Swede midge life cycle

Understanding the swede midge’s life cycle is the first 
step on the way to controlling this new pest

Swede midge adults emerge from mid-May to mid-June from pupae that have overwintered in the soil in cocoons. At least, this is the experience in Ontario — a great deal of research is still required to understand the insect and its life cycle nuances in Saskatchewan.

The adult is a very small, delicate, light- brown/grey fly, smaller than the orange wheat blossom midge fly. Also unlike orange wheat blossom midge, swede midge is active during the day. Male swede midge will live about a day, whereas females will live up to three days, using their long ovipositor to insert eggs into rapidly growing areas of the host plant — in between bud leaves, in very young flower buds and similar places. The female lays about 100 eggs in clumps of two to 50.

The eggs hatch in about three days and larvae start feeding cryptically within whatever structure they are located. Feeding can last up to three weeks, depending on temperature. Swede midge larvae are described at gregarious, meaning they live in close proximity without competition during feeding. The larvae have no external chewing mouth parts and feed by regurgitating salivary fluids on the plant to liquefy the tissue, then slurp it up.

The plant responds to this feeding activity by developing malformed and misshapen structures. Complete cessation of growth can occur if the growing tip is destroyed. Typical symptoms include twisted plant structures, premature bolting, swollen or closed buds and fused/bottle-shaped flowers. Depending on when the adults emerge and the eggs are laid, there may be no flowers at all formed and/or deformed racimes. “For example, in 2012 infestation in Saskatchewan occurred in mid-to late June,” says Dr. Julie Soroka, Agriculture and Agri-Food Canada entomologist. “The canola was in early flower at that point and the flowers showed damage as a result of the larval feeding.” In 2013, however, swede midge emergence was earlier and early canola growth was deformed and in some cases no flowers were formed at all.

Swede midge larvae are capable of jumping, which they will do if disturbed while feeding or when they are ready to pupate in the soil. There are several generations of swede midge per season, with a generation occurring every 21 to 44 days in the summer. When days get shorter in the fall, larvae enter the soil and diapause in a cocoon over the winter similar to wheat midge. Pupation occurs when temperatures warm in the spring, and adults emerge when the soil reaches adequate moisture levels.

This article appeared in the May 20 issue of Grainews.

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