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Bean seed fly

1. Bean seed fly adult ©FLPA / Nigel Cattlin
2. Larvae feeding on an onion plant base ©Tom Will Vegetable Consultancy Services (UK) Ltd.
3. Wilting onion plants ©Tom Will Vegetable Consultancy Services (UK) Ltd.


Bean seed flies are closely related to the onion fly and the cabbage root fly. The adult fly is up to 6mm long and the larvae are legless, white and up to 8mm in length.


The larvae can attack the underground parts of vulnerable plants resulting in plant death either before or after emergence. After feeding on one plant the larvae can move along the crop row destroying more seedlings. One of the early symptoms of pest activity is patchy crop emergence. For larger seedling plants, such as peas, beans or cucurbits, the larvae can tunnel into the seeds, roots and stems as well as damaging the growing tip leading to distorted growth. The flies are widespread and damage can be worst with fields with a high organic content or with plant debris from a previous crop.


The bean seed fly overwinters as a pupa in the soil and the adult emerges in late spring to lay eggs on the surface of freshly disturbed soil. Within a few days the maggots emerge to feed on organic matter in the soil or to attack the seeds, cotyledons, roots or stems of vulnerable plants. After feeding for up to three weeks the larvae pupate for a further period of up to three weeks and then the next generation of adult emerges. Bean seed flies are active from April through to September with peak activity in May. Given suitable conditions there can be as many as five or six overlapping generations per year. Water traps are used to monitor population activity of adults.


An economically important pest of many different crops. A pholyphagous pest of over 40 host plants including: legumes (peas and beans), onions, brassicas, ornamental bulbs, lettuces, sweetcorn and cucurbits. There have also been reports of damage to establishing cereal crops especially after the application of farmyard manure where cereal seeds and seedlings have been damaged. Depending on crop, control measures include the use of horticultural fleece, insecticide seed treatments or insecticide soil drenches.


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