Often confused with the common house fly, cluster flies are roughly the same size. Some characteristics that differentiate the cluster fly: they fly somewhat more slowly than the house fly, they almost always fly toward windows on the warm side of a structure and their wings overlap almost completely, when at rest. Cluster flies are most common along the northern part of the US and in other countries around the world. They will appear inside homes to overwinter only during the cool fall, winter or spring months.
Cluster flies should not be confused with other medium to large size flies which may appear suddenly. Those flies may appear when a small animal such as a mouse, rat, squirrel or bird dies within a wall, ceiling or floor void. You may or may not detect an odor. Such flies will find the hidden carcass and lay eggs on it. The eggs will hatch into larvae (maggots) which feed on the carcass. Soon, the larvae enter the pupae (cocoon) stage and then eventually hatch out as adults (flies), appearing around windows (just like cluster flies). This process may take from a few days to up to two weeks to complete and for the flies to stop appearing. Until then, open windows or doors to let the flies out or vacuum them up as needed.
Cluster flies breed in the ground outside of buildings during the warm weather (late Spring into early Summer) using earthworms as a food source for the immature larva (maggots). The flies later pupate (go into the cocoon stage), then hatch as adult flies. In temperate areas, often in late August or early September, these flies begin to migrate indoors finding any small cracks or crevices that permit entry into a structure. These include areas around window frames, door frames or eaves. Entry tends to be on the same, warm, sunny side (often the southern or western exposure) of the structure that the flies later emerge from.
Unlike other flies species, cluster flies only overwinter inside of a structure; they do not breed there. During the fall, winter or spring months, these flies may emerge, particularly on warm, sunny days thinking that spring has arrived. The flies appear at windows buzzing and "clustering" around those areas to the dismay of the occupants. This fly can become a problem in virtually any structure and they have been a problem in sensitive areas such as hospitals, where they are especially unwelcome.
Stop them on the Outside
One of the best ways to minimize entry of this pest is by applying a repellent, residual insecticide within a week or two of the time when these insects are known to begin migrating indoors. Within the US, Cooperative Extension is a good source to check with for timing information for your immediate region. In other countries, check with the entomology department of the local university. These residual insecticides are applied outdoors to window frames, door frames, soffits and eaves as well as any other areas that are vulnerable to entry.
Some have suggested that application to the surrounding soil may also help minimize cluster fly populations and later entry, but since the flies may migrate from other adjacent areas, treating the soil is a futile attempt to control the problem.
Because some of the treatment areas are inaccessible without power equipment, calling in a professional, commercial applicator to do the application is recommended. Products used by commercial applicators (common name in parentheses) include Prelude or Dragnet (permethrin) and Demand CS (lambda-cyhalothrin).
For existing infestations indoors, insecticides may be used selectively to control this pest. A total release fogger such as Pro Control Plus may be used in an attic or other room to control flies that may have migrated in.
In some cases, the use of electronic fly traps (light bulbs with a "stun" unit that disables the flies) in attics and drop ceilings have been helpful. These do require electricity and maintenance - things not always available in vacation homes.
Another solution is the Cluster Buster, a cluster fly trap. These traps attach to the inside glass of any window. These white plastic traps are filled with super finely ground (powdered) egg shell. A large cutout across the top of the trap combined with light coming through the translucent plastic attracts the flies into it. Once inside, they land on the powder which is so light and fluffy (as light as dust) that the flies actually sink into the powder, providing a "quicksand" effect. The powdered egg shell kills the fly by clogging the sphaeracles or openings into their respiratory system. These traps represent the most conservative approach to control and are virtually non toxic. These non refillable traps require no energy to operate and each hold up to 1000 flies or more. When flies begin to enter and then leave the trap, it is time to replace it.
We have sold the traps for several years now and have had excellent feedback from customers. The manufacturer guarantees the traps to work for cluster flies. They also recently introduced a Lady Bug Buster trap which looks very similar and will help with invading ladybugs.
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