This fly species is much smaller than the previous two species. Grass flies are about 3 mm long and have a yellow body with three black stripes at the top of the breast shield. The somewhat rounded wings are fairly large in proportion to the body. The female of these species lay their eggs in the soil in meadows etc. The larvae parasitize on certain types of aphids. The grass fly too has the habit of overwintering in buildings en mass. They seem to prefer church towers.
Prevention / control
To prevent a fly plague in the fall, it must first be made impossible to enter the building; screens in front of windows and doors, weather stripping, sealing holes and cracks in exterior walls and sealing ventilation openings with fine mesh. The (partial) removal of creepers can help as well. Because grass flies prefer ivy and Virginia creeper. Sealing is not always effective. The flies can also enter a building through the roof riles.
If there already is a fly plage, there are different methods to resolve it. If the cluster place of the flies is tracked and well-accessible, the swarm can be removed using a vacuum cleaner. In the spring, it is often enough to open windows against each other. Sometimes the cluster place is hard to reach or inaccessible. In that case, an insecticide is required. EWS can advise you on this.
If, regardless of all control measures taken, the flies keep returning every year, EWS can carry out a preventive treatment early in the fall, with a residual-acting insecticide. The agent is sprayed in seems and cracks, on and near entrances of the flies and – optionally – on walls and frames.