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Have you noticed flies swarming by your windows now that it’s getting cold? Here’s what’s going on.

  • Author: Jeff Lowenfels
    | Alaska gardening
  • Updated: October 16, 2019
  • Published October 17, 2019

You can probably tell that I only have a few rules when it comes to writing these columns. One, it is perfectly acceptable to write a column on a previously covered subject, but it is not okay to rerun a column. Two, the subject matter has to be related to gardening, indoors or outdoors. That is about it. So, stick with me on this one. We are between seasons.

During the past week, a couple of readers noted the swarms of flies that seem to magically appear every fall around windows. They want to know what they are and where they come from. Is there reason to worry?

Each described the same phenomena. Fall hits and all of a sudden there a dozen or so flies buzzing around windows during the day. They are slow fliers and easy to swat. They do not have the characteristic, shiny green coloring of so-called blow flies Alaskans know from cleaning fish.

How many years have I noticed the very same invasions both at home and in offices. Those very picture windows that I now worry will kill birds seem to be magnets for these critters. Time to do the research to get some answers. Oh, and there is a garden connection, for sure.

My guess is we are seeing the so-called, and for good reason, cluster flies, aka Pollenia rudis. To put you at ease immediately, these do not eat carrion or animal waste. In fact, they can’t, as they only posses a sucking tube, which they use to drink nectar from your summer flowers. They are actually pollinators, so there is no need to look for dead, rotting meat (or animal wastes) around your house.

Nor is there a need to squish one and sniff it to sense their characteristic, dark honey smell; just take my word for it; I did the research for you. It is not the description I would give it.

Next let’s answer this: what these flies are doing? Well, they were living outside, but now they are are looking for a nice warm spot, a dark one, where they can go into suspended animation without being disturbed until next spring. After they wake, they will mate. I am not sure if this is outside, but they eventually get there the same way they came in. And how do they do that? They come through small cracks around door jams and windows and the like.

Okay, here is the garden connection: Once outside and pregnant, the female fly finds an area where there is at least one earthworm. I am not making this up! It lays the egg near a worm. I am not sure what happens to the female fly, but presumably she dies having fulfilled her duty.

The egg hatches and the larva finds and invades the worm. It is parasitic, which is not good for the worm because the worm is what the larva eats. After a month or so of growth, the worm is dead, but a cocoon and then a new fly is the end product. There can be three or four series of flies produced during a season.

When it gets cool, the flies seek warmth, and you now know the rest of the story. They sneak inside and look for their winter roost. Those that survive swatting will congregate in an attic or crawl space and walls too.

Since we are not talking about flies “that spread germs’” there is no need to panic. Yes, rodents might seek them out for winter food, but still, no pesticides please. Go ahead and seal things up, though now it sounds like waiting until they can leave in the spring. That is not to say you can’t vacuum them off windows.

The good news, is that the presence of these flies means you have earthworms out there. That is really great. And, it is even better knowing that they are working those leaves while you read.

Jeff’s Alaska garden calendar

Alaska Botanical Garden dino night: 6-7:30 p.m. Tuesday, Oct. 29. Make a dinosaur planter and listen to dinosaur stories. Ages 6+ with an adult. $20 for an adult and child, plus $10 for every additional child. alaskaabg.org

Driveways: Mark yours yet?

Houseplants: Water. The heat is on.

Mountain ash: Did I say they were evergreen? They are not. Obviously.

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