Fresh off our unexpected success at finding Prionus integer in the shortgrass prairie of southeastern Colorado, field mate Jeff Huether and I made our way down into northeastern New Mexico to see if the prionic acid lures we used to attract that species would bring us success with other species of Prionus. Several species in this genus are known to occur in the Great Plains, and we had records of three from the area around Gladstone (Union Co.). Based on our experience from the previous day, we stopped at several sites that had sandy/loam soil and searched exposures along the roadsides for burrows that bore any resemblance to those that we had found associated with P. integer. Frustratingly, we saw no such burrows or evidence of the presence of Prionus beetles in any of the spots that we searched.

Eventually, with day’s end drawing near, we decided to play a hunch and set traps at two sites with soil exposures that seemed most similar to those seen the day before. As Jeff set the last pair of traps in place, I occupied myself photographing a dusty hognose snake (Heterodon nasicus gloydi). While photographing the animal, I happened to look down to my side, and what did I see but a male Prionus crawling through the vegetation! I recognized the species immediately as P. fissicornis—represented in my cabinet by a single specimen. This species is distinguished by the highly segmented antennae (up to 30 segments or more) that place it in the monotypic subgenus Prionus (Antennalia). I called out to Jeff, and for the next half an hour or so we scoured the surrounding area; however, our searches were in vain as we failed to find another individual.

Prionus (Antennalia) fissicornis male | 24 June 2014, Union Co., New Mexico

We cast an eye towards the north and watched late afternoon thunderstorms roll across the expansive landscape. Rain is normally a downer on an insect collecting trip. However, considering the numbers of P. integer that came to the traps after rain the previous day, we actually found ourselves hoping that the skies would open up. Whether they would and if this would lead to success with P. fissicornis (or other Prionus species), however, would have to wait until the next morning when we returned to check the traps.

Thunderstorms over shortgrass prairie.

Around 10 a.m. the next morning our reward was revealed—traps brimming with P. fissicornis males! Each trap contained more than a dozen individuals, and males were still actively crawling around in the vicinity of the traps as well.

Prionic acid-bated traps filled w/ Prionus fissicornis males.

Each trap contained more than a dozen males.

We spent about an hour at the site determining the extent of their occurrence, and like P. integer the previous day the males were found almost exclusively in a small area adjacent to (and upwind from) the traps. As we searched, we also found a few females, one of which seemed to be in the act of ovipositing into the soil at the base of a plant. I carefully excavated the hole into which the ovipositor had been inserted once the female withdrew it, but I was unsuccessful in finding an egg. Perhaps she was only probing to search for a suitable oviposition site and did not find this one to her liking.

Prionus (Antennalia) fissicornis female | 24 June 2014, Union Co., New Mexico

There are some interesting similarities and differences with this species compared to our experience with P. integer the previous day. Males of both were highly attracted to the prionic acid lures that we used and were effectively trapped in cans over which the lures were suspended. It is also interesting that both species were found in rather large numbers directly after a rain event, and it seems logical to presume that rain might trigger adult emergence—as is the case with many insects that inhabit dry habitats with seasonal rains. However, we did not find P. fissicornis adults associated with burrows as with P. integer, making me even more intrigued with the burrowing behavior in the latter and its possible function. Also, it is interesting that we found P. fissicornis adults most active during the morning hours and P. integer most active during early evening, but since we did not visit both localities at both times of day it is not possible to say for sure that the two species differ in their adult daily activity.

© Ted C. MacRae 2014

Our quick stop in Hardtner, Kansas to see “Beetle Bill” Smith at the beginning of our Great Plains Collecting Trip had already produced one unexpected success—the long-sought-after Buprestis confluenta. However, our trip didn’t really begin in earnest until noon the next day when field mate Jeff Huether and I stopped in the middle of a vast, dry grassland north of Las Animas in southeastern Colorado. Even at this point we hadn’t gotten to the first of our planned localities a little further south where we were hoping to find longhorned beetles in the genus Prionus, the main objective of the trip, using recently developed pheromone lures. Jeff had, however, collected an interesting blister beetle at this spot on an earlier trip and wanted to take another look since we were in the area. I looked out across the desolate landscape and wondered what on Earth I could find here that would be even remotely interesting to me.

Shortgrass prairie, 10.5 mi N of Las Animas, Colorado.

I was convinced there was no point for me to look around, but I wanted to let Jeff have some time to look for his blister beetle, so I milled around the vicinity of the car to look for any flowers that might be in bloom and hoped to maybe see a tiger beetle or two. As is often the case, plants were a little lusher and more diverse along the roadsides due to rain shedding by the road, so I decided to walk down the roadside and see what I might find. I hadn’t gone too far when I saw a big, brown beetle crawling along the edge of the road. My first thought while approaching it was ground beetle, but I quickly decided it was not that. Only when I crouched down and looked at the beetle, crawling with some urgency but making no attempt to fly, did I realize that it was a female Prionus (later identified as P. integer LeConte, 1851 in the subgenus Homaesthesis). I had, in fact, only once before ever seen a female Prionus, that being the ginormous species P. heroicus in southeastern Arizona almost 30 years ago. Incredible! We hadn’t even gotten to the first spot where we planned to look for Prionus, and I’d already found one. And a female, no less!

Prionus (Homaesthesis) integer LeConte, 1851 (female).

