Why are there so many dead flies in my window?

I was destined to write this article. I look at fly-infested windows with people every day.

My son-in-law Kevin Blauth investigates proteins by playing with the genetics of fruit flies as a doctoral candidate in a laboratory at the University of North Carolina. My youngest daughter’s boyfriend Karl Wyant studies insect habitats in the soils of Arizona as a doctoral candidate at Arizona State University. And most compelling, my marvelous (yet now perturbed) Maggie came to me this morning wondering why so many dead flies congregate in the window sills of her new town-home.

So that Maggie’s builder might get through next week unscathed, and you might be well-served, I thought I’d look into flies at windows. I’ve always wondered where they all come from anyway.

You bought a window with a screen, right?

Maggie can’t understand why the builder wasted good money on them. But she also realized that the darn water weeps that drain rainwater away from the troughs in the window frame let the little creeps walk right on in. And if window frames with weeps (typical of some vinyl windows) are on the south or west side of a house, they provide a bit of temporary shelter in cool weather. Winter is a different story. If you don’t have a barn, crevices in eaves and siding leading to attics, ceilings and false ceilings are the likeliest points of entry for flies seeking shelter in your home for the winter. They journey as far in as possible, to the warmest place they can find, huddle in droves, and sleep it off. Warm spells will awaken them, and they’ll get out and fly around, but they’ll fly slower, lending easy targets for your swatting pleasure.

Know thy enemy: all about flies.

The first thing I found out about flies was that the various species seem to share some characteristics:

• They tend to like to be around humans and animals, because we’re so messy, and build sewers, garbage bins, fruit orchards, and horse barns to safely handle our wastes, food, and valued possessions.
• Some species like meat in a restaurant, some over-ripe fruit in an orchard, and some that other stuff.
• Their life cycle is pretty short, at around three weeks or so, larvae through pupae through dead adult at your window sill. Some, like horse flies, bite, and they’ll bite you too. The little buggers are smart, yes they are.
• They find food (most drink liquids only, converting solids to liquids by means about which I’m not about to elaborate), find matter on which to deposit eggs (again, no elaboration) and warm shelter to survive winter. That shelter is usually our houses (though Maggie would allege it’s only hers).

Let me leave fly biology for a moment, and address the nuisance in question. What to do? Well, as the saying goes, they were here before we were, and they’ll be here after we’re gone. So miracle cures don’t exist. We can only try to control them, and trap or kill the ones we did not control.

Best defense? Seal spaces and gaps.

Basically, our best defense is to prohibit their ingress as autumn comes by sealing spaces, gaps and crevices through which they might squeeze. Assisting us are insecticide dusts and sprays that adhere to surfaces, under eaves for instance, where flies seek entry. We can clean drains and surfaces where they breed, and remove all organic deleterious debris from our yards and property.

We can knock them down as they fly outdoors with fogging machines and insecticides; trap them indoors with electric-lighted glue-pad devices or tub traps which are vast improvements over the old nasty fly strips; and spray them with knockdown sprays as they fly by and killer sprays where they rest.

So why die at the window sill? It’s simple really. They’re attracted to light. Our angst is avenged. They die of thirst or old age, or maybe, and I don’t have any scientific evidence to back this up here, but if it’ll make Maggie or you feel better, of injuries sustained banging their little heads against hard glass.

For pest control products, we visited doyourownpestcontrol.com. For information on those pesky flies by species, with discussion and control alternatives by species, visit their informational page on flies.