Blow Fly & Bottle Fly

📐 FIELD GUIDE ILLUSTRATION

Blow fly (Calliphoridae) identification illustration with labeled anatomical features — PestControlBasics.com

Original illustration by PestControlBasics.com. Use the labeled features above to confirm your identification.

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PestControlBasics Editorial Team

Reviewed by Derek Giordano · Updated 2026

🔍 Identification

8-15mm; metallic blue-green or grey; shiny; distinctly larger than house flies. Found congregating at specific windows in a specific room — this location clustering is diagnostic of a nearby organic source. Multiple species: bluebottle (Calliphora): metallic blue. Green bottle (Lucilia): brilliant metallic green. Grey blow fly: less metallic but larger.

🧬 Biology & Behavior

Blow flies are attracted to and breed in decomposing animal material — they're used in forensic entomology to estimate time of death. Finding large numbers inside a building clustered at one location almost always means a dead animal (mouse, rat, squirrel, bird) in a wall void, chimney, or attic. They're also significant maggot therapy insects used to treat necrotic wounds.

⚠️ Damage & Health Risk

Nuisance swarming at windows; the dead animal source creates ongoing odor problem; finding and removing the animal is the only real solution.

🔧 DIY Treatment

Find the dead animal. Inspect the area where flies are congregating — check for any openings to wall voids, attics, or crawl spaces. If the animal can be accessed, remove and seal the area. If it cannot be reached: apply odor-eliminating products, increase ventilation, and wait 2-3 weeks for the carcass to dry and odor to dissipate. Treat the room with fly traps.

👷 When to Call a Pro

For inaccessible wall void carcasses, pest control professionals may be able to drill access holes, remove the carcass, and treat for secondary pests (dermestid beetles that colonize carcasses).

❓ FAQ

Why are blow flies clustering at my window?

Blow flies orient toward light after emerging from a dark breeding site (like a wall void). The window is where they concentrate trying to exit. The breeding source (usually a dead animal) is typically within 15-30 feet of where the flies are found — and in the direction away from the window.

How long will the dead animal problem last?

A mouse in a wall void causes noticeable odor for 1-2 weeks; a rat for 2-4 weeks. Fly activity peaks within the first 5 days. After the carcass dries, blow fly activity stops. If the dead animal is inaccessible, patience and ventilation are often the only options — but secondary pests (dermestid beetles) will colonize the dried remains.

🪰 Blow Fly & Bottle Fly