|Photograph by R.H. Scheffrahn, University of Florida|
1. How do I tell that it's Pheidole megacephala?
You can ID it by using this guide.
This guide will work for most places except in the islands neighboring Madagascar/Seychelles and perhaps Southern African areas, where newly-delineated related and non-invasive species are present such as Pheidole decepticon and Pheidole megatron (in addition to P. megacephala itself).
2. Where can I find Pheidole megacephala?
P. megacephala is a tropical species with a global distribution. It can be found in sub-tropical environments as well.
You can find a global distribution map here.
You can find local colony clusters of this species here.
3. Where do Pheidole megacephala nest?
P. megacephala is primarily a ground-nesting species. They can normally be found nesting in soil; under objects such as rocks and logs and pots; around the bases of trees, etc.
P. megacephala tends to be more common in open, disturbed habitats with weedy vegetation that can support high densities of plant-feeding Hemiptera, which the ants tend for honeydew. Typically, P. megacephala is largely absent in intact natural forest, even in Hawaii, which lacks any native ant competitors (Wetterer JK, 2012).
4. Where did Pheidole megacephala originate from?
This species is currently assumed to be native to Southern Africa (Fourniet et al, 2009).
5. How big can a colony get?
P. megacephala is a unicolonial species, which means that it forms expansive polygynous populations within which individuals mix freely among physically separate nests. Unicolonial ant populations are the most extensive cooperative units known in nature, forming networks of interconnected nests extending sometimes hundreds of kilometers.
In the case of P. megacephala, a supercolony in Australia was found that extended for 3000 km (Fournier et al, 2009; Helanterä et al , 2009).
6. What is the proportion of majors in a colony cluster?
Some studies have found that it varies between 5% (in Florida) to 25% (in South Africa), with most in the 20% range (Wills et al, 2014).
7. How does Pheidole megacephala reproduce?
The primary way this species reproduces is by budding of groups of workers and queens from a main colony cluster. However, there are reports that nuptial flights of winged alates can occur in Florida.
8. Does this species ALWAYS have a negative effect on the environment?
Although the overall effect of P. megacephala incursions into non-native areas may be deleterious (especially to the native ants), there have been studies that show some positive aspects to their presence.
The predatory efficiency of the species can benefit host plants and people. For example, Bach (1991) noted that:
Predator introduction experiments showed that P. megacephala removes predatory coccinellid larvae, even when they are covered with a protective coating. Host plants from which ants had been removed had significantly higher degrees of honeydew accumulation, which resulted in greater colonization by sooty mold and greater rates of leaf death and abscission. Ants also removed herbivorous lepidopteran larvae from plants.
The role of P. megacephala as a super-dominant species also allows it to exclude ants that are even more damaging. For example, local people in New Caledonia (where P. megacephala and Wasmannia aurupuntata form mutually exclusive territories) have been using P. megacephala propagules to try to exclude the little fire ant W. aurupunctata, which stings and can cause blindness in people and their pets (Chazeau et al, 2002)
9. Is Pheidole megacephala able to expand into natural environments, or is it restricted to human-disturbed areas?
Yes, it is able to expand into natural undisturbed environments.
However, the usual invasive form of P. megacephala has some requirements that make some natural areas (e.g. canopied areas like rainforests or very arid zones) non-optimal for it. Even in this case, it is still possible for an invasion front to spread from colony clusters of this species that are in disturbed areas into adjacent undisturbed areas (Callan and Majer, 2009; Hoffman et al, 1999).
10. Will aggressive native ants be able to stop the spread of this invasive species?
Most likely not.
For example, there had been some hope that large aggressive species of Iridomyrmex might prevent P. megacephala from incursions into natural undisturbed environments in Australia, but latter studies showed that P. megacephala invasion fronts can extirpate such ants and expand into undisturbed habitats...and in fact that these large aggressive ants are the first to be displaced from baits and the first to disappear from invaded areas (Callan and Majer, 2009; Hoffman et al, 1999;Hoffman et al 2008; Vanderwoude et al 2000).
In Kenya, the colonies of very aggressive native Crematogaster ants occupying and protecting Acacia trees were destroyed, whereas colonies of Tetraponera ants were mostly spared because the latter avoided conflict with the invading P. megacephala and instead hid in their domatia until the invaders had gone back to the ground (Riginos et al, 2015).
