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Section 15.6.8 Swede Midge

Contarinia nasturii

Time for Concern: May 15 through September 30. Several weeks after transplanting through harvest.
Key Characteristics: Adults are tiny (1.5-2mm) light brown flies indistinguishable from many other midges except by a specialist. Females lay clusters of microscopic eggs in the growing tip of young plants or shoots. Larvae are small maggots initially about 0.3 mm in length before reaching their final size of 3-4 mm They are lemon yellow at maturity. Larval feeding results in leaf and flower galls and a misshapen growing point of the plant. The growing tip of the plant may become distorted and produce several growing tips or none at all, young leaves may become swollen, crinkled or crumpled and brown scarring may be seen on the leaf petioles or stems. For the most current information on swede midge management (including insecticides and fact sheets), see Swede Midge Information Center for the United States.

Management Option Guideline
Scouting/thresholds

Swede midge (SM) injury is difficult to diagnose because there are similar abnormalities that can be caused by other factors including mechanical injury, insect and animal feeding, nutrient deficiencies, herbicide injury or heat or cold stress. To confirm injury is due to SM feeding, the larvae should be dissected from the plant tissue. To scout, carefully examine young plants for unusual growth habits. The main growing point and any side shoots should be examined for damage and larvae. Brown, corky scarring is the key indication of the insect’s feeding activity. In cabbage, it is easiest to find larvae prior to head development. Injury most commonly occurs at the borders of the field, especially near sheltered areas such as tree lines, so special attention should be paid to those areas. Plant parts suspected of infestation can be examined with a hand lens for the presence of larvae. Placing the growing tips in vials of alcohol or in plastic bags in the sun will force the larvae out of the tissue. No thresholds have been established.

Crop rotation By the end of one field season, one pair of adult SM can result in approximately 86,000 larvae. The larvae will then over-winter in the soil and emerge next spring, resulting in more than 3 million larvae feeding on plants in the first generation. This life cycle means crop rotation should be one of the most important management tools for growers. SM may survive in the soil for 2 or more years; therefore, a crop rotation that does not include crucifer crops is essential. How far away does the field need to be? No one can be sure, but far enough (~ 1 to 2 km) to prevent the possibility of SM being carried into the field by a light wind. Without a host crop, the SM female will not be able to find a place to lay her eggs in her short 3-4 day life span. Planting in isolated areas, up from prevailing winds may help decrease the chance of spread.
Resistant varieties Broccoli and gai lan (Chinese broccoli) are the most susceptible crops. Based on research in Canada, Paragon broccoli consistently sustains high levels of swede midge damage. Eureka and Packman are also quite susceptible. Cauliflower, Brussels sprouts, Chinese cabbage, and red cabbage are also highly susceptible. Green cabbage appears somewhat less susceptible, but heavy infestations can cause severe damage.
Site selection Avoid planting in sheltered areas such as near trees, hedgerows, and buildings. Swede midge also prefer moist soils.
Planting and harvest dates Planting only early season crucifer crops is another control strategy to reduce damage levels and population growth. The amount of damage is directly related to the plant growth stage at the time of attack. The younger the plant the more severe the damage. As plants grow, damage becomes more evident. The first emergence of SM occurs at the end of May – beginning of June. Damage to early plantings will be less severe than to late plantings because the plant development will be advanced by mid-July when high populations occur. Harvesting of early season crucifers begins in July, which minimizes the amount of damage to the crop. Avoidance of late season crops will also help to reduce the size of the overwintering population in your fields. This strategy will not work for long season crucifer crops like Brussels sprouts.
Sanitation Control cruciferous weed species such as wild mustard, shepherd’s purse, pennycress and wild radish because they are hosts to swede midge. Deep plowing of infested crop residue will help interrupt the insect’s life cycle.
Pesticides Cornell Integrated Crop and Pest Management Guidelines for Commercial Vegetable Production or Organic Production and IPM Guide for Cole Crops