Scientific name: Agromyza frontella
Order: Diptera
Family: Agromyzidae

Info

Biological Description

• Fly: The adult fly is black, approximately 1/8 inch long, and hump-backed.
• Maggots: Small and pale yellow

Economic Importance

• Yield loss is not expected unless significant leaf drop occurs.
• Susceptible crops:
  • Alfalfa

Life Cycle

• Alfalfa blotch leafminer overwinters in Wisconsin as a pupa in the soil.
• Adults emerge in the spring and become active by the time alfalfa reaches 6" in height.
• Females lay eggs on the undersides of leaves. One to three eggs are laid per leaf.
• After eggs hatch, larvae feed in the middle layer of the leaf creating semi-transparent mines.
• Fully developed larvae drop from the leaf to the ground to pupate.
• Generations:
  • There are three to four generations per year in the Upper Midwest.
  • The generation time is about 30 days.

Damage/Symptoms

• Damage is a result of both larval and adult feeding
• Larvae feed in the middle layer of alfalfa leaves, creating semi-transparent tunnels that form a curved, comma-shaped path that widens on one end.
• Adults feed by puncturing tiny pinholes through the leaves and then feeding on the sap that exudes.
• Larval feeding can be confused with the serpentine leafminer (Liriomyza brassicae); however, the serpentine leafminer feeds in a thin, winding path.

Scouting Procedure and Economic Threshold

• Decisions to treat must be made during the adult pinhole feeding stage.
• Scout fields on a weekly basis to determine the percentage of leaves with pinhole feeding.
• It is still unclear if spraying insecticides will improve yield or quality; however, some say that control may be necessary 30-40% of the leaflets show adult pinhole feeding.

Integrated Control

• Cultural control: Early cutting can be used to reduce damage and would be most beneficial during first cut. The harvest should be immediately removed from the field, as waiting 5 days may result in 30-50% of pupae to survive. Subsequent cuttings may not coincide with peak larval damage.
• Biological Control: The parasitoid Dacnusa dryas (Nixon) is the dominant regulator of alfalfa blotch leafminer populations.
• Chemical Control: For current Wisconsin recommendations, consult University of Wisconsin-Extension Bulletin #A3646, Pest Management in Wisconsin Field Crops.

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Insect Images

• Alfalfa pest images

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