Ruud Kleinpaste: Queries about cluster flies in houses

Saturday Morning with Jack Tame

A common problem in our homes, especially in the cooler areas of Aotearoa: gazillions of flies settling into your spare room, on the ceiling of a quiet place, or in the roof cavity.

The flies arrive at your place in autumn and find a good spot to hibernate – just like they do in their countries of origin: Sweden, Finland, Netherlands, Germany, the UK, the USA, and Canada.

Pollenia pediculata is the cluster fly species we discovered on the North Shore for the first time in 1984. It may have arrived in some containerised luggage or cargo (we were not sure when we found it), it’s now well-spread, often in the cooler spots of our country.

It is quite a gorgeous fly species, with golden hairs on its back – easy to spot as it often sits quietly.

Flies have just one pair of wings whereas other insect orders usually have two pairs of wings in the adult stage. The second pair of wings is turned into small “halteres”: modified hind wings that provides stability when in flight.

Flies are real acrobats: they can fly up-side-down onto the ceiling

Our cluster fly lives a peculiar life as a parasite of earthworms.

The female fly lays eggs in dense grass habitats (paddocks and lawns) and often near or in earthworm tunnels.

The larvae (aka "maggots") hatch, find themselves some way of getting into the soil (gaps around plants or through established earthworm tunnels) and gain entry into the body of an earthworm.

Inside the worm it feeds on the internal body fluids and organs.

Gross? Yep! But these flies do minimal damage to earthworm population densities.

Most animals on the planet have "parasites" that can cause damage (humans have parasites too). In spring and summer, I often see them pollinating flowers (the name “Pollenia” is a nice indicator!).

When temperatures get cooler in autumn, the cluster flies usually look for a suitable hibernation place: holes in the ground, under bark of trees, under mulch layers, etc. But if there’s a nice warm human house nearby, they will try to gain entry – by the thousands!

They crawl through small holes (ill-fitting window frames, etc) and mark their entry by leaving a residue of pheromone scent – this means that other Pollenia flies simply follow the trail and join the others.

The pheromone is rather sticky and smells somewhat of Buckwheat honey (hence the name buckwheat fly in the USA).

Cluster flies are not of medicinal importance (like some blowflies) but are hard to remove due to their messy pheromones. Best “prevention” is to ensure there are no entrance holes around the home.

A very residual aerosol can (available from SafeWorx – safety company) sprayed at the flies’ entry and exit holes, will stop them from getting inside.

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