Steinernema feltiae
Entomopathogenic nematode- Rate
- 250k–500k /m²
- Temp
- 10–30 °C
- Best for
- Cabbage and onion maggot; on the tray or as a drench at planting
Pest
Root maggots
Delia spp. & Chamaepsila rosae
The short answer
Four closely related problems share one answer. The cabbage maggot, onion maggot and seedcorn maggot are the larvae of small grey Delia flies that look like little house flies, and the carrot rust fly is a separate small dark fly. All four lay eggs at or near the base of the plant, and the legless, pale maggots feed in the roots. They kill seedlings and transplants and tunnel into radishes, turnips, onion bulbs and carrots. The fixes are row covers put on at sowing, crop rotation, planting after the first flight, and letting organic matter rot down before you sow. By the time a plant wilts, it is too late for that planting.
hero macro · maggots and pupae on brassica roots
Delia radicum larvae and pupae on the roots of a young brassica. Photo: pending.
Identification
The three Delia flies are delicate, slightly hump-backed grey-brown flies about 1/4 inch (5–7 mm) long, and cannot be told apart by eye. UMass notes each turns up mainly near its own host crop. UC IPM finds the seedcorn maggot fly smaller, one-half to two-thirds the size of the onion maggot fly. Their maggots are white, legless and headless, pointed at the head end, and up to about 0.4 inch (10 mm). Telling the species apart takes a microscope, but the crop usually tells you which one it is.
The carrot rust fly is shinier, about 1/4 inch long, with a dark body and a yellow head and legs. Its maggots are slimmer and creamy yellow, up to about 1/3–3/8 inch (9 mm), and they leave rust-brown tunnels in the surface of the root. Pupae of all four are small, oblong and brown, and are found in the soil around the roots.
Easily confused with
- Seedcorn maggotAlso a pestThe maggots look the same without a microscope, but seedcorn maggot attacks seeds and seedlings only, rarely tunnels in bigger roots, and is active two or more weeks earlier in spring.
Damage symptoms
Maggots feed on the roots, so the first sign is a plant that wilts on sunny days and yellows or purples before it collapses. Seedlings and recent transplants are killed outright. Larger brassicas often survive, because their bigger root systems can tolerate the loss. Where the root is the crop, as in radish, turnip, swede and carrot, the tunnels make it inedible and let rots in. Seedcorn maggot eats the seed itself and the germinating seedling, and it favours large seeds such as corn, beans and peas. Its damage is easily mistaken for damping-off or wireworm.
- Transplants or seedlings wilting on sunny days, then yellowing, purpling and dying
- Patchy or failed emergence from direct-sown seed, especially corn, beans and peas
- Tunnels and maggots in radish, turnip and swede roots
- Onion seedlings dying, and later rotting bulbs in store
- Rust-brown scars ringing carrot and parsnip roots, with slender yellow maggots inside
- White legless maggots and brown pupae in the soil around the roots
Lifecycle & degree days
All four spend the winter mostly as pupae in the soil (carrot fly also as larvae). The Delia flies are active early, because they develop from about 40 °F (4 °C) rather than the usual 50–55 °F. In the North, the first spring generation does the most damage, because it meets the youngest plants. In California, UC IPM finds summer and autumn brassica plantings suffer more. Cabbage maggot has 3–4 generations a year in central New England, with a late-summer flight that hits autumn root crops. Onion maggot has three, and seedcorn maggot several. UMass gives the peak flights in degree days (base 40 °F; the seedcorn model counts from 1 January): seedcorn 360, cabbage 452 and onion 735 for the first generation. Cabbage and onion maggot eggs die after several days in soil above 95 °F. In the UK, the RHS finds carrot rust fly eggs laid in late May–June and again in August–September, over two or three generations. Its advice is to sow after mid-May to miss the first and harvest before late August to miss the second.
- Base temperature
- 40 °F (4 °C)
- 1st peak flight
- Seedcorn 360 · cabbage 452 · onion 735 DD °F
- Generations / yr
- Cabbage 3–4, onion 3 (New England); carrot fly 2–3
- Overwinters
- Pupae in the soil (carrot fly also larvae)
J
F
M
A
M
J
J
A
S
O
N
D
Host crops
Cabbage, broccoli & cauliflowerRadish, turnip & swedeOnion, shallot & leekCarrot, parsnip & celeryBeans & peas (seedcorn maggot)Sweet corn (seedcorn maggot)
Each fly keeps largely to its own crop family. Cabbage maggot takes brassicas, including radish, turnip and swede. Onion maggot takes onion, shallot, leek and garlic. The carrot rust fly takes carrot, parsnip, parsley, celery, celeriac, dill and wild relatives. Seedcorn maggot is the generalist. It attacks the seed and young seedlings of corn, beans and peas first, and also spinach, onion, brassicas, tomato and cucurbits. It is drawn to freshly tilled soil and rotting organic matter.
