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Cabbage, onion or seed corn maggot?

Posted: April 19, 2012

With the warm weather insects are early this spring. Growers have seen damage from onion, seed corn and possibly cabbage maggot. Onion maggot damage of 10-20% has been confirmed in two fields in the Shippensburg area. Bean seed maggot was confirmed in Northampton County yesterday.

These maggots attack seeds and small seedlings. While cabbage maggot and onion maggot attack only the seeds and seedlings of their namesakes, seed corn maggot will attack many plant species.

Why are they early this year?

The flies over-winter in the soil as pupae. Insect life cycles speed up with warmth and adults emerge from the soil after sufficient heat units or degree days have accumulated. For example in Easton PA the seasonal accumulation of degree days is 607 already (base 40). Onion maggot first peak flight is at 700. Usually we would not have this many degree days until mid May. See Tom Butzler’s recent article on early insect emergence.

Cabbage, onion, or seed corn maggot – they are 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?

Monitoring: 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). PA is not in their model. To calculate local growing degree days go to http://www.weather.com/outdoors/agriculture/growing-degree-days/ select Jan 1 to current date and base 40 F for seed corn maggot (50 for cabbage and onion). You can also see a map of base 50 GDD in Pennsylvania at PAPipe. Compare your local GDD to projected flights (NEWA & below). For example peak flight for seed corn maggot is 360 GDD (base 40 F) [1].

Controls

  1. 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.
  2. 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.
  3. 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. This year with early flights it may be worth it to hold some back [2].
  4. 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 [3].
  5. Soil Drench. 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.
  6. 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.
Generation Seedcorn Onion Cabbage
1st Peak 360 450-540 450

 

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.
1st Peak 700
2nd Peak 1960
3rd Peak 3240

 

Cabbage Maggot Flights (Model by: J.L. Jyoiti and A.M. Shelton) converted to Fahrenheit
Stage Accumulated Degree Days

1st Emergence

288 +/- 15
25 percent 366 +/- 5
50 percent 452

 

+/- 14
75 percent 547 +/- 66
95 percent 697 +/- 14
Overwintering generation 809 +/- 3

     
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
F1 915 +/- 59
F2 838 +/- 38
F3 718 +/- 6

Based on Growing Degree Days base 40 F. From NEWA.

  1. 1.Cullen, E. Threshold Temperature and Heat Unity Accumulation. University of Wisconsin.
  2. Hazzard, R. Cabbage and Onion Maggot Flies. UMASS.
  3. Hazzard, R., Emergence of Spring Maggot Flies is Approaching. UMass Pest Alert, 2012.
  4. 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.
  5. 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.