Bean seed fly
Delia platura and Delia florilega
Seed flies resemble houseflies, are gray in color with dark, interrupted stripes on the dorsal shield. The male flies - recognizable by the eyes meeting at the vertex - have a row of comb-like bristles on the splints of their last pair of legs (= hind tibiae), which allows them to be distinguished from similar species.
The larvae have no legs and the head capsule is recurved so that the anterior end is pointed. Inside, mouthparts ("nail hooks") shine through in black. The posterior end appears as if cut off at an angle, and in the center of this "oval" lie the so-called posterior stigmas (often misinterpreted as eyes). The edge is occupied by about 15 small papillae, the shape of which can be used for species identification.
The eggs are white, pencil-shaped and 1 mm in size.
Seed flies belong to the fly family of flower flies (Anthomyiidae). Seed flies are two closely related species with a similar way of life. In addition to the name seed fly, the name bean seed fly is also common.
The flies lay their eggs on germinating host plants. However, there are also known cases where eggs were laid on bare soil during plowing. The flies are also attracted to unrotted plant matter, for example, fields fertilized with solid manure or fields after grassland or intercropping are particularly attractive to the fly.
Fly larvae hatch from the eggs after a few days. The larvae feed partly on bacteria, which proliferate under unfavorable germination conditions for the host plants, and partly they actively penetrate plant tissue, which in turn promotes bacterial growth. After passing through three larval stages (the time required for this depends on temperature), the larvae leave their host plant and pupate in the surrounding soil at a shallow depth of a few centimeters.
After a short pupal rest, adult flies hatch again, completing the circle.
In this way, several generations of the fly are produced, while damage is only done to young plants in the spring. Finally, overwintering takes place as puparium (barrel pupa) in the soil.
An infestation can initially be recognized externally by missing areas in the rows of emerging crops, germinating host plants often remain behind in growth. Miner galleries may appear in cotyledons of bean, and in pumpkin the stems are often completely hollowed out, in which fly maggots can be found in larger numbers.
Propagation and transmission
Seed fly maggots are not primary plant pests, as their main role in the soil is to break down organic matter. However, since rot-dwelling microorganisms develop on seedlings under unfavorable germination conditions, these also provide an ideal food source for seed fly larvae.
Prevention and control
- In general, rather early sowing is recommended and the previous year's infestation must be taken into account.
- For catch crops, select plants that will freeze off in winter.
- Avoid incorporating poorly rotted plants or immature compost into the soil (especially spinach). If incorporation is necessary, it should be done in cool weather when flies are not active.
- For asparagus, cover the ridges with plastic sheeting.
- Replanting infested seedlings: Common especially for cucumbers and pumpkins.
- Turn over the infested crop and replant, preferably with a crop that does not require heat.
Last updated: 07.12.2021