Multiple Megarhyssa males

Today while hiking at Hilda Young Conservation Area (north-central Jefferson County, Missouri), I encountered a declining sugar maple (Acer saccharum) with lots of woodboring insect holes in the trunk. As I approached I noticed numerous giant ichneumon wasps in the genus Megarhyssa flying about the trunk and resting on its surface. Giant ichneumons belong to the family Ichneumonidae and are, as the name suggests, the largest members of the family in North America. Interestingly, all of the wasps that I initially saw were males. I have never seen male giant ichneumon wasps before, and certainly not in such numbers, so this was quite exciting. We have two species of giant ichneumons here in Missouri—M. atrata and M. macrurus, the females of which I have seen only rarely, but I couldn’t immediately decide which of these two species the males represented. I looked up higher on the trunk, and there I saw a female M. macrurus in the act of oviposition, so I decided that the males must also represent this species. However, one of the males was smaller and differently colored than the others, having more brown than black on the body and the wings clear with a well developed spot on the costal margin. The other males were noticeably larger and had more black than brown on the body and the wings smoky with only a narrow spot on the costal margin. After a little bit of digging, I know believe that the smaller male is also M. macrurus—the same species as the ovipositing female, while the larger males all represent the larger species M. atrata.

Megarhyssa macrurus (male) | Hilda Young Conservation Area, Jefferson Co., Missouri

Megarhyssa macrurus (male) | Hilda Young Conservation Area, Jefferson Co., Missouri

Megarhyssa atrata (male) | Hilda Young Conservation Area, Jefferson Co., Missouri

Megarhyssa atrata (male) | Hilda Young Conservation Area, Jefferson Co., Missouri

As I watched the males that had landed on the trunk of the tree, I observed both the M. macrurus male and one of the M. atrata males to bend their abdomen forward beneath their body rub the tip of the abdomen against the bark, a behavior called “tergal stroking”, and at times inserted the tip of the abdomen into cracks in the bark in an almost prehensile-looking manner. These behaviors belong to a suite of behaviors exhibited by male Megarhyssa aggregations. Previously thought to be function in early insemination of as-yet-unemerged females, the precise function of these behaviors remains unknown but seems somehow related to enabling sex discrimination of emerging wasps and/or increasing the rate at which virgin females are encountered (Matthews et al. 1979).

All species of Megarhyssa parasitize the woodboring larvae of Pigeon horntails (Tremex columba) (order Hymenoptera, family Siricidae), which the females reach by inserting their long, thin ovipositor deep into the wood where the horntail larvae live. Multiple species of giant ichneumons occurring in the same area at the same time and utilizing the same resource seems to violate a basic ecological concept; the competitive exclusion principle, which states that two species competing for the same resource cannot coexist at constant population values because one species will always eventually outcompete the other. In the case of Megarhyssa, it seems that size differences between the species allow them to share a common resource (horntail larvae), as females of the larger M. atrata have longer ovipositors than the smaller M. macrurus, thus allowing them to penetrate deeper into the wood to parasitize horntail larvae that M. macrurus females cannot reach. By the same token, M. macrurus females tend to parasitize horntail larvae tunnel at shallower depths and that tend not to be utilized by M. atrata females.

REFERENCE:

Matthews, R. W., J. R. Matthews & O. Crankshaw. 1979. Aggregation in male parasitic wasps of the genus Megarhyssa: I. Sexual discrimination, tergal stroking behavior, and description of associated anal structures behavior. The Florida Entomologist 62(1):3–8 [pdf].

