How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

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.

DAMAGE

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.

MANAGEMENT

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.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Insects and Mites
R. L. Coviello, UC Cooperative Extension, Fresno Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
S. Orloff, UC Cooperative Extension, Siskiyou Co.
G. J. Poole, UC Cooperative Extension, Los Angeles Co.

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