Dogbane Beetles (Chrysochus auratus, family Chrysomelidae) are truly Nature’s eye candy. The same beetle will present a kaleidoscope of colors when viewed at different angles to the sun. They are arguably one of the most beautiful beetles found in Ohio.
Their light-blending artistry makes these shimmering living gems stand out on the dark green foliage of their namesake host. Male and female beetles are identical in size and shimmering luster.
The beetle's scientific name, Chrysochus auratus, loosely translates to "made of gold." In fact, gold is only one of a medley of colors displayed by these gorgeous native beetles. As you change your viewing angle, the iridescent beetles glisten with mixed shades of green, copper, blue, red, and of course gold.
The myriad display of colors is more than just light reflecting off a shiny surface. Many insects are shiny, but they don't display the range and depth of colors observed with the dogbane beetle.
So-called "chemical colors" are produced by the absorption of light by pigments; the colors are defined by chemical composition. The various colors of house paints are due to the chemical composition of pigments in the paint. However, although color pigments are involved, the range of colors is more than can be explained by chemical pigments.
Dogbane beetles owe their luster to what are called "structural colors." These colors are not produced by pigments alone but originate in the scattering of light produced by microstructures such as butterfly scales. For example, iridescent structural colors are displayed on the wings of our native Red-Spotted Purple (Limenitis arthemis astyanax).
The structural colors on dogbane beetles originate in multilayered structures in the exocuticle starting with a Plexiglas-like translucent surface layer. Beneath this rests stacks of tiny slanting plates that cover color pigments.
Light rays striking the surface of the plates are reflected as a shimmering sheen, while light rays that bounce off the pigments produce various colors. The result is a lustrous mix of ever-changing hews across an optical range that is almost unmatched in the insect world.
The beetle's colorful display is like a neon sign flashing "don't mess with me" to predators. Using bright colors to send a warning message to enemies is known as "aposematic coloration."
Dogbane (Apocynum spp.) is the representative species for the dogbane family, Apocynaceae, which includes milkweeds and other plants that ooze sticky white sap ladened with poisonous alkaloids (cardiac glycosides). Indeed, the genus name Apocynum translates to "poisonous to dogs," or "dog killer." Sap from dogbane is reported to have been used at one time against ravenous feral dogs.
Dogbane beetles feed on the three dogbanes found in North America: common or hemp dogbane (A. cannabinum), fly-trap or spreading dogbane (A. androsaemifolium), and intermediate dogbane (Apocynum × floribundum). Although there are reports in the literature that the beetle feeds on various milkweeds (Asclepias spp.), I've scoured milkweeds in Ohio without ever finding this beetle. I've wondered if perhaps the reports are referring to the Cobalt or Blue Milkweed Beetle (C. cobaltinus) that feeds on western milkweeds, but this beetle has not been reported in Ohio.
Dogbane beetles ingest the poisonous cardiac glycosides in dogbane sap, store the chemicals in specialized glands, and then they secrete the noxious chemical brew when threatened by predators. Their bright coloration advertises their nasty chemical defense strategy.
However, the chemical defense acquired by dogbane beetles from their sappy hosts isn't without risk. Aside from the toxins, the sap is also very sticky and is intended to clog the mouthparts of insects that take a bite. The dogbane beetle deals with this challenge by occasionally walking backward while dragging its mandibles on the surface of a leaf to scrape away the sticky goo.
Dogbane beetle females lay their eggs in mid-to-late summer on the roots of their namesake host. The resulting larvae feed on the underside of the roots throughout the rest of the season. They spend the winter in the larval stage and pupate in the spring. There is one generation per year.