Soil caterpillars

Posted: Wednesday 7 December 2011
by Richard Jones

I recently intercepted a tweet from Harriet Rycroft [...] asking whether anyone could identify the creatures she was unearthing among the dahlia roots.

Caterpillars of common swift moth on a hand, courtesy of Harriet Rycroft

It is a gross simplification, promulgated by children’s books in particular, that caterpillars eat leaves. Some certainly do, and their chewings are all too obvious at spring leaf-burst. But others, of course, eat buds, fruits, flowers and stems. Now is the time to find the soil-dwelling critters that feed on the non-deciduous parts of the plant — the roots.

I recently intercepted a tweet from Harriet Rycroft, head gardener at Whichford Pottery in Warwickshire, asking whether anyone could identify the creatures she was unearthing among the dahlia roots. Harriet had found the long, skinny, sparsely bristled and anaemically maggot-pale caterpillars of the common swift moth, Hepialus lupulinus.

Living a subterranean life, it does not need the predator protection afforded by long dense hairs, camouflaged body tones or the bright warning colours of distasteful species. And living in the soil it is protected from the extremes of the weather; during snow-free winters it feeds continuously, pushing a little further down to avoid frost. Nevertheless root-grinding is obviously pretty poor nutrition, and most take two years to complete their growth.

The adult swifts will emerge in May or June, and although varying from drab grey to brown, some are prettily marked with cream splodges. They don’t live very long, though, lacking any functional mouthparts, so they cannot feed. Two years as an underground maggot has to see them build enough body food stores to last the days or weeks during which they fly, mate and lay eggs.

Although sometimes regarded as a minor agricultural or horticultural pest, swift caterpillars are secretive animals. Despite their distinctive form and large size, they have no ‘folk’ common name like the leatherjackets or wireworms that also do root damage. Harriet was reassuringly blasé when the caterpillars’ potentially sinister identity was suggested. “Don’t seem to have done much damage, and we have plenty of predators”. I wonder if that will change if I coin the name ‘dahliaworms’ for them in future?

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Harriet Rycroft 07/12/2011 at 20:22

Quite hard to get in focus because they were lively little beasties, just as fast in reverse as going forward - which I suppose is a useful skill underground!

Emma Crawforth 08/12/2011 at 10:54

Hello Harriet,

Thanks for the photo and for bringing this caterpillar to everyone's attention. These soil-dwelling creatures are fascinating, and can cause panic until they are identified!

Emma team