Saturday, 20 April 2013

Leaf-miners ...

Stigmella aurella

The large group of moths known as the Nepticulidae are all extremely small, and because of the manner in which their larvae feed internally within the leaf structure, they are known as leaf-miners. The shape of the pale gallery or blotch created by the larva as it feeds, is usually a much better means of identification than by comparing the adult insect.

Stigmella aurella, pictured below, is probably the commonest and most widespread in the British Isles, its whitish larval mines are a familiar sight on bramble leaves everywhere. In addition to bramble, the larvae will also feed in the leaves of wild strawberry, agrimony, dewberry, water avens, and wood avens. The adults have a wingspan of only 6mm, have a metallic sheen, and fly in May and later in the summer.

Miners are generally restricted to a certain range of food plants, so the identification of a miner is greatly assisted by correctly identifying these plants. The shape of the mines (gallery or irregular blotch) and the patterns of the droppings (frass) within, in addition to the characteristics of the larvae and pupae, can all be key diagnostic features. Lepidoptera tend to construct galleries or blotches with single lines of frass. Diptera characteristically make twin trails of frass. Hymenopteran mines have characteristically more abundant and generally darker frass, particularly when compared to dipterous mines. Several species, including certain Coleoptera, make cut-outs in the leaf and the structure of these can help aid correct identification.

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