Larval feeding can destroy the root systems of young plants and create wounds on bulbs that can serve as sites of infection for bulb-rotting bacteria and fungi. The first generation of larvae usually feed on seedlings through the three to five leaf stage.^2,3 Tunneling from the feeding of first generation larvae can result in wilting and seedling death. A single larva can kill several onion seedlings, resulting in gaps without plants in a section of row.1 Feeding on bulbs can result in affected plants wilting and becoming chlorotic.⁴ Less damage is caused by second and third generation larvae, as developing bulbs on older plants are less affected than younger seedlings. It is more difficult for larvae to penetrate older bulbs, but larvae can enter bulbs through pre-existing wounds. Feeding damage on the basal plate can result in distorted growth. The greatest damage from second- and third-generation larvae is the resulting bulb decay.^1,3

The presence and numbers of onion maggot adults can be monitored using yellow sticky cards placed around the edges of fields just above the canopy. Fly counts on the cards should be done twice a week.^2,3 The dates of emergence of adults can also be estimated using growing degree day (GDD) models.

A base 39.3°F (4°C) degree model should be used because fly development can occur down to that temperature. In New England, the first-generation adults usually start to emerge when the GGD total reaches 735, second-generation adults at 1752 GGD, and third generation adults at 2975 GGD.² Rotating fields to non-Allium crops can help lower onion maggot damage, but--as mentioned above--the adult female flies can fly up to a mile from where they emerge to find suitable host plants on which to lay their eggs. Avoid planting onions near fields that were planted to Allium species the previous year, fields with high levels of undecomposed organic matter, or onion cull piles. Plant onions at least three-quarters of a mile away from these potential sources of onion maggot adults.^1,2,3

When possible, use GDD totals to delay planting onion sets until after the emergence of the first generation of onion maggot adults to avoid that initial window of egg laying. Planting after soil temperatures in the first inch of soil are above 95°F will also help reduce levels of damage from larval feeding.^1,2,3 When direct seeding, use a press wheel or chaindrag behind the planter to cover the seed row, or consider using no-till seeding to help discourage egg laying near those plants.

Floating row covers or screen cages can be used as a physical barrier to help keep egg-laying female onion maggots away from young seedlings. The edges of the cover material need to be covered with soil. Covers or cages should not be used in fields planted with onions or other Alliums in the previous season, as they can trap emerging adults with the plants. Remove the covers after the flight of the first generation adults or once upper soil temperatures have reached 95°F.^2,4 Minimize any mechanical injury to the plants, such as during post-planting cultivation or ground moving.

Deep plowing of onion fields after harvest can help reduce the survival of overwintering pupae; however, it can have negative impacts on erosion and other aspects of soil health. Remove and dispose of any culls and volunteer onion plants.^1,2,3

Natural enemies of onion maggot include ground beetles, rove beetles, parasitic wasps and flies, entomopathogenic fungi, and nematodes. These organisms can help reduce the number of certain onion maggot developmental stages and lower damage levels. However, beneficial insects can be harmed by some standard insecticide applications. So, the dates of application and specific products used should be chosen to help minimize harm to these populations.^1,2

Some regional pest management guides recommend treating seeds with insecticides before to planting to help manage onion maggot.^1,3 The seed treatment products recommended depend on maggot species that are present (onion, seed corn, or both), as some of the products may be effective against one species but not the other. Products containing spinosad, cyromazine, thiamethoxam, clothianidin, and imidacloprid are registered for control of maggot species on onions.^5,6,7 Spinosad (active ingredient in LumiverdTM Insecticide) is active against both species, but reports from New York indicate some decline in effectiveness.5 Cyromazine (Trigard® OMC Insecticide) is reportedly more effective against onion maggot than seedcorn maggot. Thiamethoxam (Cruiser®70 WS Insecticide) may no longer be highly effective for either species.^5,6,7 Seed treatments that combine two or more of these products can help to control the first generations of both onion and seedcorn maggots.

Preplant soil applications of insecticides are recommended in some management guides. The insecticides are most effective when applied in-furrow or moved into the root zone with irrigation water after narrow-band, over-the-row applications.^1,2 For transplants, applications should be directed at the base of plants shortly after transplanting. Check regional management guides for products recommended in your area. Foliar insecticide applications to older, standing onion crops are generally not considered to provide adequate control of onion maggot. Insecticides to manage second and third generations of onion maggot are often unnecessary and not adequately effective.^1,3 Always consult current product labels for application instructions and restrictions.