Country diary: The caterpillar that sways to the rhythm of the trees

·2 min read

I can hear the sound of the aspen, like fast-flowing water rippling over a gravelly stream bed, long before it heaves into view around the bend in the footpath. Populus tremula is a restless tree with a distinctive voice, never silent, even in the lightest airs. In today’s blustery conditions, it shivers and whispers in the lulls, then rises in a crescendo to a rushing torrent with every gust.

Long, slender leaf stalks are flattened vertically, so the foliage can only swing freely from side to side, colliding, chafing, rustling. The leaves “make a great noise by being beaten one to another”, wrote John Gerard in his 1597 Herbal, “yea though the weather be calme, and scarce any winde.”

What appears to be a grove of a dozen trees here, beside this former railway line, is likely to be one genetic entity. Aspen sends up suckers from surface roots, so a solitary individual soon becomes a copse of clones. As the roots creep outwards, a pioneering advance party of smaller scions threatens to invade the footpath.

A poplar hawk-moth caterpillar, Laothoe populi, is feeding on one of these. Plump, almost as long as my thumb, with a distinctive but harmless spike on its tail, it is a lepidopteran defoliating machine, relentlessly demolishing leaf after leaf. Soon, fully fed, it will crawl down the trunk, wriggling under dead grass and decaying leaves, to pupate throughout winter.

The cryptic green colouration of the larva, which matches the leaves it consumes, should make it hard to spot, but the fickle wind betrays its presence. When surrounding foliage shimmies from side to side, the rhythm of the caterpillar’s dangling body moves with it, out of synchrony, swinging slowly like a heavy pendulum on a long leaf stalk.

There is an urgency in its feeding: time is running out; already some leaves are taking on their autumn hue and fluttering down on to the path. A few more chilly nights and the tree will stand, bare-branched, in a dazzling lemon-yellow puddle of its own making. The aspen will have lost its voice, a sound I will miss when I pass this way again in winter.

• Country Diary is on Twitter at @gdncountrydiary

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