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Beginner

Scientific Name
Contarinia nasturtii

Identification

• Clusters of eggs are laid on the youngest plant tissue
  
• The translucent to creamy yellow white larvae are 3 mm in length when mature
  
• When disturbed, larvae jump off plants
• Adults are tiny, 1.5-2 mm long, light-brown flies
• Damaged seedlings often appear twisted and may have a noticeable brown scar or a gall at the growing point
  
• If damage occurs before the plant reaches the button stage, the plant will be barren
  
• Later feeding injury results in twisted and distorted heads
  
• Also look for brown, corky scarring along the leaf petiole

Often Confused With
Bolting
Genetic disorders
Molybdenum deficiency

Period of Activity
First-generation swede midge adults emerge from overwintering pupa from mid-May to the beginning of June. It appears that there are four to five overlapping generations of swede midge in Ontario. Research into the biology of the swede midge in Ontario is still ongoing.

Scouting Notes
Swede midge adults are not strong fliers and prefer areas of low wind movement, resulting in more damage in sheltered areas along field edges and buildings. Attention should be paid to these areas when scouting. Examine young plants for unusual growth habits, with emphasis on the growth point and any side shoots.

Thresholds
None established.

Advanced

Scientific Name
Contarinia nasturtii

The swede midge, a gall midge native to Europe and Asia, was first found as a pest of plants in the Brassicaceae family in Ontario in 2000. The positive identification represented the first occurrence of this pest in North America. The swede midge is now widely distributed in Ontario and Quebec and has been detected in Nova Scotia, Saskatchewan and several U.S. states.

Identification
The swede midge adult is a tiny, light-brown fly (1.5- 2 mm, 1/16- 1/12 in.), difficult to distinguish from many other closely related midge species present in Ontario. New occurrences in unregulated areas require confirmation by a qualified taxonomist.

The eggs are small (0.3 mm, 1/100 in.) and transparent in colour when first laid, becoming creamy white just prior to hatching.

Larvae are small, gregarious maggots, initially 0.3 mm (1/100 in.) long and transparent, typically feeding in groups near the growing point. At maturity, they are 3- 4 mm (1/8- 1/6 in.) in length, yellow in colour and visible to the naked eye.

Damage symptoms are a direct result of larval feeding. Larval feeding changes the physiology of the plant and results in the formation of swollen, distorted and twisted tissue, including leaf stalks. Death of the main shoot or growing point may result in the formation of a blind head.

Heart leaves become crinkled and crumpled. Flower buds remain closed and become swollen. Heads are deformed, asymmetrical and disjointed. Brown corky scarring is typically observed along petioles and stems. Where the main stem has been destroyed, plants may compensate by producing secondary stems, resulting in multi-stemmed or multi-headed plant.

Often Confused With
Bolting
Genetic disorders
Molybdenum deficiency

Biology
First-generation adults emerge in the spring. Adults mate soon thereafter. Once the female finds a suitable host, she lays 20- 50 eggs in clusters on the youngest, actively growing vegetative tissue, commonly near the growth point (apical meristem).

Each female will lay approximately 100 eggs during her short (1- 4 day) lifetime. After 3 days, the larvae hatch and begin to feed on plant tissue. Depending on climatic conditions, the larvae can complete their development in 7- 21 days.

Research in Ontario indicates there are four to five overlapping generations. Pre-pupae of the last generation go into diapause, overwintering in cocoons in the soil and pupating the following spring; however, a few individuals may overwinter a second season before becoming adults.

Swede midge adults are not capable of long distance flight, but movement over several hundreds of metres does occur, and therefore, natural spread from one infested field to a new planting is possible.

Larvae require a moist environment. During periods of drought, they may enter a period of dormancy with growth resuming after a rain or irrigation. When mature, they drop or “jump” to the ground, tunnelling below the soil surface to spin cocoons and pupate. Most cocoons are located within the top 1 cm (2/5 in.) of the soil surface. Adults will emerge from the soil in 7- 14 days, depending on climatic conditions.

Larvae produce a secretion that breaks down the plant cell wall, allowing larvae to feed on the liquid contents. Secondary bacterial infections are common. Severity of damage is directly related to crop development at the time of attack. If the plant is attacked at the seedling/transplant stage, a gall is evident at the growth point, resulting in no marketable yield, while newly or lightly infested plants may be asymptomatic.

Period of Activity
First-generation adults emerge in the spring from mid-May until mid-June, with peak emergence usually occurring in the first week of June. Swede midge adults may be present until early October and larvae may be found on plants until mid-October.

Scouting Notes
Monitoring is best achieved using commercially available swede midge pheromone traps. Check pheromone traps and count midges two or three time a week to optimize timing of insecticide applications.

Once a suspect plant is found, examine new growth carefully for presence of larvae. Larvae can be seen with the naked eye or a hand lens. If larvae are not found, place the suspected plant material in a black plastic bag and place the bag in the sun for several hours. The high temperatures will cause the larvae to leave the plant where they can be readily observed against the black plastic.

Thresholds

	Thresholds (males per trap per day)
CABBAGE	5- 10
BROCCOLI	1 in regions of low swede midge pop. 1- 5 in regions of high swede midge pop.

Management Notes

• Once on your farm, this insect will be virtually impossible to eradicate.
  
• No single strategy will provide 100% control of this serious pest.
  Insecticides alone will not provide adequate control of moderate to high populations of swede midge.
  
• Always start off with clean transplant material.
  
• Crop rotation is the single most effective way to reduce swede midge populations in the field. With multiple generations and a high reproductive potential, swede midge populations can build up very quickly under continuous production of a host crop.
  
• Swede midge may survive in the soil for 2 or more years; therefore, a crop rotation that does not include crucifer crops is essential.
  
• Cruciferous weeds, including field pennycress, wild mustard, wild radish, shepherd’s purse, common pepper grass, yellow rocket and others are also hosts and may act as reservoirs for swede midge populations in the absence of brassica crops or canola.
  
• When leaving an infested field, make sure all soil is washed from boots and equipment. Pupae may be transported in soil, resulting in new infestations.
  
• Planting only early-season crucifer crops is another control strategy to reduce damage levels and population growth. If fields are planted prior to adult emergence in early June, damage will be less severe than in later plantings. Avoiding late-season crops will help reduce the size of the overwintering population in your fields.
  
• Given that there are multiple generations, several applications of insecticides will likely be required.
  Rotate insecticides to avoid the development of resistance to available insecticides.