Maggots

Adults of Delia species are small gray flies. When at rest, they keep their wings folded one over the other. Onion maggot flies are similar in size to house flies. Seedcorn maggot flies, however, are significantly smaller, ranging from between 1/2 to 2/3 the size of onion maggot flies.

Larvae (maggots) are creamy-white, legless maggots that are about 0.4 inch (10 mm) long. Microscopic examination is required to distinguish between species.

Female flies lay white, elongated eggs on or near the soil surface near the base of the onion plant. Maggots prefer cool, moist soils heavy in organic matter, where they can survive and move to seeds. Seedcorn maggots more commonly move to seeds than do onion maggots. Mature larvae pupate in the soil.

Maggots have several generations per year. Seedcorn maggots occur throughout California garlic and onion production areas, while onion maggots are more restricted to cooler coastal climates and the intermountain region. Maggots are generally not a problem to onion production in the low desert region.

Larvae feed on the epicotyl (developing shoot) and young roots of the developing seedling. Seedcorn maggots feed on the seed and may damage onion plants as late as the 3- to 4-leaf stages.

The first generation of onion maggots primarily feeds on seedlings, up to the 3- to 5-leaf stages. The second and third generation of onion maggots will feed on the expanding bulb of the maturing plant, which can increase bulb rot during storage.

Maggots are serious pests of onions in California, but do not generally cause economic damage to garlic. Without proper planning and insecticide controls, these pests cause onion growers to lose more than 50% of their yields.

See Identifying Pests of Onion and Garlic and Their Damage for photos of maggots and their damage, as well as identification tips.

Prevention is the key to avoiding damage from onion and seedcorn maggot, and insecticides should be applied when maggot infestations are expected.

Delaying planting is an important cultural practice to reduce damage from maggots, though this may be less feasible in cooler climates with shorter growing seasons. The amount of time to delay planting differs for onion maggot and seedcorn maggot.

For onion maggot, wait to plant until later in the spring, after the first generation of adult flies has emerged. This will avoid the first generation of egg-laying by adult flies that overwintered in the soil during the previous year. Choose when to plant by calculating the accumulation of degree-days of average daily air temperature:

• Estimate the date when 50% of overwintering adult flies have emerged based on the accumulation of 792 to 812 degree-days, with a developmental threshold of 39.2°F.
• Plant after the estimated date.

For onion maggot, take the following cultural measures in addition to delayed planting:

• Remove and dispose of onion culls and volunteer onions.
• Avoid planting successive onion crops without rotating to other crops.
• Avoid planting onions near fields where onions were recently grown, or fields that are located near onion cull piles.
  • These fields most likely harbor overwintering onion maggot pupae.
  • If possible, plant onions no less than 3/4 of a mile near fields that were previously planted to onion.

For seedcorn maggot, use degree-day accumulation to determine amount of time to delay planting after tilling or soil cultivation:

• Assume that adult seedcorn maggot flies will begin laying eggs in the field immediately after the soil is disturbed.
• Estimate the date of the first adult emergence by tracking the accumulation of 421 to 459 degree-days Fahrenheit (with a developmental threshold of 39.0°F) after tillage.

In the intermountain region, delaying planting 13 to 21 days after cultivation, tillage, or bed-shaping can reduce seedcorn maggot infestations. Soil conditions after tillage attract seedcorn maggot flies to lay eggs. Waiting at least 13 days will allow the first generation of seedcorn maggot to pupate and minimize their damage to onions. However, waiting as long as 21 days further reduces developmental time for onion bulbs, which may significantly reduce yields.

For seedcorn maggot, take the following measures in addition to delaying planting after disturbing the soil:

• Avoid planting onions after a rotation of legume crops, especially alfalfa. Adult seedcorn maggot flies are especially attracted to these decaying residues.
• Plant seeds at a shallow depth (but no less than 1/2 inch below the soil surface).
• Thoroughly incorporate organic matter such as manure, crop residue, weeds, and cover crops into the soil well in advance of planting (3–4 weeks prior).
  • In soils amended with animal manures, wait for the manure to break down completely before planting.

Cultural methods for both onion maggot and seedcorn maggot include the following:

• Use a press wheel, chain drag or similar implement behind the drill to cover the seed row when planting.
• Plant in conditions that favor rapid onion emergence, such as when the soil is warmer. This will allow onions to quickly grow to a stage that is harder for maggots to damage.
• Avoid fields that have high amounts of undecomposed organic matter, such as fields just coming out of pasture, alfalfa, or recently tilled or cultivated weeds.
• For direct-seeded crops, use a higher seeding rate.
• Consider no-till seeding.

Monitor adult activity by placing yellow sticky traps around the field edges just above the growing onion foliage. Use degree-day models mentioned above in the Cultural Control section to determine when to plant.

Pesticides are ineffective or impossible to apply to maggots attacking a standing crop. The combination of cultural methods and insecticide-treated seed is critical to avoiding intolerable yield loss. Consider planting insecticide-treated seed, especially for fields that are likely to be infested with maggots (see the Cultural Control section) or fields that have previously had problems from maggots.