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Broad bean seed beetle (Bruchus rufimanus)

Symptoms

Holes occur in the seed coat of dried broad bean seeds saved from a previous crop. Four millimetre long beetles, mottled with black, brown and white scales, may be found with the seeds. The species most frequently associated with broad bean seeds in Britain is Bruchus rufimanus. Other species may be associated with the dried seeds of other legumes.

Cause

Bruchid beetles are generally only seen by gardeners who save their own seed. The adult beetles lay eggs on broad bean pods in early summer. After hatching, the grubs bore into the developing seeds but go unnoticed in beans picked for eating. However, the grubs continue to feed in those beans allowed to develop as dry seeds. In the warmth of a house, adult beetles emerge during the winter, leaving tell-tale circular exit holes in the seed coats.

Control

There is no control available to amateur gardeners. Fortunately, affected seeds will often germinate as the grubs feed in the cotyledon part of the seed and the embryo is undamaged.