The weather we are having is perfect for several soil pests in growers’ vegetable fields. This sudden cool, wet spring is good weather for pests of seeds, transplants, and bulbs—like the seed corn maggot Delia platura (SCM) and other seed maggots such as Cabbage maggot Delia radicum (it prefers to feed on cole crops) and Onion maggot Delia antiqua (it primarily feeds on crops in the onion family). The seed corn maggot is one of the earliest seed maggots in a field and it has a huge host range of seeds and plants that it attacks. Seed maggots overwinter as pupae in the soil, usually in wooded areas where the maggots feed on decaying plant material. In early spring the adults emerge. Adults are elongated and dark greyish-brown with wings that overlap their bodies when they are at rest. Large swarms of flies can be seen in the spring flying over newly tilled fields. The flies mate within 2-3 days after emerging and lay eggs in soil that has a great deal of decaying organic matter including any rotting vegetation or manure, as well as bulbs, germinating seeds or newly set transplants—SCM flies are not finicky and will target the artificial medium in the root ball of transplants. That is why even fields that have been fumigated can still have problems with SCM.
Maggots will move into small stems and move up the plant causing a swelling of the stem just above ground level, while also causing root collapse and decay (Figs 1, 2 and 3). If these stems are split you will usually find the white cylindrical larvae. The larvae develop over a large temperature range of 50o-90o F. However, I have found that the flies do not like to lay eggs in soil that has reached 71oF at a 2-4 inch soil depth for 2-3 days in a row. Larvae or maggots are yellowish-white, about ¼ inch in length, legless with head-ends that are wedge-shaped (Fig. 2). The maggots complete their entire development within the soil by burrowing into seeds or bulbs or feeding on cotyledons or young transplants. The pupae are brown, oval-shaped capsules 1/5 inch in length. Generally, seed corn maggots complete their life cycle within three weeks and have 3-4 generations. It is usually the first generation that causes the most crop damage in our area, but when we have prolonged cool wet conditions the 2nd generation also may cause problems.
Although it can take 5 maggots per large seed like a snap bean to cause significant damage, once the seed has been opened up by the maggots it becomes much more susceptible to invasion from soil borne pathogens and under the
cool moist conditions a damaged seed or root is much more susceptible to attack. The maggots also can burrow into the bulb or stem of transplants such as watermelon or cantaloupe as well as cole crops, garlic, onions, etc.
In a few days or so when we have more consistent warm wet weather the adult flies are often found dead, stuck to vegetation. These flies have been infected by a fungus, Entomophthora sp. These infected flies usually will be found at the top of a tall object in the field such as a grass seed head or a wire field-flag (Fig. 4). Just before the fungus kills them the flies cement their body via their mouthparts to a tall object and die. If you look closely you’ll see the body is filled with the white fungus that has ruptured between the segments (Fig. 5). Being on a tall object allows the spores of the fungus to move longer distances and infect more flies than if the fly had died on the ground. Unfortunately, the infection rate is not enough to reduce the seed maggot population and stop infestations.
Management: As most of you know there is no rescue treatment once maggots are found in the seed or plant. Fields with moist, heavy-textured soil usually have the worst problems. To reduce the appeal of a field to egg-laying adults, disc or plow early in the season to incorporate residues from the previous crop and allow time for residues to completely decompose before planting. Destroy any weed growth. Avoid planting a crop following root crops or cole crops such as cabbage and cauliflower or after fall tomatoes. Later-season plantings tend to avoid the early season infestation of this pest. For crops like onions or garlic row covers can be used as soon as transplants are put in the field. Plants can remain covered until the ground warms. Organic medium seems to be particularly attractive to seed maggots. The use of treated seed or in-row banding of an insecticide gives some control of seed maggots, however, replacing dead transplants is the only solution after maggots kill a plant. Once seed maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventive.
One odd thing I have seen on a couple of farms has been collapsed plants, usually a broccoli or cabbage transplant, and I expected to find maggots or cutworms when digging up the plant, but instead found ants. On some of these farms the ants had attacked the maggot or cutworm doing the damage, but on many of the farms it was the ants that were doing the damage. I have seen this before almost always early in the season usually during wet cool periods. Not sure if the ants are there because a nest was ripped apart during tillage operations and the ants are getting the nest back together or if it is a new nest trying to expand. Normally the ants tend to only disturb a few plants, but on a few farms 20-30 percent of the plants were damaged. Most of the ant activity is below ground so control is difficult if needed. If cutworms or cabbage maggots are found with the damaged plants then the control recommendations in the 2016 Mid-Atlantic Commercial Vegetable Production Recommendations guide for them can be used and this will help with the ants too. The 2016 Guide can be found on-line at: http://extension.umd.edu/mdvegetables/2016-commercial-vegetable-production-recommendations-eb-137 .
Organic growers are in a greater bind for control of the ants. I have seen controls such as diatomaceous earth, mixtures of garlic and hot pepper, drenches of pyrethrums, boiling water poured onto the soil, and others, but none work very well if at all. One thing that seems to work but is labor intensive is using some sort of ring that goes around the base of the plant. This ring could be some old PVC pipe that is cut to about 2-4 inches in length and is about 1-3 inches in diameter. Some growers use Vaseline at the top of the ring to further hinder the ants from entering the ring. If the ant subterranean it will be difficult to get the ring deep enough to keep the ants out without restricting the roots of the growing plant too much.