© Ted C. MacRae 2015

Alkali Tiger Beetle

Eunota togata globicollis - Salt Plains NWR, Oklahoma

I haven’t written much about my early October trip to Oklahoma, where I had hoped to confirm a hunch that the gorgeous Cicindela pulchra (Beautiful Tiger Beetle) would be found in the red clay/gypsum hill habitats of Woodward and Major Counties (the same place where I had found the much rarer Cylindera celeripes the previous June).  Unfortunately, a sudden cold snap and overcast skies conspired against me for the duration of that short, 5-day trip, reducing tiger beetle activity to near zero and sending me back to Missouri with little to show for my efforts — save a scorpion, a torpid Cicindela splendida, and some very beautiful ladie’s-tresses orchids in peak bloom.  I did have one moderately successful day, however, when I returned to Salt Plains National Wildlife Refuge in north-central Oklahoma, a place where I observed seven species of tiger beetles during my June trip.  An eighth species that I did not see on that trip, but which I had observed in previous years, was my goal this time, and despite the cold temperatures and cloudy skies I was fortunate to find several individuals of Eunota togata globicollis.  Occurring primarily on saline flats in the central and southern Great Plain, this subspecies was called the Alkali Tiger Beetle¹ by Erwin and Pearson (2008), who reserved for the nominate form (found in salt marshes and tidal flats along the Gulf Coast) the more descriptive name White-cloaked Tiger Beetle².  A third subspecies, E. togata fascinans (Salt Flat Tiger Beetle) is restricted to salt flats in central New Mexico and west Texas (Pearson et al. 2006) (you may remember this subspecies from my habitat partitioning post last month).

¹ In reality, I have come to consider the term ‘alkali’ as a bit of a misnomer, as it is saline soils specifically — not just those with high pH (alkaline) — that the species is fond of. Moreover, there are many species of tiger beetles in addition to this one that are associated with saline soils.

² Okay, I might as well just get all this off my chest. Pearson et al. (2006) gave common names to each species of tiger beetle in the U.S., but not subspecies. I think most non-taxonomists probably consider this a good thing, although it is not without its problems (some species already had multiple common names applied to them, forcing choices that are sure not to please everyone). Erwin and Pearson (2008) took this further and came up with common names for all of the subspecies as well, and like any good taxonomist they steadfastly applied existing common names only to nominate forms. Eunota togata, however, is an example where the original common name would have been better applied to one of the non-nominate subspecies. The species epithet togata means “cloaked” (being derived from the Latin word toga — a reference to the broad white band running along the elytral margins). Each of the two non-nominate forms are distinguished by the white band being more broadly expanded (indeed, almost entirely covering the elytra in subspecies fascinans), yet it is the nominate subspecies — the least “cloaked” of the three — that retains the original common name. A silly argument I suppose, but if we start applying the “prinicple of priority” to common names in the same manner as scientific names, then what have we gained? Of course, I am of the opinion that most insect groups are too diverse and their taxonomy still too unstable to warrant a rigid system of “official” common names. Is it really any easier to learn White-cloaked Tiger Beetle than Eunota togata? How about Mount Ashland Night-stalking Tiger Beetle instead of Omus cazieri? And this is not even considering what happens when category-level shifts occur. For example, the genus Tetracha was formerly called the Big-headed Tiger Beetles; however, its former subgenera were recently elevated to genus level. Erwin and Pearson, accordingly, applied the common name to the entire subtribe containing Tetracha and its relatives and applied a new common name, Metallic Tiger Beetles, to the new, more limited concept of Tetracha. Thus, in an ironic case of common name instability despite no change in scientific name, the Virginia Big-headed Tiger beetle (Tetracha virginica) became the Virginia Metallic Tiger Beetle. Are your eyes bugging yet? Common names may be appropriate for higher vertebrates, but can they really be used effectively for beetles and other insect groups where the increasing use of molecular tools is sure to result in additional, perhaps radical, shifts in taxonomy? There — I said it, and I feel a lot better!

This species is restricted to saline flats in the central/southern Great Plains.

