Contarinia nasturtii (Kieffer) - Swede Midge

BACKGROUND

The swede midge, Contarinia nasturtii, was first detected in Southern Ontario in 2000. Significant losses, which were at first attributed to nutrient deficiencies, had been observed in broccoli fields in the province since 1996. When swede midge was confirmed in Ontario, it became clear that these losses were due to swede midge damage.

HOSTS

The hosts of C. nasturtii include a wide range of species within the family Brassicaceae: broccoli (B. oleracea var. italica), cauliflower (Brassica oleracea var. botrytis), cabbage (B. oleracea var. capitata) and radish (Raphanus sativus).

DISTRIBUTION

• Asia: Turkey
• Europe: widespread distribution
• Distribution - In North America, established populations of the swede midge have been reported on plants belonging to the family Brassicaceae in the Canadian provinces of Saskatchewan, Manitoba, Ontario, Quebec, Nova Scotia and Prince Edward Island, and in the U.S. States of New York, Connecticut, Massachusetts, New Jersey, and Vermont.

BIOLOGY

Overwintering adults emerge during May-June. The females oviposit on the first available plant, the eggs are laid in strings or clusters of 15-20 eggs on the youngest parts of the plant, e.g., on flower buds, or at the base of leaf stalks. Each female lays about 100 eggs. After three days, the larvae start to hatch from the eggs and feed mainly near the growing point, but the larvae can live on almost any part of the plant within an almost liquid environment. After 2-3 weeks the larvae are fully grown, jump/drop to the ground, and spin cocoons in the soil in which they pupate. Two weeks later the next generation of flies appears.

In Europe, there are up to 4 overlapping generations per year; the number varies with the climate. In Ontario, three generations per year have been observed. Adults of the overwintering generation emerge from the end of May until the beginning of August, with a peak in June; a second (summer) generation emerges from mid July to the first part of August. The third generation emerges from the end of August into early September.

Females become inactive at temperatures below 20°C. During periods of drought, the larvae may enter a period of dormancy, but growth resumes after a rainfall. Under drought conditions mature larvae make spherical cocoons deeper than usual in the soil and, after the drought, they return to the soil surface, only to reenter the soil and make new, ovoid cocoons in which to pupate.

DETECTION & IDENTIFICATION

Symptoms

• The symptoms of attack are easy to recognise. Flower buds remain closed and become swollen, heart leaves are crinkled and crumpled, and the young shoots and leaf stalks may be swollen, distorted and twisted, resulting in the death of the main shoot or in the development of secondary shoots. In addition, inspectors should look for the tiny yellowish "jumping" larvae in all areas showing signs of damage. In England, the crumpled-leaf and many-necked conditions in swedes and turnips are known comprehensively as "cabbage top". On swedes, the rot that usually sets in after midge attack, especially in damp weather, is a secondary symptom. On cauliflower, "blindness" refers to the complete destruction of the inflorescence. In Ontario, "blindness" and distortion of leaves has also been observed in seedlings prior to transplanting.

Identification

• Eggs: The eggs are 0.27 mm long and 0.08 mm wide, with a pedicel measuring 0.06 mm. Larva: The full-grown larva is 2 mm long and lemon-yellow in colour, and the terminal abdominal segment bears four pairs of unequal "legs". Prepupa: During studies in Belgium two types of cocoon were found. The first were large and oval, containing large whitish larvae extended to their full length, the second type were small and round, containing smaller, thinner yellow or orange curled larvae. Adult: The adult is a light-brown fly, about 1.5-2 mm long excluding the antennae, with very hairy wings. The female's ovipositor is long, retractile, and needle-shaped.

Fig. 1

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Fig. 7 (32K)

Related Sites:

Ontario Ministry of Agriculture and Food
Institut National de la Recherche Agronomique

Text: Plant Health Survey Unit.
Photo Credits: Fig. 1, Fig. 3., Fig. 4. Fig. 5. and Fig. 6. Dr. R.H. Hallett, Dept. of Environmental Biology, University of Guelph, Guelph, Ontario. Fig. 2. and 7, Institute National de la recherche agronomique, Paris, France.