Cactus beetle redux!

Moneilema armatum LeConte, 1853 | Vogel Canyon, Otero Co., Colorado

On my most recent Great Plains collecting trip, cactus dodger cicadas weren’t the only residents of the cholla cactus (Cylindropuntia imbricata) that studded the open grasslands in southeastern Colorado—cactus beetles (Moneilema spp.) also were found, though in lesser numbers than their frenetic, screeching neighbors (perhaps the reason for their scarcity?!). I’ve covered cactus beetles before, posing the question, “How do you photograph cactus beetles?” The answer was, of course, “Very carefully!” That is certainly true in most cases, but not all.

The clumsy, flightless adults rely on their host’s vicious spines for protection.

Cactus beetles are found almost exclusively on opuntioids (chollas and prickly pear cacti), and while most opuntioids are rather viciously spined the plants themselves vary tremendously in structure. Prickly pears (Opuntia spp.), on which I previously photographed these beetles, generally grow in low, dense clumps, their flattened pads often forming a tangle of well-armed hiding places for the beetles. Such is not the case with chollas, which are generally taller, more erect, and have a much more open structure of well spaced, cylindrical stems. The beetles on these plants still enjoy a great amount of protection by the long, barbed spines that cover the stems, but to entomologists/photographers like me they are still much more easily collected and photographed. In this particular case, no special techniques were needed to get nice close-up photos against a clear blue sky other than crouching down a bit and being careful not to lean too close to the plant. That is not to say, of course, that photographing insects on cholla is completely without risk, as this photo showing the spines impaled in my flash unit afterwards will attest (but better the flash unit than poking the lens!).

The hazards of photographing cactus beetles.

The cactus beetle in these photographs appears to be an armed cactus beetle, Moneilema armatum, by virtue of the small but distinct lateral spines on its pronotum, lack of pubescence on the elytra, and relatively smooth pronotum lacking large punctures except along the apical and basal margins. These shiny black beetles occur in the western Great Plains from Colorado and Kansas south through New Mexico, Oklahoma, and Texas into northern Mexico. Adults and larvae seem to prefer chollas over prickly pears—adults feeding on the surface and larvae tunneling within the stems. I suspect the adult feeding helps provide nutrition for egg maturation in addition to creating an oviposition site, and plants infested with larvae often appear unthrifty and exhibit black masses of hardened exudate along their stems (Woodruff 1966).

Hardened black masses of plant exudate indicate larval feeding within the stems.

REFERENCES:

Linsley, E. G. and J. A. Chemsak. 1984. The Cerambycidae of North America, Part VII, No. 1: Taxonomy and classification of the subfamily Lamiinae, tribes Parmenini through Acanthoderini. University of California Publications in Entomology 102:1–258 [preview].

Woodruff, R. E. 1966. A cactus beetle new to the eastern United States (Coleoptera: Cerambycidae). Florida Department of Agriculture, Division of Plant Industry, Entomology Circular No. 53, 2 pp. [pdf].