Of the eight tiger beetle species that I’ve now observed at Salt Plains NWR, half of them (Cicindela fulgida, C. nevadica knausii, E. togata globicollis, and Habroscelimorpha circumpicta johnsonii) are true saline habitat specialists.  One of the other four species (Cicindela tranquebarica kirbyi) is also fond of saline habitats but also occurs commonly on dry, sandy soils as well, and two show a high affiinity for nearly any moist (Cicindela repanda) or moist to dry (Cicindela punctulata) soils with little regard for salinity.  Only Cicindela formosa, a denizen of dry, deep sands seems a little out of its element on the moist, salty mud at Salt Plains NWR — perhaps the few individuals I’ve observed here are incidental visitors, mistaking the white, barren expanses of salt-encrusted soil for the dry sand the species prefers during disperal searches.  This again brings up the question of habitat partitioning for competition avoidance among tiger beetle species sharing the same habitat.  Eunota togata globicollis is active during the spring and fall and, thus, temporally isolated from C. nevadica knausii and H. circumpicta johnsonii (both summer-active species).  The other saline specialist at Salt Plains NWR (C. fulgida) is active during the same seasons as E. togata globicollis; however, in my observations that species prefers the sparsely-vegetated zone at the edge of the saline flats, while E. togata globicollis prefers to stay out in the more open areas.  These observations mirror those of Melius (2010) for E. togata fascinans and the other seven species he noted in the Laguna del Perro area of New Mexico, and of Willis (1967), who recorded as many as 11 sympatric tiger beetle species in saline habitats in the central U.S.

Saline flats at Salt Plains NWR are home to eight species of tiger beetles.

Microhabitat selection and seasonal occurrence are not the only isolating mechanisms that can minimize interspecific competition among the different tiger beetle species at Salt Plains NWR.  Cicindela tranquebarica kirbyi is also a spring/fall species and doesn’t appear to display a preference for open versus vegetated areas, potentially allowing it to compete directly with both E. togata globicollis and C. fulgida.  However, C. tranquebarica kirbyi is a decidely larger species, while the other two are smaller, and correlated with such differences in overall size is the size of their mandibles.  Mandibular size directly correlated to prey size in a number of tiger beetle species (Pearson and Mury 1979), thus providing another mechanism for avoiding competition between these three co-occurring species. 

Photo details:
Beetles: Canon 100mm macro lens w/ 68mm Kenco extension tubes on Canon EOS 50D (manual mode), ISO 100, 1/250 sec, f/18-20, MT-24EX flash 1/4 power w/ Sto-Fen diffusers.
Landscapes: Canon 17-85mm zoom lens (22mm) on Canon EOS 50D (landscape mode), ISO 100, 1/100 sec, f/10, natural light.

REFERENCES:

Erwin, T. L. and D. L. Pearson. 2008. A Treatise on the Western Hemisphere Caraboidea (Coleoptera). Their classification, distributions, and ways of life. Volume II (Carabidae-Nebriiformes 2-Cicindelitae). Pensoft Series Faunistica 84. Pensoft Publishers, Sofia, 400 pp.

Melius, D. A. 2009. Post-monsoonal Cicindela of the Laguna del Perro region of New Mexico. CICINDELA 41(4):81-89.

Pearson, D. L., C. B. Knisley and C. J. Kazilek. 2006. A Field Guide to the Tiger Beetles of the United States and Canada. Oxford University Press, New York, 227 pp.

Pearson, D. L. and E. J. Mury. 1979. Character divergence and convergence among tiger beetles (Coleoptera: Cicindelidae). Ecology 60:557–566.

Willis, H. L.  1967.  Bionomics and zoogeography of tiger beetles of saline habitats in the central United States (Coleoptera: Cicindelidae).  The University of Kansas Science Bulletin 47(5):145-313.

Copyright © Ted C. MacRae 2010

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Habitat Partitionining in Tiger Beetles

Cicindela willistoni estancia

Cicindela willistoni estancia. Photo by David Melius.

ResearchBlogging.orgThe latest issue of CICINDELA (December 2009, vol. 41, no. 4) contains an interesting paper by David A. Melius titled, “Post-monsoonal Cicindela of the Laguna del Perro region of New Mexico.” This paper continues a theme that I have touched on a few times in recent posts regarding the partioning of resources by multiple species of tiger beetles utilzing the same habitat. The author reports on the results of two visits to the Laguna del Perro salt lake region of New Mexico (Torrance County) in July 2009, during which time he recorded a total of eight tiger beetle species in the area. As in many other parts of the arid west, tiger beetles in this region are highly dependent upon summer monsoonal rains to trigger adult emergence (Pearson et al. 2006), resulting in multiple species occupying a given habitat during the relatively short post-monsoonal period. However, according to the competitive exclusion principle (Hardin 1960), two species cannot stably coexist in the same habitat and compete for the same resources—one of the two competitors will always overcome the other unless resources are partitioned to avoid competition.

