​What is Pheidole rhea?

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Pheidole rhea is the largest known species of Pheidole in the world, and one of a small handful that is really trimorphic. Most of the 1,100-plus described Pheidole species have two worker castes, minors and soldiers, with a narrow size range in each. Only seven described species have polymorphic soldiers, and rhea is one of them (Huang and Wheeler 2011).

What that means for you is a colony with minor workers at 3 to 4.5mm, majors at 5 to 10mm, and supermajors at 11 to 13mm, alongside a queen at 16 to 18mm. That queen is Camponotus-sized. The supermajors are bigger than most ants!

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They're fast, they're aggressive, they harvest seeds, and a lot of keepers call them their dream species. You can find Pheidole rhea for sale in the US here.

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What does Pheidole rhea look like?

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The minors are unremarkable at a glance, small reddish-black Pheidole. The soldiers are where it gets interesting. Both majors and supermajors have the enormous blocky head that gives big-headed ants their name, and in rhea the propodeal spines on both the major and the minor are long and sharp.

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The two soldier classes aren't just big and bigger. Huang and Wheeler (2011) measured head allometry across the caste system and found that rhea soldiers show extreme positive allometry, meaning their heads grow disproportionately faster than their bodies, while the supermajors show negative allometry. The two classes are built on different growth rules, not just different scales.

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Where is Pheidole rhea found?

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Southern Arizona, southern New Mexico, and northwestern Mexico, in the Sonoran and Chihuahuan desert regions and the semi-arid grasslands around them. Creighton (1950) recorded them on plateaus and among the foothills at the base of mountains across an elevational range of roughly 1,100 to 2,100 meters, with an apparent preference for the lower end of that range.

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Stefan Cover's field notes, published in Wilson's Pheidole monograph (2003), describe enormous colonies under sloppy craters or beneath rocks, chiefly on open rocky slopes. Mature colonies run well-developed trunk trail systems and harvest seeds in quantity, but they're active predators too. Minors and majors do most of the foraging. The large majors and supermajors are rarely seen outside the nest in the wild, but in captivity, they're known to be more confident, especially as the colony grows.

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Is Pheidole rhea good for beginners?

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Yes, in some ways. It might be best considered intermediate.

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On one hand, they're resilient, fast-growing ants that are unlikely to give out on you in good conditions. On the other hand, they're extremely fast-growing and voracious, and might be a little quick for the unwitting antkeeper. 

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Founding and the queen

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Pheidole rhea queens are fully claustral. She raises her first workers from her own reserves and shouldn't be fed until those workers hatch.

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At 16 to 18mm she is quite large for a Pheidole and carries the reserves to match, which is part of why founding colonies of this species get going so quickly once they're warm. The queen can often achieve more than 20-30 nanitics grown solely from her reserves!

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They're monogynous, and will fight if confined together. Never put these queens together!

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Founding needs heat. This is the single most common way keepers fail with this species. Incubator is best, and sitting right around 82 is my recommendation. At that temperature, it shouldn't be much more than 4 weeks from egg-worker. 

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Growth rate and colony size

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This is a fast species. Brood development is 3 to 6 weeks depending on heat and caste, with the majors of course taking longer than the minors. A well-fed, properly heated colony can pass 5,000 workers in its first year, easily.

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Mature colonies get very large. Gordon and colleagues (2017) ran their behavioral study on a laboratory colony of roughly 30,000 workers, which gives you a sense of the ceiling.

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Huang and Wheeler (2011) reared colonies of several Pheidole species side by side in the lab and counted them at 15 months. Pheidole rhea averaged 33,574 workers, the largest average colony size in the study. By comparison, the dimorphic Pheidole spadonia averaged 1,221 over the same period under the same conditions.

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What that means for you is that the first year is not the end of it. A colony that passes 5,000 workers in twelve months is on track, not exceptional, and it will keep accelerating. Plan your nest space accordingly, because this species will outgrow whatever you thought was big enough.

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That also means that these ants can get insanely large colonies. If they have 33,000 workers after just over 1 year, think of 2 or 3 years!

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Temperature and humidity

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Pheidole rhea requires heating. They will not grow at room temperature, and this is not a species where you can compromise on it at all.

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Keep the nest around 82 to 88°F in the warm section, with a gradient providing cooler areas so the colony can regulate. Pupae and larvae/eggs want different things: the older brood does best warm, while the smaller larvae and especially eggs may prefer cooler areas.

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For humidity, they prefer a medium-humidity nest overall, with the eggs and larvae wanting more moisture than the pupae. Give them a gradient there too and let them sort the brood where they want it. Lots of the nest should be humid, while other little bits dry (especially for a seed granary).

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Do not let this species get anywhere near freezing. There is no diapause requirement and no benefit to cooling them. They might be able to withstand it for short periods, and some colonies even diapause in the wild, but it's not required in captivity and probably not worth risking the colony unless you really know what you're doing!

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Choosing a nest

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They enjoy a rough nest that they can grip onto. A nest with ultracal/hydrostone is one they enjoy. 

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Don't oversize the nest. As with any species, a colony spread thin across chambers it can't fill will use the empty space as a dump. With rhea that's less of a long-term problem than with slower species, because they grow into their space fast and always have a seed pile, but it still applies in the first few months.

