Biology

Swede midges spend the winter as pupae that reside in the top 1-5 cm of the soil. As temperatures become favorable, adults emerge and make their way to the soil surface. Research in Ontario, Canada indicates that the entire overwintering population of swede midge does not emerge synchronously in early spring, but rather in three distinct peaks from mid-May through June. In Ontario, there appear to be 3 to 5 main population peaks (flights) per year. Adults emerge continually from the end of May through the end of September or early October, thus ensuring a long-term battle with the pest. Temperatures between 68-77°F and adequate rainfall provide perfect conditions for development.

Mating occurs soon after emergence and females then begin to look for suitable hosts. Eggs are laid in clusters of 2-50 eggs, primarily on the newest growth points of the plants and leaf axils. Each female will lay about 100 eggs during her short (1-5 day) lifetime. Eggs are very small (0.3 mm) and transparent at first, but change to a creamy white color as they mature.

Swede midge eggs hatch into larvae within a few days, depending on the temperature, and begin to feed on plant tissue. Larvae are small maggots, initially about 0.3 mm in length before reaching their final size of 3-4 mm. Larvae can be seen with the naked eye, but it is best to use a hand lens. They are transparent and become increasingly more yellow with age. The larval stage is the only life stage that can damage cruciferous plants. Larvae produce a secretion that breaks down the plant cell wall allowing larvae to feed on the liquid contents. Larval feeding changes the physiology of the plant and results in the formation of swollen, distorted and twisted tissue.

Depending on the temperature and host plant quality, larvae can complete their development in 7-21 days. Mature larvae have the unique capability of flipping themselves off of the plant when disturbed and use this tactic to leave the plant to pupate in the soil. If conditions are favorable, new adults will emerge 7-14 days later, depending on the temperature, and the cycle will continue. During periods of drought, the pupae in the soil may remain dormant, but may emerge following rainfall or irrigation. The literature suggests that some pupae may remain in the soil for more than one winter.