Bean stem fly
A tunnel hidden inside the young stem kills the growing point before you ever see the fly — the most dangerous pest of the seedling stage.
- Pest type
- Fly maggot (stem borer)
- Attacks
- Seedling stem, growing tip
- Worst in
- Seedling stage, first 3 weeks
- Main risk
- Dead seedling; costly resowing
Overview
Bean stem fly is a tiny black fly that lays eggs on young leaves of soybean and rajma seedlings. The real damage is done by the pale, legless maggots that hatch and mine down inside the stem toward the base, leaving a thin dark tunnel and sometimes a swelling where they fed.
This pest is most dangerous in the first two to three weeks after the seedlings come up. A heavily mined stem can snap or wilt and kill the plant outright, and a bad, early attack across a field can even force a costly resowing. Older, established plants tolerate the same damage far better.
How to identify it
- Tiny black flies seen around young seedlings; the damage is done by pale, legless maggots inside the stem, not by the adult fly itself.
- Split a wilting seedling’s stem open — a thin dark tunnel running down toward the base, sometimes with a swelling where the maggot fed, confirms the pest.
- Affected seedlings show wilting of the growing tip, and a badly mined stem can snap or die outright.
- Worst in the first two to three weeks after emergence; older, established plants tolerate it much better.
The damage it does
- A heavily mined stem kills the growing point, and the seedling wilts and dies.
- An early, widespread attack can force a costly resowing of the whole field.
- Established plants that survive an attack usually yield less than an undamaged crop nearby.
When to act
- Watch the crop closely in the first three weeks after emergence — this is the only window the pest really threatens.
- Check wilting seedlings by splitting the stem, not just looking at the leaves, to confirm stem fly rather than another cause.
- Treat seed before sowing rather than waiting for symptoms, since damage inside the stem cannot be undone once it happens.
Organic & cultural control
- Sow at the recommended time for your area so seedlings pass quickly through the vulnerable early stage.
- Good, healthy seed and timely sowing shorten the window the pest can attack, which matters more than any spray once seedlings are up.
- Earth up soil around the stem base to encourage new roots above the damage on affected plants.
- Remove and destroy badly wilted seedlings so they do not carry the pest to healthy plants nearby.
Chemical control
- A seed treatment with imidacloprid or thiamethoxam before sowing is the most effective control, since it protects the seedling from the moment it emerges.
- If damage is still spreading after emergence, a foliar spray of a systemic insecticide can help, but seed treatment works better because it acts before the maggot ever gets inside the stem.
- Confirm the current recommended product and dose with your local KVK.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My soybean/rajma seedlings are wilting for no clear reason. What should I check?
Split open the stem of a wilting seedling near the base. A thin tunnel running down the stem, sometimes with a swelling, means stem fly maggot. This pest hits mainly in the first three weeks after the seedlings come up.
Can I still save the crop once I see wilting seedlings?
A seedling with a badly mined stem usually cannot be saved. Focus on protecting the rest of the field — a seed treatment before the next sowing is far more effective than trying to treat damage that has already happened.
Why does this pest matter more for young plants than older ones?
The maggot tunnels toward the growing point, which is what actually kills the plant. In a young seedling that growing point is small and close to the damage, so it dies easily. An older, established plant has more stem and root reserves and shrugs off the same damage.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.