A sudden burst of large blue flies around a window is one of the most common fly calls we receive.
One day there appears to be no issue. A short time later, large numbers of flies may be gathering around windows and light sources.
Many people assume the problem is the flies themselves. In reality the important question is usually: what are the flies breeding in?
A few flies at an open window in summer can just be flies. A sudden appearance of dozens of blowflies or bluebottles usually means there is a source nearby, such as a dead rodent, bird, animal carcass or decaying food.
Metallic green flies, often called greenbottles, are blowflies too. We approach them in the same way.
Tell us what you have found and where you are seeing the flies. We will explain the most sensible next step. Prices are plus VAT.
What have you noticed?
Common signs include:
- Large blue or metallic-looking flies appearing indoors
- Metallic green flies appearing in the same way
- Flies gathering around windows and light sources
- A sudden increase in fly numbers over a few days
- Flies appearing in lofts, upper floors or around chimney breasts
- Repeated fly activity around bins or waste storage areas
- An unpleasant smell in a room, loft or void
- Flies emerging from an area that is difficult to access
- Maggots crawling across floors, dropping from a ceiling or appearing away from the source
Blowflies at a glance: usually metallic blue, blue-black or green; larger than many common flies; often called bluebottles; strongly attracted to light after they emerge indoors.
A small number of flies can be normal. A sudden appearance of dozens usually suggests a breeding source nearby.
Why is it happening?
Blowflies are among the first insects to find dead animals and other decaying material. They can locate a source before people notice a smell.
Female blowflies lay eggs on that material. The eggs hatch into maggots, which feed and then pupate before emerging as adult flies. When fully grown, the maggots may crawl away from the breeding material to find a suitable place to pupate. This is why they can appear some distance from the original source. In warm weather this can happen quickly, which is why a property can go from no obvious problem to dozens of flies at the windows.
After emerging, the adults head towards light. That is why they are so often found on windows.
The flies are usually a symptom. The source they are breeding in is the problem.
Common breeding sources include:
- Dead rats
- Dead mice
- Dead birds
- Food waste
- Forgotten meat or food products
- Waste bins
- Animal carcasses in lofts, chimneys, wall cavities or floor voids
A dead rodent after a rat treatment is one of the more common causes of a sudden indoor blowfly problem.
How Guardian approaches a blowfly problem
Our first objective is to identify the source. Killing adult flies without finding why they are there often provides only temporary relief.
Finding and dealing with the breeding source is usually more important than treating the flies on the glass.
Our approach follows three practical stages.
1. Find the source
The most important stage is identifying what the flies are breeding in.
Common locations include lofts, wall cavities, chimneys, voids, waste areas and around appliances.
2. Remove the source
Where a dead animal or other breeding source is accessible, we will remove it.
This is not always possible. A dead rodent inside a wall cavity, for example, may not be accessible without significant building work.
3. Control remaining activity
Where appropriate, insecticides may be used to reduce the remaining adult flies and to treat accessible areas where larvae or pupae are present.
We then explain what has caused the problem and give practical recommendations to reduce the chance of it happening again.
What happens when we visit?
The technician will inspect the affected area and discuss where the flies are being seen.
Particular attention may be given to loft spaces, chimneys, wall voids, waste storage areas, kitchens, utility rooms and areas where rodents or birds may have been present.
If the source can be identified and safely removed, that is normally the preferred option.
After the visit you will receive a report explaining what was found, what was done and any practical recommendations. Domestic customers and one-off jobs receive an electronic report. Planned commercial contracts receive electronic visit reports together with the agreed site records or pest management folder.
Will one visit be enough?
Often, yes.
If the breeding source can be identified and removed, the problem is frequently resolved quite quickly.
Even after the source has been removed, some adult flies may still emerge from pupae that have already developed.
Where the source cannot be reached, flies may continue emerging for a period until the breeding cycle has finished.
We do not guarantee that all activity will stop immediately. We will explain what is realistically achievable in your situation.
Commercial considerations
Blowflies can create hygiene concerns in food businesses, hospitality premises, care settings, commercial kitchens, food storage areas and waste handling areas.
A single blowfly is not always evidence of a major problem, but repeated activity should be investigated.
Where ongoing protection is required, Guardian can supply, fit and service fly screens and insect light traps, and advise on waste management, hygiene measures and proofing.
Commercial blowfly work can normally be discussed over the phone. Prices are plus VAT.
Frequently asked questions
Why are all the flies sitting on the windows?
Adult blowflies are strongly attracted to light. After emerging from a breeding source they often move towards windows and light fittings.
Are blowflies and bluebottles the same thing?
Most people use bluebottle for the larger metallic blue blowflies. Metallic green flies, often called greenbottles, are also blowflies and are dealt with in the same way.
Why have dozens appeared all at once?
A sudden increase in numbers often means a breeding source such as a dead animal or decaying material nearby.
Does a blowfly problem mean I have rats?
Not necessarily. A dead rodent, including after a rat treatment, is one of the more common causes of sudden indoor blowfly activity.
Can you always find the source?
No. We always try, but sources inside wall cavities, beneath floors or in other inaccessible areas cannot always be located or removed.
Should I just spray the flies?
Spraying may reduce the number of adult flies, but if the breeding source remains, more flies are likely to appear.
Are blowflies a sign of poor hygiene?
Not always. Many blowfly problems are caused by a dead animal in a loft, chimney or wall cavity and have nothing to do with how clean the property is.
Tell us what you have noticed
Tell us where the flies are appearing, whether they are indoors or outdoors, and how long the activity has been present.
Photographs can be helpful, particularly if you are unsure whether you have blowflies, cluster flies or another species.
We will explain the most sensible next step.
