

Cluster flies are common flies in Delaware homes during fall and

early spring. Slightly larger than house flies, cluster flies are

awkward, noisy fliers. They are attracted to lights and windows.

They blunder into structures and fall to the floor, where they spin

noisily on their backs only to recover and fly about again.



They are called cluster flies because they aggregate in attics,

empty rooms, and closets. They enter buildings from the outside,

apparently seeking warmth and shelter. Inactive in colder weather,

cluster flies may suddenly reemerge on warm spring days. They don't

harm people or home furnishings. They are considered a nuisance



Description and Life History



Cluster flies appear more compact than houseflies, and they are not

as brightly metallic in color as blow flies, another common fly in

homes. Cluster flies are dark gray to black in color The top of the

thorax, where the wings attach, lacks obvious striping. Young

cluster flies have distinct yellow, tawny, or golden hairs on the

back of the head and top of the thorax, but these become darker

with age. Wings at rest overlap, giving a scissor-like appearance.



Cluster flies are parasitic on earthworms. Females lay their eggs

on the soil near earthworm burrows during the active season when

they can be found only outside. The fly maggot isn't associated

with garbage or other kinds of food. Maggots feed inside an

earthworm host for two to three weeks, after which they leave to

pupate in the soil. Egg to adult development takes four to six

weeks. In Delaware, they have two or three generations a year.



As temperatures drop in the fall and the earthworms needed to rear

more flies go deeper into the soil, surviving adult cluster flies

migrate inside. Wall voids, attics, or other areas of the home

provide perfect shelter. They easily gain access through cracks and

crevices. In more rural areas, they overwinter in barns or outside

in tree holes or around plant debris. Isolated homes in the country

and suburbs may be especially inviting to hibernating cluster flies

because these structures offer the best shelter around.



Wintertime cluster fly activity is temperature related. As the

temperature increases, they wander from hibernation site to nearby

surfaces. They show up on the south (warm) sides of homes, in

windows that face the sun, or in response to heat in a house.



In early spring, adult flies feed on nectar in flowers or other

CLUSTER FLIES
sugar sources. They leave hibernation sites as the temperature
warms and earthworms return to the soil surface.

Control

Cluster flies enter buildings through small openings so they are
difficult to keep out. Screens are not always an effective
deterrent since they enter by walking. Reducing possible entry
areas where cluster flies are a continual problem will help provide
some relief. Seek out their routes of entry, and close them with
caulking or exclusion materials before they enter.

For flies inside, an aerosol for flying insects will remove those
flies that become a nuisance. If you discover their clustering in
an attic, unused room, wall void, closet or similar location, you
can spray the flies before they disperse into other areas. Sprays
of synthetic pyrethroids or the Vapona strips of DDVP are also
effective cluster fly killing agents.

Use a vacuum to pick up flies that come out of hibernation. They
may accumulate on window sills, behind drapes or on the floor below
windows. Do not spray dead flies with an insecticide since other
organisms will feed on the dead bodies and eventually remove them.

Efforts to control cluster flies with applications of insecticides
to lawns and gardens are not successful. Such materials may harm
other organisms or the earthworms that are the food of the fly
maggots. Differences in infestation from year to year may depend
upon soil or weather conditions, especially as they affect soil
moisture and the population of earthworm hosts.

Dewey M. Caron
Extension Entomologist
7/94
.
