Cabbage, onion, or seed corn maggot - are they all the same right?
When you see little wrigglers in your plant roots it is disheartening and you might not care which maggots are causing the problem. In general you are right. A maggot is a maggot. But there are a few differences that may make it worth figuring out which problem you have. Cabbage and onion maggot stick to their namesakes. Cabbage maggot attacks all mustard family crops (cabbage, broccoli, mustards, turnips, as well as weedy crucifers) and onion maggot attacks alliums. When overwintering pupae emerge as adult flies they look for their host and lay their eggs at the base of the plant. Cultural controls such as rotation and floating row cover may work for cabbage and onion maggot. In contrast seed corn maggot has a wide host range and seems to be attracted to freshly turned organic matter. Seed corn maggots might already be there waiting when you transplant. Historically we have seen significant damage from seed corn maggot in sweet corn and beans planted from seed, and muskmelon placed in the ground as transplants.
Is your planting at risk?
Check to see if the adult flies are in your field. They look like houseflies, but about one-third the size of houseflies. The flies are much easier to identify and monitor than the eggs or maggots. Flies are attracted to bright yellow colors. Yellow sticky cards (3X5 inches) are inexpensive and easy to use; you can purchase small wire stakes made for this purpose, or clip to a wooden stake. Place near the soil. Check and change traps twice weekly to record changes in fly activity. Leaving them for a whole week usually results in a card and flies covered with blown soil, and is a less accurate measure of flight activity (sources: Great Lakes IPM, Gemplers). Yellow pan traps filled with water and a drop of soap also work.
Using Growing Degree Days
The beginning and peak of each fly generation can be identified using Growing Degree Day (GDD) accumulations. To check for ongoing GDD updates, visit the Northeast Network for Environment and Weather Applications (NEWA). Choose a monitoring location near you and the appropriate crop. You can also see a map of base 50 GDD in Pennsylvania. Compare your local GDD to projected flights (NEWA & below). For example peak flight for seed corn maggot is 360 GDD (base 40 F) .
Wait for green material to decay
If you are incorporating green cover crop let it sit as long as possible. That way the maggots can feed on the green material first.
Row cover can exclude the flies
But if the problem is corn seed maggot, they might already be there feeding on decaying organic material. Use in a rotated field, as flies overwinter in soil.
Avoiding damage by later planting
The first flight and egg-laying period is generally the most intense. After the first flight is over, and as soils heat up, perhaps fewer eggs are laid and those that are laid are less likely to survive. Also, plants that are growing rapidly, in warm soils, have a much better chance of out-growing feeding. You may want to watch the growing degree days and wait to transplant till the first flight has peaked. This is often later than optimal planting time. 
Monitoring cabbage for eggs
If you have transplants hardening off in a cold frame or outdoors, flies may find them and lay eggs in the flats. To check for eggs in the field or in flats, look for the 1/8-inch long, torpedo-shaped white eggs that are laid along the stem, or on the soil next to the stem of young transplants. Often eggs are laid in neat rows, or inserted into the soil. They may be under a small clod of dirt near the stem. A pencil point or knife helps stir the soil to look for them. A reliable field scouting method is to check 25 or more plants, in groups of 2-5 plants, scattered around the field. If you find an average of 1 egg/stem or more, it is likely to be a damaging population and a banded soil drench is recommended. Eggs may be more abundant in wetter areas of the field. Egg numbers may build up rapidly after the first eggs are seen .
Check the Commercial Vegetable Production Guide for products labeled for the crop you are working with. Note Lorsban has documented resistance in many fields in New York according to Dr Brian Nault, Cornell. As a foliar Lorsban is labeled but often has low efficacy according to Nault. For organic growers there has been some efficacy using spinosad products. Entrust and Seduce are possibilities as an in furrow soil drench at planting or a transplant dip. If a planting is already infected typically treatment will not be affected.
Nematodes for biological control
One alternative method that has shown promise but has not been widely field-tested is soil application of entomopathogenic nematodes, especially Steinernema feltiae [4, 5]. It is important to match the life history of the nematode with the pest. For example 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 hand over a than 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.
Comparison of peak flights (GDD base 40 F).
Onion Maggot Flight Peaks
|Flight Peak||Accumulated Degree Days|
Based on Growing Degree Days base 40 F. From NEWA.
Cabbage Maggot Flights
(Model by: J.L. Jyoiti and A.M. Shelton) converted to Fahrenheit
|Stage||Accumulated Degree Days|
After completion of spring emergence, accumulated degree days need to be reset to zero. Degree days necessary to complete F1, F2 and F3 generations are shown below. After each generation, the accumulated degree days need to be reset to zero.
|Generation||Accumulated Degree Days|
Based on Growing Degree Days base 40 F. From NEWA.
- Cullen, E. Threshold Temperature and Heat Unit Accumulation. University of Wisconsin.
- Hazzard, R. Cabbage and Onion Maggot Flies. UMASS.
- Hazzard, R., Emergence of Spring Maggot Flies is Approaching. UMass Pest Alert, 2012.
- Chen, S.L., X.Y. Han, and M. Moens, Biological control of Delia radicum (Diptera : Anthomyiidae) with entomopathogenic nematodes. Applied Entomology and Zoology, 2003. 38(4): p. 441-448.
- Schroeder, P.C., et al., Greenhouse and field evaluations of entomopathogenic nematodes (Nematoda: Heterorhabditidae and Steinernematidae) for control of cabbage maggot (Diptera: Anthomyiidae) on cabbage. Journal of Economic Entomology, 1996. 89(5): p. 1109-1115.