As pointed out by the authors: Based on the strong dominance hierarchy among the
four native acacia ants (Palmer et al. 2000), we initially
predicted that the competitive dominant C. sjostedti
would best be able to withstand a P. megacephala
invasion and that the competitive subordinate T. penzigi
would least be able to persist. Instead, we found that T.
penzigi was best able to persist, likely because of its
nonaggressive interspecific behavior
The primary limit to the spread of P. megacephala seems to be abiotic factors, although in non-optimal conditions its spread is also limited by other dominant (mostly exotic and similarly unicolonial) ants.
11. Will P. megacephala have an adverse effect on native ant species where it is present?
Almost all studies show that as the density of P. megacephala increases, it extirpates and excludes virtually all other ant species in the area (e.g. Majer 1985; Heterick et al 2000; Hoffman et al. 1999; Hoffman et al 2008; Callan and Majer, 2009).
There are exceptions at moderate levels of P. megacephala, which is accomplished mainly by the avoidance of conflict with the invasive ant.
One exception is due to competitor release. For example, Tetraponera penzigi occurs in higher densities once P. megacephala has removed its normal Azteca spp competitors (Riginos et al 2015).
Another exception involves species which forage at temperatures and conditions inimical to P. megacephala. For example, Melophorus sp. can forage at temperatures that would kill the Pheidole workers, and thus can avoid direct conflict with them.
Some other species are cryptobiotic, and thus avoid meeting the invasive species altogether, while some may have some natural defensive abilities that allow them to withstand extirpation - for example, Brachymyrmex cf. Obscurior (Wetterer and Wetterer 2004).
12. Is P. megacephala a threat to undisturbed rainforest?
Maybe, probably not.
The normal invasive form of the species is optimized for living in urban like conditions, and even in areas where P. megacephala has dominated for more than a hundred years (e.g. Hawaii), their penetration into the interior of the islands has been negligible (Wetterer 2007). Other invasive species such as Wasmannia aurupunctata, which can thrive in rainforest and heavily canopied areas, are significantly more of a threat in this regard.
13. Is Pheidole megacephala a threat to native invertebrates?
Maybe not, but more likely yes.
Studies have shown the efficacy of the predatory abilities of P. megacephala (Dejean et al 2007; Dejean et al 2007).
Several other studies have shown significant declines in the populations of invertebrates where P. megacephala density is high, such as for stink bugs, Hawaiian crickets, Hawaiian endemic spiders, land snails, and various other invertebrates (Jones et al 2001; LaPolla et al 2000; Rosemary et al 1993; Uchida et al 2016; Milligan et al 2016).
On the other hand, a survey by Burwell et al (2010) on several Capricornia islands in Australia showed only medium effects on beetle and fly populations, and almost no effect on other invertebrate groups.
14. How successful are eradication efforts against Pheidole megacephala?
There have been a few successful eradications of this species, the largest of which was from 30 hectares in Kakadu National Park in Australia (Hoffmann and O'Connor, 2004).
It should be noted that removal of P. megacephala sometimes is followed by unintended consequences, including the arrival of other dominant exotic species, which had previously been excluded by P. megacephala (Plentovich et al, 2011).
15. Can I ethically keep this species?
Maybe, maybe not.
If you live in an area where P. megacephala has a long history and has become almost naturalized, then it's probably fine. Otherwise, there are many other species of ants, some probably local to your area (and larger and more colorful), that may provide for your hobby needs.
References:Bach, C.E. 1991. Direct and indirect interactions between ants (Pheidole megacephala), scales (Coccus viridis) and plants (Pluchea indica). Oecologia 87: 233-239
Burwell CJ, Nakamura A, Lambkin CL, Raven RJ, Wright K (2010) Invertebrates of the Capricornia Cays: human impacts, pest species and their interactions with native fauna. Queensland Museum Report for the Queensland Government, Department of Environment and Resource Management, Queensland Parks and Wildlife Service
CALLAN, S.K. & MAJER, J.D. 2009: Impacts of an incursion of African big-headed ants, Pheidole megacephala (FABRICIUS), in urban bushland in Perth, Western Australia. – Pacific Conservation Biology 15: 102-115.
Jean Chazeau, Tana Potiaroa, Lydia Bonnet de Larbogne, Djoël Konghouleux & Hervé Jourdan (2002). Expression de Wasmannia dans les milieux cultivés en NouvelleCalédonie: le cas de la caférie familiale. Etude de l'invasion de la Nouvelle-Calédonie par la fourmi pionnière Wasmannia auropunctata (Roger) : modalités, impact sur la diversité et le fonctionnement des écosystèmes, moyens d'une maîtrise de la nuisance. Programme INWASCAL Rapport final
A. Dejean, M. Kenne and C. S. Moreau (2007). Predatory abilities favour the success of the invasive ant
Pheidole megacephala in an introduced area. J. Appl. Entomol. 131(9-10), 625–629
Alain Dejean, Corrie S. Moreaub, Pierre Uzac, Julien Le Breton, Martin Kenn (2007). The predatory behavior of Pheidole megacephala. C. R. Biologies 330 (2007) 701–709
Fournier D, de Biseau J-C, Aron S (2009) Genetics, behaviour and chemical recognition of the invading ant Pheidole megacephala. Molecular Ecology 18, 186-199.
