Every fall, thousands of these large, slow-moving, kind of nondescript flies appear in clusters on the siding of all of our buildings, trying to find a way in to any crevice that they can.

They aren’t really after anything, other than a place to hibernate for the winter. Once they find a spot, they settle in until spring, at which point they once again appear in large numbers as they emerge, mate, and fly off.

They normally sit with their wings folded on top of each other, which makes the veins hard to make out. And, since wing venation is a key feature used to identify fly species, they are hard to identify down to species from most photographs. But, if they are nervous and getting ready to fly off, they may partially open their wings, and the veins will stand out, like so:

Not that seeing the veins helps me so much, but this will at least give a fly expert a fighting chance at a positive ID. I think you can also tell the sex by looking at the eyes, at least you can if they are like most of the other vaguely-housefly-like flies: the eyes of males nearly touch across the forehead, while the eyes of females are clearly separated.

Which means that the cluster fly I posted a fairly bad picture of way back in 2007 was most likely a female.

These are pretty clearly Cluster Flies in the genus Pollenia, which have the following differences from houseflies:

– They are somewhat larger than houseflies
– They rest with their wings fully closed over their backs
– The thorax has no distinct stripes, but does have a scattering of yellow hairs

BugGuide also says that they have an “odor of Buckwheat honey” when crushed, but (a) I don’t know what Buckwheat honey smells like, and (b) I haven’t really gone around crushing flies and sniffing them in the first place.

The adult cluster flies are pretty innocuous. Most information sites don’t seem to know (or, maybe, care) what the adults eat, although it looks like they just drink flower nectar. Unlike most of the other flies that resemble houseflies, these do not :
(a) carry human diseases,
(b) hang around garbage,
(c) leave flyspecks (basically fly vomit) all over everything,
(d) bite to suck blood, or
(e) do most of the other things that we find annoying about flies.

Their main annoyance factor is the way that they get into houses in large numbers looking for hibernation spots. And, if they actually get into the heated interior of a house, they will be flying around in some numbers for the whole winter, which is irritating. They are particularly obvious if you are taking old siding off of a house, as you will frequently find huge numbers of dead flies that didn’t make it through some winter in the past. In addition to being unsightly, their corpses provide food for Carpet beetles, which I understand a lot of people kind of object to.

Locally, a lot of people confuse these with the “Friendly Flies” or “Government Flies”, Sarcophaga aldrichi, which parasitize tent caterpillars, and tend to land on people and animals a lot without biting (not so much “Friendly”, as “Annoyingly Clingy”, I think). But, the Friendly Flies look a lot more like houseflies, with stripes on their thorax.

When Cluster Flies emerge from hibernation in the spring, this MSU site says they lay eggs in the soil, which hatch out into maggots that parasitize passing earthworms. This is usually fatal to the earthworm. They have multiple generations over the summer, probably depending on the length of the season. The MSU site also says that they generally have four generations per year, but I expect that up here they probably only manage three. Still, this is enough to produce a heck of a lot of them, and most houses around here have a lot of them getting inside every winter. The only real way to keep them out is to seal the house very thoroughly. Pesticides aren’t really notably effective and aren’t worth the effort, expense, and possibility of poisoning yourself, family, and pets. And, since sealing your house is also good for keeping down your heating bills, well heck, why not do it?[1]

We have found one constructive use for them, by the way: S_ has some tarantula spiderlings that are still pretty small, and can’t handle full-sized crickets. Cluster flies are an ideal size to feed them, at least for now.

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[1] One thing to watch out for when you seal up your house, though, is ventilation. If you don’t ventilate, the air in a well-sealed house can get pretty stale, to the point that people in it don’t feel so good[2]. And yet, if you ventilate by just exhausting stale air and pulling in fresh air, then during the winter the exhaust air will be warm and the fresh air will be cold, and whoops,there goes your heating efficiency from sealing your house! A solution to this is to use one of several kinds of Heat Recovery Ventilators (HRVs), such as an air-to-air heat exchanger. This is a ventilator that uses the heat contained in the exhaust air, to preheat the incoming air. These can recover as much as 80% of the heat in the exhaust air, at a considerable energy savings. I like ours, it gives us very good indoor air quality. In every other house I have lived in, I would start to feel a bit sick in the winter if I couldn’t get outside for at least a few hours every day, but that hasn’t been a problem since we moved into this house with its HRV.

[2] When this happens in commercial or public buildings, it gets referred to as Sick Building Syndrome. Basically, buildings with bad ventilation accumulate all sorts of crud in the air, so that people who work in them or visit them get to feeling seriously ill. I have noticed this in a few buildings, most noticeably in the mall south-west of town. Whenever I go there, I almost immediately start developing a kind of low-grade nausea, which quickly turns into a headache. I first realized this was happening back when the K-mart that used to be their anchor store started having money troubles (mainly because of competition from the Wal-Mart that opened next door). I think they cut out maintenance on the ventilation system at that time as a cost-saving measure. Unfortunately for them, I expect that most people were affected the same way as me. Which means that anytime somebody came into the store, they would feel bad, and then feel better as soon as they left again. So, even if people did not consciously notice this, the fact that when they went into K-mart they felt bad, but when they went into Wal-mart they felt good, would subconsciously condition everybody to preferentially go into Wal-mart whenever they had a choice. Just like you can condition mice to not go into one side of their cage by making them feel vaguely uncomfortable on that side with a weakly-electrified grid. While I’m sure it wasn’t the only cause, I think that the bad air in their store was a strong contributing factor to them going out of business.

And now, with all but about 7 stores gone from the mall and the other 3/4 of the building being vacant, I expect that the mall can barely afford to keep the ventilation working at all. The only saving grace is that very few people go there these days, and so there is less actual need for a ventilation system. I still feel sick when I go there for more than maybe 20 minutes, though.