Divisi
Bean fly
Ophiomyia phaseoli · Insect
Photo: Lonsdale O, Murphy ST, Scheffer SJ · CC BY 4.0
Bean fly (also called bean stem maggot) is a tiny shiny-black fly whose maggots are the deadliest enemy of young beans, cowpeas and soybeans. The female pricks the first leaves to feed and lay eggs; the maggot that hatches tunnels down inside the leaf and into the stem, eating its way to the base at the soil line. Because it works hidden inside the stem of a seedling too young to withstand it, a single maggot can kill a plant — and a bad attack in warm, dry weather can take almost the whole stand.
Howard F. Schwartz, Colorado State University, Bugwood.org · CC BY 3.0 us 
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Why it matters
Bean fly is the number-one killer of young beans, cowpeas and soybeans. Its maggots tunnel down inside the stem of your seedlings and rot the base, and a bad attack can wipe out up to 9 in every 10 plants — often before the crop is three weeks old. The worst part is there is no cure once it is in: the maggot hides inside the stem where no spray can reach it, and by the time the leaves go yellow and wilt the plant is already dying. With bean fly, everything depends on stopping it before it starts — protect the seed and the seedling, because you cannot save a plant once it is mined.
When to act
Any yellowing or wilting of seedlings within three weeks of emergence, or larval mines seen when stems are split; act early because damage is fatal to young plants.
This guidance is general, compiled from published agronomic references including FAO and CABI. Thresholds and timing shift with your crop, your variety and the season.
In much of the region a spray is simply called mushonga — medicine. But a chemical group you cannot name is one you cannot rotate, and a product you did not write down has no pre-harvest interval you can observe. Mushonga, and other farming words →
The full bean fly field guide
- Identify it
- Scout for it
- Control it
14 field-guide sections in the app
The full guide to bean fly — every symptom to check, how and when to walk your field looking for it, and the controls that actually work, ranked. Built into your crop plan, so you are told to look before it costs you a harvest.
Crops it affects
Frequently asked questions
How do I identify bean fly?
The first sign is the seedlings themselves: within two to three weeks of coming up, the lower leaves turn yellow and the young plant wilts in the heat even though the soil is still moist. Look closely at the first leaves and you may spot tiny yellow or whitish dots near the base — the pinpricks where the fly fed and laid its eggs.
When should I take action against bean fly?
Any yellowing or wilting of seedlings within three weeks of emergence, or larval mines seen when stems are split; act early because damage is fatal to young plants.
How do I control bean fly?
Prevent it first — this is the whole battle. Plant early, as soon as the rains are steady, into a warm, well-prepared seedbed so your seedlings grow away fast and pass the tender three-week stage before the fly builds up. Sow a bit thicker (a higher seed rate) so that even if you lose some seedlings you still keep a full stand. Spread mulch and give the crop enough manure or fertiliser — a strong, fast-growing seedling shrugs off an attack that would kill a starved one.
Which crops does bean fly affect?
Among the crops covered here: Common bean, Cowpea, Soybean.
Catch it before it costs you a harvest
Divisi builds scouting into your crop plan — it tells you when to walk your field, what to look for at that stage, and what to do when you find it. Free for farmers, works offline.
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