- Created: Monday, 05 October 2015 14:41
In July, 2016, Scott Meers did a quick survey for Swede midge in Alberta. He targeted these counties and MDs: Beaver, Flagstaff, Lamont, Minburn, Paintearth, Provost, Smoky Lake, St Paul, Special Area 4, Two Hills, Vermilion River and Wainwright.
Please note that the numbers were very, very, very low. Click the map below for more information.
by Lars Andreassen, Agriculture and AgriFood Canada, Saskatoon, Sask.
The swede midge, Contarinia nasturtii, is relatively new to North America, having been discovered in Ontario in 2000. Upon its discovery, the Canadian Food Inspection Agency initiated a monitoring program across Canada, which detected the midge near Melfort and Yorkton in northeastern Saskatchewan and Portage la Prairie in Manitoba in 2007. The CFIA program ended at this time. In 2012 injury to canola plants was noted in northeastern Saskatchewan, near Nipawin. A new monitoring program was then initiated by entomologists at AAFC Saskatoon, with assistance from a large network of collaborators. Additionally, we have been conducting research since 2014, with a focus on planting dates, insecticidal seed treatments, and the identification of resistant varieties. The experience in Ontario, where the midge has provoked a three year moratorium on canola production, has shown that this insect is potentially devastating to canola production, and prairie growers must watch closely as the midge spreads. This article will summarize the midge life cycle, its appearance and symptoms, and some control options.
The swede midge life cycle is quite flexible, permitting rapid population growth in favourable conditions, while protecting the midge in a cocoon in soil during unfavourable conditions. The key environmental factor is moisture. Winter is spent as a larva in diapause in a spherical cocoon, up to 15 cm deep in soil. In spring, if soil is moist enough, the larva leaves this first cocoon and moves near the soil surface, where a second oval-shaped cocoon is created, in which the midge pupates. About 12 days later, the adult midge emerges. Male midges are attracted to the female sex pheromone shortly after emerging. Mated female midges locate a host plant and lay eggs in clusters of two to 50 in crevices associated with meristematic tissue (Figure 1). Midges in dry air live a day or less, while in humid air they may live up to about five days, and it on the second day of adult life that egg production really increases. Additionally, the eggs are very susceptible to drying out.
Midge larvae have very rudimentary mouthparts, and drink rather than eat their host. A large proportion of the newly-hatched first instar is composed of the salivary glands, the contents of which interact with the plant meristematic tissue. The plant tissue closest to the larva becomes “nutritive tissue”; plant cells break themselves down into nutrients easily digestible by the midge, and the cell walls become porous and leak this liquid. Cells outside the region of nutritive tissue continue to grow, resulting in abnormal leaves or buds that may appear twisted or crinkled. Having initiated the abnormal plant growth, the first instar molts to a second instar, which grows rapidly but is still quite small. The third instar is more readily observed. Each of these stages lasts about 3 days, with the mature third instar dropping or leaping to the ground.
Each generation, a proportion of the larvae enter diapause and rest until the next season, and this proportion increases as the season progresses and days become shorter. The fate of the other larvae depends on soil moisture; in moist conditions, pupal cocoons are created and a new cycle begins, while in dry soil the larvae become quiescent, resting until the soil becomes moist again. In Ontario, there may be four or even five generations, with the first emergence of adults in late May or early June. In northeastern Saskatchewan, in both 2014 and 2015, the first emergence was in early July, and there seemed to be just two generations, with the first infesting the crop and the second in volunteer canola and cruciferous weeds. Weed hosts include shepherd’s purse, stinkweed, and wild mustard.
Current and expected distribution
Figure 2 illustrates the expected suitability of different regions in Canada to swede midge, based on climate. Green areas are considered unsuitable, yellow areas are suitable, orange areas are favourable, and red areas are considered highly favourable. The moratorium on canola production in Ontario is near Temaskaming, in the northern part of the agricultural area, which the model describes as favourable. Although the model is based on incomplete information about the midge, growers in Alberta will notice that a large part of that province is considered similarly favourable. The midge could therefore have severe impacts in Alberta, particularly if the midge starts emerging earlier than it does in northeastern Saskatchewan. The model takes no account of irrigation, which would increase favourability substantially.
A network of pheromone traps to attract and count male swede midges has been in place since 2012. In 2015, traps were spread from the British Columbia Peace River country to Steinbach in southeastern Manitoba. Of 114 locations, only five had any swede midge. In Ontario, there can be hundreds of male midges on each trap in a matter of days, whereas on the prairies the record number was 36 over the whole season. Because midge larvae and injury to plants is found where trap catches are zero, a more reliable method of following the spread was judged to be examining fields for symptoms and larvae. Figure 3 illustrates the survey results for 2015. The midge was detected in fused canola flowers south of Lloydminster, at very low levels. Illustrating the influence moisture can have on the midge, none were found in some parts of Saskatchewan in 2015 even though they had been there the previous year, such as Shellbrook and Lac Vert. Interestingly, although Yorkton was one of the original positive locations in 2007, midge larvae remain undetectable there.
Figure 4 illustrates the fused flowers that are typical of injury to canola on the prairies. Inside are the swede midge larvae, perhaps as many as 25 in each flower. If the flower is opened, the larva will squirm, being legless, or leap by curling itself in a circle and then releasing the tension. Thrips are about the same size and colour, but these have legs and run or walk, instead of crawl or leap.
Matador and Coragen are registered for control of swede midge in canola. Researchers at the University of Guelph are working to develop action thresholds based on catches of male midges in pheromone traps. The nominal action threshold for 2015 was a total of 20 males on four traps for the first application, and then 5 males/ trap/ day for up to two subsequent applications, with the active ingredients being alternated. No prairie locations reached these thresholds in either 2014 or 2015, since the one field with 36 midges caught these males in August, when the crop was no longer susceptible. A problem with the thresholds, at least on the prairies, is that males of other midge species are frequently caught in numbers that would be high enough to signal a spray is recommended, and these are indistinguishable from swede midge without a microscope. More research is required in this area.
An idea is to bury the pupal cocoons by tillage, since the adults have a difficult time moving through soil. Since the larvae can move through soil, though, this would have to be timed in spring some time after the larvae leave their overwintering cocoons and the pupal cocoons are created. As this would in most cases involve tilling a crop that is already developing true leaves, it may not be feasible for canola. Vegetable growers may find tillage more useful.
We have compared 14 varieties of glyphosate-tolerant canola, and found them all to be equivalent in susceptibility to midge. We are working to screen different older varieties and cruciferous weeds to determine if any of these are less susceptible based on physical structures like trichomes, glucosinolate contents, or phenolic compounds like those responsible for resistance to the orange wheat blossom midge. Several promising lines have been identified for further study, but resistance varieties are some years away from commercialization.
Finally, there are at least two species of parasitic wasps attacking the midge on the prairies that are not present in Ontario or Quebec. One of these attacks eggs, the other attacks mature larvae, and both emerge from the pupal cocoons. We are working to put names to these new species, and identify their original hosts in order perhaps to introduce them to eastern Canada, and to enhance their effects on the prairies. Levels of parasitism are already relatively high, approaching 40 % in some fields, and may increase further still as the wasps evolve to adapt to swede midge. Figure 5 illustrates how small these wasps are, and how they appear when attacking midge larvae within a fused flower.