On our insect collecting trip to the western Great Plains last June, Jeff Huether and I encountered several species of beetles in the genus Prionus—longhorned beetles (family Cerambycidae) known commonly as “prionid root borers”. I’ve already recounted our experiences with two of them—P. integer in southeastern Colorado and P. fissicornis in northeastern New Mexico, both of which we encountered unexpectedly and by the “bucketload” using prionic acid baited traps in shortgrass prairie habitats. The third species we encountered was also rather unexpected, so much so that we were not even sure about its identity until we examined the collected specimens more closely. Unlike the two previously mentioned species, however, the behavior of this third species—P. heroicus, especially regarding its attraction to prionic acid, was quite different, and one of the localities where we found it raises some interesting questions regarding its recorded distribution.
Prionus heroicus is among the largest species in the genus, and we first encountered it in northeastern New Mexico while searching for suitable habitat around the small town of Mills (Harding Co.) where several grassland-associated species in the genus had been found by other collectors. While we were searching we were intrigued by the USDA Forest Service road sign pointing to “Mills Canyon” and decided to head down the gravel road to see what it looked like. A few miles down the road the endless shortgrass prairie quickly changed to sandstone canyonland with oak/pine/juniper woodland. By now it was late afternoon, and the rich, pulsing buzz of cicadas filling the air as we drove slowly through the area suggested the potential for good insect activity. We stopped to take a closer look at things, and within minutes an enormous prionid beetle came flying up to the car—apparently attracted to the prionic acid lures inside! We netted the beetle, and shortly afterwards another beetle flew into the vicinity… and another! We hurriedly set out some lure-baited traps, and over the next hour or so we collected close to a dozen beetles. Our first thought, based on their large size and the presence of 12 antennal segments, was that they might be P. californicus. This was a bit of a letdown, as our goal was to find little known and uncommonly encountered Great Plains species, not common and widespread western species like P. californicus. However, the blackish rather than reddish color and relatively broad pronotum eventually convinced us that we had actually found P. heroicus—not a rare species, but also not something we expected to see at this relatively northeastern location compared to its usual haunts in southeastern Arizona.
Interestingly, none of the beetles came directly to the lure-baited traps but, rather, flew to the vicinity and then either circled broadly in the air or landed some distance from the trap and ran rapidly on the ground as if searching. We missed a few that we tried to net in mid-air, which responded by immediate straight line flight far away into the distance. Those that we watched while on the ground searched for up to several minutes but seemed unable to locate the lure, eventually taking flight and flying away. This suggests that prionic acid is more important as a component of the female’s calling pheromones for long distance detection, but that other components might be necessary to allow the males to locate females in the immediate vicinity. We returned to the site the following day around noon and were surprised to see males flying to our lures immediately after our arrival, since we had observed P. integer and P. fissicornis activity to occur almost exclusively during the morning and evening hours. Linsley et al. (1961), however, also note that adults of this species fly during the day, as well as at night. Again, none of the beetles seemed capable of finding the precise location of the lure.
The larval habits of P. heroicus are not known, but the closely related P. californicus feeds on roots of living oaks or occasionally in dead oak logs or stumps (Linsley 1962). Prionus heroicus may have similar habits, since adult females have been found in leaf litter at the bases of living oak trees and stumps (Hovore & Giesbert 1976, Skiles 1978). Presumably the Mills Canyon population is associated with Gambel oak (Quercus gambelii), which was the only oak species noted at this location. An interesting defensive behavior was observed for an adult beetle that I kept alive and photographed later in the hotel room. I had put the beetle on a light-colored tray, and while I was getting the camera ready to take some shots the beetle began crawling away. I picked it up and put it back into position, and for a brief period it assumed the rather comical-looking “standing-on-its-butt” pose shown in the accompanying photograph. It maintained this pose for some seconds and then gradually lowered itself in the front before beginning to crawl again. If I poked it before it finished lowering itself it immediately pushed its front all the way back up again and held the pose briefly before beginning to lower itself again. I did not encounter this behavior in the field—beetles on the ground immediately tried to flee when I handled them and took flight if allowed to.