Cicindela willistoni estancia

Cicindela willistoni estancia. Photo by David Melius.

Tiger beetles that occupy the the same habitats employ a variety of mechanisms for avoiding direct competition. One of these is partitioning the environment into different “microhabitats.” One of the earliest reports of this was by noted American ecologist Victor Shelford, who reported that adult tiger beetles on the southern shores of Lake Michigan occupied different habitats from water’s edge to oak forest floor (Shelford 1907). Similarly, Choate (2003) found three sympatric species of tiger beetles in a coastal mudflat region in South Carolina, each of which utilized a different portion of the salt marsh. I myself have noted multiple species occupying the same habitat in Oklahoma’s Salt Plains National Wildlife Refuge, on a coastal salt marsh in Florida, and in the White River Hills of southwestern Missouri.

In the present study, the author noted distinct preferences among the eight species for different microhabitats within and adjacent to the salt flats, including 1) thick, wet mud immediately adjacent to the water, 2) damp, soft sand 10-20 m from the water and devoid of vegetation, and 3) dry to damp sand further away from the water with salt-tolerant plants. Nearby roadside habitats were also noted as an additional microhabitat. The species found and their preferred niches were:

  • Cicindela fulgida rumppii, exclusively in vegetated dry sand areas around the salt flats.
  • Cicindela (Cicindelidia) nigrocoerulea, mostly 10-20m from the water’s edge, a few also in roadside habitat.
  • Cicindela (Cicindelidia) punctulata chihuahuae, exclusively in roadside habitats.
  • Cicindela (Cicindelidia) willistoni estancia, mostly along the water’s edge.
  • Cylindera terricola cinctipennis, exclusively in dry grassy areas away from the water.
  • Ellipsoptera nevadica, exclusively along the water’s edge.
  • Eunota togata fascinans, unvegetated areas near and 10-20m from the water’s edge.
  • Habroscelimorpha circumpicta johnsoni, limited to roadside habitats and vegetated dry sand areas around the salt flats.

These microhabitat partitions can be visualized below. Note that although eight total species were collected, only 2-4 occur within each particular microhabitat and that all eight species were limited to just 1 or 2 microhabitats, resulting in unique species-guilds for each.

Some differences were also noted in species present during the different trips, suggesting that species occurring within the same microhabitat are also utilizing differences in temporal occurrence to further minimize competition. Differences in size among the different species were noted as well – for example, of the four species occurring in the vegetated, dry-damp sand microhabitat, Cylindera terricola is notably smaller and Habroscelimorpha circumpicta notably larger than the others. Since mandible length of adult tiger beetles is highly correlated with preferred prey size (Pearson et al. 2006), this likely results in utilization of different prey, further partioning resources within the different microhabitats.

I thank David A. Melius (Albequerque, New Mexico) for allowing me to include his stunning photographs of Cicindela willistoni estancia in this post.

REFERENCES:

Choate, P. M., Jr. 2003. A Field Guide and Identification Manual for Florida and Eastern U.S. Tiger Beetles.  University Press of Florida, Gainesville, 224 pp.

Hardin, G. 1960. The competitive exclusion Principle. Science 131:1292-1297.

Melius, D. A. 2009. Post-monsoonal Cicindela of the Laguna del Perro region of New Mexico. CICINDELA 41(4):81-89.

Pearson, D. L., C. B. Knisley and C. J. Kazilek. 2006. A Field Guide to the Tiger Beetles of the United States and Canada. Oxford University Press, New York, 227 pp.

Shelford, V. E. 1907. Preliminary note on the distribution of tiger beetles (Cicindela) and its relation to plant succession. Biological Bulletin of the Marine Biological Laboratory at Woods Hole 14:9-14.

Copyright © Ted C. MacRae 2009

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