Bean Seed Fly

If you grow beans on your allotment (particularly runner and French types), keep an eye on your bean patch for signs of bean seed fly. The maggots of these flies can damage and in most cases kill your crop before they get a chance to grow.

Bean seed fly maggots eating a runner bean seed

Adult bean seed fly looks very similar to the the common housefly, so not exactly easy to spot. Active from May onwards they lay their eggs in soil and are particularly attracted to fresh compost.

Bean seed fly larvae (maggots) hatch after a few days and feed on organic matter or their favourite – bean seeds. The maggots eat away at the bean and the roots, so even if beans manage to reach seedling stage they will not thrive and fail to climb up the poles as they should.

Bean seed fly maggots eat the roots of developing bean seedlings causing them to fail and die

Signs to look out for:

  • Patchy germination or no beans showing at all.
  • Seedlings coming up blind (no growing tip) or distorted.
  • Brown discoloration to the stem, seedlings eventually die.
Bean seedling with no growing tip. Note the brown discoloration to the stem from maggot damage.

There are many ways to avoid this happening to your beans, here are some tips:

  • Adult flies are attracted to fresh compost, add organic matter in the autumn rather than spring.
  • Sow beans in warm soil and good weather to increase the chances of fast germination, cold and wet soil will slow the process down giving the maggots adequate time to take hold.
  • Alternatively, germinate bean seed in pots to avoid the most vulnerable stage rather than direct in the ground, transplant once seedlings are established.
  • Use nematodes 
  • Protect seeds and seedlings with an insect barrier mesh to prevent flies from laying their eggs in the soil.

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