Invasion of the fatty cluster flies

02:48, Aug 07 2009
INVADED BY FLIES: A day's collection of cluster flies in on disgruntled householder's lounge.

So you have cluster flies - and they are driving you nuts. Every day you vacuum and there they are again the next day, in their thousands.

These little greasy flies have invaded parts of the central region of the North Island in plague-like proportions this autumn.

Cluster flies have been in New Zealand since at least the 1980s when they were spotted in Auckland. They moved south along the east coast. Wairarapa had a big infestation, so bad at one stage a reservoir had to be closed as they were fouling the water.

South Island residents have put up with them for some time, and now they have reached other areas of the North Island.

There is no easy way to deal with the flies and many people have resorted to having the vacuum cleaner permanently on standby. One woman fills the vacuum bag with the flies and then burns them. Judy Jack, of Rewa, near Feilding uses the long nozzle on her cleaner for those on the ceiling, and said she has to cover her food.

"Spending hours every day vacuuming up flies is not a productive use of time," she said.


The name cluster fly or loft-fly, which comes from the Northern Hemisphere, refers to Pollenia rudis Fabricius, and P. pseudorudis that are parasitic on four species of earthworms.

Dr Allen Heath, senior scientist at AgResearch said they don't bother all earthworms and there seems to be a balance as the earth worm population has not decreased.

Various websites offer advice on how to deal with the flies. The experts say the best way to prevent the flies moving into your home is by treating the larvae in the soil. A slow release insecticide can be used for removing pest insects from the soil. It will kill the worms, but the fly larvae are doing that anyway.

MAF entomologist, Maurice O'Donnell said MAF gets quite a few calls on their 0800 line about cluster flies.

"It is a major problem for those affected.

"It anyone came up with a solution (for getting rid of cluster flies) they would probably win the Nobel prize."

Flies in the house like rooms that are shut up. Check the guest rooms. In warm dark dry corners, behind pictures, under mats and bedding, try a spray, and for areas that are hard to access, use a bug bomb. Treat the grass and the house before the next season, as they will come back to the same places.

One theory is that the flies form flight paths. One house can be infested, while a neighbour is not affected.

Overuse of insecticides both inside and out is not a particularly safe option for humans or other animals, warns Dr Heath.

The flies have abdomens full of fat globules, so it's not a good idea to swat them. Dr Heath said this is possibly the remnants of the larval fat and may provide food for the flies during their winter hibernation.

Dry warm houses are their first choice, where they collect in roof spaces, but they will hide under any object that affords shelter - between the carpet and skirting boards and in the gaps between windows and frames.

Wool sheds, tool sheds, hay barns and stables provide other wintering-over spaces.

Dr Heath said the flies lay their eggs on grass on the soil surface and then move some distance away and deposits another batch, laying about 100-130 eggs. Three days later the eggs hatch and the larvae move into the soil by following natural spaces such as gaps between plant stems and soil and worm burrows, to find a host.

Some species of earthworms emerge from their burrows at night and move freely about on the ground, easy pickings for the larvae, which emerge as flies to start the cycle all over again.

Research done by Dr Heath indicates the flies are sometimes attracted to horse dung, meat and fruit, with banana reported to be especially good bait.

"Remove the breeding source," is Dr Heath's solution.

* If you have a solution, write to Editor, Central Districts Farmer, PO Box 190, Feilding. Or email

Central Districts Farmer