Nola Robertson, of Lawrence, asks :-

How does one get rid of cluster flies?

Scott Hardwick, a biosecurity entomologist at Agresearch, responded.

The frustration of cluster flies in your house cannot be underestimated and I understand the desire to control them. Unfortunately this is not easy.

Cluster flies that you see in houses during autumn are looking for somewhere to hibernate during winter. They have developed from larvae that feed on earthworms during spring and summer. Various control methods target different life stages of the cluster fly. Use of a “bug bomb” in the ceiling space during autumn targets cluster flies that are entering your home to overwinter. This will kill any cluster flies that are present at the time of treatment. However, unless the “bug bomb” contains a residual insecticide it will not kill cluster flies that enter the ceiling space after its use. As cluster flies continue to enter a house over a number of months that more than one “bug bomb” will be needed to be used over autumn.

Treating lawns with an insecticide during late summer targets the cluster fly larvae and their earthworm host. This control method should not be used as it is not effective at preventing cluster flies entering nearby houses. This is because cluster flies entering a house to overwinter can originate from over one and a half kilometres away. Also it isn’t environmentally acceptable to apply insecticides to kill off earthworms as they are beneficial creatures that play an important part in the cycling of organic matter in soil.

A third control option is to use a licensed pest controller to treat the exterior walls and ceiling space of your house with a residual insecticide in autumn. For people who are uncomfortable with the use of insecticides around their house trapping cluster fly with fruit baits is possible. However, this will not necessarily reduce their numbers below nuisance thresholds.