Swede Midge

Table of contents

    Swede Midge (Contarinia nasturtii)

    Background

    Swede midge - adult male on fused canola flowers

    Tiny swede midge larvae on canola leaf

    Swede midge, Contarinia nasturtii, is a tiny gall midge originating from Europe and Asia which has been found in Canada and has the potential to do serious harm to canola production. Host plants for the minute fly include canola and mustard as well as cabbage, cauliflower, radish and a wide range of other crops and weeds from the family Brassicaceae.

    Swede midge populations have caused substantial damage in other areas of the world and have the potential to cause mild to severe damage in Canada’s canola production regions.

    Range

    The first Canadian confirmation of this pest was in Southern Ontario in 2000. It heavily infested Ontario’s cruciferous crops and caused yield losses up to 85% in some areas. Since then, the insect has made its way into Quebec (2003), Nova Scotia and Saskatchewan (2007) and, more recently, PEI and Manitoba (2008). It has also been reported in New York, Connecticut, Massachusetts, New Jersey and Vermont in the United States of America.

    It could potentially become established in any province under preferential conditions of high moisture and warm temperatures throughout the growing season. Unlike many non-native species, cold winter temperatures aren’t a limiting factor due to its Scandinavian origins.

    Biology

    Swede midge is a short-lived, multi-generational insect which doesn’t have a single flush of emergence after overwintering. Overwintering adults emerge during May-June in at least two peaks, separated by one to two weeks (which acts as insurance for the species, in case one cohort emerges into unfavourable spring conditions and doesn’t survive). Each generation also contributes a small percentage of pupae to the overwintering population to guarantee survival into future years.

    The number of generations varies with climate, with three to five generations per growing season in Ontario, overlapping in emergence so that all stages can be found at the same time. Depending upon conditions, the lifecycle can range from approximately 20 to 80 days, with eggs hatching in 1 - 10 days, 7 – 21 days in the larval stage and 10 – 48 days for pupation. The last generation in the fall waits underground through the winter in a cocoon, emerging in the spring to start a new cycle.

    Swede midge larvae on canola stem

    Swede midge larvae about to jump off canola stem

    Throughout their lifecycle, females will lay batches of two to 50 very small eggs in clusters of 15-20 eggs on the youngest parts of the plant. The eggs are approximately 0.27 mm long and 0.08 mm wide and hatch into tiny (~0.3mm) transparent larvae. At full size, larvae will be 2 to 4 mm long and an opaque yellow colour with four pairs of unequal "legs" at the end of the abdominal segments. As they near maturity, the larvae spring themselves off of the plant and onto the ground before spinning cocoons in the soil to pupate. They emerge as small adult flies no more than 2-3 mm long (excluding antennae) with very hairy wings.

    The small midge is a fairly weak flier but may be carried longer distances by the wind, and it is small enough to make detection difficult. Swede midge has further adaptations for unsuitable conditions. For example, the length of time in diapause (while overwintering) can vary from year to year. Swede midge will “wait” as pupae until there is sufficient soil moisture before emerging as adults. There are different shapes of cocoons for pupation and overwintering, with spherical ones being used for overwintering and ovoid ones used for pupation.

    Diagnosing damage

    Swede midge larvae feed protected by surrounding plant tissues, and damage symptoms may take a week or more to develop. When scouting plants, a hand lens is recommended to detect larvae and early damage symptoms. Damage symptoms are easily mistaken for heat or cold stress, fertility issues, mechanical damage, herbicide damage, drought or other insect feeding. The insect itself can also be mistaken for other midges, so confirming the diagnosis with an entomologist is recommended.

    Adult swede midge don’t directly cause any damage to crops as they only feed on the nectar of flowering plants. Females lay their eggs on the youngest parts of the plant (e.g. meristematic tissues), developing flower buds and in leaf axils. While flower buds are a potential target, females appear to prefer laying eggs on plants in the late vegetative to early bud stage, if possible (as seen in southern Ontario). In western Canada flowers tend to be the target, possibly due to most midges not emerging until after the plant has bolted.

    Swede midge damage prior to bolting

    Characteristic fused petals from swede midge

    Photo by Lars Andreassen

    Characteristic bottle cap flowers from swede midge


    Opened floret with swede midge larva

    Photo by Tyler Wist, AAFC

    Characteristic damage - bouquet appearance

    Swede midge trap

    The larval stage is the only stage that causes damage to canola. Larvae eat at the growing points (apical meristem), which may kill the growing point and cause plants to produce compensatory branches. Conversely, if only the flowers are attacked, the plant is not a complete loss.

