Thursday, 11 September 2014

The root of the problem

While scanning the ground the other morning my attention was caught by a large black weevil, lumbering slowly across the sandy substrate. Thickset, and with a broad nose, it reminded me of the vine weevils that I sometimes encounter in the garden at home. It didn’t look quite ‘right’ for a vine weevil – and I was on stretch of sand dune rather than in a garden – so I popped it in a pot to take a closer look. There was no question that this was one of the broad-nosed weevils, belonging to the family curculionidae; the question was, which one?

The weevils are well represented within the UK, with several hundred species on the latest checklist, and they come in various colours, sizes and shapes. These, however, are based around a common body plan that is characterised by an elongated ‘snout’ – the rostrum – which projects from the front of the head and ends with modified mouthparts. Most weevils use these mouthparts to cut into plant tissue, providing access to the site in which the eggs will be laid. The resulting larvae feed inside the plant on its tissue and it is the diversity of plant species available to the weevils that may help to explain the diversity of species seen within the weevil family itself.

The broad-nosed weevils are, therefore, unusual in not using their modified mouthparts to access the internal tissues of plants for egg laying; it is also why they have a broad rather than narrow rostrum. Instead, they lay their eggs on or near the soil surface and the resulting larvae feed on roots or root nodules without penetrating the plant itself. The resulting damage is still sufficient to see many of the broad-nosed weevils listed as significant plant pests, as the demise of a favourite pot plant to vine weevil larvae quickly demonstrates. The adult vine weevils may also cause damage to the plant, leaving characteristic semicircular feeding notches on the edge of the plant’s leaves.

Putting aside their economic impacts in the worlds of horticulture and agriculture, weevils are rather smart creatures. This particular individual has still got me puzzled, however, and it will be a case of taking it home and looking at it under the microscope to determine its identity. 

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