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How to Manage Pests

UC Pest Management Guidelines


Onion and Garlic

Leafminers

Scientific name: Liriomyza spp.

(Reviewed 1/07, updated 5/10)

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

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.

MANAGEMENT

Biological Control
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.

Cultural Control
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. Neem products are allowed as restricted use materials.

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.

Common name Amount/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. CYROMAZINE
  (Trigard) 2.66 oz 12 7
  MODE OF ACTION GROUP NUMBER1: 17
  COMMENTS: For use on onion and garlic. Do not make more than 6 applications/crop.
 
B. PERMETHRIN*
  (Ambush) 6.4–12.8 oz 12 1
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: For use on onion and garlic. Do not apply more than 2 lb a.i./acre/season. May cause secondary outbreaks of spider mites.
 
C. CYPERMETHRIN*
  (Ammo 2.5 EC) 2–5 oz 12 7
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: For use on onion and garlic. May cause secondary outbreaks of spider mites.
 
D. PYRETHRIN and ROTENONE
  (Pyrellin EC) 1–2 pt 12 12 hours
  MODE OF ACTION GROUP NUMBER1: 3/—
  COMMENTS: For use on garlic. This material is helpful in reducing adult pressure when heavy migrations into the field are occurring.
 
E. AZADIRACHTIN#
  (Neemix) Label rates 12 0
  MODE OF ACTION GROUP NUMBER1: un
  COMMENTS: For use on onion and garlic. A restricted use material in an organically certified crop.
 
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until the harvest may take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on organically grown produce.
1 Rotate chemicals 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; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Insects and Mites
S. Orloff, UC Cooperative Extension, Siskiyou County
E.T. Natwick, UC Cooperative Extension, Imperial County
G. J. Poole, UC Cooperative Extension, Los Angeles County
Acknowledgment for contributions to Insects and Mites:
R. L. Coviello, UC Cooperative Extension, Fresno County
W. E. Chaney, UC Cooperative Extension, Monterey County

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