Bean Seed Fly is a common pest that affects french beans and runner beans.
What does it look like?
The adult is a small, dark fly about 6mm long. It's the larva that does the damage: a white, legless, headless maggot that grows up to 8mm long as it feeds in the soil.
What causes it
Cold, wet soil is what turns this from a minor nuisance into real losses, because it slows germinationA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition → and leaves seed sitting exposed to maggots for longer. Adult flies are also drawn in to lay eggs by fresh organic matter freshly worked into the soil, so a bed dug over and sown straight away carries more risk than one left to settle first.
Is this pest active now?
Watch from May through the summer, with the first generation in May typically the most damaging one for spring-sown beans.
🌱 Plants at risk
Symptoms
The maggots damage germinating seeds. Seedlings can be killed, or emerging shoots come up distorted and blind, with no growing point. Phaseolus beans generally do not send up replacement shoots, so a blind seedling is usually lost (PGRO). Seeds sown in cold, wet soil often suffer greater losses because germination is delayed.
Life cycle
The flies overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → as pupae in the soil and emerge in early spring, becoming active from May onwards, when they lay eggs in the soil. There are three to five generations during spring and summer, overlapping from late spring until early autumn. The larval stage lasts one to three weeks before pupation, and it is the first generation in May that is often the most damaging.
Why this matters in Scotland
Cold, wet soil that slows germination makes losses worse, and that can describe a Scottish spring. No Scottish incidence figures were found.
Because the first generation in May is often the most damaging one, it is worth building a fast start into a normal bean-sowing routine rather than treating it as an occasional problem.
Controls: what works
Raise seedlings indoors before transplanting rather than sowing direct
Seed germinates in warm, controlled conditions rather than sitting exposed in cold outdoor soil, cutting out the extended vulnerability window that cold, wet ground creates.
Insect barrier netting or horticultural fleece over direct-sown seed
A physical barrier over the sown row stops adult flies reaching the soil to lay eggs in the first place, useful where seed is sown direct rather than raised indoors.
Avoid working fresh organic matter into the seed bed right before sowing
Adult flies are attracted to soil containing fresh compost, and live green organic matter or manure worked into spring soil attracts egg-laying females. Dig organic matter in the previous autumn where beans are going to be grown, and leave at least seven days between the latest cultivation and sowing (PGRO).
Biological control: pathogenic nematodes watered into the soil
Sold as Fruit and Vegetable Protection, this drench targets the maggots directly in the soil rather than trying to keep adult flies away.
Myths about bean seed fly
“Raising bean seedlings indoors and transplanting them avoids bean seed fly entirely”
Raising seedlings in trays or pots and planting out once they've germinated is described as a way the fly can be 'largely avoided', not eliminated. The method works by skipping the exposed germination window in cold, wet soil, which is where most losses happen, but nothing in the guidance claims a transplant is immune once it's back in open ground. Treating it as guaranteed immunity oversells what the advice says.
Our read Neither source we checked makes any claim, positive or negative, about risk to the plant after it's transplanted outdoors. The gap between 'largely avoided' and 'entirely avoided' is the whole basis for calling this one busted: it's a real reduction in risk, just not the total one the claim describes.
What to do: Raise seed under cover as the first line of defence against this pest, but keep an eye on seedlings for wilting or collapse for a while after planting out rather than assuming the risk is over.
1 source
- RHS, Bean seed flyThe fly can be largely avoided by raising seedlings in seed trays or pots and planting out once they have germinated.
“Bean seed fly is the same insect as onion fly”
Bean seed fly (Delia platura, with a close relative Delia florilega) and onion fly, also called onion maggot (Delia antiqua), are separate species in the same fly family, closely related enough that their maggots are hard to tell apart without close inspection. Onion fly specialises on onions, garlic, leeks and other alliums, while bean seed fly is a generalist that also attacks beans, peas, sweetcorn and more, and it can turn up in onion beds too. That overlap on alliums is a likely source of the confusion, but it doesn't make them one species.
