How to Manage Pests
UC Pest Management Guidelines
Onion and Garlic
Scientific name: Liriomyza spp.
(Reviewed 1/07, updated 5/10, pesticides updated 6/16)
In this Guideline:
DESCRIPTION OF THE PESTS
Adults are small black and yellow flies. Females puncture the leaf to feed on plant sap and to lay eggs within the leaf tissue. Eggs hatch within 2 to 4 days and the small 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 pupate in the soil or in the leaf axils on plants. Many generations occur each year.
Damage caused by leafminers is primarily cosmetic in green bunching onions; contamination by pupae and larvae, however, is a problem. Damage in dry onions and garlic is of little concern unless populations become so high as to prematurely kill foliage.
Natural enemies, especially parasitic wasps, are commonly found reducing leafminer numbers. These parasitic wasps are very susceptible to insecticide sprays, however, and may not be important in fields where insecticides have been used.
Leafminers attack a wide variety of crops in coastal California. Close proximity to crops such as lettuce, celery, or spinach will increase the potential for damage by leafminers in onions. It is also important that fields being planted to onions that were previously in one of these susceptible crops be worked thoroughly and that sufficient time be allowed to pass before planting into these fields to allow pupae in the soil to emerge.
Organically Acceptable Methods
Biological controls are often effective in controlling this pest in organically grown onion and garlic crops. Supplemental or inundative releases of parasites are rarely economically justified. Cultural controls as described above are critical. Azadiractin products are allowed.
Monitoring and Management Decisions
There is no established threshold for leafminer in onions. Because large populations of adults do not always lead to large larval populations, make your management decisions based on the level of larval infestations on the plants.
UC IPM Pest Management Guidelines:
Onion and Garlic
Insects and Mites
S. Orloff, UC Cooperative Extension, Siskiyou County
Acknowledgment for contributions to Insects and Mites:W. E. Chaney, UC Cooperative Extension, Monterey County
R. L. Coviello, UC Cooperative Extension, Fresno County
G. J. Poole, UC Cooperative Extension, Los Angeles County