🐛✗ Tested and false“A broad bean seed with a small round hole in it has been killed by seed beetle and won't grow, so you should bin it”
The exit hole marks where an adult beetle has already left, not where the seed died. The grub spends its life feeding on the cotyledon, the seed's stored food reserve, and leaves the embryo, the part that actually has to sprout, undamaged. Affected seed usually still germinates.
What to do: Sow holed seed as normal rather than discarding it. If you want reassurance before committing a whole row, sow a few extra alongside your unaffected seed and thin out once germination confirms which have come up.
In Scotland: The holes typically show up over winter, sometimes indoors where household heat lets the trapped adults finish emerging, which is exactly when a seed-saver is looking over their stored seed and can mistake normal life-cycle completion for spoiled stock.
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- RHSAffected seeds will usually still germinate because the grubs feed in the cotyledon rather than the embryo, so a hole in the seed coat doesn't mean a dead seed.
“Sowing broad beans in autumn for an early crop also means they escape seed beetle damage”
Overwintered broad beans are often sown in autumn to dodge blackfly and get an early harvest, but that timing works against a gardener where seed beetle is concerned. Adult beetles emerge and become active from around March once temperatures reach the mid-to-high teens, then lay eggs on flowering pods. Autumn-sown crops flower and set pods earlier in the season, which puts that vulnerable stage right in the middle of the beetles' peak activity window, whereas a later spring sowing flowers after more of that window has passed.
Our read This guidance comes from field bean trial data on commercial Vicia faba crops rather than a domestic vegetable patch, but it's the same species, the same beetle and the same biology, so the timing logic carries over directly to a garden crop being grown on for seed.
What to do: If avoiding seed beetle matters more to you than an early harvest, favour a spring sowing over an autumn one. If you sow in autumn anyway for the earlier crop, treat any pods being saved for seed as higher risk and check them for exit holes before storing.
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- AHDB (Agriculture and Horticulture Development Board)Later sowing of spring crops can reduce the risk of damage, while autumn crops are at increased risk because flowering and pod set are more likely to coincide with adult beetle activity.
- RHSAdults are most active from March to July, feeding on pollen and laying eggs on the developing pods.
“Broad bean seed beetle grubs found in stored seed will keep breeding and spread the infestation to other stored seed or pulses if you don't deal with them.”
Bean seed beetle (bruchid beetle) completes a single generation per year in the field, laying eggs on developing pods in summer. Grubs already inside seed at harvest will finish developing and adults may emerge from the seed later, but the pest does not breed again once beans are in storage, so a holed batch of seed cannot seed a growing infestation in the store or spread into other stored produce.
What to do: No need to isolate, fumigate or bin a whole batch of home-saved seed because a few beans show exit holes. Any grubs present will finish their single annual cycle without multiplying in storage; pick out visibly holed seed if you prefer, but the rest of the batch and any other stored pulses nearby are not at risk of a spreading infestation.
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