It's surely true that the only people who don't mind insects are those who study them. For everyone else, the response to creepy-crawlies and flying pests can range from indifference to annoyance to displays of sheer terror. When it comes to flies, well, they pretty much buzz around the top of life's list of super annoyances. If they're swarming your porch, you'll need to figure out what's attracting them and eliminate the problem.
It Depends on the Fly
Different types of flies have different reasons for hanging around a porch area, but those reasons usually involve food.
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If the flies are small, they may be fungus gnats attracted by soil mixtures of potted plants and flowers on your porch. They're also found on compost and wet mulch piles.
Flies swarming a side or back porch near a kitchen or other food preparation or storage area may be fruit flies, a species of the common housefly. Big fans of ripening and rotting produce, they'll be itching for access to your garbage disposal and trash cans — ready to make a mad dash inside when the door opens.
Common houseflies will also buzz your porch waiting for their chance at that shriveling fruit on the counter. The difference is that they don't have the discriminating palate of the fruit fly. These persistent little disease-carriers buzzing your front door while you're carving that expensive cut of meat may have just explored the dog feces in your garden or hung out on yesterday's roadkill a couple of streets over.
A housefly will go for anything — food and filth alike. Even if your kitchen is clean, they'll be out there licking their chops over the cat box you forgot to change or the dirty diaper pail. And the attraction may be outside — a dead animal beneath your porch or nearby pet feces.
Don't Make Them Welcome
It makes sense that, before you resort to using biological control agents or insecticides, you should simply take steps to remove what's attracting the flies to your porch area in the first place.
If your visitors are fungus gnats, you may want to consider relocating your porch plants and compost pile and getting rid of excess piles of mulch or any other organic debris near your porch that can become and remain damp.
Keeping fruit flies away involves rethinking how you manage food storage and disposal. Store all foods in well-sealed containers. Don't keep rotting produce around, don't allow food matter to remain in your garbage disposal, and keep a tight lid on your kitchen composting receptacle. Seal all trash securely in bags and dispose of it in outdoor trash cans with tight lids that aren't located near your porch and door.
If houseflies are dive-bombing your doors and windows trying to get inside, rethink your approach to sanitation. Follow the same cleaning procedures you would for fruit flies, but also focus on removing filth. Routinely scoop and change cat litter and empty diaper pail contents into a tightly sealed bag. If these things can't be disposed of immediately outside, make sure the kitchen trash can you use inside has a tight lid. When the problem is outside and involves animal feces or the rare occurrence of a decaying animal carcass, you should be able to quickly locate it by the odor and simply remove it.
About Cluster Flies
In the fall season, flies you notice hanging around a porch may be cluster flies looking for a place to spend the winter — specifically, inside your house. They're harmless, they're not attracted by smells, and they generally aren't disease carriers. They're just sort of pesky and very good at gaining entrance through cracks in siding and around windows. The best thing you can do to keep them out is make sure things are well sealed around your house.