Leafminers

Adults are small black-and-yellow flies. Females puncture the leaf in a linear fashion to feed on plant sap and lay eggs within the leaf tissue. Both male and female leafminer flies feed on the sap from these punctures. The stippling of the puncture wounds becomes more visible over time as the damaged tissue dries and turns to white scar tissue. Eggs hatch within 2 to 4 days, and the small (less than 2.5 mm), white-to-yellow larvae tunnel within the leaf tissue. Larger larvae may feed inside the hollow leaves of onions or garlic, but still produce the characteristic "mines" visible from the outside of the leaf. Larvae exit the leaf upon completion of their development, and either pupate in the soil or in the leaf axils. Many generations occur each year, and the specific number of generations per year varies between growing regions. Leafminers may also overwinter as pupae in the soil and emerge the following spring.

Damage caused by leafminers is primarily cosmetic; however, contamination by pupae and larvae is a marketing problem for green bunching onions. Damage in dry onions and garlic is rarely a concern unless leafminers become so numerous that they prematurely kill foliage.

Leafminers attack a wide variety of crops, including vegetables such as lettuce, celery, and spinach. The potential for damage to onion and garlic will be higher when other hosts are grown nearby.

Natural enemies, especially parasitic wasps, can reduce leafminer numbers. However, these wasps are very susceptible to insecticide sprays, so they may not be reliable or effective in fields where insecticides have been used.

To culturally control leafminers:

• Thoroughly work fields previously planted with susceptible crops before planting onion.
• Allow at least two weeks for leafminer flies to emerge from pupae in the soil before planting onion in a field that previously had a leafminer problem.

Biological control by naturally occurring predators and parasites is often effective in controlling this pest in organically grown onion and garlic crops. However, supplemental releases of commercially available natural enemies are rarely economically viable. Cultural controls as described above are critical. Azadirachtin products and the Entrust formulation of spinosad can also be used.

There is no established threshold for leafminers in onions. Because large numbers of adults do not always lead to large larval infestations, make your management decisions based on the number of larvae on the crop and the type of onion crop.

See Identifying Pests of Onion and Garlic and Their Damage for leafminers photos and identification tips.