Wheat bulb fly is a local and sporadically important pest of winter wheat; infestations also occur on winter barley and on early-drilled crops of
Fig. 266 Tubercles on the anal segment of Delia larvae: (a) onion fly, D. antiqua; (b) wheat bulb fly, D. coarctata; (c) bean seed fly, D. platura; (d) cabbage root fly, D. radicum.
spring barley or spring wheat (usually crops sown before early March), but not on oats; Elytrigia repens is a wild host. Larvae cause the yellowing and death of central shoots ('dead-heart' symptom). Individual larvae typically move from one shoot to another, and overall effects on yield may be considerable, especially on untillered crops. On heavy land, attacks are usually most severe on crops which follow fallows or bastard fallows (leys ploughed-up in July); on lighter-land sites, the risk of attack is greatest when host crops follow pea, potato and root crops such as sugar beet. In the British Isles, infestations occur mainly in the midland and eastern counties of England (including Yorkshire).
Adults occur from June onwards. Eggs are laid on bare soil in July or August. The females select either fallow land, recently cultivated fields or bare soil beneath the leaf canopy of existing crops such as potato or sugar beet. Sites with well-structured soils are preferred. The eggs hatch in the following January or February. Larvae die within a few days in the absence of suitable host plants. On sites where cereals have been drilled, each larva bites its way into the base of a plant, just below soil level, and enters the central shoot to begin feeding. Larvae develop through three instars, and older larvae move
from tiller to tiller, or from plant to plant, and thereby increase the extent of crop damage. The base of each infested shoot bears a ragged hole, surrounded by discoloured tissue. Larvae are usually fully grown by mid-May. They then enter the soil to pupate, and adults emerge a few weeks later. There is just one generation annually.
Adult female 6-8 mm long, dull yellowish-grey; legs partly brown. Adult male 6-8 mm long, dark brown; legs black (Plate 9e). Egg 1.8 x 0.3 mm, elongate-oval, white and ribbed longitudinally. Larva up to 12 mm long, whitish to creamish-white, pointed anteriorly and blunt posteriorly (Plate 9f); head retracted into prothorax; anterior spiracles fan-like, with seven to eight lobes; posterior spiracles prominent; posterior papillae prominent (Fig. 266b) (cf. yellow cereal fly, Opomyza florum, p. 183; late-wheat shoot fly, Phorbia securis, p. 201). Puparium 68 mm long, brown and barrel-like; posterior papillae and spiracles prominent (Fig. 267).
Delia echinata (Seguy) Spinach stem fly
This species is a locally important pest of spinach. Infested foliage becomes distorted, the petioles and mid-ribs split open, and shoots become blackened. Larvae may also occur as contaminants in crops sent for processing. In the British Isles, attacks occur most frequently in southern and southeastern England.
Adults first appear in the spring, and eggs are then deposited singly along the veins on the upper surface of young leaves. The eggs hatch within a few days. Larvae immediately burrow into the leaf tissue to feed within expansive blotch mines; fully fed individuals eventually pupate in the soil. Occupied mines occur from May to late September or early October, and there are three or four generations annually.
Adult 4-5 mm long, yellowish-grey with a slightly darker median stripe on the thorax and abdomen; legs black; wings hyaline, iridescent, with yellow veins. Larva up to 7 mm long, white, with plant sap often congealed on the body to form distinctive black rings; head deeply retracted into prothorax; anterior spiracles 8-lobed and fan-like; posterior spiracles relatively large and prominent (cf. mangold fly, Pegomya hyoscyami, p. 200).
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