Distribution. Turnip root maggot is found in northern regions of North America, Europe, and Asia. It is common in Canada, including northern areas, but is infrequent in the United States. Turnip root maggot greatly resembles seedcorn maggot, Delia platura (Meigen), and in northern locations many records probably represent mixed populations.
Host Plants. The host range of turnip root maggot is quite similar to that of cabbage maggot, Delia radi-cum (Linneaus), but turnip maggot is more abundant in northern latitudes and where soils are light. Turnip maggot is known principally for its damage to rutabaga and turnip, which explains its common name. However, it damages most crucifers, including weed species. Brooks (1951) considered it to be a truly phytophagous insect, like cabbage maggot, rather than being attracted to stressed or plant disease-infected plants, like seedcorn maggot.
Natural Enemies. The biology of this insect is not well known, and the natural enemies are particularly poorly studied. It appears that predators and fungi associated with other Delia spp. also affect turnip root maggot. In Norway, Andersen and Sharman (1983) reported that over 40% of turnip root maggot eggs were consumed by predators in only 2-3 days. Plots with numerous carabids and staphylinids (both Coleoptera) had greater egg mortality, and insecticides interfered with predation.
Life Cycle and Description. Most information on turnip root maggot biology was gathered in Europe, but it seems consistent with the limited data available from North America. There is only one generation per year. Turnip root maggots overwinter as pupae, and flies from the overwintering population emerge throughout the summer months. Late July and early August are peak emergence times.
Egg. The eggs of turnip root maggot are white and elongate. One side is concave while the opposite side is convex. They are marked with longitudinal ridges, and measure about 1.1-1.2 mm long and 0.280.38 mm wide. They are laid in July and August, and are usually deposited in clumps in the soil around the base of plants. Females normally deposit about 200 eggs during their adult life of about 40 days (Havukkala and Virtanen, 1984). Eggs hatch 8-9 days after being deposited.
The biology of turnip root maggot was given by Brooks (1951) and Miles (1952,1955a). Brooks also provided characters to distinguish the adult, larval, and egg stages of this insect from related species. Rearing procedures for cabbage maggot (Finch and Coaker, 1969) also are suitable for turnip root maggot (Finch and Collier, 1989). An interesting account of Delia ecology, and implications for management from a British perspective, was found in Finch (1989).
Turnip root maggot can be quite damaging. Beirne (1971) reported that in Canada's Prairie Provinces up to 40% of commercial turnips and 80-90% of rutabagas had been made unsuitable for consumption owing to feeding damage by this insect. Larvae generally confine their feeding to roots, but sometimes work their way up into petioles of the lower leaves. They normally scarify the surface of the root, but rarely penetrating very deeply. For crops where the root is not harvested, such as cauliflower, the damage is much less, owing to both the indirect nature of the injury and the fact that larvae tend to occur so late in the season that the plants are fairly mature.
Sampling. Yellow-sticky traps can be used to monitor abundance of adults. Flies are more likely to land on horizontal surfaces than vertical surfaces, however, so water-pan traps might be more suitable
(Finch and Collier, 1989). Allyl isothiocyanate is an important element in host acceptance, functioning as an oviposition stimulant (Havukkala and Virtanen, 1985), and has potential use in monitoring.
Insecticides. Management of turnip root maggot often depends on use of insecticides, usually applied as a granular formulation at planting to protect against larval injury.
Host-Plant Resistance. Variation in susceptibility between and among crops was studied by Alborn et al. (1985). In general, fast-maturing varieties seem especially prone to damage. Chinese cabbage, mustard, and some cauliflower cultivars were especially susceptible to injury, whereas kale and radish were quite resistant, and broccoli was intermediate. In an evaluation of 22 cauliflower selections, significant differences in susceptibility to injury were identified. There was a positive correlation between plant damage and number of eggs deposited by flies. Shaw et al. (1993) determined that turnip cultivars with relatively higher percent dry matter were more resistant to damage, but this conclusion is disputed (Finch and Thompson, 1992).
Cultural Practices. Weather and soil conditions affect damage potential. As is the case with seedcorn maggot, damage is worse in cool and wet years. Also, Beirne (1971) reported that in the Prairie Provinces, damage by turnip root maggot occurred principally on farms that were irrigated. In Norway, turnip root maggot was the principal crucifer pest on light soils, whereas cabbage maggot was the major pest on heavy soils (Alborn et al., 1985).
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