What Are Cluster Flies?
Cluster flies are large, slow moving, hairy, flies around 10-15mm with a dark gray to black non-metallic abdomen. They lay eggs on grass lawns and pasture in spring. The larvae of the flies feed on earthworms in grass pasture or lawn areas. Then in autumn and early winter the adult flies will move into homes and other buildings to survive the winter.
They are named cluster flies because of their habit of congregating in large groups or swarms, often in dark places such as attics, corners of dark rooms, 'nooks and crannies' and 'cracks and crevices'. Each fly releases a pheromone (smell) that attracts others. This pheromone will linger on, even after all the flies have been destroyed and removed. Thus the pheromone will continue to attract cluster flies to that area and clusters will keep recurring. It is therefore important that treatments are continued even when there are no flies present.
Why Are They a Pest?
The flies congregate to survive through the winter and once warmer weather arrives in spring they emerge from their 'hibernation' to lay eggs on surrounding grassland, pasture and lawns. So there tend to be two times of the year that these flies bother us in homes and other buildings; autumn when they congregate and spring when they emerge from their hiding places to lay eggs.
How To Stop Clusters
Kill the adults to prevent them laying eggs in surrounding grass. Kill the larvae in the grass to prevent the adults clustering in your buildings.
If you, or a neighbour, have had the problem in the past treat the likely areas of clustering in early autumn to prevent clusters forming. Spray with residual surface spay as described.
If you have the adult flies swarming and clustering in the house or other building you need to carry out a full program of treatment.
- Check around your house/buildings for clusters of flies in warm, dark, dry corners specially your roof void and eaves.
- If these can be accessed, spray the clusters with a residual insecticide containing deltamethrin or permethrin. If the clusters cannot be found or accessed use a bug bomb or borer fumigator in the voids.
- Once the flies in the cluster are dead, clear away as many bodies as can be reached. If you use a vacuum cleaner ensure the bag is disposed of immediately. Spray the surfaces they were clustering on with long life surface spray containing an active ingredient such as deltamethrin or permethrin to prevent new clusters forming where the pheromone lingers.
- Treat the surfaces around the home where flies have been seen and entry points to the roof void around the eaves etc. with the surface spray.
- Treat again prior to the next season to prevent new clusters collecting. Given a chance they will use the same areas to form clusters.
- Prevention - If possible it is best to prevent the problem of these flies moving into your home by treating the larvae in the soil before they become adult flies. Use soil insecticide such as diazinon lawn prills which is a slow release insecticide in the form of granules. This is an ideal product to protect you from cluster flies and other soil pests such as grass grub and porina. Sprinkle the granules on grassy areas within 100-200m of your home and water in thoroughly. These flies rarely travel further than this to infest a home.
Stay Cluster Fly free this season.