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How to Manage Pests

UC Pest Management Guidelines

Onion maggot.

Onion and Garlic


Scientific names: Seed corn maggot : Delia platura
Onion maggot : Delia antiqua

(Reviewed 1/07, updated 6/08)

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. Seed corn 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 seed corn 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 seed corn 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/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. DIAZINON* 14G 24–28 lb 12
  COMMENTS: Registered for onions only. Broadcast and incorporate into soil 3–4 weeks before planting.
  . . . or . . .
  DIAZINON* AG500 1 qt 24
  COMMENTS: Registered for onions only. Apply in sufficient water to drench seed furrow at planting. Avoid drift and tailwater runoff into surface waters.
  (Lorsban) 4E 1.1 oz/1000 row ft at 18-inch spacing 24
  COMMENTS: For dry bulb onions only. Apply as an in-furrow drench in 40 gal/acre. Incorporate into soil 1–2 inches. Do not make more than 1 application/year. Avoid drift and tailwater runoff into surface waters.
  . . . or . . .
  (Lorsban) 15G 3.7 oz/1000 row ft at 18-inch spacing 24
  COMMENTS: For dry bulb onions only. Apply in-furrow. Do not make more than 1 application/year.
+ 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.
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). For additional information, see their Web site at http://www.irac-online.org/.
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
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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