Captain Stan’s ‘Creature Features’ Volume 7 – Cluster Flies November 2022

Welcome to the seventh edition of ‘Captain Stan’s Creature Features,’ where we are looking at some of the more interesting animals in the pest management universe. This month, we learn about the cluster fly, one of our so-called ‘fall invaders.’ Some interesting facts follow:

• During cool months, some insects seek to enter structures for temporary shelter. The most common of the flies that do this are known as cluster flies.
• Cluster flies are often problematic in areas of irrigated turfgrass, near streams or rivers, and other sites where soils are moist.
• Cluster flies are dull grayish-brown and about 3/8 to ½ inch long, which is slightly larger than the house fly. They can be distinguished from similar flies by the golden, tangled hairs on the thorax (the body region right behind the head).
• Cluster flies are not a type of ‘filth fly’ that develop in carrion, decaying plant matter, or garbage. Instead, the larvae of most cluster flies in North America feed on earthworms, of all things! Yummy! They find an earthworm, penetrate the body, and the resultant feeding kills the earthworm. Adult flies feed on water and other available liquids, such as nectar.
• As cooler, shorter days approach, cluster flies (and other fall invaders) can be seen resting on sun-warmed (mainly south and west) walls of buildings. They will then make use of any available entry sites to go in the structure.
• Once in the structure, the flies will cease reproduction, slow down their metabolism, and enter a dormant period. They will remain in this inactive state but may become active during the occasional warm, sunny day. At this point, they may wander into living areas and become a nuisance.
• The basic premise, and the really only effective method to prevent cluster fly problems is exclusion; exclusion BEFORE they enter the structure. If all potential entry points are not sealed, it is next to impossible to control cluster flies inside a structure.
• The value of insecticides in managing cluster fly problems is limited and involves two primary uses. Residual insecticides can be applied as sprays targeted to cracks and crevices on the building exterior in late summer. Be sure that the label allows for such applications. Desiccants/dusts can also be injected into voids or other areas behind walls where cluster flies aggregate.
• Cluster flies are not attracted to things such as fly paper or traps with lures. However, they are attracted to certain types of light traps placed strategically within structures or in living areas where cluster flies are seen. Finally, simply vacuuming the flies is also effective.