Broad bean seed beetle

Broad bean seed beetle can be a problem in dry seed, it is most likely to be seen by gardeners who save their own seed.

Broad bean seed beetle

Quick facts

Common name Broad bean seed beetle
Scientific name Bruchus rufimanus
Plants affected Broad bean seeds
Symptoms Circular emergence holes in dry bean seeds
Most active March-July

What is broad bean seed beetle?

The adult insect is a small beetle that belongs to a sub-family of beetles that are known as bruchid beetles, seed beetles or seed weevils. It is the larval stage feeding inside seeds that causes the damage.

Symptoms

Keep an eye out for the following symptoms or signs of broad bean seed beetle activity;

  • Circular holes, just over 1 mm diameter, occur in the seed coat of dry broad bean seeds saved from a previous crop
  • The maggot like grubs reach approximately 5 mm in length and feed on the cotyledon of the seed
  • The adults are 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 bruchid species are associated with the dry seeds of other legumes

Control

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

Biology

Typically, the life cycle of broad bean seed beetles is as follows;

  • Adult broad bean seed beetles lay eggs on broad bean pods in early summer
  • After hatching, the grubs bore into the developing seeds but are too small to be noticed in beans picked for eating. However, the grubs continue to feed in those beans allowed to develop as dry seed
  • Pupation occurs within the seeds 
  • In the warmth of a house, adult beetles can emerge during winter, leaving tell-tale circular exit holes in the seed coats

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