Cooperative Extension at Highmoor Farm

Garlic IPM Newsletter No. 3 — May 1, 2025

Disease Forecast Alert: Conditions in Bangor, Lewiston, and Milton, NH may be favorable for onion downy mildew and Botrytis leaf blast development in coming days. See NEWA onion diseases for details – more on these in a future newsletter.

Onion Maggot

Onion maggot (Delia antiqua) can be a serious pest of allium crops. Overwintered pupae emerge as adults in spring, constituting the first of three generations per year. These adults, which look like small house flies, lay eggs at the base of allium plants or sometimes on their young leaves. The resulting larvae move into the soil and feed on belowground roots and bulbs. Larvae feed there for 2-3 weeks before they pupate in surrounding soil to cycle into the next generation. For images of onion maggot, go to the University of California IPM Maggots webpage.

Affected plants become stunted, wilted, yellow, and may die. This will cause larvae to move to nearby plants. Young and/or injured plants are especially favored by the maggots simply because they are easier to penetrate. The overwintering generation is the most damaging, as it can reduce stands by more than 50%. Still, damage from second and third generations can distort growth, enable pathogen entry, and render crops unmarketable.

The overwintering generation of onion maggot begins to emerge at 390 GDD_40°F, with “peak flight” (50% emergence) occurring at 735 GDD_40°F. NEWA has a tool specifically for onion maggot – choose the location that best reflects your weather conditions to see how many GDDs have accumulated near you, as well as the 5-day forecast. Near the top of the page, you can click on “More Info” to learn about the pest and model. GDDs for more Maine sites are on the Maine Climate Office page; be sure to look at the map labeled for base 40°F. Current information from both sources is combined below, showing the forecasted GDDs where possible from NEWA.

GDDs from NEWA and Maine Climate Office. Values with an * indicate the 390 GDD threshold for onion maggot emergence is met.

Durham and Rochester, NH, will accumulate sufficient GDDs by early next week for onion maggot emergence to begin, and locations in southern Maine will soon follow. You can see in the NEWA charts that 10-20 GDDs easily accumulate daily with the warm weather we’ve been having.

NOTE: Just because GDDs are sufficient for pest development does not mean that the pest is present in your field. Anticipate potential infestations based on problems in previous years both in your own field as well as the greater area.

Controls

There are no known allium cultivars that are resistant to onion maggot, and no controls will be effective in-season once damage has been detected. Therefore, preventing infestation and population buildup is key!

• Crop rotation: Plant 0.5 – 0.75 miles from previously infested sites where possible.
• Delay planting: This is not possible for garlic or other overwintered alliums. For spring-planted alliums, however, time planting to avoid coinciding with emergence in your area – either well before or after.
• Row cover: Protect young plants with row cover to keep adults from laying eggs by the crop. This must be in place before peak flight (preferably before emergence). This will not help if planting in a previously infested area.
• Minimize physical damage: Avoid damaging plants when cultivating, etc., to reduce easy entry points for maggots.
• Beneficial insects: Parasitic wasps and predatory beetles and flies are all natural enemies of onion maggot. Promote these populations with habitat-building practices (see Cornell University’s bulletin for suggestions).
• Destroy culls: Remove all culls from the field and either bury or compost them in deep piles to reduce surface area from which spring emergence may be possible. Plow fields in fall to reduce overwintering flies.
• Chemical control: Consult the New England Vegetable Management Guide for options.

For more information on this pest, see resources from Cornell, Univ. of Massachusetts, and the Univ. of Wisconsin.

Peyton Ginakes, PhD
Research Associate
University of Maine Cooperative Extension
Highmoor Farm
52 US-202
Monmouth, ME 04259
(207) 933-2100

This project is funded by a Specialty Crop Block Grant through the Maine Department of Agriculture, Conservation, and Forestry. Funding for the Maine 2024 Specialty Crop Block Grant Program was made possible by a grant/cooperative agreement from the USDA Agricultural Marketing Service. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USDA.

Where brand names or company names are used, it is for the reader’s information. No endorsement is implied nor is any discrimination intended against other products with similar ingredients. Always consult product labels for rates, application instructions and safety precautions. Users of these products assume all associated risks.

The University of Maine System is an equal opportunity institution committed to nondiscrimination.