[Pest Spotlight] Recreational Pursuits of Flies

If you haven’t seen it yet, a silly new e-mail is going around. The premise of the e-mail concerns what you can do when you’re bored at the office (this never happens in my office of course). According to the e-mail, all it takes is a few dead flies, some paper and pencil, a little imagination and a digital camera. At left appear a few of the nine examples.

The e-mail (which I predict may go viral) ends with a question: "Makes me wonder, though: where does someone work that there are this many flies???"

The entomological answer appears to be: "almost everywhere but where I live." The flies in the pictures appear to be cluster flies, Pollenia rudis. These rather large, sluggish flies are identified by the sharp bend in the medial vein on the wing, and the characteristic golden hairs visible on the thorax in some of the pictures. Cluster flies are common in northern areas including Europe, Canada and the northern United States. I first encountered cluster flies as a graduate student at Purdue University in Indiana. They were common there in the fall and winter, sneaking in around the drafty windows of my office in old Entomology Hall. Cluster flies are best known for their annual fall migrations into homes and offices, where they "cluster" in windows and attics similar to the behavior of paper wasps, Asian multicolored lady beetles and other fall invaders in Texas. Curiously, I’ve not seen a single cluster fly since moving to Texas more than 25 years ago.

One of the things that always fascinated me about cluster flies is that they are parasites of earthworms. Who knew that earthworms even needed parasites? Don’t they have enough to worry about with robins and moles and other predators? And what harm could earthworms possibly cause anyway?

I learned from my mailbox recently that some people, at least, think that earthworms are still a little too abundant, even with all their plagues and problems. A recently published paper by Dan Potter et. al at the University of Kentucky reports on problems with over-abundant earthworms in golf courses and sports fields. Too many earthworms can lead to uneven playing surfaces and messy turf areas. In Texas we get occasional calls from homeowners complaining that earthworms turn their lawns into mush, making it difficult to push a lawn mower and even walk over the spongy ground. As Potter and colleagues pointed out in their article, earthworms can also pose a hazard at airports by attracting flocks of birds to feed on the oozing populations that creep on runways, especially after a heavy rain.

People ask me all the time, "So what good is that insect?" The cluster fly shows us that even the most humble insect can be invaluable to society. Fighting earthworms and boredom at the same time is no small accomplishment.

The author has been an entomology specialist for Texas AgriLife Extension since 1989. Readers can contact him via e-mail at mmerchant@giemedia.com.