A handful of flies in July is normal. Twenty flies on the kitchen window every morning is not — it means something nearby is breeding them, and the lifecycle from egg to adult can be as short as seven days in summer heat.
Houseflies vs blowflies
Houseflies are dull grey and breed in decaying organic matter — wheelie bins, food waste caddies, pet waste, compost. Blowflies (bluebottles and greenbottles) are larger and metallic, and they're almost always breeding on protein — a forgotten bin liner, a dead bird in a chimney, a mouse that died behind a kickboard.
Find the source first
- Sudden cluster of bluebottles in one room with no obvious bin — check the chimney, loft and wall cavities for a dead bird or rodent.
- Houseflies focused on the kitchen — pull out the bin, clean the lid and runners, and check the food waste caddy seal.
- Flies in a utility or boot room — check pet bowls, litter trays and any damp mop heads.
- Flies emerging from a floor drain or gully — that's drain flies (Psychoda), not houseflies. Different treatment.
When to call a professional
If you can't find the source within a day, or the flies are coming from an inaccessible space like a chimney or sealed cavity, a technician can fog the void to break the cycle and locate the carcass. We also fit and service UV fly units for commercial kitchens across Hertfordshire.