DTY Cluster Flies
All Warmed Up And No Place To Go
Cluster fly treatments mean looking in attics and eaves


Unwanted overwintering pest guests are anywhere from annoying to alarming. One such pesky perpetrator is the flying freeloader the cluster fly, also known as the attic fly, Pollenia rudis, roughly meaning, “powdery condition and bad mannered.”

Adult flies emerge during late summer to autumn, cryptically encroaching structures en masse to enter dormancy. Cluster flies prefer inaccessible areas such as attics, under insulation, within wall voids and inside plumbing chases; evicting cluster flies is challenging for the savviest of service professionals.

The cluster fly is a northern hemisphere species producing several generations in a season. It is an urban pest within areas of managed turf grass containing soil rich in organic matter, which sustains larval food sources.

The major food sources for cluster fly larvae are the earthworm species Aporrectodea caliginosa, Aporrectodea chlorotica, Eisenia lucens, Lumbricus rubellus, and Lumbricus terrestris. Immediately after hatching, cluster fly larvae forage for earthworms as a food source as well as a host for larval development.

Adult cluster flies resemble house flies, but are more robust. Adults are ~7 mm long and recognized by distinct lines or stripes behind the head. The thorax possesses yellow, curly setae or hairs and the abdomen is pollinose and non-metallic. Adults are typically slow moving.

Cluster fly fast facts
  • Cluster fly females place eggs on moist soil.
  • Three larval instars.
  • First instar larvae penetrate the earthworm’s epidermal integument.
  • Larvae position spiracles outside of the earthworm.
  • Larvae feed inside the earthworm until ready to pupate.
  • Life cycle is ~30 days.
  • Three to four generations per year is common.
  • Adult flies are generally herbivores (feeding upon plant secretions and feces).
  • Adult flies are attracted to malt extracts, acetyl acetate, and carrion proteins.

The last seasonal generation of adult flies enters structures through exploitable small openings (not doors and windows). Large aggregations are formed within attics and structural voids. Although generally winter-dormant, intra-structural thermal variances may cause flies to become active and fly to lights and windows, both annoying inhabitants and dying in large numbers.

For seasonal cessation, service professionals can use Zenprox EC. For example, apply Zenprox EC diluent at a rate of 0.25% or 2 fl. oz. per gallon of water per 1000 ft2. For crack and crevice and spot treatments, use a coarse, low-pressure spray or pin stream. Treat entry points such as around water pipes, doors, windows, and eaves. Treat areas where pests normally feed or hide such as baseboards, corners, around water pipes, cracks and crevices. Spot treat floor rugs, beneath furniture, in closets and storage areas.

For broadcast treatment, use a coarse, low-pressure fan spray to apply the dilution uniformly over surfaces to be treated. Apply to carpets, floors, cabinets, cove base, and other areas where pests are located. Repeat treatments to infested areas as necessary, but not less than 14 days after the initial treatment. Always read, understand, and follow all product label directions.

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