Tawny (a.k.a. Rasberry) Crazy Ants in West Houston, Texas

This article by Jerry Cates, with contributions of specimens from Thang Dang, et al., first published on 11 September 2012, was last revised on 28 December 2022. © Bugsinthenews Vol. 13:09(01).

001. The Rasberry Crazy Ant

First came an invasion of Odorous House Ants…

In late August 2012 I was called to a large nursing home in the western portion of the city of Houston, Texas [the nature of the facility, and its exact location, are not divulged in this article in order to protect the privacy of facility staff and patients], to deal with an invasion of ants that were gaining access to the facility’s interior. The ants involved at that time were darkly colored and of two sizes. Worker ants were tiny, only a few millimeters in length, but they were accompanied by larger reproductive ants that had wings. The ants did not bite or sting [aggressive fire-ant control measures were in place and appeared to be working properly], but were present in sufficient numbers to alarm the nursing staff.

I first collected a few specimens for examination at the lab. At the time my portable microscope wasn’t with me, but shortly afterward I made it a habit to carry the microscope wherever I go, so specimens can be examined microscopically and identified to genus and species at time of collection. The ants were gently plucked from the floor, walls, and furniture with an ethanol-soaked swab, and preserved in glass vials of ethanol. One kit of specimen collection vials was provided to Thang Dang, the maintenance supervisor at the facility, and another was given to the facility nursing director, so that additional specimens could readily be collected by them or members of their respective staffs if ants were observed later. The kits contained five one-ounce vials of ethanol, a non-sterile swab, a set of immersible, archival-quality linen vial labels, and a disposable India ink pen with which to inscribe the labels.

After performing a treatment regimen known to resolve occasional ant invasions of this sort, I inspected the grounds  in the vicinity of the invasion to confirm that fire ants were not involved, then inspected the remaining grounds surrounding the facility to ensure that no additional ant issues were in evidence.

The landscaping at this facility was manicured, in good condition, and both adequate and appropriate for the setting. By that I mean that the landscape was sufficiently populated with botanicals to be esthetically pleasing, but that nothing in its architecture provided for unusual harborage conditions particularly attractive to invasive ants, such as expanses of thick jasmine ground cover, effusive shrubbery that obscures the foundation, or botanical arrangements that impinge on the structure’s exterior walls. Unfortunately, however, the property was bordered on at least one side by a dense natural wilderness area of mixed hardwoods and conifers underlain with a thick, impenetrable undergrowth of low trees, shrubs, and vines. This wilderness area, which was separated from the facility grounds by a curbed asphalt drive and a narrow grassy border, could not be treated for pests, yet provided excellent harborage for all the insect, arachnid, and mammalian organisms typically found here, at the southern extremity of the transition between the Texan and Austroriparian biotic provinces of Texas (as described by Blair, 1949). As I gazed once more at this wide expanse of wild, untamed forest, I wondered afresh how its proximity to this facility would impact and drive my efforts to keep this facility free of pests.

Finally, my work on this visit now complete, I documented the  treatment program that had been carried out and returned to my lab in Round Rock.

At the lab the collected ant specimens were examined under a dissecting microscope to identify their distinguishing anatomical characters. Each had a twelve-segmented antenna without a club, and a single-noded petiole that was concealed from view, dorsally, by the first tergite of the gaster. None possessed a visible postpetiole. All were absent a sting but lacked the round, hair-ringed orifice (acidopore) of ants in the Formicinae subfamily. These characters — in combination — typify ants in the subfamily Dolichoderinae, specifically odorous house ants (Tapinoma sessile Say). These ants are annoying, and when crushed they exude an odor similar to rotten coconuts (Mallis, 2011), but because they do not bite or sting, and have not been shown to carry or spread disease, they do not pose imminent medical risks to humans. Most important, they are amenable to control measures of the kind performed on the treatment visit described above. However, they forage tirelessly, night and day, over hundreds of yards from their nests. Nothing in the facility grounds appeared suitable for nesting activity, so chances are they are foraging from nests in the nearby wilderness area. That means they will return, repeatedly, unless extensive habitat modifications are carried out to eliminate all conditions in the grounds and facility interior that attract and nurture them.

I took photos through the microscope lens for documentation purposes, and began searching the literature to confirm their identity to subfamily and further identify them to genus and species. That work, which required detailed mensuration of a number of critical anatomical characters, was soon interrupted by more urgent projects, and was put aside until additional time could be made available.

