Risk factors and crop damage caused by bean seed flies

Bean seed flies can be a pest of several crops, including field beans, peas, alliums, sweetcorn, asparagus, cereals, and vegetable brassicas. The first sign of damage is usually patchy emergence of the crop, as larvae feed on seeds and cotyledons.

• Eggs are preferentially laid in freshly disturbed soil, especially near vegetable or farmyard manure residues
• Any factor that slows down the speed of germination and shoot vigour increases the risk of damage. These factors include low temperatures and excessively deep sowing
• High levels of moisture are also thought to increase the risk of damage
• Crops sown in later spring or early summer are at greater risk

Scientific name: Delia platura and D. florilega

Adults are small, greyish–black fly, 4–5 mm long.

Eggs are white, elongated and about 1 mm long.

Larvae are white maggots, which reach 5–8 mm in length.

Pupae are reddish-brown and about 5 mm long.

Jan–Feb: Bean seed flies overwinter as pupae in the soil.

Mar–Sep: Adults emerge, and females lay eggs just below the soil surface – generally singly – and up to 40 eggs in one day. Typically, a period of several days’ elapses before a further batch of eggs is laid. Females prefer to lay their eggs on freshly disturbed soil, particularly where a large amount of organic matter is present. 

Mar–Sep: The larvae feed on the buried seed or the cotyledons of the seedling (prior to sprouting). In the absence of a suitable host plant, larvae feed on decomposing organic matter.

Mar–Dec: The larvae pupate in the soil at varying depths.

Bean seed flies can complete between three and six generations per year, depending on temperatures.

In all crops, damage mainly appears as patchy emergence or seedling death.

Severe attacks on runner beans or French beans can result in loss of the growing point. This causes seedlings to emerge in a twisted condition – known as ‘snake head’ – and die.

In onion, plants are often killed at the ‘loop’ or ‘crook’ stage.

The pest can cause hollowed grains in cereals.

Damage to newly transplanted cucurbits can occur within days of planting and cause complete plant collapse. Later attacks cause plants to wilt, especially during dry weather.

On asparagus, attacked spears are deformed. They often split and have a bitter taste.

Non-chemical control

The presence of organic matter in the soil is an important stimulus for egg laying. Burying organic debris from previous crops should reduce risk.

Natural enemies include generalist predators, such as certain species of beetle, spiders, insect-pathogenic fungi and parasitoids (beetles and wasps). Two species of Aleochara (rove beetle), parasitise bean seed fly pupae and may also predate eggs and larvae.

Monitoring

Monitor adults throughout the season with sticky and water traps. Yellow, blue, and white traps are all effective, but white and some blues may be most effective.

Thresholds

There are no established thresholds.

Insecticide resistance

None known.