Frit fly is a major pest of oats but will also cause significant damage to barley, maize (including sweet corn), rye, wheat and various cultivated grasses, especially Italian and perennial rye-grass. Damage in the British Isles is most significant on grassland and oats, although economically important attacks on sweet corn are also frequent. Infestations are usually most severe on lowland pastures and on crops direct-drilled into grass swards or on crops following a grass ley, as the larvae transfer readily from the old sward or the ploughed grass to recently germinated crops. First-generation (spring) and third-generation (autumn/winter) larvae: these attack young plants to feed on the growing points of the central shoots, which then wither and die ('dead-heart' symptom); each larva is capable of attacking several shoots and damage is particularly severe if crops have not begun to tiller. The most important damage to cereals occurs in plants up to the fourth-leaf stage, with 'dead-heart' symptoms on winter wheat evident from
Fig. 264 Puparium of gout fly, Chlorops pumilionis (xfO).
November to mid-February - earlier than damage caused by wheat bulb fly, Delia coarctata (p. 197) and yellow cereal fly, Opomyza florum (p. 183). On maize and sweet corn, young plants usually survive attacks but emerging leaves or shoots are twisted and ragged, and cobs will be of poor quality. Second-generation (summer) larvae: these occur mainly on the ears of oats and cause the developing grains to become shrivelled ('fritted grain'); attacks launched before ear emergence may result in 'blind' shoots; very severe crop losses are reported.
Adults appear in the spring, usually from late April onwards. Eggs are then laid at the base of host plants, especially spring oats. They hatch 3A4 days later. Larvae feed within the shoots and take about 2 weeks to develop. They then pupate in situ and adults of the second generation appear about 2 weeks later (usually in June and July). Their eggs are usually deposited on the heads of flowering oat plants, in the shelter of the glumes. Eggs may also be deposited on the ears of wheat plants. Second-generation larvae eventually pupate within the grain and third-generation adults appear in the autumn (usually from August to October). Eggs of the autumn generation are laid on young grasses (especially rye-grass) and on volunteer cereals. The resulting larvae feed throughout the autumn and into the winter, and eventually pupate in the spring. Larvae transfer readily from plant to plant, so that attacks may appear on later-drilled crops, but they will not do so at soil temperatures below 8°C.
Adult 1.5 mm long, stout-bodied, shiny black (Fig. 265); legs mainly, if not entirely, black (cf. Oscinella vastator, below). Egg 0.7 mm long, broadly elongate, white, ridged longitudinally. Larva up to 4 mm long, narrow-bodied, creamish-white and translucent; no posterior papillae (Plate 9c) (cf. wheat bulb fly, Delia coarctata, p. 197); anterior spiracles fan-like and inconspicuous, each with four to seven lobes (cf. yellow cereal fly, Opomyza florum, p. 183); posterior spiracles borne on small tubercles.
This species, along with various other close relatives, is associated with grasses such as fescues, rye-grasses and timothy. The larvae may, on occasions, transfer from grasses to cereals (apart from spring oats) and cause death of the central shoots but economically important attacks are rare. Adults of Oscinella vastator often require separation from those of O. frit in water-trap collections used to monitor frit fly numbers; O. vastator is distinguishable by, for example, its noticeably short, stubby, more brownish and more strongly veined wings, the longer antennal filament and usually more obviously banded (black and yellow) legs.
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