The Tawny (a.k.a. Rasberry) Crazy Ants were not far behind…

Approximately two weeks later I received another report that this facility had again been invaded by ants. Immediately I cleared my schedule for the day and returned to Houston. This time the ants were of a different species. That was not surprising, as the bordering wilderness area likely harbored a variety of ant species, most or all of which could be expected to show up at this facility from time to time if given the opportunity. The ants collected on this visit were lighter in color than those collected earlier. They also had a reddish hue, and were somewhat larger than the tiny worker ants observed before. None had wings. As before the ants did not bite or sting, but — again, as before — were present in alarming numbers. I collected specimens, performed a standard ant control regimen similar to that done on the previous visit, examined the grounds again for evidence of additional ant sources, and — finding none — documented my work.

Before departing the facility I examined the newly collected specimens using the portable microscope I now carried with me. Under magnification the ants presented an intriguing picture: their legs were somewhat longer than usual, but not as long as those of the common longhorn crazy ant, Paratrechina longicornis. Their petioles (the stem connecting the thorax to the abdomen) were comprised of a single node as with the ants collected on the previous visit, but was not obscured by the abdomen, could easily be discerned without manipulating the specimens, and included an obvious postpetiolar structure visible dorsally. As with the ants collected on the previous visit, these ants carried no visible stingers, but — unlike the ants collected earlier — in place of stingers each had a ring of hairs surrounding an open, circular, posterior orifice known as an acidopore.

These characters, in combination, were suggestive of a new (to Houston) species of ant discovered here by a local exterminator, Tom Rasberry, in 2002. That ant, known today as the Tawny crazy ant (Nylanderia fulva, Mayr 1862), was previously — until the ESA formally recognized it under that common name (Ferro, 2013) — known as the Rasberry crazy ant. It is also now formally recognized under the taxonomical name Nylanderia fulva (Mayr), replacing the temporary taxon Nylanderia sp. nr. pubens, which was awkward at best. This ant is a highly invasive species capable of creating huge colonies of millions of worker ants. Though they do not sting, their minuscule bites — while not known to produce medically significant wounds (if they break the skin at all) — can be disconcerting to those who find themselves covered with thousands of these ants at a time.

The Tawny crazy ant was, at the time, one of the most troublesome ants found along the Gulf Coast. The species soon afterward began showing up in other regions, some distance from the coast in Texas, Louisiana, and Mississippi, most likely after being transported inland in shipped goods, timbers, turf, etc. Whether they would thrive far from the Gulf Coast was an open question, but the risk appeared significant that they would. As a consequence, at least one pesticide manufacturer began work on producing a pesticide specifically targeting the species. The challenge, to me and all who sought to control this pest, was to find a way to deal with them decisively without exposing ourselves and others to pesticidal toxicants that might prove, later, to be worse than the ants themselves. This article chronicles the work done by others, as well as the work I carried out at this and other sites in Texas, to deal with this vexatious and interesting ant.

My research on optimal pest management techniques for invasive pest organisms has always focused first on habitat modification, leaning heavily on the use of insect growth regulators and mechanical modifications of landscape architecture. Although I take advantage of all the pesticides available for such purposes, particularly when the need to bring immediate cessation to an existing infestation is indicated. I have learned — in the course of more than 44 years of active involvement in this field — that habitat modification is always the most effective, safest, and longest lasting approach of all. There was no doubt in my mind that it remained the best approach to dealing with the Tawny crazy ant as well, but it took time to formulate proper procedures and to implement them once they were established.

My research into the biology of this ant included a PhD thesis prepared by a graduate student at Texas A&M University. Within this voluminous work was the student’s interesting discovery that the species enjoyed a close relationship with the red imported fire ant (Solenopsis invicta, Buren 1972). That relationship was so intimate that, one gathered from the research reported in the aforementioned PhD thesis, in the absence of the fire ants, the amazing fecundity of Nylanderia fulva would be stunted, if not extinguished. Acting on this, I carried out an unusually aggressive fire ant control program at the affected nursing facility. Within a matter of weeks all evidence of the Nylanderia fulva infestation ceased. As long as I continued that fire ant control program, the tawny crazy ant infestation remained in abeyance.

