move towards and into bulbs and leaf sheathes where they pupate. 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. Both the leaf punctures and mines serve as entry routes for bacterial and fungal pathogens. High rates of infestation have been reported: from 20 to 100 pupae per plant, and 100% of plants in fields. The literature suggests organic production and market garden production systems tend to be most at risk, perhaps due to insecticidal control in conventional production systems. Leafminers as a pest in Allium crops has rapidly increased following introduction of allium leafminer.

Allium Leafminer Damage 1

Allium Leafminer Damage 2

Allium Leafminer pupa embedded in leaf tissue from a field in Lancaster Pa.  Photo L. Donovall.

Allium Leafminer Damage 7

Feeding punctures along side of leaf

Distribution

The allium leafminer was first described in 1858 from Poland, and is native to Poland and Germany. Recently, the geographic range has been rapidly expanding. It is now present throughout Europe, reaching the United Kingdom in 2004. It has recently been reported in Asia, Turkey, and parts of Russia and Turkmenistan.

Identification

  • Adults: Small (~ 3 mm) long grey or mat-black colored flies with a distinctive yellow or orange patch on the top and front. Yellow color also present on side of abdomen. Wings held horizontally over abdomen when at rest. Legs with distinctive yellow “knees” (at femur-tibia junction). White halteres. Although adults are fairly distinctive, male genitalia are required to confirm identity.

Allium Leafminer Adult

Adult Allium Leafminer

  • Eggs: White, 0.5 mm long, and slightly curved.
  • Larvae: White, cream, or yellowish maggots, headless, up to 8 mm long at their final instar.
  • Pupa: Dark brown, 3.5 mm long, with a pair of posterior spiracles with 18-20 bulbs per spiracle.

Allium Leafminer Damage 5

Allium Leafminer pupa. Photo by Donovall.

Life history

Allium leafminers overwinter as pupae in plant tissue or surrounding soil. Adults emerge in late winter (March) into spring (throughout April, perhaps into May), and lay eggs at the base of plant stems. Larvae mine leaves, and move downward into the base of leaves or into bulbs, where they pupate. Pupae may move into soil. These 1st generation pupae undergo a diapause or aestivation period which lasts throughout the summer, and develop into adults that emerge in the autumn (September / October). This 2nd generation of adults lay eggs into Allium spp., which develop through the larval and into the pupal stage. These 2nd generation pupae will overwinter. Egg-to-adult development is estimated to require 1,090 degree-days using a 5.1°C/41.2°F threshold, or 1,225 degree-days using a 3.2°C/37.8°F threshold.

Monitoring and management

Adults have been captured using yellow sticky cards or yellow plastic bowls containing soapy water.

Covering plants in February, prior to the emergence of adults, and keeping plants covered during spring emergence, can be used to exclude the pest. Avoiding the adult oviposition period by delaying planting (after mid-May in Poland) has also been suggested to reduce infestation rates. Covering fall plantings during the 2nd generation flight can be effective.  Growing leeks as far as possible from chives has been suggested. Continuous cultivation of Allium species (such as chives) provides the pest with a continuous food source.

Systemic and contact insecticides can be effective. EPA registrations vary, however, among Alliumcrops. Check labels to ensure the crop is listed, and for rates and days-to-harvest intervals. Options labelled for leafminers or dipteran leafminers that may be effective include azadirachtin (Aza-Direct or other formulations), cyromazine (Triguard), dinotefuran (Scorpion), lambda-cyhalothrin (Warrior II or other formulations), spinetoram (Radiant), spinosad (Entrust or other formulations), and zeta-cypermethrin (Mustang or other formulations). Other materials labelled for Liriomyza leafminers or thrips that may be effective include abamectin (Agri-Mek or other formulations), acetamiprid (Assail), and cyantraniliprole (Exirel). Among these, the Entrust formulation of spinosad is allowable for certified organic production if allowed by your certifying organization.

Taxonomy

  • Scientific Name: Phytomyza gymnostoma Loew (Family Agromyzidae)
  • Common Names: Allium leafminer, onion leafminer
  • Recent Synonyms: Napomyza gymnostoma Loew

Reporting a possible detection

If you suspect believe you may have observed damage or a life stage of the allium leafminer, please contact a plant inspector in the regional Department of Agriculture office or an Extension Educator or Diagnostic Laboratory in the local Cooperative Extension Office. In Pennsylvania, contact the plant inspector in your regional office of the Pennsylvania Department of Agricultureor an Extension Educator in the local Penn State Extension office.

Source: Penn State Department of Entomology


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