Tuesday, 1 May 2007

Moth follows hedgerow decline

Despite setting off under the sun and spring warmth of inland Norfolk, I arrived at Holme NWT reserve to find a light fog drifting in off the sea and a chill in the air. This served to deaden the sounds of waders feeding just beyond the dunes and lowered my expectations of being able to photograph newly arrived migrants in good light. My spirits lifted somewhat with sight of my first wheatear of the year, a male posturing in typical head-up pose. However, it was not the spectacle of this bird, or of the ring ouzels that mingled with a late-departing fieldfare, that made my visit memorable. Instead, it was the discovery of a larval web of an increasingly uncommon moth, the small eggar, on a small hawthorn bush just a few dozen metres from the visitor centre.

While the adults of this species are on the wing between February and March, they are rarely seen and only appear at light traps occasionally. Quite thickset, these squat little creatures belong to a group of moths known as the Lasiocampidae, which includes more familiar species like fox moth, drinker and the lackey.

The eggs of the small eggar are laid in clusters on blackthorn or hawthorn bushes (occasionally elm, grey willow and spindle, deposited by the female and covered with fine hair-scales dropped from the tip of her abdomen. From these eggs, tiny greyish-black, red-haired, caterpillars emerge. Almost immediately, the caterpillars spin a larval web made of fine silk, which appears to offer some protection from the unwelcome attentions of predators – though possibly not parasites; the species is known to suffer high levels of parasitization. As they grow, the caterpillars alternate between bouts of daytime sunbathing within the web and nocturnal excursions in search of food. Each web will support many dozens of caterpillars all sheltering within its confines through until just before pupation. Interestingly, some individuals entering pupation may remain within the resulting chrysalis for two or even three winters, rather than simply emerging the following year. This could be a mechanism by which the impacts of parasites on subsequent generations are reduced.

The webs themselves would once have been a familiar sight over much of southern Britain. However, the widespread removal of traditional hedgerows has resulted in a very large decline in the small eggar population, to the extent that conservationists are now concerned for its future. Indeed, many local Biodiversity Action Plans include the species in their list of objectives, Even in those counties, like Norfolk, where the species still occurs, this moth can best be described as decidedly uncommon. Finding a larval web, then, is something memorable.

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