At an annual meeting of the Entomological Society of America not long afterward, I bumped into a colleague who was employed by a major pesticide manufacturer. We were good friends, and had known each other for over a decade. We exchanged war stories, as entomologists are wont to do,  I happened to mention the tawny crazy ant infestation I’d worked on in Houston. He brightened up, and informed me that his company was developing a special pesticide to combat that species, and would let me know as soon as it was available. When I told him how I’d cured the infestation at the Houston nursing facility, his composure evaporated. “That’s absurd!” he exclaimed and, visibly upset, hurried away. The pesticide he mentioned never came to market. Although the tawny crazy ant has continued to spread throughout Texas, it has never managed to produce the frightful infestations predicted soon after its initial discovery. I suspect the natural predators imported to combat the red imported fire ant have also managed to combat this ant as well. Chalk one up for Mother Nature…

Fig. 001 is a photograph taken through the lens of the dissecting microscope at the EntomoBiotics Inc. lab, of one of the ant specimens recently collected at the medical facility in west Houston. Additional photographs, showing details of the anatomical structures that characterize them as Rasberry crazy ants, have been processed, and are posted below.

Note that all photographs on this and other postings on BugsInTheNews can be enlarged for more detailed viewing by hovering your cursor over the photo and clicking:

100. Frontolateral head

101. Frontal head

102. Lateral head

103. Head & antenna

104. Dorsum, annotated

105. Posterior abdomen

106. Acidopore

107. Acidopore

108. Protibial spur

109. Abdomen, annotated

110. Mandible, annotated

General Anatomy: These ants present with anatomical characters consistent with workers in the subfamily Formicinae (from Bolten, 1994), and the genus Paratrechina/Nylanderia. For more information on these taxonomical units, see the section, below, on taxonomy.

Anatomical Characters of ants in the subfamily Formicinae (adapted from Bolten, 1994):

• The clypeus is broad, from front to back, so that the antennal sockets are well behind the anterior margin of the head (figs. 100-103). The median portion of the clypeus does not extend backward between the frontal carinae, but a postclypeal frontal triangle is present, projecting back between the frontal carinae/antennal sockets (fig. 101).
• The antennal sockets are inclined such that the portion of the socket margin and torulus nearest the head’s dorsal midline are on a higher level (fig. 101).
• The frontal carinae are present as simple structures that only partially conceal the antennal sockets (fig. 101).
• The narrow neck that joins the condylar bulb of the antennal scape to the scape proper is straight (fig. 101).
• Eyes are present (figs. 100-103), and the antenna has 8-12 segments (fig. 103).
• A promesonotal suture is present (fig. 104).
• Metanotum present (fig. 104); note that the metanotum is present in the form of a metanotal groove separating the mesonotum and propodeum, the last vestige of the mesonotum on the dorsal alitrunk.
• Propodeal lobes are absent (fig. 104).
• Waist, or petiole, is of a single node (fig. 104).
• Helcium tergite dorsally with extensive U-shaped emargination of its leading edge, reaching back the major length of the sclerite (fig. 104).
• Abdominal segment 4 = gastral segment 2, without differentiated presclerites (fig. 109).
• Pygidium, i.e., tergite of abdominal segment 7 = gastral segment 5, large and simple (figs. 105, 106, 107 & 109).
• Hypopygidium, i.e., sternite of abdominal segment 7 = gastral segment 5, with a U-shaped to almost circular acidopore apically, which acidopore is equipped with a circlet of hairs (ibid).
• Sting absent, replaced by acidopore (ibid).

Taxonomy:

• Kingdom Animalia (ahn-uh-MAYHL-yuh)  — first described in 1758 by the Swedish taxonomist Carolus Linnaeus [23 May 1707 – 10 January 1778], using the Latin word animal = “a living being,” from the Latin word anima = “vital breath”, to refer to multicellular, eukaryotic organisms whose body plans become fixed during development, some of which undergo additional processes of metamorphosis later in their lives; most of which are motile, and thus exhibit spontaneous and independent movements; and all of whom are heterotrophs that feed by ingesting other organisms or their products;
• Phylum Arthropoda (ahr-THROPP-uh-duh) — first described in 1829 by the French zoologist Pierre André Latreille [November 20, 1762 – February 6, 1833], using the two Greek roots αρθρον (AR-thrawn) = jointed + ποδ (pawd) = foot, in an obvious reference to animals with jointed feet, but in the more narrow context of the invertebrates, which have segmented bodies as well as jointed appendages;
• Class Insecta (Linnaeus, 1758) — named using the Latin word insectum, a calque of the Greek word ἔντομον ( EN-toh-mawn) = “(that which is) cut into sections”; comprised of arthropods with chitinous external (exo-) skeletons, a three part body composed of a distinct head, thorax, and abdomen, the midmost part having three pairs of jointed legs, and the foremost part having a pair of compound eyes and antennae;
• Order Hymenoptera (hye-muhn-OPP-turr-uh) — first described in 1758 by the Swedish taxonomist Carl Linnaeus (1707 – 1778), who combined the Greek words ὑμήν (pron. humēn) = “membrane” + πτερόν (TARE-awn) = “wing”, thus ὑμενόπτερος (hew-men-OPP-tehr-ose) = “membrane-winged” to refer to insects with membranous wings, specifically the sawflies, wasps, bees, and ants; this is one of the largest orders of insects, and includes over 130,000 species;
• Family Formicidae (fohr-MISS-uh-dee) — first described in 1809 by the French zoologist Pierre André Latreille (1762 – 1833), from the Latin formica = “ant” to refer to hymenopteran insects that have elbowed antennae and a narrow waist that separates the thorax from the abdomen with a node-like petiole; at present, 20 distinct subfamilies of ants are recognized:
  • Subfamily Aenictogitoninae: a subfamily comprising a single genus, Aenictogiton, with seven known species of rarely collected ants found in Central Africa with morphological and phylogenetic affinities to the army ant genus Dorylus; only males have been collected, and nothing is known about their workers, queens or behavior;
  • Subfamily Agroecomyrmecinae: characterized by the following derived traits (see Bolton 2003): mandibular masticatory margins oppose at full closure but do not overlap; eye at extreme posterior apex of deep antennal scrobe; antennal sockets and frontal lobes strongly migrated laterally, far apart and close to lateral margins of head; abdominal segment IV with complete tergosternal fusion; sternite of abdominal segment IV reduced, tergite much larger than sternite and strongly vaulted;
  • Subfamily Amblyoponinae (including the subfamily Apomyrminae): mostly specialized subterranean predators, comprised of a single genus of two species native to California; characterized by the following traits (see Bolton 2003): workers of this ant subfamily — which was formerly considered a tribe within the subfamily Ponerinae — exhibit the following characters: eyes small or absent, if present situated behind the mid-length of side of head; anterior margin of clypeus with specialized dentiform setae; the promesonotal suture is flexible; the petiole is broadly attached to abdominal segment 3 and is absent a distinct posterior face; the postpetiole is absent; a sting is present and is well developed.
  • Subfamily Aneuretinae: this subfamily is comprised of a single extant tribe containing a single extant genus and a single extant species (several extinct tribes, genera, and species have also been described), namely the Sri Lankan relict ant (Aneuretus simoni); this is one of the few ant species considered endangered;
  • Subfamily Cerapachyinae: a subfamily of 5 genera and 217 recognized species, distributed throughout the tropics; they possess spines on the pygidium; their antennae are short and thick; and they lack dorsal thoracic structures; they prey on other ant species;
  • Subfamily Dolichoderinae: presently not divided into tribes, but comprised of 24 genera, including the Argentine ant (Linepithema humile), the erratic ant (Tapinoma erraticum), the odorous house ant (Tapinoma sessile), and cone ants in the genus Dorymyrmex; these ants are distinguished by having a single petiole, absent a post-petiole, and lacking a sting but possessing an apical slit-like orifice at the posterior abdomen instead of the round acidopore encircled by hairs typical of the Formicinae subfamily;
  • Subfamily Ecitoninae (incl. “Dorylinae” and “Aenictinae”): New World and Old World army ants; in the New World, these ants are found in the tribes Cheliomyrmecini (containing the single genus Cheliomyrmex) and Ecitonini (containing the four genera Neivamyrmex, Nomamyrmex, Labidus, and Eciton); the genus Neivamyrmex — the largest of all army ant genera — contains more than 120 species, all native to the United States; the predominant species of the genus Eciton, E. burchellii, has been given the common name “army ant” and is considered the archetypal species; Old World army ants are usually divided into two tribes, Aenictini and Dorylini, but are often treated as a single tribe, Dorylini, alone; each contains a single genus; the genus Aenictus contains over 100 species, and the genus Doryus contains the aggressive “driver ants”, of which 70 species are known;
