Bean Seed Flies

Delia platura und Delia florilega

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Changed on: 09.11.2018
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Bean Seed Flies

Saatenfliege
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Bean seed fly

Bean seed flies are two closely related species with similar life patterns. Delia platura (MEIGEN) is found worldwide and frequently at temperate latitudes, while Delia florilega (ZETTERSTEDT) is limited to Central and Northern Europe and is not as common. Bean seed flies are sometimes also referred to as seed flies. Bean seed flies can be found in large numbers in our fields every year, but they only become pests for crop plants that prefer warmer temperatures during cooler weather periods. This is probably the case because the bacteria germinating on the host plants during cold periods are an important food source for the larvae.

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Saatenfliege
caption
Bean seed fly

Bean seed flies are two closely related species with similar life patterns. Delia platura (MEIGEN) is found worldwide and frequently at temperate latitudes, while Delia florilega (ZETTERSTEDT) is limited to Central and Northern Europe and is not as common. Bean seed flies are sometimes also referred to as seed flies. Bean seed flies can be found in large numbers in our fields every year, but they only become pests for crop plants that prefer warmer temperatures during cooler weather periods. This is probably the case because the bacteria germinating on the host plants during cold periods are an important food source for the larvae.

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Symptoms

Symptoms

Damage

Gaps in the rows of germinating crops, germinating host crops show growth deficits, tunnels in cotyledons (seed leaves) of beans, stem (hypocotyl) of pumpkins and squashes is often completely hollow and holds a large number of maggots.

Hosts

Various species of arable crops preferring warmer temperatures such as maize, spinach, cucumbers, pumpkins, squashes, beans, asparagus… Other species may also be affected under specific conditions.

Oviposition

The flies’ rod-shaped, 1 mm long, white eggs are deposited on germinating host crops. However, cases have been recorded where they have already deposited their eggs on the bare soil during ploughing: apparently, they are lured by unrotten plant material, e.g. during the ploughing in of the spinach leftovers or unripe compost.  

Hatching of Larvae

The larvae hatch only a few days after the eggs have been deposited. They have no legs and the front of their head capsule is pointed. Their internal mandibles shine black through their bodies. Their posterior abdomen appears to be cut off at an angle – in the middle of this “oval” are the hind stigmata (often misinterpreted as eyes). The sides are covered with approximately 15 small papilla, the shape of which is used to determine the species. 

 

 

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Tunnels in the cotyledons
Miniergänge in den Keimblättern
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gaps in the rows of germinating crops, germinating hosts remain
Fehlstellen in den Reihen auflaufender Kulturpflanzen, keimende Wirtspflanzen bleiben zurück

Further Development

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Stems (hypocotyl) are often completely hollow, housing a large number of fly maggots.
Der Stängel (Hypokotyl) ist oft völlig ausgehöhlt - darin sind Fliegenmaden in größerer Zahl zu finden

The larvae feed partially on bacteria that proliferate under germination conditions that are adverse for the hosts, but they also make their way into the plant tissue, furthering bacteria growth.

Once they have gone through three larval instars (duration depends on temperature), they leave their host and pupate in the surrounding soil just a few centimeters below the surface.

The adult flies hatch after a short pupa period.

 

 

Flies

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Male hindleg with bristles
borstges Hinterbein des Männchens

These new generation flies cause basically no damage, as the host rapidly develops subsequently to the already higher temperatures. This way, several generations are produced, but only the first causes damage as a result of the colder temperatures. The flies overwinter as puparium in the soil.



Control

Control

  • Avoid ploughing in plants into the soil that have not properly rotted or unripe compost (spinach, in particular). If ploughing-in is necessary, it should be done during cool weather conditions, when the flies are not active. 
  • For asparagus, cover the ridges with plastic foil. 
  • Prophylactic application of insecticide agents. Please refer to the Register of authorized plant protection products in Austria to see whether such agents are authorised for use for the crop in question. 
  • Curative spraying of affected plants – only agents indicated as “fressende Schädlinge” (feeding pests) for the crop in question. 
  • Replace affected seedlings: common practice with cucumbers and pumpkins or squashes
  • Conversion of the affected crop and replanting – ideally planting a crop that does not need warmth
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