Swede midge (Contarinia nasturtii) is a significant pest of spring canola in Ontario. Enzymes in the saliva of larvae break down plant tissue, resulting in swollen and distorted leaves, shoots and flower buds and can potentially prevent bolting.
Swede midge can reproduce and cause injury in winter canola but because winter canola typically bolts and begins branching in early May before adult midges emerge from the soil, economic damage by swede midge is not anticipated in winter canola.
Identification
The adult swede midge is a tiny light brown fly roughly 1.5-2 mm. It is difficult to properly identify from other closely related midges. Larvae are small (0.3-3 mm), transparent, off-white to yellow maggots that are hidden within the growing point of the plant.
It is unlikely that adult midges will be observed while scouting the crop, and larvae will only be found by carefully searching within growing points and looking closely or using a magnifying glass.
Figure 1. Adult swede midge on a sticky trap (insect is not fully intact, damaged was sustained on the sticky trap).
Figure 2. Swede midge larvae are very tiny and hide within tissue at growing points.
Life Cycle
The crop is monitored from emergence to full bloom. The first adults emerge in mid- to late-May and there are 4 to 5 overlapping generations through to October. Each generation can take 24 to 31 days to complete, depending on temperature.
Swede midge overwinters as a larva in a cocoon in the soil, pupating in the spring before emerging as an adult. First adult emergence is in mid- to late May, though not all swede midge emerge at the same time. There are two main emergence phenotypes that have their first peaks about 10-14 days apart in late May to early June. Rain events that moisten the soil to the depth of 2.5 centimeters trigger emergence. Adults live for only 1 to 3 days and, although considered to be relatively weak fliers, they can move several hundred meters and can be carried much further by wind.
Females are ready to mate on the same day they emerge, laying their eggs in clusters of 20-50 eggs on the youngest, most actively growing portions of the host plant. Larvae hatch from the eggs and feed in clusters on the growing point of the plant. Larvae may feed for 1 to 3 weeks, depending on temperature. Once mature, the larvae drop to the top few centimeters of soil to pupate for two weeks until emerging as an adult. Some larvae of every summer generation will enter diapause (a period of arrested development) with increasing numbers as day length shortens in late summer. Some midges (2-10%) remain in diapause for two years, possibly more.
Crop Damage
The greatest damage is caused when adults lay eggs on canola seedlings prior to bolting. Enzymes in the saliva of larvae break down plant tissue, and when the main growing point at the center of the rosette is damaged the canola may not bolt. This results in 100% yield loss for plants that do not bolt, which is commonly observed along field edges or in a small portion of the field. Eggs are laid on growing points, and a bolted canola plant has many growing points including branching points and flower buds. Damage causing reduced branching can result in some yield loss and can result in pods forming low on the plant, requiring a low cutting height at harvest time. Injury to individual flowers or bud clusters formed later in the season has little or no economic impact.
Figure 3. Damage to the main growing point at early growth stages can prevent bolting and cause deformed and necrotic tissue.
Figure 4. Swede midge damage at growing points can cause “witches broom” at the main growing point (top of the plant) and can prevent elongation of secondary branches.
Scouting
Larvae are very difficult to see because of their small size and because they hide within the plant growing points. Larvae may have exited the plant before damage is observed. Adults are monitored using pheromone traps. Begin trapping in early May or as soon as plants emerge and continue until the crop is in full bloom. Swede midge numbers can increase to threshold levels quickly, so it is important to check traps regularly (every 2 days) to determine the number of adults captured per trap per day.
Two to four traps per field are recommended. Trapping supplies include swede midge pheromone lures, Jackson traps with sticky card liners, and stakes to hang traps from.
Traps should be placed at least 200 feet apart from each other along the field perimeter, no higher than 25cm above the canopy. Lures should be replaced every 4 weeks. Sticky card liners should be replaced every time the trap is checked. Count and record number of adults on the sticky card and record the date each time traps are checked.
Figure 5. Adult swede midges on one square inch of a sticky trap.
Thresholds
Once seedlings have one true leaf, begin counting the number of midges captured in each trap and add them together. When a total of 20 adults have been captured from the start of trapping, the first insecticide application is warranted. Subsequent insecticide treatments may be necessary if an average of 5 adults per trap per day are caught and the canola is still in pre-flowering stages. To determine this, count the total number of midges captured per trap and divide by the number of traps and the number of days since you last counted. Make the insecticide application as soon as possible once the threshold has been reached. Do not use damage symptoms to time spray applications.
Management with Insecticides
When using insecticides to control swede midge, follow the provided scouting and threshold information. It must be noted that insecticides are not 100% effective and crop injury may still be observed when insecticides are applied at threshold.
Insecticides registered for swede midge control can be found on the Crop Protection Hub.
Using high water volumes and smaller droplets may ensure good coverage and penetration of crevices where larvae are feeding. Consult insecticide labels to confirm whether surfactants are needed. Multiple insecticide treatments may be necessary. Leave a minimum of 7 days between treatments. Rotate chemistries to avoid resistance.
Cultural Management Strategies
Crop rotation is very important. In fields with known infestations, rotating out of canola and other Brassica crops for at least 4 years is the best strategy. Other canola fields within 2km may contribute to local infestations.
Control all weed and cover crops in the Brassica family found in and along the field perimeter that can act as alternative hosts, including mustard (ie. wild, white, black, brown, garlic, hedge etc.), hoary alyssum, stinkweed, penny-cress, wild radish, tillage radish, shepherd’s purse, yellow rocket, pepper-grass and volunteer canola.
The crop is most vulnerable during the vegetative (rosette) stage to the green bud stage when tiny flower buds are developing in the centre of the plant, as well as during secondary bud development. Early planting and/or rapid vegetative growth where plants have developed beyond vulnerable stages in early June (when midge emerge) can help avoid significant damage. This may be achieved with early planting; however, planting into cold soils can cause slow early growth. Try to avoid slow emergence and growth caused by poor fertility, cold soils, crusted or compacted soils, and seeding into dry soils. When canola is injured by flea beetles it may recover and grow slowly and may be at risk of swede midge damage.
Beneficial Parasitoid
Surveys have confirmed the presence of a small wasp, Synopeas myles, that has been observed parasitizing swede midge larvae. S. myles is not native to Canada and is not a specific parasitoid of swede midges but will lay eggs within the larvae of multiple types of midges. Females deposit one egg within one midge larva, and the larva continues to develop with the wasp growing inside. The larva drops to the ground to form a cocoon as it normally would, but an adult wasp emerges rather than an adult midge.
Surveys have shown a wide distribution of S. myles in spring canola fields across Ontario. At one site, in one Ontario study, a parasitism rate of over 20% was observed but the average rate was much lower. S. myles may not provide significant protection to canola fields because swede midge larva cause plant damage while carrying a developing wasp egg, and because one wasp egg prevents just one adult midge from developing. However, the presence of the wasp may contribute to suppression of swede midge populations in Ontario.