The story does not end here, however, as this would not be our last encounter with the species. While we were traveling back towards the east, we decided to visit the area around Black Mesa in the extreme northwestern corner of the Oklahoma panhandle. I had been to this site the previous summer, and although conditions were very dry at the time the presence of Gambel oak in the area—perhaps the easternmost outpost of this western species—had me thinking about the possibility of the area serving as an unknown eastern outpost for western species of jewel beetles (family Buprestidae). Conditions were once again dry, and no jewel beetles were found, but this quickly became a moot point as not long after our arrival a large prionid beetle came flying up to me—again in the same manner as P. heroicus at Mills Canyon and in the middle of the afternoon. We decided these, too, represented P. heroicus and remained in the area for the rest of the day, counting at least a dozen males attracted to the vicinity of the lures that we’d set out. As with the males we observed in New Mexico, none seemed able to find the precise location of the lure, and some circling only briefly and then flew off without even landing. Unlike New Mexico, however, we did succeed in trapping a few males in lure-bated traps that we set out overnight and checked the next morning.
Our capture of P. heroicus at Mills Canyon in northeastern New Mexico and near Black Mesa in northwestern Oklahoma not only seems to suggest an association of this species with Gambel oak, but also confirms the previously questionable occurrence of the species in Oklahoma. Alexander (1958) reported the species (under one of its synonyms, P. tetricus) from Payne Co. in north-central Oklahoma. This record is dubious, since Payne Co. is well east of the 100th meridian and has a decidedly eastern flora, including its oaks. That record has not been mentioned by any subsequent authors addressing the overall distribution of P. heroicus (Linsley 1962, Chemsak 1996). The presence of P. heroicus in the Black Mesa area, with its decidedly western flora, does not necessarily give credence to the Payne Co. record, but it does bring up the question of just how broadly distributed P. heroicus really is. Most records of this species are from mountainous areas of southern Arizona and New Mexico, but more recently it has been recorded from Texas (Big Bend National Park) by Van Pelt (1999, 2007) and extreme northwestern Colorado (Moffat Co.) by Heffern (1998). The latter record is particularly interesting, as this represents a considerable range extension from any previously recorded locality and begs the question regarding its occurrence in other parts of Colorado.
Alexander, D. R. 1958. A preliminary survey of the cerambycids (Insecta: Coleoptera) of Oklahoma. Proceedings of the Oklahoma Academy of Science 38:43–52 [pdf].
Chemsak, J. A. 1996. Illustrated Revision of the Cerambycidae of North America. Volume I. Subfamilies Parandrinae, Spondylidinae, Aseminae, Prioninae. Wolfsgarden Books, Burbank, California, x + 149 pp., plates I–X [description].
Heffern, D. J. 1998. Insects of Western North America 1. A Survey of the Cerambycidae (Coleoptera) or Longhorn Beetles of Colorado. Gillette Museum of Arthropod Diversity, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, i + 32 pp., 1 color plate [summary].
Hovore, F. T. & E. F. Giesbert. 1976. Notes on the ecology and distribution of western Cerambycidae (Coleoptera). The Coleopterists Bulletin 30:350 [JSTOR].
Linsley, E. G. 1962. The Cerambycidae of North America. Part II. Taxonomy and classification of the Parandrinae, Prioninae, Spndylinae, and Aseminae. University of California Publications in Entomology 19:1–102, 1 plate [OCLC WorldCat].
Linsely, E. G., J. N. Knull & M. Statham. 1961. A List of Cerambycidae from the Chiricahua Mountain area, Cochise County, Arizona (Coleoptera). American Museum Novitates 2050:1–34 [pdf].
Skiles, D. D. 1976. Taxonomy and description of two prionine Cerambycidae from southern Arizona: a new species of Stenodontes and new status for Neomallodon arizonicus (Coleoptera). Proceedings of the Entomological Society of Washington 80:407–423 [Biodiversity Heritage Library].
Van Pelt, A. F. (ed.). 1999. Inventory of insects of Big Bend National Park, Texas. Big Bend Natural History Association, Big Bend National Park, and Texas.
Van Pelt, A. F. (ed.). 2007. Inventory of insects of Big Bend National Park, Texas. Report to Big Bend National Park, 204 pp.
© Ted C. MacRae 2015