Family Limnephilidae

Limnephilus lunatus Curtis

This caddis fly is a frequent pest of watercress. The larvae feed on the leaves, roots and stems, and often cause considerable damage. Infestations in watercress beds are especially harmful during the winter months, when pieces of plant severed by the larvae float away and are lost.

BIOLOGY

Adults appear from late May or June onwards, with some individuals surviving well into

November. Jelly-like batches of eggs are laid in the autumn on the leaves and stems of watercress plants, each placed a few centimetres above the surface of the water. The newly laid egg batch soon swells, following the absorption of water, to become a colourless, gelatinous mass up to 10 mm in diameter, in which the individual eggs (of which there may be several hundred) are arranged in distinct rows. The eggs hatch in about 3 weeks; however, development of the embryos may be delayed if the water temperature drops below 10°C. The case-inhabiting larvae feed for several months and pass through five instars. The cases of first-instar larvae are usually formed from sand grains; those of subsequent instars are constructed from pieces of watercress leaves and petioles, but fourth- and fifth-instar larvae usually revert to using sand grains. Most larvae are fully grown by the following May or June. Each then pupates within its case. There is just one generation annually.

DESCRIPTION

Adult 28 mm wingspan; forewings angular, yellowish-brown, suffused with brown and partly transparent centrally; hindwings mainly transparent; fore tibiae each with a single spur; abdomen often appears green (in male) or yellow (in female), owing to the underlying fat-body tissue. Egg 0.4 x 0.3 mm, elliptical, translucent-yellow. Larva up to 17 mm long; head yellowish-brown with darker markings; thorax pale chestnut-brown to yellowish-brown, with darker markings; abdomen white. Case up to 23 mm long; composed either of sand grains or of cut-out pieces of watercress, cemented together.

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