Insect of the Month - Swede Midge

  Hort Snacks - March 2018
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 Contarinia nasturtii

Crops Affected: cruciferous vegetable (Cole) crops – broccoli, cabbage, cauliflower, Brussels sprouts, kale, kohlrabi, collards, rutabaga, turnip, radishes, ethnic Cole crops; canola, field mustard, Cruciferous weeds

Life Cycle:

  • Adults are tiny, light-brown flies, which resembles most other midge species
    • Midges have long, delicate legs and long, beaded antennae, as well as wings with short, fine hairs on them
    • Adults are not strong fliers, preferring areas with low wind movement
  • In areas where a population is established, adults emerge in the spring, over a period of approximately 1 month
  • After mating, females locate a host and lay eggs in clusters of up to 50 eggs on very young, actively growing vegetative tissues, typically near the growing points
    • Females lay approximately 100 eggs in their 1-4 day lifespan
  • Larvae (maggots) emerge within a few days to feed in groups on host tissues
    • Larvae feed and develop for 1-3 weeks, depending on weather conditions
    • Larvae secrete a toxic saliva to break down tissues, resulting in swollen tissues and abnormal growth
    • A moist environment is required, therefore dry conditions can result in short periods of dormancy
    • Larvae are yellow at maturity
    • Mature larvae exit the plant and pupate shallowly in the soil
    • End of season larvae go into diapause, to overwinter into cocoons in the soil, surviving up to 2 years
  • In some regions, there can be as many as 4-5 overlapping generations, with each generation lasting 3-5 weeks
  • Tissues infested with larvae are distorted and watery, due to toxic nature of larval saliva
  • New tissues are affected by feeding resulting in:
    • Leaf and stem tissues that are swollen, distorted, twisted and scarred
    • Blind heads and/or multi-headed plants can result from the death of a growing point or main shoot
    • Plants that do not form heads
    • Flower buds do not open and become swollen
    • Internal leaves may become crinkled or crumpled
  • Transplants are unmarketable
  • Plants become less susceptible as they get older, resulting in reduced symptoms in later infections
  • Symptoms may resemble nutrient deficiencies, mechanical damage from cultivation, other insect feeding, herbicide injury, etc.
Swede midge adult
Swede midge trap
Photo by Susan Ellis, USDA APHIS PPQ,
Photo by Robert Spencer

  • Confirm injury due to Swede midge by observing larvae on or within tissues
    • Depending on the size of the larvae, they can be seen with the naked eye or a hand lens
    • Tissues may also be placed in a black, plastic bag in the sun for several hours, which will cause the larvae to exit the plant, allowing detection
  • The use of pheromone traps may be used to detect low levels of a population within a field
    • Adult males are drawn to the trap
    • Place traps along field edges, and/or in sheltered areas
    • Traps are hung on stakes, approximately 30 cm (12in) above the soil
    • About 4 traps are recommended per field, at least 50 m (150-200 feet) apart (fewer can be used in smaller fields)
  • Established populations are very difficult to control – avoid introduction
  • Ensure that transplants are clean and free from infestation
    • If growing your own transplants, ensure good greenhouse monitoring and sanitation practices are followed
    • Systemic insecticide applications in greenhouses can be effective
  • Ensure a good crop rotation is followed, with a minimum of 2 years between host crops
  • Crops such as broccoli, ethnic Cole crops, Brussels sprouts and cauliflower are more susceptible, resulting in higher populations
  • Ensure at least a 1 km distance between new and previously cropped fields, as males can fly 300m and females further than that
  • Good field sanitation
    • Control Cruciferous weeds to reduce out-of-crop host options
    • Avoid deep tillage; clean off equipment and boots when leaving infested fields
  • Planting early maturing crops prior to early adult emergence can reduce damage; avoid late maturing crops to reduce overwintering
  • The application of registered chemicals based on monitoring may reduce damage and population build up
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on February 27, 2018.