Latin name: Delia platura
Plants affected: Seedling French and runner beans
Main symptoms: Seedlings killed or shoots distorted and blind
Caused by: Larvae of a fly
What is bean seed fly?
- As the fly only affects young seedlings rapid healthy growth soon after germination is key to controlling this insect
- Adult flies are attracted to soil containing fresh compost, so dig in organic matter the previous autumn where beans are going to be grown
- The soil should be well prepared in order to encourage rapid germination
- The fly can be largely avoided by raising seedlings in seed trays or pots and planted out once they have germinated
- Garden-sown beans can be protected insect barrier netting, horticultural fleece is especially valuable as it provides extra warmth that is highly beneficial to tender crops as well as excluding bean seed fly
- A mixture of pathogenic nematodes, sold as Fruit and Vegetable Protection, can be watered into the soil which may control the young larvae, this is available by mail order from biological control suppliers
- Damage from this insect can be avoided by non-pesticide methods
- None of the pesticides available to home gardeners are suitable for use against bean seed fly
Biological control suppliers (pdf document)
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.
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