Author – Paul Pritchard –

A number of weeks ago I had an interesting call from Wayne at Wairarapa Carpet & Pest Services. He had been called to a fly treatment job involving the “Cluster Fly”. Later I received a call from Chris at Makebrite Carpet and Pest Services concerning an unusual fly treatment job in Clevedon (south of Auckland). We were able to identify this job as “Cluster Fly” as well.

MAF first became concerned about the “Cluster Fly” some years back due to the parasitic nature of their larvae which feed on earthworms. Fortunately, their impact on the earthworm (and ultimately the N.Z. economy) has been minimal. However, the adults remain a nuisance for some rural householders.
The most interesting feature of the “Cluster Fly” is its ability to hibernate in clusters (hence the name) and survive up to 2 years, as opposed to only 2-3 months like most of its cousins.

Below is some information courtesy of the University of Vermont.


  • Size: 6-10mm overall body length
  • Markings: Reddish eyes and a thorax covered in golden hairs
  • Location: Numerous locations, but usually in lofts and roof spaces. South-facing (Northern Hemisphere) walls are often favoured.

Several species of flies enter buildings in the fall to find a protected place to spend the winter. The Cluster Fly is the most common and annoying of these overwintering flies. On warm days throughout the winter and spring, these flies emerge from hiding and plague the householder.
Cluster Flies often enter buildings in large numbers. Although they resemble the housefly, these flies are not at all related nor alike in habits. Cluster flies do not breed in buildings!

Cluster Flies belong to the family Calliphoridae, the blow flies and flesh flies. The larvae (maggots) are parasitic on earthworms, particularly where manure is spread. The Cluster Flies lay their eggs on the soil. Upon hatching, the maggots attack and parasitize several species of earthworms.
When fully mature, the larvae leave the earthworm host and pupate in the soil before emerging as adult flies.

Cluster Flies are not attracted to foods in the home. They are extremely annoying with their habit of flying aimlessly about lights at night whenever the temperature is above 12 degrees C. During the day, the flies usually crawl about the floors, walls, and windows in a sluggish manner. On warm days or when the sun is on their side of the house, the flies often emerge by the hundreds only to disappear later in the day when the sun drops and it gets cooler. As soon as cool weather approaches, they seek protected nooks in which to hibernate, entering buildings by squeezing through any tiny crack that may be found around doors, windows, vents, under clapboards, or in attics. They enter and congregate in partitions despite ordinary precautions effective against houseflies. Later in the winter, they make their way into the heated rooms where they often are attracted to and fly erratically around the lights. They may persist until late in the spring. When the air is chilly, they gather in clusters, thus the name Cluster Flies.

Cluster Flies are also called “buckwheat flies” because they give off an odour of buckwheat honey when crushed. Buildings surrounded by lawns and pastures are most frequently selected for hibernation. Houses surrounded by large trees are seldom heavily infested.


Prevention:

If Cluster Flies can be denied access to a building, they will not be a nuisance or require control. Often certain buildings or portions of buildings are preferred. Make sure of the following:

  • All windows, louvres, air vents, etc., are tightly screened
  • All cracks around windows, chimneys, and trim are kept tightly caulked
  • All splits and holes in the siding are kept caulked and painted over

Although caulking and careful carpentry, which limit the entry of flies, are time-consuming, expensive, or impractical, and futile, they should be seriously considered when alterations, remodeling, etc., are being planned. Tight, full-coverage polyethylene vapor barriers, including
window casings and door jamb areas, can be very effective if installed during remodeling.

Chemical Controls:

  • Inside: Residual and space sprays used within the home can provide temporary relief from the flies.
  • Outdoors: A residual spray applied under the eaves, around windows and to other outside areas where flies are seen congregating will reduce the number of flies entering the house. Residual sprays should be thoroughly applied to exterior surfaces. Residual control and the period of needed protection will vary with the weather and building surfaces. Depending upon the structure(s) and the weather, it may not be possible to control all the flies.

Important Note: If you come across these flies please keep us informed.