Allium Leafminer

Appearance

• Adults: Small (~ 3 mm) long grey or black flies with a distinctive yellow or orange area on the top and front of head. Wings are held horizontally over abdomen when at rest. Legs have distinctive yellow “knees” 
• 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 (larval growth stage)
• Pupa: Dark brown, 3.5 mm long

Life cycle/habits

• This European pest was first recorded in North America in Lancaster Co. PA in 2015. Allium leafminer (ALM) was first observed in Maryland in 2017 and is now widespread through Central and Northern Maryland
• ALM overwinters as pupae in plant tissue or surrounding soil. Adults (flies) emerge in late winter (March) into spring (throughout April, perhaps into May), and mate
• Female flies puncture leaf surfaces with their ovipositor to lay eggs in rows. Male and female flies feed on the plant sap at the puncture sites
• Larvae hatch, mine leaves (feeding between upper and lower leaf surfaces), and move downward into the base of leaves or into bulbs where they pupate. Pupae may also move into the soil
• The 1st generation pupae undergo a dormant phase that lasts throughout the summer and develops into adults that emerge in fall (September/ October). The 2nd generation of adults lays eggs on Allium species, larvae feed on available Allium plants and then develop into pupae that overwinter in plant tissue or soil
• Spring and fall adult flight occurs over a 5-7 week period

Host plants

Allium genus (onion, garlic, shallots, leeks, chives, green onions; also, wild and ornamental alliums) 

Signs/symptoms

• Small round white dots (ovipositing/feeding scars) in a row that appear on the middle towards the end of leaf blades 
• Larval feeding may cause leaves to be curled or wavy 
• Monitor all alliums during expected flight periods for small white dots 
• 1st generation feeds on overwintered alliums and those planted in early spring
• 2nd generation feeds on leeks as well as alliums planted late summer and early fall
• Plants can become heavily infested with >20 larvae or pupae per plant
• Both the leaf punctures and mines serve as entry routes for bacterial and fungal pathogens

Prevention/control techniques

• Remove and discard all allium plant debris after the growing season to eliminate overwintering sites
• Row cover or insect netting can exclude flies and prevent egg-laying
• Apply the row cover or netting over plants in February, prior to the emergence of adults; keep plants covered during spring emergence. When covering a fall-planted crop, like garlic, in early spring, make sure there are no ALM in the crop, otherwise, you’ll trap the flies under the row cover
• Cover fall plantings during the 2nd generation flight 
• Consider delaying spring planting until mid-May to avoid the 1st generation egg-laying period and 
• Planting garlic in late October to avoid ALM if it’s active in your area
• Organic insecticides: Neem oil or spinosad. The latter is absorbed into leaf tissue so will control larvae after they hatch and start to feed. Apply spinosad after white dots (oviposition marks) appear. Make a second application two weeks later. Since allium leaves are waxy it is recommended to add a surfactant (e.g. insecticidal soap) to help insecticides stick to leaves