| 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
Symptoms:
- 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.
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Swede midge adult | Swede midge trap |
Photo by Susan Ellis, USDA APHIS PPQ, Bugwood.org | Photo by Robert Spencer |
Monitoring:
- 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)
Management:
- 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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