Broad bean seed beetle
Broad bean seed beetle can be found in dry broad bean seeds, it is most likely to be seen by gardeners who save their own seed.
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 5mm long beetle that belongs to a sub-family (Bruchinae) of leaf-beetles (Chrysomelidae) that are known as bruchid beetles, seed beetles or seed weevils. It is the larval stage feeding inside seeds that can cause damage.
There are nearly 20 species of seed beetle found in Britain. The adults feed on pollen whilst the grubs feed within seeds. They can be specific to one species or several related species of plants, and several species are only known from stored products in the UK. Bean seed beetle larvae feed within a range of bean (legume) seeds. More information on seed beetles can be found at UK Beetles.
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 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 seed beetle species are associated with the dry seeds of other legumes
This beetle can usually be tolerated, 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. There are no controls available for this beetle.
Typically, the life cycle of broad bean seed beetle 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 heated building, adult beetles can emerge during winter, leaving tell-tale circular exit holes in the seed coats
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