Cluster Flies and other “Winter Flies” Fact Sheet No. No. 5.618 Insect Series|Home & Garden W. Cranshaw1 During the cool months some insects There are some other kinds of flies Quick Facts have the habit of moving into buildings that will be found indoors and these are • Cluster flies frequently use as a temporary winter shelter. Common reviewed in Fact Sheet 5.502 Flies in the buildings for winter shelter examples of such cool season "nuisance in- Home. Among these are some other "win- and can be the most common vaders" are boxelder bugs, western conifer- ter flies" that use homes for winter shelter, seed bug, and elm leaf beetles. Cluster flies particularly the “blue bottle flies” (Calliphora fly found indoors during the (Pollenia species) are the most common of species) and the black blow fly (Phormia cool season. the flies that often use buildings as a tem- regina). The habits of these differ in some • Cluster flies develop by porary shelter through winter. important ways from those of cluster flies feeding on earthworms. and are discussed below. Cluster flies tend to be particularly com- • Cluster flies found in buildings mon in higher elevation communities, but are in a dormant state and Life History and Habits of problems with cluster flies within buildings cannot reproduce indoors. can occur statewide. Most often they are a Cluster Flies problem where there are nearby areas of ir- • Sealing buildings by prior to rigated turfgrass, near streams or rivers, and Cluster flies are not a type of "filth fly" when cluster flies enter in late other sites where soils are moist. that develops in carrion, decaying plant summer can limit nuisance matter or garbage. Instead the young stag- indoor problems the flies in Cluster flies are dull grayish-brown flies es are predators of other small animals and winter and early spring. about 3/8-1/2-inch long, slightly larger than most, if not all, cluster flies in North America • Other flies that will use a house fly. They may be noticed within feed on certain earthworms (e.g., Apor- buildings from early autumn through rodectodea rosea, Allobophora chlorotica). buildings for winter shelter spring and sometimes occur in large num- Although these earthworms are introduced include blue bottle flies and bers. When active the flies are in a semi- European species, not native to Colorado, the black blow fly. dormant state and fly lazily and somewhat these are now widely found in irrigated erratically. More often cluster flies are noted lawns and pastures and along waterways. when they are dead or dying, typically around windows. Three species of cluster flies are com- mon in Colorado: Pollenia pediculata, P. angustigena, and P. rudis. All are slightly similar in size and are a dull grayish-brown but many have irregular light patches on the abdomen. A feature that can be used to distinguish cluster flies from some similar flies (e.g., blue bottle flies, black blow fly) is the presence of golden, tangled hairs on the thorax (the region of the body that includes the wings and legs). The adult flies feed on water and other available liquids, such as nectar. ©Colorado State University Extension. 5/18. Fig. 1. Cluster fly Active cluster flies can be found out- extension.colostate.edu doors from early spring through much 1Whitney Cranshaw is an Extension Specialist and of autumn. They are most often present Professor of entomology at Colorado State University. on turfgrass and other sites where earth- 5/18. worms are present. Female flies will lay moving through available openings, their eggs in soil crevices and when the such as in cavities behind walls and egg hatches the tiny larva (maggot) that areas provided above a suspended emerges will migrate into the soil seeking ceiling. an earthworm. When they locate a host earthworm, the larva of the cluster fly For the cool season months, be- will penetrate the body and feed within ginning in early autumn, cluster flies the earthworm, ultimately killing it. The remain in a dormant state, moving developing larvae become full-grown in little if at all, usually clustered together. about 2 to 3 weeks, then pupate in the During this time cluster flies cease soil. Adults subsequently emerge to pro- reproduction, slow down their metabo- duce a second generation, repeating the lism, and are in a dormant (diapause) cycle. As many as four generations may period that allows them to survive be completed during the growing season. through spring. They will remain in this Fig. 2. Mass of dead cluster flies on an attic floor inactive state for many months, but will to eliminate points where insects may Cluster flies shift their behavior as stir a bit during warm, sunny periods. enter. Particular attention should be day length shortens in late summer. At this time some of the dormant flies given to sealing sites on sun-exposed Adults cease reproduction and instead may incidentally wander into the