There are three to five generations of bean seed fly during the spring and summer but it is the first generation in May, that is often the most damaging.
Adult bean seed flies are 6mm long and resemble house flies. They become active from May onwards and deposit eggs in the soil. The eggs hatch within a few days. The larvae are white, legless and headless maggots that reach 8mm long. They feed on the seeds and roots of beans and sometimes other plants such as brassicas, cucurbits, lettuce, onion and sunflower. They can kill young seedlings. The larval stage lasts one to three weeks before they pupate. New adults emerge after three weeks.