A downy woodpecker shreds the hollow stems of poison hemlock to feast on tumbling flower beetle larvae. Click here for larger image. Photo by Jeff Mitton.
By Jeff Mitton
On a grey day in November, a downy woodpecker tapped furiously, thrashing tall, hollow stems in a dense stand of poison hemlock. I followed the sound to its source and found a woodpecker hammering at the soft material, sending shreds of stems flying.
The woodpecker was so intent that it ignored me and continued its drilling. I was only 15 feet away, so I was able to discover why it was so intent — it was harvesting a larva every 20 or 30 seconds. The bird thoroughly examined each stem, taking three to six grubs from each.
Curious to know whether this feeding activity was a rare episode or sustained foraging, I returned to the hemlock patch two more times in the next 10 days. Each time I arrived, I could hear a Downy hammering, and was able to approach closely. This large patch of weeds is immediately east of the parking lot for the White Rocks Trailhead on Valmont Road.
Hoping to discover what the Downy was eating, I took a few fragments of stems to my laboratory and peeled them apart. I anticipated that these would be the young of poison hemlock moths, which occur in high densities on poison hemlock, but I was wrong.
I found small, yellow larvae with three pairs of prolegs at the anterior end. To me, this suggested that these were beetle larvae. But even if my hunch was right, I had not narrowed the field much, for the number of known beetle species is between 350,000 and 400,000.
I enjoyed following my curiosity. I thought that woodpeckers foraged exclusively on trees, but now I know that Downy woodpeckers are resourceful foragers.Knowing the difficulty of identifying beetles from larvae, I decided to keep some of the hemlock stems in my laboratory. With a little luck, larvae would survive and develop and I could capture some adults for identification. In March, I found adult beetles that I had not seen before. They were small, 0.1 to 0.3 inches long, and wide at the thorax and narrowing to a pointed abdomen that extended beyond the wings. The beetles were dark but had yellow-green iridescence and short hairs. The shape of the head was distinctive — the back of the head ended abruptly in a flat, vertical surface.
When it comes to flies and beetles, our local expert is Professor Boris Kondratieff, curator of the C. P. Gillette Museum of Arthropod Diversity at Colorado State University. He identified the beetles as tumbling flower beetles in the genus Mordellistena, but explained that over 130 species were known in North America, and identification to species would require substantial work by an expert on that genus.
Adult tumbling flower beetles are seen most commonly at flowers, and larvae develop either in rotting wood or the stems of herbaceous plants. The common name is taken from the extraordinary, frantic efforts to escape predators. They have strong back legs that drive them into the air, spinning around the long axis of the body dozens of times per second, and tumbling head over tail at a lower rate. Most of the tumbling beetles are presumed to be herbivores, eating the plant stems from the inside, but some are known to hunt and eat the larvae of insects that make galls. Little is known about most species of tumbling flower beetles.
I enjoyed following my curiosity. I thought that woodpeckers foraged exclusively on trees, but now I know that Downy woodpeckers are resourceful foragers. Apparently, plant stems are home to many species of insects. I also learned just a bit about tumbling flower beetles, which are both common and diverse.
My prolonged inquiry also altered my appreciation of a local habitat. A very scruffy stand of tall weeds was initially uninteresting, but now it is the base of a cryptic community with an unlikely top predator. Woodpeckers eat tumbling flower beetles, which eat both the stems of herbaceous plants and the galling insects living on the same plants.
Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.
May 13, 2016