How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic


Scientific names: Seedcorn maggot : Delia platura
Onion maggot : Delia antiqua

(Reviewed 1/07, updated 6/08, pesticides updated 6/16, corrected 1/17)

In this Guideline:


Adults of Delia species are small gray flies that are somewhat smaller than house flies. When at rest, they keep their wings folded one over the other. Larvae are creamy white, legless maggots about 0.4 inch (10 mm) long. Microscopic examination is required to distinguish between species. The flies lay eggs in the soil surface near the germinating plants. Larvae feed on the developing seedling and, in the case of the onion maggot, on the expanding bulb. Mature larvae pupate in the soil. There are several generations per year. Maggots prefer soils heavy in organic matter where they can survive and move to seeds. Seedcorn maggots occur throughout California garlic and onion production areas. Onion maggots are more restricted to cooler coastal climates. Maggots are primarily a pest of onions and do not generally cause economic damage to garlic.


Larvae of seedcorn maggots attack germinating seedlings, feeding on the developing roots and epicotyl. Their damage is usually restricted to the very early seedling stage. Onion maggots inflict similar damage but can continue to feed on the expanding bulb during later stages of growth. This results in increased rot in bulbs held in storage.


Cultural Control

Avoid planting in soils that are high in undecomposed organic matter, such as fields just coming out of pasture or very weedy situations. In soils amended with animal manures, allow adequate time for the manure to break down before planting. Avoid planting successive rotations of onion crops. Early spring-planted crops are more likely to be damaged when the soil is too cool for rapid germination and emergence. If serious infestations are expected, wait until the soil warms up in spring, or if feasible, plant in fall while the soil is still warm. When planting, use a chain drag or similar implement behind the drill to cover the seed row.

Monitoring and Management Decisions

No specific monitoring methods have been developed. However, estimates of adult fly activity obtained from the use of yellow sticky traps have been used in other parts of the country to assist in determining the necessity and timing of treatments. The use of yellow sticky traps may also be helpful in California growing areas where onions are planted in summer or fall. Treatments for onion and seedcorn maggot are preventative and should be considered for fields that are high in organic matter or undecomposed organic material, or that have had previous maggot problems.

Common name Amount per acre REI‡ PHI‡
(Example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
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. Not all registered pesticides are listed. Always read the label of the product being used.
  (Sepresto 75WS) bulb: 0.0074–0.011 oz/1000 row NA NA
    bunching: 0.0056–0.0065 oz/1000 row NA NA
  COMMENTS: Only available on Nunhem's onion varieties. Seed treatment. Avoid planting when pollinators are nearby; pesticide may become airborne.
  (Lorsban 4E) 1.1 oz/1000 row ft at 18-inch spacing 24 NA
  COMMENTS: For dry bulb onions only. Apply as an in-furrow drench in 40 gal/acre. Incorporate into soil 1 to 2 inches. Do not make more than one application per year. Avoid drift and tailwater runoff into surface waters.
  . . . or . . .
  (Lorsban 15G) 3.7 oz/1000 row ft at 18-inch spacing 24 NA
  COMMENTS: For dry bulb onions only. Apply in-furrow. Do not make more than one application/year.
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 until the harvest may take place. In some cases the REI exceeds the PHI 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.
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 the 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).
NA Not applicable



[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
L. D. Godfrey, Entomology, UC Davis
S. K. Dara, UC Cooperative Extension, Santa Barbara 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

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