Delia radicum L Cabbage root fly

The cabbage root fly is a major pest of brassica crops (Brassicaceae) and causes considerable damage to summer cabbage and autumn cauliflower; infestations are also of significance on Brussels sprout, calabrese, Chinese leaf, radish, swede, turnip and ornamentals such as wallflower. Most damage results from eggs laid by first-generation flies in late April and May. Seedlings or recent transplants wilt and die, as the fibrous roots are eaten away and much of the tap root destroyed; damaged root systems are also liable to subsequent attack by fungal pathogens. Older or less heavily infested plants survive, but are usually stunted, and the outer leaves frequently become discoloured (often blue-green or reddish) and may wilt. Brussels sprout: larvae often feed within the buttons to cause extensive damage, and much of the inner tissue turns brown. Such larvae, and the damage they have caused, often remain undetected until after harvest; this is a particular problem on crops sent for freezing where only a small proportion of affected buttons may be sufficient for consignments to be rejected. Cauliflower: light attacks are sufficient to reduce curd size and, hence, yield of autumn cauliflowers but winter crops are rarely affected. Chinese leaf, swede and turnip: larvae cause minor damage to roots but may also feed within the growing points to produce multiheaded plants. Radish: fibrous roots are destroyed, weakening host plants, and the bulbous tap root may become riddled with tunnels (Plate 10a), rendering the crop unmarketable; superficial root damage may be sufficient to make crops unsuitable for human consumption. Wallflower and other ornamentals: attacked plants wilt in warm, dry weather and make poor growth; small plants may collapse and die.

BIOLOGY

Individuals overwinter as pupae within puparia. Adults emerge in the spring, from mid-April onwards, but the precise timing of their appearance depends upon temperature. Eggs are deposited in the soil close to the stems of host plants; the period of egg laying often coincides with the commencement of flowering of the common field-side weed Anthriscus sylvestris. Eggs hatch 3-7 days later, and the larvae immediately attack the roots of nearby host plants. They feed for 3-4 weeks and then, when fully grown, move away through the soil for a few centimetres before pupating. Adults of the second generation appear in late June and July, and those of the third from mid-August onwards. However, the two generations tend to overlap so that subsequent egg laying can occur at virtually any time from July to September. Although most eggs are laid in the soil, a few may be placed on the lower leaves of host plants. Larvae emerging from the latter may attack the stems and growing points or feed in the major leaf veins. Eggs are also deposited between the outer leaflets of developing Brussels sprout buttons, usually on those close to the base of the stems of early-maturing cultivars.

DESCRIPTION

Adult 6-7 mm long, mainly grey to blackish. Egg 1mm long, elongate-oval, white and ribbed longitudinally. Larva up to 10 mm long, creamish-white; head deeply retracted into prothorax; posterior papillae prominent, the middle pair beyond the spiracles distinctly bifid (Fig. 266d) (cf. Delia platura, p. 199, etc.). Puparium 6-7 mm long, elongate-oval, reddish-brown.

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