Heteropeza pygmaea Winnertz Mushroom cecid

This midge is generally common and widespread in decaying vegetation and rotting wood, and has become an important pest of cultivated mushrooms. The larvae contaminate the sporophores; they also introduce a bacterium that produces brown longitudinal stripes on the stipes and tiny black globules of liquid on the gills. Crop losses, especially of the later flushes, are often heavy.

BIOLOGY

Breeding populations in mushroom beds consist of paedogenetic larvae, which feed on fungal mycelium and also invade the sporophores. The larvae become fully grown in about 5 days. The larval gut then opens to expel a faecal tube about 10 mm long (cf. mushroom midge, Mycophila barnesi, p. 175), after which the larvae (known as 'mother' larvae) moult into sedentary 'hemi-pupae' within which several embryos develop; new individuals emerge about 2 days later, each as a "daughter' larva, about 1mm long. Under ideal conditions. 14 'daughter' larvae are produced from each 'mother' larva in just under a week, and larval populations build up extremely rapidly; many thousands can occur in a mere handful of casing material. Under dry conditions, the larvae will clump together to form sticky seething masses, often measuring many millimetres across. These larvae are frequently transported to previously clean mushroom beds on workers' clothes, equipment and tools, and are a major source of new infestations; larvae may also be introduced into mushroom houses in contaminated peat. If breeding conditions become unfavourable, fewer than normal individuals are produced within the 'mother' larvae, and the embryos develop into thick-walled resting stages, capable of surviving for well over a year. Larval infestations in mushroom beds may continue to develop for many generations but, eventually, usually about 2 months after spawning, 'imago' larvae appear. These pupate after a single moult and, about 5 days later, adult midges emerge. The adults often occur in vast numbers but are harmless. Also, most are female and few become fertilized, so the risk of their initiating further infestations is slight.

DESCRIPTION

Adult female orange, weak-bodied, with sclerotized spermothecae visible towards the

Fig. 247 Hessian fly, Mayetiola destructor (xl5).

Fig. 247 Hessian fly, Mayetiola destructor (xl5).

tip of the abdomen; wings 0.8 mm long. Paedogenetic larva up to 3 mm long, white, tapered anteriorly, but somewhat rounded posteriorly, with a pair of anterior eye-spots meeting to form an 'X'. 'Imago' larva similar in appearance to paedogenetic larva but with a sternal spatula and separated eye-spots.

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