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Swede midge

Agronomists are finding swede midge in Prairie canola fields. Add this new pest to your 2013 field-scouting list

The Swede midge, a gall midge native to Europe and Asia, was first identified in Ontario in 2000. In Europe, Swede midge is a common threat to cruciferous vegetable crops like broccoli, cabbage and cauliflower. Its first discovery in Ontario was in vegetable crops, and it has been closely monitored since because of its potentially devastating impact on marketability of produce.

Over the past decade, Swede midge has become widely evident in Ontario and Quebec and has been identified in other provinces, including Saskatchewan.

Although most commonly found in vegetables, canola is another potential victim for this cruciferous-loving pest.

Swede midge sightings

Canadian Food Inspection Agency personnel caught small numbers of adult Swede midge in pheromone traps in the northeast (Nipawin and Melfort) and east central (Yorkton) areas of Saskatchewan in 2007. Jumping larvae were first sighted in northeastern Saskatchewan in 2007 and 2008, but the first documented damage in Saskatchewan that resulted in a financial impact was in 2012.

“The fact that damage was found on fall-seeded canola may indicate that timing is a factor,” says Scott Hartley, Saskatchewan Agriculture. “Swede midge over-winters in cocoons in the soil, similar to wheat midge, so timing and weather conditions both play big roles.”

Swede midge is a Hessian fly, from the same family as the more familiar wheat midge. The adult is a tiny, light-brown fly, often hard to distinguish from other closely related midge species. First-generation adults typically emerge in the spring from mid-May to mid-June, with peak emergence in the first week of June. Females will lay anywhere from two to 50 eggs in clusters, usually near the growth point of the youngest, actively growing vegetative tissue. During their short life span of two to four days, each female will manage to lay approximately 100 eggs.

Larvae hatch after three days, small maggots about 0.3 mm long that start feeding near the plant’s growing point. Depending on the weather conditions, larvae can reach full development within three weeks. At maturity, they can reach three to four millimetres in length and are a bright yellow colour, easily visible.

Research done in Ontario indicates four to five overlapping generations. Pre-pupae of the last generation overwinter in cocoons in the soil and hatch the following spring, but a few individuals may overwinter a second season before becoming adults. Swede midge adults can be seen until early October and larvae may be in evidence on plants until mid-October. The adult Swede midge isn’t capable of long distance flight but movement over several hundreds of metres does occur, so spread from an old infested field to a new planting is a very real possibility.

The potential for survival in the soil for two or more years makes crop rotation the most effective preventative measure. With multiple generations and a high reproductive rate, midge populations can grow quickly under multiple plantings of the same crop. High numbers of overwintering larvae could cause significant problems for the next growing season.

Swede midge damage

“Damage is usually done to the growing point of the plant, resulting in a ‘bouquet’ of pods,” says Hartley. “But it’s important to remember that some types of herbicide damage can result in a similar bouquet effect, so you have to be able to distinguish between them.”

Other symptoms can include aborted flowers or flowers with petals fused or glued together, and deformed, stunted or missing pods. Experience in Ontario, where the pest has been monitored for much longer, indicates that evidence of damage depends on the growth stage of canola during peak feeding, as well as the intensity of feeding. If pre-bolting canola comes under attack the growing point may be damaged to the point that bolting doesn’t occur. Damage to a bolting stem is what causes the “bouquet” effect, a shortened raceme crowned with a bouquet of pods radiating out from one point rather than along a typical raceme. Canola yields are not affected after full flowering.

Documented damage in 2012 was represented by fused florets. When the florets were separated, larvae were exposed. The fused florets died, resulting in reduced yield.

“High moisture, existing populations and favourable temperatures are all factors,” says Hartley. “There’s been limited evidence in Saskatchewan and even less in Manitoba so far, so we’ll have to wait and see what happens. A lot will depend on the weather.”

Plans are in place for increased monitoring in Saskatchewan in 2013, to measure both presence and impact. So far, the movement pattern has been more easterly than northern-moving. The bug’s inability to fly long distances should dictate a relatively slow advance, but there’s just not enough evidence yet to reliably predict its habits and movement patterns. While the impact of a large infestation could be devastating, Hartley counsels a wait and see attitude.

“It’s been here for five years and to date, financial losses have been very small,” says Hartley. “Total eradication just isn’t realistic, but it will be possible to keep it below the economic threshold. Decisions will have to be based on the same kind of logic applied to other pesticide treatments for things like wheat midge and bertha armyworm. You’ll never eliminate them completely, so you have to weigh benefits against cost of treatment and make the best decision.”

Learning to read the signs accurately will also be key. Some of the newer chemical formulae need to consume tissue in order to be effective, resulting in small amounts of tissue damage.

“Don’t automatically assume tissue damage is from midge,” cautions Hartley. “Treatments that act on tissue are very different from some of the old stand-by products that worked quickly and effectively with no tissue damage. Get to know the difference in plant reaction so you make a better diagnosis.” †

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