Why Did My Bean Seeds Never Come Up After Sowing?

A row of bean seeds that simply never emerged — with no indication of germination and no sign of the seed when you dig to investigate, or with a hollow, excavated shell of seed remaining — may have been attacked by bean seed fly (Delia platura). This small, grey fly is closely related to the cabbage root fly and causes similar underground damage: the female lays eggs in the soil near freshly disturbed or freshly sown ground, and the cream-coloured larvae bore into germinating seeds and developing seedlings, destroying them before they can emerge. The larvae are small (about 8 mm) and feed entirely underground, making the damage invisible until you dig.

How to confirm bean seed fly damage

Dig at one end of the non-emerged row and examine the seed. Bean seed fly damage is distinctive: the germinating seed shows a hollow interior where the cotyledons (seed halves) have been completely excavated, or the emerging radicle and shoot have been eaten off at the base. The seed coat (testa) remains but is an empty shell. You may find the small, yellowish-white, legless maggot inside or in the soil immediately around the seed. This is distinct from mice damage (where the whole seed is removed) or rot (where the seed is soft and discoloured brown).

Conditions that favour bean seed fly

Bean seed fly is most active in spring, particularly in May and June — coinciding with the main bean sowing window. Freshly cultivated soil, ground recently incorporated with green manure or fresh compost, and beds where the soil has been disturbed attract egg-laying females. Cold, slow-germinating conditions that keep seeds in the ground for a long time increase exposure to larval attack — another reason why waiting until soil is warm enough for fast germination reduces losses.

Prevention

Starting beans in individual pots under cover and transplanting established 15 cm seedlings outdoors bypasses the seed fly window entirely — larvae cannot attack established plants with woody stems. For direct sowing, delay until soil is above 12°C so germination is fast (fewer days at risk). Avoid incorporating large amounts of fresh organic matter immediately before sowing; green manures and fresh compost are better incorporated in autumn to decompose over winter before spring sowing.