Latin: Pollenia Rudis
The cluster fly is a small insect that is generally considered to be reasonably clumsy and harmless. The cluster fly got its name as Danes could observe them clustering together inside before it was normal to have all the rooms in houses heated all the time. The cluster flies settled in tightly packed groups in rooms where there was no heating.
Although the cluster fly is a species in the family of the housefly, it does not look very much like its family members. Spit flies are typically recognized by their shiny appearance, almost like metal. The cluster fly, on the other hand, is not metallic, but instead golden and hairy.
Appearance
Cluster flies are slightly larger than ordinary houseflies. Instead of the metallic, shiny appearance that houseflies have, the cluster fly has a distinctive forebody with small, golden-yellow hairs. Together with a hind body that has light but irregular drawings on it, this gives the cluster fly a lighter appearance than the darker, shiny flies that you typically see.
You can also tell the difference between the cluster fly and the housefly by the fact that the wings of the cluster fly almost completely cover each other when standing still, almost like when a human is sitting with its legs crossed.
Biology and behavior
Like all other flies, the first life stage of the cluster fly is like eggs, then larva and finally adult fly. Adult cluster flies lay their eggs individually in small cracks in soft soil. When the eggs hatch after a few days, the newly hatched larvae seek against earthworms. Unlike other types of housefly larvae that live in carrion, cluster fly larvae live in earthworms as parasites. This stage typically takes a couple of weeks, after which the larva seeks away from the earthworm again and pupates, while still living in the ground. A few weeks later, the full-grown cluster fly emerges.
The behavior of cluster flies depends a lot on the season. In summer they live outside and you can typically find them on flowers as they feed on nectar. Likewise, they also suck juice from plants, especially from fruits.
The cluster fly is also a very tough species of flies, which overwinters in Denmark as a full-grown fly. It does this by searching indoors in the fall, where it will then try to stay all winter. During the day one can find large groups of cluster flies on exterior walls where they sunbathe. When the sun goes away, the cluster flies search inside through cracks and crevices in the wall. Moreover, their behavior in winter depends on the heat. If their residence is very cool, they sit quieter, but if you heat the room, they will become more active.
Damage
Overall, cluster flies are neither dangerous nor harmful. They do no harm to people and they do not destroy woodwork, go into food, or do any other harm to people’s homes. Like larvae, they live outside, where they also stay all summer. Only in winter do they seek warmer (but not too warm) surroundings, and this is where you can experience larger groups of them in your home. In general, cluster flies are more of a nuisance than a harm to humans.
Prevention and pest control
Although they are not harmful to one’s person or property, cluster flies can still be a nuisance to humans. This is especially true as one has been able to observe that cluster flies search towards the same location year after year, so if one does not get them removed one winter, they are likely to return next winter.
By removing their entrance to one’s home, one can easily prevent against cluster flies. You must therefore check your outer walls for cracks and fissures, which you can then seal. Keep a particularly close eye on windows as well as under the roof.
To control cluster flies in your home, you can either use insect spray or powder, possibly both. On house walls, where cluster flies have been observed during the autumn, one should powder. If you only detect them once they have entered, they can be controlled individually with insect spray. If you have found a larger cluster of them together, you can take them with a vacuum cleaner. Also note that they will be easier to fight if they are cold, as they will then sit more quietly. It is only possible to prevent and control adult cluster flies, as it is simply not possible to find them as eggs or larvae.
"Pests in House and Home"
"Bed Bugs - Bites, Stings and Itches"
"Food Pests"
"Husets dyreliv" (Insects Around the House - Only danish)
"Skadedyr i træ" (Timber Pests - Only danish)
"Stuefluen" (Common Housefly - Only danish)