Bean Seed Fly is a common pest that affects french beans and runner beans in Scotland. Here is what it looks like, how to spot the damage, and which controls have been tested.
What does it look like?
The adult is a small, dark fly about 6mm long. It's the larva that actually 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.
Symptoms
Maggots tunnel into germinating seed and the base of emerging shoots. Seedlings that survive come up distorted and 'blind', with no growing point, while more heavily fed-on seed is killed outright before it ever breaks the surface.
Life cycle
Pupae overwinterKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → in the soil, with adults emerging in early spring to start three to five overlapping generations through the growing season. Each generation runs its own cycle from there: one to three weeks as a feeding larva, then pupation, then around three more weeks before the next adult emerges.
Why this matters in Scotland
The exact condition that makes this pest worse, cold and wet soil slowing germination, is closer to the Scottish spring norm than the exception.
That's not a one-off risk tied to a single bad spring, it's a recurring feature of gardening this far north, so it's worth building into a normal sowing routine rather than treating as an occasional problem.
Controls: what actually works
All the methods below are backed by real evidence, not just repeated folklore.
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
Freshly dug-in green matter or manure is what draws egg-laying adults to a bed in the first place, so digging it in well ahead of sowing, or leaving at least a week between cultivation and drilling, removes the attractant before seed goes in.
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
4 common claims about bean seed fly, checked: 3 busted1 disputed0 untested0 hold up
“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 actually 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 genuinely 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 actually 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 actually 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's actually 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 actually 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 genuinely 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?
Start with raise seedlings indoors before transplanting rather than sowing direct. After that, insect barrier netting or horticultural fleece over direct-sown seed. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
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 are bean seed fly most active in Scotland?
Check the seasonal activity section above. Scottish timings can run a few weeks behind the UK-generic figures.
Sources
4 sources, recorded with what each was used for
- 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