Monitoring & action thresholds
Action threshold
1 egg per stem (cabbage maggot)
UMass: check 25 plants and look for the white eggs at the stem base. If you find an average of one egg per stem or more, significant damage is likely. It is a commercial figure. There is no garden threshold for the others, because once the maggots are feeding nothing can be done for that planting. UC IPM says pesticides are ineffective or impossible to apply to maggots in a standing crop.
Yellow sticky cards set just above the crop or near the soil catch the flies and show when a flight has started; UMass suggests changing them every 3–5 days. Cabbage maggot flight begins as yellow rocket (wintercress) comes into bloom, and onion maggot starts when cabbage maggot is at its peak. For brassica transplants, stir the soil at the stem base with a pencil point to look for the 1/8-inch white eggs. Check each successive sowing, not just the first.
Beneficial
Biological control
Soil-dwelling ground beetles and rove beetles eat maggot eggs, larvae and pupae and can cause high mortality. One rove beetle, Aleochara bilineata, also develops as a parasitoid inside the pupae, and in Alberta its adults emerged in early summer, beginning shortly after the root maggot flies started laying eggs. These beetles are susceptible to broadcast soil insecticides. The nematode Steinernema feltiae has worked against cabbage maggot in trials, even in soil as cool as 50 °F (10 °C). UMass reports that 100,000–125,000 infective juveniles per transplant were needed to reduce damage. In Cornell greenhouse trials, doses below 100,000 per plant did not prevent significant root loss, and a Belgian trial found spraying the plant trays placed it better than drenching the soil. The RHS points gardeners to S. feltiae in spring against carrot fly, but gives no tested rate.
Staphylinidae
Rove beetles- Kind
- Wild — attract it
- Best for
- Eat eggs and maggots; Aleochara parasitises the pupae
Carabidae
Ground beetles- Kind
- Wild — attract it
- Best for
- Eat eggs, maggots and pupae at the soil surface
Cultural controls
The extension advice is the same for all four: keep the fly off the crop, or plant when she is not laying. The University of Minnesota notes there is no home-garden insecticide for use before planting.
- Cover seedbeds and transplants with row cover or insect mesh on the day of sowing or planting, with the edges sealed with soil
- Rotate: never cover ground that grew the same crop last year, or flies emerge under the cover
- Plant after the first flight, from late May into June in the North
- Let cover crops, manure and other organic matter break down for 3–4 weeks before sowing
- Fit a 3¼–6 inch (8–15 cm) collar of card or underlay round each brassica transplant
- Sow carrots thinly to avoid thinning, and try less susceptible cultivars such as 'Resistafly' and 'Fly Away'
- Grow onions from sets, and clear cull onions and crop debris after harvest
Chemical, last resort
No home-garden insecticide is available for root maggots. Prevention is the controlCheck any product before it goes on. Open the compatibility matrix →
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UMass Extension Vegetable, Cabbage Root Maggot (R. Hazzard, updated 2025)
UMass Extension Vegetable, Onion Maggot (updated 2016) and Seedcorn Maggot (R. Hazzard, 2015)
UMN Extension, Cabbage and onion maggots (Schuh, Hahn & Wold-Burkness, reviewed 2022)
UC IPM Pest Management Guidelines: Cole Crops — Cabbage Maggot; Seedcorn Maggot (2020)
UC IPM Pest Management Guidelines: Onion and Garlic — Maggots
WSU Hortsense, Carrot: Carrot rust fly (reviewed 2025)
RHS Advice: Cabbage root fly; Carrot fly; Onion fly
Schroeder et al., Entomopathogenic nematodes for cabbage maggot, J. Econ. Entomol. 1996
Beck et al., S. feltiae against Delia radicum: dosage and timing, Pest Manag. Sci. 2014
Filgueiras et al., Entomopathogenic nematodes for onion maggot field control, Insects 2023
Broatch et al., Emergence and seasonal activity of Aleochara bilineata, Environ. Entomol. 2008
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
Suggest a correction →Root maggots by crop
The same pest behaves differently on different hosts. Each page below gives the host-specific control, rate and threshold.