    Larvae secrete salivary fluids onto the plant, which break down the cell walls and allow them to suck up the nutrients released by the digested cells. This feeding can leave behind brown scarring and causes the plant tissues to twist and swell, and potentially form a simple gall around them. The gall protects the larvae from predation or insecticide, allowing them to feed undisturbed.

    Depending on the stage of the plant at the time of feeding, the larvae can cause crinkling and crumpling of the leaves, fused (vase-like) petals and flowers that don’t open and may become swollen in earlier emerging plants. If larvae attack the growing point of the plant (apical meristem) before bolting then the plant will not recover.

    Damage to pre-bolting plants can prevent the main raceme from even forming. The young shoots and leaves may become swollen, distorted and twisted. Damage to the terminal meristem after primary bud formation may prevent regular elongation of the raceme and cause a bouquet- or palm-tree-like appearance.

    Death of the main growing point, which results in the lack of head formation in the cruciferous vegetable crops, is referred to as "blindness" formation. Death of the apical meristem may also cause the production of secondary compensatory shoots, resulting in multiple small vegetable heads that are unmarketable.

    The amount of insect pressure can impact the damage caused by swede midge. Flower buds infested by larvae may remain unopened, with petals fused together, and fail to produce a pod. Moderate midge pressure may result in reduced or abnormal growth and under high midge pressure, growing points may completely fail to develop.

    Environmental conditions can indirectly impact the amount of damage too, since losses are most significant under favorable conditions (warm, wet weather) when midge emerge early, have good survival and produce multiple generations.

    Swede midge management

    Prior to considering any potential insecticide application, an accurate assessment of the population should be completed. Looking for damage can be a useful way to scout, but the use of pheromone traps is the best method to monitor the presence and population of the insect. These traps, which can be set up on each of the four sides of the field, attract only male swede midge adults if they are emerging from fields near the traps.

    Checking every two weeks is adequate to determine the presence of the insect, but increased frequency (at least two to three times per week) is needed in preparation for any management or control. Pheromone lures should be replaced in traps every 4 weeks. Samples can be sent in to AAFC Saskatoon for identification as this insect is very small, delicate and hard to identify.

    Do not base insecticide applications on the presence of swede midge damage alone – as it takes some time for damage symptoms to develop, larvae may have already left the plant before symptoms are detected. Timing your applications at adult population peaks based on pheromone trap monitoring should help to increase efficacy, however appropriate pheromone action thresholds have not yet been established, so intensive monitoring and consultation are recommended to determine whether or not an insecticide application is needed. Under moderate to high populations, numerous – and therefore costly – insecticide applications timed according to pest emergence may be needed to protect yields.

    As for crop management, planting early can ensure your crop is past the most susceptible stages by the time the adult swede midge emerges. Maintaining proper fertility management always increases the plant’s potential to withstand a pest. If swede midge is found in your field, a minimum three year crop rotation can help prevent populations from increasing rapidly. However, in areas with high amounts of canola in the landscape, swede midge can easily move from a previous field in which they overwintered to a new field the following year. Subsequent fields should be placed as far apart as possible from the previous year’s canola fields.

    In the Prairies, swede midge population growth and range expansion will be limited in dry years. However in Ontario and eastwards, conditions are very suitable for population growth in most years.

    Additionally, swede midge parasitoids have recently been discovered in Saskatchewan, Quebec and Ontario, which may also help to curtail populations to some extent.


    References and Further Reading

    Chen et al. 2011. Swede Midge (Diptera: Cecidomyiidae), Ten Years of Invasion of Crucifer Crops in North America. J. Econ. Entomology. 104(3): 709-716.

    Hallett et al. 2007. Monitoring and detection of the swede midge (Diptera: Cecidomyiidae). Can. Entomology. 139: 700-712.

    Olfert et al. © Agriculture & Agri-Food Canada. 2006. Potential distribution and relative abundance of swede midge, Contarinia nasturtii, an invasive pest in Canada. Entomologia Experimentalis et Applicata 120: 221–228. Journal compilation © 2006 The Netherlands Entomological Society

    AAFC Field Crop and Forage Pests and their Natural Enemies in Western Canada: Identification and Management Field Guide (2015)

    CFIA Swede midge fact sheet: http://www.inspection.gc.ca/plants/plant-pests-invasive-species/insects/swede-midge/fact-sheet/eng/1326212430732/1326212527789

    OMAFRA Swede midge fact sheet: http://www.omafra.gov.on.ca/english/crops/facts/08-007.htm