How to know if you have cluster flies.
Most flies are single flying insects and are not in a constant large number. That is the easiest way of knowing what you have, if you have house flies, blowflies or similar you may see a couple of dozen at the most. Cluster flies are hundreds at a time.
The cluster fly begins life as an egg, laid by the female, during the warmer months of the year in the castes of earthworms. Once they hatch, they work their way down the burrow until they find the worm. They enter the body of the earthworm where they become parasites until mature, at which point they leave the worm to pupate.
The size of a cluster fly population is very much dependent on the number of worm castes in the area combined with weather conditions. The warmer and drier the weather the larger the cluster fly population.
You will find cluster flies on the warmer south side of the building and when they are overwintering in the loft etc they will be clustered on the south side of the roof.
Once they have mated and the outdoor temperature begins to fall in the autumn, the flies seek refuge inside buildings, and anywhere they can survive a cold winter.
Cluster flies do NOT spread diseases like normal flies, but they can be a nuisance, especially if they are in the loft area. Once the hatch is open, they will fly towards the heat of the property and flies will instinctively fly towards the window/s.
Before the introduction of double-glazed windows and doors, cluster flies were common in the sash window frames and holes around window and door frames.
Cluster flies leave a pheromone behind after they have landed and clustered and as such the next generation knows where it is safe to over winter.
What to do and how to sort out the problem. here are several ways that the DIYer can carry out this simple treatment at very low cost, and bearing in mind that the problem is there all year, it is just that the cold weather brings more together to huddle in a group to survive the cold. Hence, they are more noticeable in cold winter months.
Ideally the area needs to be sprayed with a long-term residual flying insect spray, these are available to the DIY market, or a self-locking on aerosol insecticide spray. The use of smoke bombs works well with cluster flies in the loft, if they are placed on a non-flammable surface, bearing in mind they are a firework carrying chemicals. Loft areas are the only place I would ever use them out of choice and even then, it would be my last choice due to my arsenal of pest control apparatus, which the DIY person does not have. The kit we used was expensive but constantly in use where as you as the DIYer will only tackle the problem when required. For the sake of approx. £10 you can fumigate your loft. The best time to gain greater results is always after the first frost and or before the last frost of the season. That stops the flies from mating and tends to help reduce the numbers.
If they are over wintering in sash window frames a light dusting with a synthetic pyrethroid dust will suffice. These are generally in the form of ant powder or similar, but they all have the same ingredient. They work well but puffing into the hole where the cord enters the frame.