Description of the Pest
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.
Organically Acceptable Methods
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.
Monitoring and Treatment Decisions
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.
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|MODE-OF-ACTION GROUP NUMBER1: 17|
|COMMENTS: Do not make more than six applications per crop.|
|(Success)||3–6 fl oz||4||1|
|MODE-OF-ACTION GROUP NUMBER1: 5|
|(AzaGuard)||10–16 fl oz||4||0|
|MODE-OF-ACTION GROUP NUMBER1: un|
|COMMENTS: A restricted-use pesticide in an organically certified crop.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|#||Acceptable for use on organically grown produce.|
|1||Rotate insecticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B. Mode-of-action group numbers (un = unknown or uncertain mode of action) are assigned by IRAC (Insecticide Resistance Action Committee).|