Onion maggots are tiny maggots that feed below ground on onion bulbs, making tunnels in the bulbs and potentially allowing disease organisms to enter. These maggots are often the most serious pests of onions, especially where continuous production is practiced. Onion maggots are highly host-specific to plants in the onion family, including onions, leeks, shallots, garlic, and chives.

Appearance

Onion maggot adults are about 1/3-inch long and resemble small houseflies. Larvae taper to a point at the head and are only ¼-inch long. Their wings overlap with their bodies while at rest. Adults lay white, elongated eggs at the base of the onion plant. Cream-colored onion maggot larvae develop over the course of 3 stages that last a total of 2-3 weeks.

Symptoms and effects

Larvae feed on the hypocotyl (below ground) tissue of seedlings, resulting in various types of damage. Damage appears as wilted and yellowed foliage, followed by collapsed leaves. Leaves can become rotten, and plants may die. Onion plants are most vulnerable during the seedling stage, and larval feeding may kill seedlings. Poor plant stands may indicate an onion maggot problem. Onion maggots can cause damage throughout the season, although they are often an early season pest during stand establishment. At the end of the season, maggot feeding can lead to storage rots. Onion types differ in susceptibility to onion maggot damage, with set onions most susceptible, followed by white varieties, yellow varieties, and finally red varieties which are least susceptible.

Life cycle

Onion maggots overwinter as pupae in the soil in onion culls or cull piles. Adults emerge in mid-May and mate over a 3-day period after which they begin laying tiny, white eggs. When larvae emerge, they crawl beneath the leaf sheath and enter the bulb. They pupate in the soil and the next generation of adults appears 3-4 weeks later.

Scouting

It is too late to attempt control after damage has been detected. Therefore, action thresholds for foliar insecticide applications are based on the emergence of adults. Peak emergence can be forecasted using degree day models. Begin accumulating degree days when the ground thaws. Using a base temperature of 40° F, peak flight for the first three generations will occur at 680DD, 1950DD, and 3230DD, respectively.

Management

Cultural control

There are several programs that can be practiced to reduce populations, avoid insecticide resistance and achieve control. Rotating onion crops at least 1 mile between seedlings and previous crops, and isolating onion fields by at least ¼ mile can reduce migration of maggot populations between fields. Avoid green manure, destroy crop debris, and remove culls from field to reduce field attractiveness to egg-laying adults. Planting onion sets one week before flies are expected to emerge can reduce damage.

Chemical control

Preventative soil insecticide applications are recommended to control the first generation larvae if damage from the previous year’s crop exceeds 5-10%. Avoid foliar insecticides, since they are often ineffective on migratory adult populations. Resistance has been documented, and new at-plant seed treatments are currently available for control of this pest.

Refer the the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for recommended insecticides and management practices.