T.E.R:R.A.I.N - Taranaki Educational Resource: Research, Analysis and Information Network

Fly (Cluster) Pollenia rudis

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Family: Calliphoridae
Genus: Pollenia
Species: P. rudis
Binomial name: Pollenia rudis
Synonyms: Musca familiaris Harris, Musca rudis Fabricius,
Common names: Cluster flies, Fatty cluster flies, Common cluster fly, Attic fly, Loft fly

Pollenia rudis is a medium to large species of fly in the family Calliphoridae. It is a European species and they have been in New Zealand since at least the 1980s. They are slow-moving, dopey, hairy, flies 10-15mm in length, which is slightly larger than the common house fly. The adult flies are a dark grey to dull, black with a non-metallic abdomen. There are golden-yellow hairs on the thorax and under their body which gives them a golden brown look. 
In New Zealand, they have invaded many parts of North and South Islands in plague-like proportions. At one stage Wairarapa had a big infestation, so bad at one stage, a reservoir had to be closed as they were fouling the water.

During the summer Pollenia rudis are outside in fields and open areas. The adult flies are, in most cases, herbivores. They feed on many types of organic matter, plant sap, fruit, flowers and faeces. During spring the female flies deposit eggs into the soil. 
The hatched larvae feed on earthworms. Once the larvae hatch, they burrow into the soil by following natural pore spaces, such as holes near plant stems or paths that earthworms have already created. By randomly moving through these pores, P. rudis larvae find their host worms.The larvae then eat their way through a section of the earthworm’s epidermis, where they feed, eventually killing the worms before pupating and emerging as adult flies. This species can have as many as five life cycles over one summer. 
When there is a sudden drop in temperature the last generation of flies migrate in large numbers into the interior of structures to hibernate in warm, dry areas over the autumn and winter months before the outdoor breeding cycle begins to spring. They congregate in large number due to each fly releasing a pheromone (smell) that attracts other flies. They are often seen “clustering” near the interior windows of houses. While in hibernation they live of their own stored body fat.
Pollenia rudis is most commonly known for being a household nuisance. They do not cause any true damage to structures, textiles, foods, or humans but piles of dead flies left in the walls can sometimes lead to secondary infestations of carpet or larder beetles and rodents.

A video of a cluster fly invasion.

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/