Helanterä H, Strassmann JE, Carrillo J, Queller DC. Unicolonial ants: where do they come from, what are they and where are they going? Trends Ecol Evol. 2009 Jun;24(6):341-9
HETERICK, B.E., CASELLA, J. & MAJER, J.D. 2000: Influence of Argentine and coastal brown ant (Hymenoptera: Formicidae) invasions on ant communities in Perth gardens, Western Australia. – Urban Ecosystems 4: 277-292.
Benjamin D. Hoffmann, Alan N. Andersen, and Greg J.E. Hill, “Impact of an introduced ant on native rain forest invertebrates: Pheidole megacephala in monsoonal Australia,” Oecologia, vol. 120, no. 4, pp. 595–604, 1999.
Hoffmann B & O'Connor. 2004. Eradication of two exotic ants from Kakadu National Park. Ecological Management & Restoration 5(2): 98-105.
Hoffmann, B.D. & Parr, C.L. An invasion revisited: the African big-headed ant (Pheidole megacephala) in northern Australia. Biol Invasions (2008) 10: 1171.
JONES, V.P.,WESTCOTT, D.M., FINSON, N.N. & NISHIMOTO, R.K. 2001: Relationship between community structure and southern green stink bug (Heteroptera : Pentatomidae) damage in macadamia nuts. – Environmental Entomology 30: 1028-1035.
John S. LaPolla, Daniel Otte and Lauren A. Spearman (2000). Assessment of the Effects of Ants on Hawaiian Crickets. Journal of Orthoptera Research, 9: 139-148
Majer J. D. 1985. Recolonization by ants of rehabilitated mineral sand mines on North Stradbroke Island, Queensland, with particular reference to seed removal. Australian Journal of Ecology 10:31–48.
Milligan, P. D.; Prior, K. M.; Palmer, T. M. (2016), An invasive ant reduces diversity but does not disrupt a key ecosystem function in an African savanna. Ecosphere, 2016, 7, 10, pp e01502
Sheldon Plentovich, Jakob Eijzeng, Heather Eijzeng, David Smith (2011). Indirect effects of ant eradication efforts on offshore islets in the Hawaiian Archipelago. Biol Invasions 13:545–557
Riginos, C., M.A, Karande, D.I. Rubenstein & T.M. Palmer. 2015. Disruption of a protective ant–plant mutualism by an invasive ant increases elephant damage to savanna trees. Ecology 96:654-661.3
ROSEMARY G. GILLESPIE AND NEIL REIMER (1993). The Effect of Alien Predatory Ants (Hymenoptera: Formicidae) on Hawaiian Endemic Spiders (Araneae: Tetragnathidae. Pacific Science vol. 47, no. 1: 21-33
Uchida, S.; Mori, H.; Kojima, T.; Hayama, K.; Sakairi, Y.; Chiba, S. (2016). Effects of an invasive ant on land snails in the Ogasawara Islands. Conservation Biology, 2016, 30, 6, pp 1330-1337
VANDERWOUDE, C., BRUYN, L.A.L. DE & HOUSE, A.P.N. 2000: Response of an open-forest ant community to invasion by the introduced ant, Pheidole megacephala. – Austral Ecology 25: 253-259.
Wetterer, J. K. 2007. Biology and impacts of Pacific Island invasive species. 3. The African big-headed ant, Pheidole megacephala (Hymenoptera: Formicidae). Pacific Science 61:437-456.
Wetterer, J.K. 2012. Worldwide spread of the African big-headed ant, Pheidole megacephala (Hymenoptera: Formicidae). Myrmecological News 17: 51-62.
Wetterer, J. K., and A. L. Wetterer. 2004. Ants (Hymenoptera: Formicidae) of Bermuda. Florida Entomologist 87: 212–221
Wills BD, Moreau CS, Wray BD, Hoffmann BD, Suarez AV: Body size variation and caste ratios in geographically distinct populations of the invasive big-headed ant, Pheidole megacephala (Hymenoptera: Formicidae). Biol J Linnean Soc 2014, 113:423-438. 81.