Delia radicum (L.) - Root Fly, Spring Cabbage Fly, Radish Maggot.
Systematic position.Class Insecta, order Diptera, family Anthomyiidae, genus Delia.
Synonym.Delia brassicae (Bouche) (= Chortophila, Erioischia, Hylemya, Phorbia).
Biological group.Oligophagous pest of cruciferous cultures.
Morphology and biology.Close to Delia floralis, differing as follows. Male 5-5.5 mm, dark-ash with wide dark stripes on mesonotum. Abdomen with wide black longitudinal stripe, all segments with a narrow cross stripe. Eyes contiguous. Frons 4-5 times narrower than eye. Female is larger (6-6.5 mm) and lighter than male. Frons nearly as wide as eye. Average fertility is 100-150 eggs. Several females may lay eggs on one plant. Eggs white, elongated, cigar-shaped, with wide and deep groove; they are laid in small groups on plant at root collar under nubbins and in fissures of soil. Females prefer lumpy or coarse-grained ground for egg laying, avoiding dust and mulch. Larvae hatch in 5-10 days. The larva is apodal, thick, white or yellow, shining. It has 2 spiracular plates on hind apex of abdomen. 12 tubercles are located around the plates, including 4 larger lower ones, 2 bidentate central, and simple lateral tubercles. Young larvae live under the thin skin of, or inside, plant root. Older larvae make numerous holes in edible roots. The larva feed for 20-30 days, molting three times. Development of pupa lasts 15-20 days in spring generation and several months in autumn generation. Larvae of 1st generation cause the greatest harm, 2nd generation damages late varieties of cabbage. Pupation occurs in ground. False cocoon elliptic, 5 mm long. Puparia wintering in ground at depth 10-15 cm.
Distribution.Occurs in Europe from Scandinavia to Spain, Madeira and Azores, in Northern Africa (Algeria, Morocco), China, and Japan; adventive species in Northern America. In the former USSR it is found everywhere except for tundra zone and desert areas of Kazakhstan and Middle Asia.
Ecology.Monovoltine in the North (Khibiny), bi- or trivoltine in Leningrad Region and in middle belt of the European part. Flight of imago in the spring is observed at ground temperature 12-13°C at depth of puparia location, which coincides with average terms of pricking out sprouts in the ground. The period of flight usually occurs during flowering of cherry and bittercress. Flight of 2nd generation occurs usually in June in southern areas and in July in middle belt. The fly appears much earlier in hotbeds. The first attribute of larvae presence is the delay of plant growth and withering of leaves, which get a bluish-leaden tint. The larvae harm increases in warm weather. A factor limiting distribution of the fly is high air temperature and low humidity. Diapausing pupae are destroyed by frosts at temperatures below -25°C. The optimum temperature for development of the pest is 15-25°C at humidity 80-100%. Eggs are susceptible to decreases in temperature.
Economic significance.Larvae damage cabbage, turnip, radish, swede, and other cruciferous crops. During mass outbreaks one plant can be populated by 300 larvae. They damage inner parts of the main root, breaking normal receipt of nutrients to overground plant organs which results in depression or destruction of plants. Measures of control include early terms of pricking out strong healthy sprouts inserted in peat-compost pots, additional fertilizing, deep autumn plowing of fields after harvest of cruciferous root crops and insecticide treatments.
Related references.Dzholova, N.G. 1965. Insect pests of vegetable crops in Baikal region. Moscow: Nauka, 80 p. (in Russian).
El.berg, K.Yu. 1981. Antomyiidae. In: Narchuk, E.P. & Tryapitsyn, V.A., eds. Insects and mites - pests of agricultural plants. V. 4. Hymenoptera and Diptera. Leningrad: Nauka, p. 188-198 (in Russian).
Rogochaya, E.G. 1974. Antomyiidae. In: Vasil.ev, V.P., ed. Pests of agricultural crops and forest plantations. V.2. Kiev: Urozhai, p. 540-548 (in Russian).
Stepanova, L.A. 1962. Experience of the ecological analysis of conditions of development of pests of cruciferous vegetable cultures in nature. Entomologicheskoe obozrenie 41(4): p. 721-736 (in Russian).
Yaremko, G.A. 1976. Some features of biology of the spring cabbage fly (Delia brassicae Bouch?) and its advanced control. Izvestiya TSKhA 5: 170-175 (in Russian).