We seem to be having an abnormal amount of gnats getting into the house this year. My screens are in good condition but are the standard mesh, 18X16 sold with the windows. These bugs are slender in width and up to ¼” long. They are attracted by light, swarm after dark, are temperature sensitive with fewer bugs on cool nights and seem to be getting in through the screens. My questions are what mesh size should I order to keep these bugs outside and does this last for the entire summer, or is this a short term problem. I live in northern Mich. on the shore of Lake Superior and in a heavily wooded area. Thank you for your help.
Without seeing a picture or an actual specimen, I can't be sure what type of "gnats" you are referring to. However, based upon the fact that you live near a large lake and are seeing large numbers of them, I suspect the gnats are most likely chironomid midges (Diptera; Chironomidae). The larvae of these flies live in aquatic habitats, most often in lakes and ponds, where they feed on plants, algae, diatoms, detritus, and occasionally on each other. They are important food items for fish, amphibians, and other insects in many aquatic ecosystems. They are often the most abundant insects in aquatic systems, and there may be dozens, if not hundreds, of species in any given lake or pond. The life cycle usually takes a few months, with several generations of adults emerging through the spring and summer. For any given species, the adults usually all emerge within a few days, usually at night to avoid predators. They typically are short-lived, surviving as adults only long enough to mate and lay eggs. Most species are readily attracted to lights. So, the midges you are seeing may only be a temporary nuisance. However, it is also possible that additional species or new generations may emerge through the summer and create additional episodes. These midges can be anywhere from almost 1/2 inch long to very tiny, depending on the species. Their long legs make it difficult for any but the smallest of them to actually get through standard window screens. However, they seem to be very adept at getting around or over them. On some older double-hung windows, they can climb up the screens and get over them between the panes of glass and then on into the house. And some of the tiniest species can actually get through the screens. Solutions? Turn off as many unnecessary porch and other house lights as practical at night. If you can close the windows in the rooms that are lighted at night that may solve most of the problem about them getting into the house. You can also try putting a bright light in the yard away from the house to attract the midges there and away from your windows. The light must be brighter than the house lights to attract them away. And you should shield the side of the light toward the lake so that it doesn't just attract more midges from that direction. If you indeed have tiny midges that are coming through the screens, you will probably need to find something like "no-see-um" netting or a very fine metal screening to cover the windows, and /or seal up any cracks or openings along the edges.
Now it is possible that you could have other types of midges or fungus gnats coming out of the woods, especially if it is moist with lots of decaying vegetation. "Controls" would be much the same as for the midges above. And finally, if you are seeing tiny blackish gnats inside the house, and have lots of house plants, the gnats may be coming from there.
If you look up chironomid midges or fungus gnats on the internet, you will find more information that may help you determine what they are and how to prevent them from being such a nuisance.