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Escape prevention matters more here than with most species. The minors are small, the colony is large and active, and they forage aggressively. Fluon or talcum powder on the outworld walls works, but an upside down barrier surrounding an opening on their lid is king. All barriers are temporary with this species, and sharp corners are avoided in an outworld if you're smart!

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Feeding Pheidole rhea

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This is a seed harvester first. Keep a substantial quantity of seed in the nest at all times, not as an occasional offering. It's a good idea to vary your seeds, so they have plenty of nutrients. Make sure any seeds you offer them are not contaminated with pesticides or any weird chemicals! They should be natural. 

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The supermajors exist largely to process seeds. Watching an 11mm ant crack large seeds is one of the best parts of this species!

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For protein, they take crickets, mealworms, and fruit flies readily, and protein is what enhances growth rate. If you want a colony that hits four or five figures in its first year, seeds with heavy protein with heat is the formula.

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Liquid sugars are optional with this species, which is unusual. They'll take sugarwater or Sunburst and it won't hurt, but unlike most of what we sell, they don't depend on it. You could even go without it entirely, if you wanted. 

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I recommend always making sure the ants have a large stockpile of seeds, and since sometimes they keep old seeds they don't plan on eating, to offer new seeds pretty frequently.

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Remove uneaten protein within 16 to 24 hours to avoid mold and mites. Seed casings can build up with this species, so make sure that you're there to remove them every now and then. It can be difficult to remove the trash without getting some ants, so it can take some time to clean out their outworld!

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Are the supermajors really for fighting army ants?

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You'll see it said that rhea supermajors evolved to defend against Neivamyrmex army ant raids. But, the evidence for that comes from a different species.

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Huang (2010) documented multi-phase defense against raiding army ants in Pheidole obtusospinosa, the other well-studied soldier-polymorphic Pheidole. It's a reasonable inference that rhea supermajors serve a similar function. But nobody has published the observation in rhea specifically.

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So: the supermajors are almost certainly defensive and almost certainly better at cracking large seeds. They may also serve to defend the colony from army ants, but this hasn't necessarily been proven yet!

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Quick summary

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Heat them properly at 82 to 88°F with a gradient, keep seeds in the nest constantly, feed protein hard, give them a healthy nest to grow in and secure your outworld. Skip diapause entirely. Do that and you'll have one of the most spectacular colonies in the hobby inside a year.

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Common questions

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How big do Pheidole rhea get? Queens get 16 to 18mm, minors 3 to 4.5mm, majors 5 to 10mm, and supermajors 11 to 13mm. It's the largest Pheidole species known.

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Do Pheidole rhea need heat? Yes. They will not grow at room temperature. Keep the warm end of the nest at 82 to 88°F with a cooler area available.

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How fast do Pheidole rhea grow? Brood development takes 3 to 6 weeks depending on heat and caste. A well-fed, well-heated colony can easily exceed 5,000 workers in its first year, and much more in some cases.

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Do Pheidole rhea need hibernation? No. They grow year round and should not be allowed near freezing temperatures.

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What is a supermajor? A third worker caste, larger than a major. Only seven described Pheidole species have polymorphic soldiers like this (Huang and Wheeler 2011), and rhea is the largest of them.

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What do Pheidole rhea eat? Seeds, mostly, but they need some protein to keep the healthy colony too.

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Can Pheidole rhea sting? No. They bite, and a supermajor bite is quite noticeable, but there's no sting and no formic acid spray.

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References

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Creighton, W.S. (1950). The Ants of North America. Bulletin of the Museum of Comparative Zoology 104: 1-585.

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Gordon, D.G., Zelaya, A., Arganda-Carreras, I., Arganda, S., and Traniello, J.F.A. (2017). Behavior, brain, and morphology in a complex insect society: trait integration and social evolution in the exceptionally polymorphic ant Pheidole rhea. Behavioral Ecology and Sociobiology 71: 166.

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Huang, M.H. (2010). Multi-phase defense by the big-headed ant, Pheidole obtusospinosa, against raiding army ants. Journal of Insect Science 10: 1-10.

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Huang, M.H. and Wheeler, D.E. (2011). Colony demographics of rare soldier-polymorphic worker caste systems in Pheidole ants (Hymenoptera, Formicidae). Insectes Sociaux 58: 539-549.

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Morgan, R.C. (2009). Natural history, field collection, husbandry, and display of Pheidole rhea: the big-headed harvesting ant. Invertebrates in Education and Conservation Conference Proceedings, Sonoran Arthropod Studies Institute, Tucson, AZ: 131-142.

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Rajakumar, R., San Mauro, D., Dijkstra, M.B., Huang, M.H., Wheeler, D.E., et al. (2012). Ancestral developmental potential facilitates parallel evolution in ants. Science 335: 79-82.

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Wheeler, W.M. (1908). Original description of Pheidole rhea.

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Wilson, E.O. (2003). Pheidole in the New World: A Dominant, Hyperdiverse Ant Genus. Harvard University Press.

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If you want specific help from other antkeepers, join the Stateside Ants Discord.

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To buy this species from us, see the product page.