Several species from the same genus (Delia) cause the damage. It appears to be either the bean seed maggot (D. florilega) or the seedcorn maggot (D. platura). Management is similar for both species, and sometimes they co-exist in mixed populations. Seedcorn maggot biology is summarized in "Vegetable Gardening: Recommendations for Home Gardeners"(below - except note that adult emergence appears to be starting earlier, in early April, at least in 2017):
Maggots pupate inside a dark brown, capsule-like puparium that resembles a grain of wheat. Seed corn maggot puparia can be found in soil throughout the year, and maggots overwinter in these puparia. The adult flies emerge from the puparia [in] late April and early May [Note: in 2017, first reports are in early April]. The adults are brownish gray flies that closely resemble common houseflies, except that they are about half the size. Tiny, white, elongated eggs are deposited among debris and around plant stems near the soil surface. Eggs hatch in a few days and the maggots work their way into the soil in search of food. Maggots (the immature larval stage) are dirty white with a yellowish tinge, legless, cylindrical, and tapered; full-grown maggots reach 0.2 to 0.25 inch in length. Maggots feed in the seed or on the underground parts of seedlings. Damaged seed may germinate, but there may be too few food reserves left in the seed for the plant to survive. The time required to grow from egg to adult is between three and four weeks. There are three to five generations each year in Pennsylvania. Populations tend to decline during the dry months of summer.
We are finding large (thus, older) maggots feeding up into the plant stem. This sounds like the adult flies laid eggs on the transplants while they were still in the greenhouse / high tunnel. Check transplants for maggots before transplanting by combing through the root ball and slicing open stems.
Classically, cool, wet, slow-growing spring weather is great for maggot pest problems. Plants are less able to outgrow the maggot feeding. Planting in warm soils is the best management option. If you have to plant in cool soils, avoid planting into an abundance of decaying organic matter. Incorporate organic matter well, several weeks prior to planting. Be prepared to replant if you have significant stand loss. Check if the maggots are young (less than 1/8 inch), and if they are, wait another week or two for them to pupate before replanting. When they reach about ¼ inch, they are ready to pupate.
Figure 1. Seedcorn Maggot (Delia platura): top image shows size of larvae in millimeters (~5mm), middle image shows late instar larvae and pupa, bottom image shows adult fly.1
Figure 2. Bean Seed maggot (Delia florilega): top image shows size of larvae in millimeters (~5mm), middle image is of larva in bean cotyledon, bottom image is of adult.2
Both seedcorn maggot and bean seed maggot have a very wide host range, and sweet corn, beans, and muskmelons (from seed or transplant) have had significant damage in the past. We are getting Delia (probably florilega) in cabbage and kale seedlings. Cabbage maggot and onion maggot have also caused problems to the crops that carry their name. Some experts can distinguish among species with the larger larval stages, using prothorasic spiracles and patterns on the rear anal plate. Adults are helpful to distinguish species.
Seed treatments (neonicotinoids, or chlorpyrifos), and pre- or at-plant soil treatments (organophosphates or pyrethoroids) are registered for sweet corn, and can provide effective control, and each crop will have different registered options. Neonicotinoids used at planting for other pests also provide some control of root maggots. Rescue treatments that occur after the maggots are feeding inside a developing seed, however, tend to not be effective.
Can we monitor with traps? Could traps provide control?
Research (Kuhar et al. 2006) suggests opportunities. They combined the traditional white or yellow sticky traps (Great Lakes IPM, or Gemplers) with a slow-release attractants ("Adult Maggot lure", from AgBio Inc., 9915 Raleigh St. Westminster, CO 80031; 303-469-9221; firstname.lastname@example.org). Trap catch was increased significantly, and sex ratio more resembled what we'd expect from a field population. Kuhar et al. 2006 suggested that a high enough density of these traps might actually control the population in very small or confined production areas, providing one option for organic growers, and anecdotal evidence suggest that this might be helpful. The concept makes sense: researchers have used olfactory attractants (blood meal, fish meal, onion pulp, yeasts, etc.) to increase infestation rates when running efficacy trials. This needs to be evaluated further; it is possible that attracting adults could make the problem worse. Also, attractants may vary for different Delia species. But it is worth trying.
Degree Days to estimate when the seedcorn maggot first adult flight tapers off.
The Northeast Network for Environment and Weather Applications (NEWA) has expanded to multiple sites in PA. The onion maggot model in on NEWA website. Select Jan 1 to current date and base 400F for seed corn maggot (50 for cabbage and onion). You can also see a map of GDD base 40 Or base 50.
Compare your local GDD to projected flights. Peak flight for seed corn maggot is 360 GDD (base 40 F), 450-540 for onion maggot (base 50), and 450 for cabbage maggot (base 50).
Nematodes for biological control.
Figure 3. A house fly, Musca domestica, larvae infected with Steinernema feltiae.3
Tianna Dupont also summarized using Steinernema feltiae [Chen et al. 2003, Schroeder et al 1996 ]. As stated in her earlier report:
"Steinernema feltiae are "cruzer foragers" says UC Davis nematologist Amanda Hodson. They forage right at or below the surface, unlike other nematodes that will stay on the soil surface. Hodson recommends applying infective juveniles in the irrigation (drip or microsprinklers). "Apply them in the morning and evening when it is not too hot and irrigate to keep soil moist," she says. Another common method is application to transplants before planting or in the water wheel transplanter. Carol Glenister at IPM Laboratories cautions that "the nematodes die in sunlight within 30 minutes, so need to be trenched or washed down into the soil." They have had over a decade of satisfaction using Steinernema feltiae on cabbage maggot. Due to similar biology corn seed maggot may also be controlled. But her customers have not had success on onion maggot. They recommend 25 million infective juveniles for a transplant dip (@200 trays) with perhaps another 25 million in the water wheel transplanter. Rates of 100,000 to 125,000 infective juveniles per transplant have been shown to be needed to achieve reduction in damage."
Chen, S.L., X.Y. Han, and M. Moens, 2003. Biological control of Delia radicum (Diptera : Anthomyiidae) with entomopathogenic nematodes. Applied Entomology and Zoology. 38(4): p. 441-448
Kuhar, T., W. D. Hutchison, J. Whalen, D. G. Riley, J. C. Meneley, H. B. Doughty, E. C. Burkness, and S. J. Wold-Burkness. 2006. Field evaluation of a novel lure for trapping seedcorn maggot adults. Plant Health Progress. Doi: 10.1094/PHP-2006-0606-01-BR.
Schroeder, P.C., et al., 1996. Greenhouse and field evaluations of entomopathogenic nematodes (Nematoda: Heterorhabditidae and Steinernematidae) for control of cabbage maggot (Diptera: Anthomyiidae) on cabbage. Journal of Economic Entomology. 89(5): p. 1109-1115.
1Figure 1 - Seedcorn Maggot top image by Jade Savage, Bishop's University; middle image and bottom image by Leon Higley, University of Florida.
2Figure 2 - Bean Seed Maggot top image by Jade Savage, Bishop's University, bottom image by Janet Graham April 2012 in Marford Quarry, North Wales.
3Figure 3. A house fly larvae by Allen L. Szalanski and C. Dayton Steelman, USDA Regional Project S-1006.