Our read Because bean seed fly is the generalist of the two and does sometimes attack onions, someone who spots maggot damage in an onion bed and calls it 'onion fly' could be looking at bean seed fly instead. That's a real identification trap even though the underlying claim, that they're the same fly, is false.
What to do: Don't assume control advice written for one pest transfers fully to the other; check which species a source is naming before borrowing its timing or netting advice.
2 sources
- Cornell CALS, Onion Maggot and Seedcorn Maggot (Delia antiqua and Delia platura)Onion maggot is a specialist that feeds only on Allium crops, while seedcorn maggot (bean seed fly) is a generalist that also attacks beans, peas, corn and other crops including alliums, and the two are closely related but distinct species.
- RHS, Bean seed flyBean seed fly damage is caused by the larvae of Delia platura and Delia florilega.
“Cold, wet soil attracts bean seed flies to lay their eggs”
Cold, wet soil raises losses because it slows germination and leaves seed exposed to maggots for longer, not because it draws in more egg-laying adults. The attractant named for adult flies is freshly worked-in organic matter such as compost or manure, which pulls flies to a bed regardless of soil temperature. Mixing the two mechanisms up could mean someone focuses on warming the soil while ignoring the bigger draw sitting right in front of them.
Our read The two effects are easy to conflate because they both make a cold spring worse for beans, but they're doing different jobs: one lengthens the seed's vulnerable window, the other brings the flies to the bed in the first place. Addressing only the first misses half the problem.
What to do: Warm the soil and speed germination as one defence, but separately leave a gap of at least a week between digging in organic matter and sowing, since that's what attracts egg-laying flies.
2 sources
- RHS, Bean seed flySeeds sown in cold wet soils often suffer greater losses as germination is delayed and the period over which damage can occur increased, while adult flies are separately attracted to soil containing fresh compost.
- PGRO, TU43 Bean Seed Fly (Delia platura)Leaving a period of at least seven days between latest cultivation and drilling may help to reduce levels of damage.
“Sowing beans as early as possible in spring is the best way to avoid bean seed fly”
The defence that is described is getting seed through germination fast, and spring is named as when most damage happens overall, which points toward timing sowing carefully rather than simply sowing at the earliest opportunity. Separate field guidance names late spring and early summer specifically, not the very earliest sowings, as the ones most at risk. Rushing seed into soil that's still cold works against the fast-germination defence, so 'earliest possible' and 'lowest risk' aren't the same target.
Our read The two sources aren't contradicting each other on the facts, RHS on when damage is worst overall and PGRO on which sowing dates carry the most risk, but they pull in different directions for what a gardener should do, which is why this is marked disputed rather than a clean bust.
What to do: Time sowing to soil temperature rather than the calendar: wait until the ground has warmed enough for quick germination, even if that means sowing a little later than the earliest possible date.
2 sources
- RHS, Bean seed flyThere may be three or four generations of larvae during the summer but most damage is usually confined to the spring.
- PGRO, TU43 Bean Seed Fly (Delia platura)Late spring and early summer sowings are most at risk from attack.
Frequently asked questions
What should I do first if I find bean seed fly in my garden?
The first thing to try: Raise seedlings indoors before transplanting rather than sowing direct. Then: Insect barrier netting or horticultural fleece over direct-sown seed.
Will bean seed fly kill my plants?
Yes, at the seed and seedling stage — this isn't a foliage pest that an established plant can simply outgrow. A germinating seed or fresh seedling can be killed outright or left with no growing point at all. Once a plant is through that early establishment stage and growing away, it's no longer at meaningful risk from this pest.
When should I watch for bean seed fly?
Watch from May through the summer, with the first generation in May typically the most damaging one for spring-sown beans.
Sources
Show the 4 sources
- RHS1: identification, symptoms, the cold-wet-soil trigger, life cycle and generations, raising seedlings indoors, barrier netting, nematode drench, absence of a chemical control
- RHS1: confirming bean seed fly directly on French beans' own growing page
- RHS1: confirming bean seed fly directly on runner beans' own growing page
- pgro.org: the fresh-organic-matter attractant mechanism, UK-specific confirmation of no licensed insecticidal seed treatment, and the wider host-range context used to test (and exclude) peas and broad beans