Swede midge
An invasive gall midge, native to Europe, that attacks broccoli, cauliflower, cabbage, kale and their relatives in the Northeast US and eastern Canada. The adult is a 2 mm fly you will rarely notice. The damage comes from its larvae, which feed hidden in the growing point: plants end up with crumpled leaves, brown corky scars, several small heads or none at all. A single larva can make a cauliflower unsaleable. There is no biological control available to buy or release, so rotation, netting and timing are the defence.
Broccoli with no crown and brown scarring where Contarinia nasturtii larvae fed. Photo: pending.
Identification
The adult is a delicate fly about 1.5–2 mm long, light brown to light orange. The males have long antennae. Adults live only one to five days and are weak fliers, so you are far more likely to see the damage than the insect.
The larvae are the stage to look for. They are 1–4 mm long, creamy white when young and lemon yellow when mature, and they live in the growing point, often in groups. Fully grown larvae fling themselves off the plant to pupate in the soil. On the plant, the giveaway is brown, corky scarring at the growing point and the bases of leaf stalks.
Easily confused with
- Herbicide injury or nutrient deficiencyHarmlessBoth can twist leaves and leave plants with odd, headless growth. Brown, corky scarring near the growing point is the feature that points to swede midge.
- Animal or other insect damageAlso a pestA growing point eaten out by animals or caterpillars can also leave a plant blind. Look for brown corky scarring and tiny yellow larvae in the tip, which only swede midge leaves.
- Aphid midge larvaeBeneficial — leave itA related gall midge, but its orange larvae sit among aphid colonies eating them. Swede midge larvae are creamy to lemon yellow, hidden in the growing point, with no aphids needed.
Damage symptoms
The larvae secrete substances that break down plant cell walls, and the growing point responds by swelling, scarring and twisting. Damage ranges from slight swelling to complete loss of the growing point. Heading crops suffer most: broccoli and cauliflower fail to form crowns or form several small ones. In one study a single larva left cauliflower unsaleable 52% of the time, and losses to broccoli and cauliflower in Ontario can exceed 85% a year.
- Broccoli or cauliflower with no head (blind heads), or several small heads
- Crumpled, puckered young leaves and twisted, swollen leaf stalks
- Brown, corky scarring at the growing point and the bases of leaf stalks
- Scarred, uneven heads and galls on buds and flowers
- Tiny yellow larvae in the growing point
Lifecycle & degree days
Swede midge overwinters in the soil, in the top 1–5 cm (½–2 in), and adults emerge from mid-May. Females lay eggs in the growing points, larvae feed there for one to two weeks, then drop to the soil to pupate. Generations overlap, so adults keep appearing until late September or early October. Some pupae stay dormant in the soil, which makes emergence hard to predict. The species was first found in North America in Ontario in 2000 and in the US in New York in 2004; it has since spread through the Northeast and as far as Minnesota.
- Generations / yr
- 2–5, overlapping (3–4 in Quebec)
- Adults active
- Mid-May to early October
- Fastest development
- 66–77 °F (19–25 °C)
- Overwinters
- In the top 1–5 cm of soil
Host crops
Only plants in the cabbage family. UMass rates broccoli, cauliflower, kohlrabi and collards the most susceptible, cabbage and kale moderately so, and Brussels sprouts, turnips, bok choy and mustards less so. Among kales, Red Russian took the most damage in a two-year on-farm trial of four varieties. Cruciferous weeds such as shepherd's purse also host it, though they suit it far less: in choice tests 97% of larvae were found on cauliflower and under 3% on wild mustard.
Monitoring & action thresholds
An Ontario threshold from pheromone traps (Hallett & Sears 2013), with sprays at least 7 days apart. It worked for cabbage but not broccoli, which stays susceptible at every growth stage, so no trap threshold protects broccoli. In a home garden there is no threshold: prevention is the whole game.
Commercial growers use pheromone traps, 3–4 per field and at least 50 ft apart, checked weekly, with lures changed every 4 weeks (UMass). The traps catch males only, so they tell you when adults are flying, not how many plants are infested. In a garden, check the growing points of young plants from late spring for crumpled leaves and scarring, and pull a damaged tip apart to look for the yellow larvae.
Biological control
There is no biological control to buy or release for swede midge. As a 2026 study of its host range puts it, the parasitoid wasp Synopeas myles has potential as a biological control agent, but no biological control programs are currently available. In laboratory tests, insect-killing nematodes, Heterorhabditis bacteriophora above all, killed the larvae and pupae in soil, and in Ontario field trials they suppressed adult emergence at one site but had no significant effect at another. None of the extension guides we checked recommends them. The honest answer is that rotation and exclusion do the work.
Cultural controls
Because the larvae feed hidden in the growing point, keeping the fly off the plants works better than trying to kill it once it is there.
- Rotate brassicas away from infested ground for 2–3 years; even one year off cuts numbers
- Grow brassicas under fine insect netting, on ground where no midges overwintered — netting over last year's brassica bed traps them in
- Remove and destroy crop residue promptly after harvest
- Control cruciferous weeds such as shepherd's purse, which can carry the midge
- Where pressure is high, favour less susceptible crops such as Brussels sprouts or mustards over broccoli and cauliflower
Chemical, last resort
- SpinosadREI 4 h
- SpirotetramatREI 24 h
- Lambda-cyhalothrinREI 24 h
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 Vegetable Program, Brassicas: Swede Midge (G. Higgins, 2020)
University of Minnesota Extension, Swede midge (Perez & Schuh, reviewed 2026)
Chen et al., Swede midge: ten years of invasion in North America, J. Econ. Entomol. 2011
Kikkert et al., Detection of C. nasturtii in New York, J. Econ. Entomol. 2006
Stratton et al., A single swede midge larva can render cauliflower unmarketable, J. Insect Sci. 2018
Hallett & Sears, Pheromone-based action thresholds for swede midge, J. Econ. Entomol. 2013
Hallett, Host plant susceptibility to the swede midge, J. Econ. Entomol. 2007
Chen et al., C. nasturtii on cruciferous weeds, J. Econ. Entomol. 2009
Corlay & Boivin, Seasonal development of C. nasturtii in Quebec, Environ. Entomol. 2008
Izzo et al., Swede midge susceptibility among kale varieties, J. Econ. Entomol. 2025
Lopez et al., Host plant range of C. nasturtii, J. Insect Sci. 2026
Evans et al., Soil entomopathogens against the swede midge, J. Econ. Entomol. 2015
Evans & Hallett, Biopesticides for swede midge, J. Econ. Entomol. 2016
UC IPM Cole Crops PMG: Relative Toxicities to Natural Enemies; Cabbage Looper (2020)
UC IPM Lettuce PMG: Lettuce Aphid; Relative Toxicities to Natural Enemies (2017)
UC IPM Cucurbits PMG: Squash Bug (2016)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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