Jeff was out of ear-shot, so I resumed my walk along the roadside to look for more. Not long afterwards I found a male carcass laying on the ground, and then shortly afterwards I found another one. One of the carcasses was in pretty good condition still, giving me optimism that I would find a live one as well. Frustratingly, however, I saw nothing more during the next hour as I continued slowly down the road to a distance of nearly one mile from the car and then along the other side all the way back. During the time that I was searching, I noticed strange burrows in the ground. They were only along the roadsides and were especially numerous in the area where I had found the female and two male carcasses. The diggings looked fresh, so I tried excavating a few. They were no more than two inches deep, and I found nothing in any of the dozen or so burrows that I excavated. Nevertheless, I was convinced that the burrows were somehow associated with the Prionus beetles that I had found (even though I had never heard of burrows associated with adult Prionus beetles, nor have I been able to find anything in the literature since)—the size of the burrow matched that of the beetles perfectly, and the presence of fresh diggings meant something had made them recently. If the Prionus beetles didn’t make them, then what did?

Prionus integer adult burrow.

All the time as I was searching, I wondered if Jeff knew about the beetles occurring here. However, when we finally met up again near the car and I showed him what I had found, I could see by the surprised look on his face that this was not the case. I also showed him the burrows that I’d found, and we both agreed they had to be connected to the beetles. I decided to try using a shovel instead of my knife to excavate more burrows, thinking that maybe I wasn’t excavating deep enough, so we got the shovel out of the truck, walked to the area where I had seen the live female, sunk the shovel deep into the ground next to one of the burrows, and pried up a chuck of the soil containing the burrow in its entirety. As we broke apart the soil there it was—another female!

A female Prionus integer dug from her burrow (shovel in lower right).

We excavated more burrows in the vicinity, preferentially choosing those with the freshest-looking diggings, and while most were empty we did find several more females. We were beginning to think that only the females created the burrows, but eventually a male was dug from its burrow as well. By now it was clear that we needed to make the first schedule change of the trip (before even reaching the first planned locality!) and decided to set out lure-baited traps and check them later in the evening before resuming our plans the next day. We expected the beetles to become active during dusk, so we went into town to eat dinner and check out a nearby locality before returning to the site. While we were gone heavy rains moved through  the area, and we weren’t sure if or how this would affect beetle activity and their possible attraction to the traps. However, as we approached the site (slipping and sliding on the muddy 2-track), we could actually see beetles crawling on the road from afar. What we found when we got out of the car was nothing short of mind-blowing—beetles were everywhere, crawling on the road, crawling through the grass, and overflowing in the flooded traps! The vast majority, as expected, were males—distinguishable by their smaller size and distinctly flabellate antennae—but we also found a fair number of the much more rarely collected females—most of them either mating or digging burrows.

A male Prionus integer attempts to mate with a female.

It was tempting to focus on just collecting individuals, as we had clearly encountered an ephemeral emergence event. However, it was soon apparent that the number of individuals milling around and flying into the area was far greater than what we could possibly put in our bottles. As I gradually realized this, my focus shifted from collecting to observing—especially the females involved in mating and digging burrows. Most of the females that I saw digging burrows had their abdomens protruding from the burrow as they used their front legs to dig soil from the burrow, but eventually I noticed a burrow that had a female sitting completely within the burrow with her head just below and facing towards the burrow opening.

A Prionus integer female sits in her burrow.

I crouched down and watched the female closely (through the camera lens) and noticed a regular, almost rhythmic movement by the beetle as she sat in the burrow. I imagined perhaps the female was in the act of oviposition, although I now think it unlikely that this is the case (it doesn’t make sense for a female to make the energetic expenditure to dig a burrow large enough to contain the adult body when a simple probe of the ovipositor into the soil surface would accomplish the same thing. It also occurred to me that the female was emitting pheromone to attract males, but this begs the same question: why call for mates while enclosed within a burrow when doing so from the surface would be just as (and possibly more) effective. At any rate, the chance to observe mating and other behaviors made the encounter far more informative than if our focused had remained strictly on collecting the abundance that we had encountered.

Prionus integer female moving rhythmically in her burrow, but why?

Barr & Penrose (1969) give the only biological notes I am aware of for this species, noting that the larvae have been associated with the roots of sagebrush (Artemisia tridentata) and yellow rabbitbrush (Chrysothamnus viscidiflorus) and on two occasions finding larvae damaging newly planted beans or cutting underground stems of potato plants in southern Idaho. In both cases the fields had been recently cleared for cultivation, suggesting opportunistic feeding by larvae that were already present in the soil and feeding on native hosts when the land was cleared. At any rate, the site where we found the species was completely devoid of any rangeland shrubs, suggesting that herbaceous plants also may serve as suitable hosts for the species. We did observe small bunch grasses that were abundant in the area where both the beetles and their burrows were most numerous—in fact (although perhaps coincidentally), the burrows seemed most often to have been dug at the base of these grasses (see 3rd and 6th photos).

The sinking sun in the west and receding storm clouds in the east created conditions ripe for rainbows—a fitting exclamation point to our first (and completely unexpected) Prionus success of the trip. As the cloak of dusk descended, we packed up our gear and headed into town full of optimism about what other Prionus experiences lay ahead in the coming days.

A rainbow hangs over the shortgrass prairie.

REFERENCE:

Barr, W. F. & R. L. Penrose. 1969. Notes on the distribution and bionomics of some Idaho Cerambycidae (Coleoptera). Great Basin Naturalist 29:88–95 [Biodiversity Heritage Library].

© Ted C. MacRae 2014