In midsummer, meadows abound in wildlife. I have been awaiting the arrival of one of the most gorgeous of all insects Chrysochus auratus. The scientific name Chrysochus auratus means “made of gold” and refers to the fantastic metallic hues of blue, green, gold, and red on surface of the beetle. This beetle has no accepted common name, but it is often referred to as the dogbane beetle due to its habit of feeding almost exclusively on plants in the genus Apocynum, herbaceous plants we refer to as dogbane. Dogbane grows along roadsides and trails near the forest edge. It is a perennial with clusters of small white flowers. If you damage a leaf or stem, it oozes a white sticky sap. There in full view of all you will find the dogbane leaf beetle. It is a wonder that an insect so conspicuous can survive in a world where birds, toads, and mice dine on insects with gusto. There is a secret to the dogbane beetle. The dogbane plant, like its relative the milkweed we met in a previous episode with its nemesis the milkweed longhorned beetle, is full of highly poisonous compounds known as cardenolides. These compounds are toxic to a variety of animals, including humans. Cardenolides are also known as cardiac glycosides and they can have profound and lethal effects on the heart if ingested. It is likely that these compounds evolved to keep leaf-munching animals from eating dogbane. The clever Chrysochus has the ability to eat the leaves of dogbane unscathed. Instead it ingests cardiac glycosides, stores them in glands, and then secretes them when threatened by its own predators. It is believed that these toxic compounds confer protection from hungry predators that would otherwise devour Chrysochus. The conspicuous color and tendency for the beetle to feed and rest in full sight of its enemies are probably the beetle's way of advertising its nasty chemical defense. The biological message is this: “That's right, I'm bad, don't even think of eating me.”
In the ongoing struggle between dogbane and Chrysochus, it looks like the beetle has the upper hand, right? Well, not exactly. Dogbane has yet another trick to foil its herbivores. In addition to nocuous glycosides in their tissues, the sap of dogbane is a sticky, white, liquid similar in consistency to latex paint. Insects attempting to eat dogbane soon find their jaws gummed-up with a rapidly hardening gob of goop. The dogbane beetle has a crafty trick to rid itself of this sticky mess. After nibbling for a while, the dogbane beetle does a moon walk while dragging its mouthparts on the leaf to wipe-off the sticky dogbane latex. Once free of the latex, it moves to a new spot to resume its feast.
When the goop is just too much to handle, a backward stroll helps the dogbane beetle to rid its mouthparts of sticky latex.
Now is a great time to explore the meadow to enjoy these marvelous beetles and witness the ‘tit for tat’ relationship they have with their chemically defended hosts. Don your boots, tuck your pants into your socks, and go into the meadow to find dogbane. You may be rewarded with a glimpse of the dogbane beetle, one of nature's most golden delights.
Information for this week’s episode came from the “The Love of Insects” by Thomas Eisner and “Deactivation of plant defense: correspondence between insect behavior and secretory canal architecture” by Dave Doussard and Bob Denno. We thank Frank for providing the inspiration for this episode.