living sides (south, west) and upper areas seek dry, sheltered areas suitable for areas, producing nuisance problems. survival through winter. Buildings pro- of the building where cluster flies are most likely to enter. vide many cluster flies the crevices and Normally, few of the cluster flies that cavities that make good overwintering spend winter in buildings end their life Chemical Exclusion. sites. by dying indoors. The great majority that spend winter within cavities of a The value of insecticides to man- Beginning as early as mid-August building ultimately move outdoors in age cluster fly problems is limited and and extending into early October, early spring, repeating their normal life involves two primary uses. Residual cluster flies increasingly can be seen cycle. insecticides can be applied as sprays resting on sun-warmed (south, west) targeted to cracks/crevices on the walls of buildings. If there are any building exterior in late summer. This entry points - cracks around window Management of Cluster application could kill some flies before or soffits, incompletely sealed ven- Flies in Buildings they move behind walls or cause them tilation openings, unsealed areas of to die shortly afterwards. This use of building siding - cluster flies will enter. Physical Exclusion. insecticides is effective as a supple- Upon entry into the building cluster ment to (not a substitution for) the pri- The fundamental step to prevent flies then find sheltered areas in wall mary management focus of sealing en- problems with cluster flies is to effectively voids or attics where they may aggre- try points used by cluster flies. Various seal building openings in a way that gate in loose clusters. (Lending them pyrethroid insecticides – containing blocks all entry points used by cluster the name “cluster fly”). Areas where active ingredients such as permethrin, flies. In the absence of a means that seals cluster flies particularly concentrate are bifenthrin, lambda-cyhalothrin, cyflu- out these insects, it is not possible to con- on the upper stories of buildings and thrin – are used for these treatments. trol cluster fly problems within buildings. “attic fly” is another common name Both over-the-counter and commer- All other methods used to control cluster applied to these winter visitors of build- cial formulations exist for all of these flies supplement this approach. ings. Once within a building cluster insecticides. However, for control of cluster flies all insecticides should be flies can spread throughout a building, Buildings must be sealed prior to only be used as targeted applications when cluster flies move into wall voids to cracks and crevices, such as around and attic areas. Cluster flies begin to the outside of windows or openings in move into buildings for winter shelter in soffits. late August, and most move in dur- ing September. Potential entry points Drying Dusts Applied behind Walls. should be sealed by mid-August and certainly no later than Labor Day. The second type of insecticide applica- When sealing buildings, all cracks tion is the use of a desiccant/drying dust and crevices around windows, un- type of insecticide that is injected into screened ventilation sites around areas (voids) behind walls and other sites eaves, and other exterior openings that where cluster flies aggregate. This can kill cluster flies (and some other insects) Fig. 3.Cluster fly sunning on a south wall on allow flies access to interior areas of late October buildings must be sealed with caulk- that have managed to find their way into ing (and/or silicone) or other methods a building, which prevents their later movement into living areas. However, any dead flies that remain behind walls Larval Control Outdoors. may later attract the attention of rodents, dermestid beetles and other scavengers. Young stages of cluster flies devel- op in soil, feeding on their earthworm Two types of desiccant dusts are hosts. It is sometimes suggested marketed, diatomaceous earth (DE) whether insecticides applied to lawns and silica aerogel (SA), both of which at this time will reduce later problems are types of silicon dioxide (the pri- with adult flies in buildings. It will not. mary constituent of sand). However, This type of insecticide use has never these drying dusts have a specific been shown to kill developing cluster physical structure, either produced flies. Also, the adult flies seeking winter as the fossilized remnants of hard cell shelter can fly long distances and the walls produced by diatoms (DE), or flies present in a building may have as a synthesized structure that is very originated at sites several hundreds of Fig. 4. Black blow fly light and highly absorptive (SA). In yards distant. powder forms these can be applied as dusts that on contact can disrupt the protective wax layer on the surface of Other Winter Flies insects, causing them to die by drying out (desiccation).