Come fall, however, and they may be taken by surprise by an equally pesky, yet mostly innocuous, household pest: cluster flies
As the days grow shorter and temperatures drop, the large, black cluster flies start entering homes in the search of protected overwintering sites, and they may be seen flying around in large numbers throughout winter and early spring. They show a landing preference for warm, sunny locations usually on the south- and west-facing walls of light colored buildings. Structures situated on open hilltops or near meadows and lawns appear more attractive due to the existence of the large populations of earthworms
, their preferred host.
They enter homes through small openings, such as gaps under eaves, as well as open windows and doors. They congregate in large numbers (thus the name 'cluster' flies) in walls, attics, and basements, waiting for spring to emerge and start a new life cycle outdoors. However, cluster flies may become active during winter warm spells and begin sluggishly moving around, clustering at windows and lights. Although they don't cause any structural damage during their stay, they tend to occupy the same building to over-winter year after year.
Why Should You Worry about Cluster Flies
At first glance, they may look much like Musca domestica
, but they are larger, darker, and slower-moving. On the bright side, they are not a filth-breeding
or pathogen-carrying fly species like the house fly, so they are not considered a health hazard. They do not breed or feed inside homes, nor do they lay eggs inside structures the way other filth flies do. They can, however, be more than just an annoyance in homes and commercial buildings. Here’s why.
They use your house as their toilet
. You will often come across “fly specks,” or small brown dark-colored spots of excrement on surfaces they land on, particularly walls and curtains. When crushed, they leave a sticky gray spray that smells like buckwheat honey.
They make a constant buzzing sound
. Cluster flies have a louder buzzing sound than the housefly, and they can be seen clumsily blundering around windows and lights or colliding with various objects in their path. They often fall to the floor on their backs and start spinning aimlessly until they are able to right themselves. They can lie motionless on their backs for up to two days
They may attract other pests
. If there is a heavy infestation and large numbers of cluster flies die within your house, this may attract pests such as carpet beetles, larder beetles, mice, and rats, causing secondary infestations.
They produce an offensive smell
. If flies aggregate in large numbers, you may be able to detect a sickly, sweetish pheromone odor, which is their way of letting other flies know they have found a warm spot for winter. They also give off a foul odor when disturbed or crushed.
They can trigger allergic reactions
. Dead bodies of cluster flies inside wall voids and vacant places around the house may trigger allergic reactions and several respiratory ailments in sensitive individuals.
Fall Is the Best Time for Effective Fly Control and Management
Controlling cluster flies after they’ve gained access inside a building is often difficult because they hibernate in inaccessible places. Hidden inside wall voids, behind electrical outlets, baseboards, or under insulation, they are immune to most treatments until they make their appearance within the living spaces of the house.
If cluster flies have not yet infiltrated the structure
, mechanical exclusion is the most effective way to prevent them from ever doing so. Seal cracks around their typical entry points - doors, windows, siding, utility pipes, chimneys - using good quality materials and inspect other openings such as dryer vents and utility boxes for holes. Damaged window and door screens should be repaired or replaced, while loose siding and weather stripping should be carefully inspected for damage. Installing insect screening over air vents in soffits will keep the annoying flies out from finding shelter in your home.
If cluster flies have already infiltrated the structure
, you will need help from a professional pest management professional
to locate the openings where the insects gain access and seal them shut. A preventative treatment with a 30-day or longer residual effect will be applied on exterior walls to ensure quick knockdown to cluster of flies that are about to enter the building. The PMP will apply crack and crevice spot treatments to adequately cover entry points, along with space applications in infested attics or void spaces where flies are congregating. Professional treatments vary depending on location, weather, and size of population.
If ignored, the clusters of flies can become enormous, with several thousand flies congregating under the same roof, buzzing and emitting foul odors. Although they do not pose health hazards to humans or cause structural damage to buildings, their presence is unsightly and can ultimately attract more dangerous pests to your property.
Using the treatment and exclusion services
of an experienced pest control company is the safest way to removing all flies that plan to over-winter in your home and prevent secondary insect and rodent infestations. Find a local pest company that uses environmentally-safe and superior treatments that eliminate unwanted visitors without jeopardizing the health of your family or the integrity of your structure.
About the Author
Daniel Mackie, co-owner of Greenleaf Pest Control, is a Toronto pest control expert well-known as an industry go-to guy, an innovator of safe, effective pest control solutions, and is a regular guest on HGTV. Mackie, along with business partner Sandy Costa, were the first pest control professionals in Canada to use detection dogs and thermal remediation for the successful eradication of bed bugs. In his free time, he is an avid gard