The Carlisle Mosquito Online

Friday, January 22, 2010


Cluster Fly

Last week I started a story of a Bald-faced Hornet nest by documenting one of its many winter residents, the Spotted Lady Beetle. In the days following, I became much more closely acquainted with some of the other occupants of the nest – but first, I’d like to back up to the retrieval of the nest itself.

Bald-faced hornet nest (Photo by Kay Fairweather)

Bald-faced Hornets usually make their nests in trees and shrubs but on occasion they make them on the side of houses and barns. This nest was at the highest point under the eaves of Katharine Endicott and Leslie Thomas’s house on Prospect Street. After many frosts and days of sub-freezing temperatures, Katharine went up the ladder to remove the nest, confident in the knowledge that the hornets would be dead. When she began dislodging the nest, it started to buzz. No-one wants to be face-to-face with a buzzing hornet nest, and least of all when they are high up a ladder.

After a careful retreat, she returned to the task somewhat less confident but fully protected in a bee suit and safely removed the nest. As she did so, she noticed hundreds of flies pouring out of the nest. When the exodus was over and the buzzing had ceased, she put the nest in a large bag securely folded over at the top and brought it to me. Leslie volunteered to help me identify any remaining inhabitants.

Flies everywhere: I left the bag on my doorstep for a day or two, looking into it now and then. I neither saw nor heard any more insect activity so I brought it into the house one evening. The next day when I was carefully taking it apart on my dining table an insect flew out. I assumed it was another fly and looked around for where it might have gone. I

Cluster fly (Photo by Kay Fairweather)

discovered it on the ceiling along with about a hundred of its friends. They must have been coming out all through the night, warmed by the indoor temperature. As I dismantled the rest of the nest (outdoors) I found groups of about 50 or more somewhat sluggish flies huddled between each of the remaining layers of paper. (There was a total of 11 layers.) There were other insects too and I saved all the examples I could of each of the different species and took Leslie up on her offer to help with identifications.

Nest inventory: The predominant species turned out to be the Cluster Fly which was sharing the nest with many smaller flies which we were unable to identify. After the flies, and not counting the dead Bald-faced Hornets, the next most populous was the Asian Lady Beetle, followed by a dozen or so Northern Paper Wasps, four Western Conifer Seed Bugs, and a single Spotted Lady Beetle. I was surprised to find no spiders.

Cluster Fly: The Cluster Fly is in the genus Pollenia and is a member of the Calliphoridae family which has about 75 species in North America and includes Bluebottles and Blow Flies. As a member of this family, it has more than its fair share of disgusting relatives but should not be tarred by the unsanitary brush of the “filth flies” who are its garbage-dwelling and rotted carrion-feeding cousins. Adult Cluster Flies feed on clean things like tree sap, flower nectar, and pollen. They lay their eggs on soil near earthworm burrows. The tiny new larvae enter the body of living earthworms where they feed and grow. The fully grown larvae leave the worm and pupate in the soil. There are four (sometimes more) generations in a season. In the fall, adults start looking for protected sites, perhaps in your house, to spend the winter.

Identification: The biggest clue in identifying the fly was its clustering behavior. We were able to confirm the identification by its size and shape, the feathery antennae which are characteristic of the Calliphoridae family, the yellow hairs on its thorax, the way its wings overlap when at rest (versus the spread wing pose of the blow fly), the patches of color on its abdomen, and its hairy legs. When crushed, it is reputed to smell like buckwheat honey – a smell I don’t know. Despite many swats on the ceiling, the dominant smell was a delicate blend of Dachshund and dead hornet.

Sources: Insects, Their Natural History and Diversity by Stephen A. Marshall; (search for Pollenia); University of Guelph at (search for cluster fly). ∆

Submissions for the Biodiversity Corner are encouraged from everyone. You can write the column or tell me what you saw and I will write it. The only requirements are that it occurs naturally in the wild and was seen in Carlisle. Send a note to

© 2010 The Carlisle Mosquito