Across the Great Plains of North America, sand dune fields dot the landscape along rivers flowing east out of the Rocky Mountains. Formed by repeated periods of drought and the action of prevailing south/southwest winds on alluvium exposed by uplifting over the past several million years, many of these dunes boast unique assemblages of plants and animals adapted to their harsh, xeric conditions. Some are no longer active, while others remain active to this day. Among the latter is Beaver Dunes in the panhandle of northwestern Oklahoma.
As I explored the more vegetated areas around the perimeter of the dunes, I spotted the characteristically hairy, fleshy, opposite leaves of Ascelpias arenaria. Known also as “sand milkweed,” this plant is associated with sand dunes and other dry sandy soil sites throughout the central and southern Great Plains. I always give milkweeds a second look whenever I encounter them due to the association with them by longhorned beetles in the genus Tetraopes. It wasn’t long before I spotted the black antennae and red head of one of these beetles peering over one of the upper leaves from the other side.
This was no ordinary Tetraopes, however. Its large size, dense covering of white pubescence, and association with sand milkweed told me immediately that this must be T. pilosus (the specific epithet meaning “hairy”). Like its host, this particular milkweed beetle is restricted to Quaternary sandhills in the central and southern Great Plains (Chemsak 1963), and also like its host the dense clothing of white pubescence is presumably an adaptation to prevent moisture loss and overheating in their xeric dune habitats (Farrell & Mitter 1998).
Tetraopes is a highly specialized lineage distributed from Guatemala to Canada that feed as both larvae and adults exclusively on milkweed (Chemsak 1963). Larval feeding occurs in and around the roots of living plants, a habit exhibited by only a few other genera of Cerambycidae but unique in the subfamily Lamiinae (Linsley 1961). Milkweed plants are protected from most vertebrate and invertebrate herbivores by paralytic toxins, commonly termed cardiac glycosides or cardenolides. However, a few insects, Tetraopes being the most common and diverse, have not only evolved cardenolide insensitivity but also the ability to sequester these toxins for their own defense. Virtually all insects that feed on milkweed and their relatives have evolved aposematic coloration to advertise their unpalatability, and the bright red and black color schemes exhibited by milkweed beetles are no exception.
As noted by Mittler & Farrel (1998), variation in coloration among the different species of Tetraopes may be correlated with host chemistry. Milkweed species vary in toxicity, with more basal species expressing simpler cardenolides of lower toxicity and derived species possessing more complex and toxic analogs. Most species of Tetraopes are associated with a single species of milkweed, and it has been noted that adults of those affiliated with less toxic milkweeds on average are smaller, have less of their body surface brightly colored, and are quicker to take flight (Chemsak 1963, Farrell & Mitter 1998). Thus, there seems to be a direct correlation between the amount of protection afforded by their host plant and the degree to which the adults advertise their unpalatability and exhibit escape behaviors. Asclepias arenaria and related species are the most derived in the genus and contain the highest concentrations of cardenolides. In fact, they seem to be fed upon only by Tetraopes and monarchs while being generally free from other more oligophagous insect herbivores such as ctenuchine arctiid moths and chrysomelid beetles that feed on less derived species of milkweed (Farrell & Mitter 1998). Accordingly, T. pilosus is among the largest species in the genus and has the majority of its body surface red. Also, consistent with it being more highly protected than others in the genus, I noted virtually no attempted escape behavior as I photographed this lone adult.
In addition to metabolic insensitivity to cardenolides, adult Tetraopes also exhibit behavioral adaptations to avoid milkweed defenses (Doussard & Eisner 1987). The milky sap of milkweed is thick with latex that quickly dries to a sticky glue that can incapacitate the mouthparts of chewing insects that feed upon the sap-filled tissues. Adult Tetraopes, however, use their mandibles to cut through the leaf midrib about a quarter of the way back from the tip. This allows much of the sticky latex-filled sap to drain from the more distal tissues, on which the beetle then begins feeding at the tip. Leaves with chewed tips and cut midribs are telltale signs of feeding by adult Tetraopes.
Chemsak, J. A. 1963. Taxonomy and bionomics of the genus Tetraopes (Coleoptera: Cerambycidae). University of California Publications in Entomology 30(1):1–90, 9 plates.
Doussard, D. E. & T. Eisner. 1987. Vein-cutting behavior: insect counterploy to the latex defense of plants. Science 237:898–901 [abstract].
Farrell, B. D. & C. Mitter. 1998. The timing of insect/plant diversification: might Tetraopes (Coleoptera: Cerambycidae) and Asclepias (Asclepiadaceae) have co-evolved? Biological Journal of the Linnean Society 63: 553–577 [pdf].
Linsley, E.G. 1961. The Cerambycidae of North America. Part 1. Introduction. University of California Publications in Entomology 18:1–97, 35 plates.
Copyright © Ted C. MacRae 2013