Cluster flies

Cluster flies in the house every autumn

Why slow cluster flies gather in sunny upstairs windows each autumn in New Zealand, where they spend the winter, and what actually reduces them the next year.

The short answer

Cluster flies are not breeding in your house. Their larvae live as parasites of earthworms in lawns and pasture, and in autumn the adults look for a sheltered place to spend the winter, which is often a roof space or the wall cavity behind a sunny window. They are sluggish and harmless but hard to shift once inside, so the real fix is fly treatment timed before they arrive, plus sealing the gaps they use.

What a cluster fly is, and how to tell it from a house fly

Cluster flies belong to the genus Pollenia. They are about 8 mm long, dark grey and not metallic, with short golden hairs on the thorax, though these rub off as the fly ages, and they are noticeably slow and clumsy on glass. A house fly is a little smaller and quicker, with four dark stripes along the thorax. A large shiny blue or green fly is a blowfly, which is a different problem entirely.

New Zealand has many native Pollenia species of its own. The autumn nuisance in houses, though, is chiefly an introduced Northern Hemisphere cluster fly, first recorded as established in Auckland in 1984 and now found through much of the country, according to a 2004 study in the New Zealand Journal of Zoology.

Why they come inside in autumn

Their young develop inside earthworms, so the adults are born outdoors, in lawns, paddocks and parks. As the days shorten in autumn they look for a dry, sheltered place to spend the winter, and gaps around the roof, eaves and window frames lead them into roof spaces and wall voids. They favour the walls that get the most sun, which in New Zealand usually means the north-facing ones.

Through winter they sit quietly in the roof or the walls. On warm days some wake, follow the light and end up on the inside of the windows, which is why the same panes fill up again and again. In spring the survivors leave to breed outside, so the season in a house runs roughly from autumn through to spring.

Why the same house gets them every year

The conditions that drew them in do not change from one year to the next: a warm, sunny wall, a roof space with gaps, and lawns or paddocks nearby full of earthworms. Houses that look onto open grass, lifestyle blocks and hillside suburbs with paddocks below tend to be hit hardest, and cluster flies commonly return to the same building year after year.

That is why sealing and timing matter far more than how many dead flies you vacuum up. Killing this year's flies does nothing about the next generation growing in the lawn, and the house that attracted them will attract the next lot unless the way in is narrowed.

What reduces them, and when

Three things, in calendar order. In late summer, seal the gaps around eaves, roof flashings, vents and window frames, and have a residual treatment applied to the sunny exterior walls before the flies arrive. Once they are inside, treat the roof space and vacuum the live and dead flies from the windows rather than spraying living areas. Each spring, check where they got through.

Nothing removes them permanently while the lawns and paddocks around you keep producing more; what you can do is cut the numbers sharply, season by season. Vacuum out the dead flies in the roof space where you can reach them, because a heap of dead insects can feed other pests such as carpet beetles, and it can smell.

If they are big and shiny, it is not cluster flies

Large, metallic blue or green flies appearing suddenly in one room, at any time of year, almost always mean blowflies breeding in something dead, most often a rat or mouse that died in a wall or ceiling void, or a bird in the roof. Spraying will not stop them. The flies keep coming until the source is found and removed, and the room smells until it is.

If there has been rodent activity, the carcass is a symptom and the rodents are the real job, which is where rodent control comes in. Trapping rather than baiting is one way to stop the same thing happening again, because the body is recovered rather than left in the cavity.

Smells, sprays and gadgets: what helps and what does not

We have found nothing reliable showing that a smell keeps cluster flies out of a roof space. Oils dabbed on window sills may move a few flies along the glass but do nothing about the ones in the wall behind it. Aerosol fly spray kills the flies you can see and none of the ones you cannot, and the next warm afternoon brings out more.

Light traps in a roof space can catch flies drawn to the light, and sticky window strips catch some as well, which helps on a bad day. Most of the work, though, is done by sealing the gaps and treating the outside before the flies arrive, which falls in the same late-summer spray window as for spiders, so one visit can cover both.

Questions

Cluster fly questions

Are cluster flies harmful?

Not to people. They do not bite, they do not breed indoors, and unlike house flies they are not associated with food scraps or rubbish. They are a nuisance, mainly through sheer numbers on the windows on warm days and the smell of dead flies building up in a roof space. Large numbers of dead flies are best vacuumed out where you can reach them, rather than left in place.

Why are the flies only on one side of the house?

Because cluster flies pick the warmest walls to spend the winter behind, which is usually the side that gets the most sun, in New Zealand mostly the north and sometimes the east or west. They gather in the roof space and wall voids on that side, so the windows on that side fill up while the shady side of the house stays almost clear.

Do cluster flies lay eggs in the house?

No. Their larvae need living earthworms, so the eggs are laid outdoors in soil, near worm burrows in lawns and pasture. Every cluster fly you see indoors arrived as an adult looking for winter shelter, which is why killing the ones on the windows never solves the problem on its own, and why the work is done on the outside of the house.

When is cluster fly season in New Zealand?

They move into buildings in autumn, sit out the winter in roof spaces and wall voids, reappear on the windows on warm days, and leave in spring. So the flies you notice in winter arrived months earlier. The time to act is late summer, before they arrive, by sealing gaps and treating the sunny walls; once they are in, the options narrow to the roof space and the vacuum cleaner.

Will a pest controller get rid of them for good?

No honest one will promise that. A roof-space and exterior treatment, with the gaps sealed, cuts numbers sharply for that season, but the lawns and paddocks around you keep producing flies, so most affected houses need treating again before the next autumn. What changes over time is how many get in, as the gaps are closed one by one.

Sources

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