  • Subfamily Ectatomminae: In North America a single genus, Gnamptogenys, is represented; that genus is not native to North America but has been introduced;
  • Subfamily Formicinae: see the detailed description of this subfamily below;
  • Subfamily Heteroponerinae:
  • Subfamily Leptanillinae: comprised of two tribes, the Anomalomyrmini (two genera, seven species) and Leptanillini (three genera, 41 species); within the tribe Leptanillini the larva provide their hemolymph as food to the queen through specialized processes on their prothorax and third abdominal segment; this behavior resembles that of the unrelated Adetomyrma, also called Dracula ants, which actually pierce their larvae to get at the body fluids; ants in the genera Leptanilla and Phaulomyrma are minute, yellow, blind, and subterranean;
  • Subfamily Leptanilloidinae: 1 tribe, 3 genera, 15 species;
  • Subfamily Martialinae: 1 genus containing a single species, Martialis heureka, discovered in 2000 from the Amazon rainforest near Manaus, Brazil, and placed as the sole member of a new subfamily (Martialinae); the generic name, which means “from Mars,” refers to its unusual “out-of-this-world” morphology; the species epithet heureka honors the surprise that accompanied its discovery; it is the oldest known extant species of ants;
  • Subfamily Myrmeciinae (incl. “Nothomyrmeciinae”): once distributed worldwide but now restricted to Australia and New Caledonia; one of several ant subfamilies which possess gamergates, i.e., female worker ants which are able to mate and reproduce, thus sustaining the colony after the loss of the queen; formerly composed of a single genus, Myrmecia, but revised (Ward & Brady 2003) to include two tribes and four genera; three additional genera, one form genus, and nine species were later described (Archibald, Cover and Moreau 2006) from the Early Eocene of Denmark, Canada, and Washington;
  • Subfamily Myrmicinae: approximately 130 genera in 23 tribes, and 10 additional genera not assigned to specific tribes, all cosmopolitan; the pupae lack cocoons; some species retain a functional sting; the petioles have two nodes; nests are permanent, in soil, rotting wood, under stones or in trees; the subfamily includes leaf cutters (tribe Attini), acrobat (tribe Crematogasterini), harvester (tribe Myrmicini), big-headed (tribe Pheidolini), and fire (tribe Solenopsidini) ants;
  • Subfamily Paraponerinae: comprised of a single genus, Paraponera, containing a single species (Paraponera clavata), known as the lesser giant hunting ant, the conga ant, or the bullet ant (so named for its powerful sting); this ant inhabits lowland rainforest, from Nicaragua and eastern Honduras, and south to Paraguay; the ant is called “hormiga veinticuatro” by locals to refer to the 24 hours of pain following each sting;
  • Subfamily Ponerinae: about 1,600 species in 28 extant genera, including Dinoponera gigantea, which is one of the largest species of ant found in the world; distinguished from other formicine subfamilies by their constricted abdomens;
  • Subfamily Proceratiinae: similar to Ponerinae but the promesonotal suture is fused and the frontal lobes, elevated rather than transverse, are frequently reduced; antennal sockets are exposed in frontal view; in most species abdominal tergite 4 is much enlarged and vaulted, while abdominal sternite 4 is reduced; these are specialized predatory ants that are represented in California by a single species;
  • Subfamily Pseudomyrmecinae: three genera of slender, wasp-like ants that forage alone and readily sting when molested;
• Subfamily Formicinae (fohr-mih-SEE-nee) — first described in 1836 by the French entomologist Amédée Louis Michel le Peletier, comte de Saint-Fargeau (1770 – 1845), usu. referred to as Lepeletier, from the Latin formica = “ant” to refer to a subfamily of ants whose evolutionary development is not as robust as most other subfamilies, e.g., they generally retain such primitive features as pupal cocoons, ocelli in workers, and a lesser tendency toward reduced palpal or antennal segmentation; all formicines have reduced stings and enlarged venom reservoirs, with a venom gland that is uniquely specialized to produce formic acid, and a one-segmented petiole having the form of a vertical scale;
• Tribe Plagiolepidini (PLAH-gee-oh-lepp-uh-DEE-nee) — the identity of the taxonomist(s) who first described this tribe is obscure, but the term appears to be cobbled together from the Greek roots πλαγιος (PLAH-gee-ose) = “sideways” + λεπιδος (pron. LEPP-eh-dose) = “a scale” to refer to the character of the one-segmented scale of the petiole;
• Genus Nylanderia (nye-lahn-durr-EE-uh) — first described in 1893 by the Swiss myrmecologist, neuroanatomist, and psychiatrist Auguste-Henri Forel (1848 – 1931), notable for his early sociological convictions, his later conversion to the Bahá’í Faith, as well as his  investigations into the structure of the human brain and that of ants; his choice of this generic name is obscure, but appears to be derived from the surname Nylander, perhaps in honor of a personage of special interest to him;
• Species Nylanderia fulva (nye-lahn-durr-EE-uh FUHL-vuh) — the specific name Nylanderia fulva was first described in 1862 by the Austrian entomologist Gustav L. Mayr (1830-1908), at the age of 32, based on the Latin fulvus, meaning tawny, yellowish-brown, or reddish-yellow.

References:

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