Seedcorn, Onion & Cabbage Maggots

by Becky Sideman, UNH Extension Professor & Specialist, Vegetable & Berry Crops

It is the time of year when various root maggots can cause damage to young vegetable transplants.

Root maggots are the larvae of several families of flies that feed on seeds and young seedlings of many crops including corn, beans, onions, spinach, cole crops, and others. The first symptoms are usually poor germination, or failure of seedlings to emerge. Transplants may turn yellow and wilt, a sure sign of root damage. Symptoms can be difficult to distinguish from other problems, like damping off due to soilborne fungi, or wireworm feeding. If maggots are the culprit, they can usually be found in the soil around and inside seedlings and seeds (see photo below). Maggots are yellow-white, ¼ inch long, legless, with a wedge-shaped head. The adults look like small houseflies.

seedcorns

Arrows point to several maggots in cold spring soil that was recently amended with soybean meal. If you look closely, you can see even more without arrows!

The most common of the root maggots is the seedcorn maggot (Delia platura). The seedcorn maggot attacks a broad spectrum of crops, and is frequently reported as causing damage to corn, bean and pea, as well as to transplants such as melons, spinach, cole crops, and onions. Onion maggot (Delia antiqua) and cabbage maggot (Delia radicum) flies look nearly identical but are likely to be found only on or near their host crop. Cabbage root maggot attacks all types of Brassica crops, while onion maggots are highly specific for the onion family.

All of these maggots overwinter in the soil as small brown pupae. In early spring, the adults emerge and lay eggs. The time it takes for them to hatch depends on soil temperature. In Minnesota, research has shown that peak emergence of the first three generations of seedcorn maggot occur when 200, 600 and 1000 degree days (°C) have accumulated. Degree days are calculated on a daily basis by using the formula: (Max temp – Min temp)/2 – base temperature. You can also refer to a handy calculator at http://newa.cornell.edu/index.php?page=growing-degree-days. This calculator tells me that, here in Durham, we reached 200 DD (°C), which equals 360 DD (°F), using a base temperature of 40°F by May 9 of this year. The first generation usually causes the most damage. The cabbage and onion maggots are a bit later than seedcorn maggots. Cabbage root maggot adults typically begin to fly right around the time that yellow rocket begins to flower.

The adults prefer to lay eggs in wet soil that is rich in organic matter. Crop seeds that germinate slowly are more vulnerable to attack. The adults are likely to be active ahead of schedule, and crops that are planted in wet soil when the soil is too cool for them to germinate quickly may be especially susceptible to damage.

Management strategies: By the time you see damage, it is too late to control the problem using either cultural or chemical methods. Prevention is the key.

  • Avoid seeding fields (especially wet fields) too early. Seeds germinate more quickly and are less vulnerable in warmer soils.
  • Disk and incorporate organic matter (such as a cover crop) at least 4 weeks before seeding to give it time to break down and make it less attractive to the flies
  • Avoid applying manure in late fall or early spring to heavy soils that you might want to plant early. Lighter, well-drained, sandy soils are less likely to have problems (because they warm up faster than others).
  • Rowcovers can help – but only if the maggot flies are coming from elsewhere, and if you do not trap flies under the rowcovers as you apply them. Damage can occur if the flies have overwintered in the soil underneath the rowcovers.
  • If you need to replant, wait at least 5 days if maggots that you find are ¼ inch long; if they are smaller than that, wait at least 10 days to make sure they have pupated and will not damage the new seeds.
  • Preventative soil-applied insecticides are an option for some. These are most effective when made prior to planting or laying plastic. See the most recent New England Vegetable Management Guide for recommended materials and timing for specific crops. Don’t have the guide? Check it out online or order one for $25 by calling your county Extension office or emailing me.
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