How P. megacephala repels Eciton army ant raids

Eciton burchelli army ant.
Image by Alex Wild (from Wikipedia)

There is a certain morbid fascination associated with watching the depredations of various army and driver ants on their hapless prey. The huge colonies of these ants and their mass raids make them formidable predators against most ant colonies. Even the very populous Atta leafcutter colonies, which are normally immune to most army ant species, can be overwhelmed by army ants specialized against them, such as Nomamyrmex esenbecki. (Swartz M, 1994)

Some invasive ant species too are prey to army ants. Colonies of the little fire ant Wasmannia auropunctata for example, are devastated by the army ant Neivamyrmex compressinodis and perhaps some other Neivamyrnex species (J. Le Breton et al, 2007).

Wasmannia auropunctata.
Image from Hawaii Invasive Species Council (HISC).

Solenopsis geminata colonies also fall prey to Eciton and Labidus army ant raids, although most times the fire ants manage to abscond from their nest ahead of the invasion (Perfecto 1992).

Pheidole ants as a rule tend to be favored prey not only for the larger Eciton army ants, but for the smaller Neivamyrmex ants as well, probably due to their smaller colony populations and ubiquity in tropical and subtropical environments. This has resulted in some very interesting social behavior, including the classic discovery of enemy specification by E.O. Wilson (Wilson E.O, 1976).

However, unlike other Pheidole ants, P. megacephala with its huge unicolonial societies and extremely aggressive workers might pose a problem for even the most voracious army ants.

Indeed, P. megacephala in Africa actually preys on the formidable Dorylus driver ants, and a study done by Dejean et al (2014) reveals that this invasive ant is able to handle the New World Eciton army ants as well.

P. megacephala minors prey on Dorylus worker.
Image courtesy of Alex Wild. Click to go to Myrmecos.net

The researchers observed interactions between P. megacephala and two Eciton species in Puerto Morelos, Mexico. They analyzed 24 raids by Eciton burchelli on a P. megacephala colony, and 11 raids by Eciton hamatum.

According to the researchers, the incoming raids were met by fierce resistance from the Pheidole, with many army ants spread-eagled by the defenders as the raiders pushed inward through the colony entrances. In addition to those being killed in the melee by the P. megacephala, outgoing army ants after seizing booty or escaping from the Pheidole nest were in turn attacked by their own nestmates!

During each raid, an average of 1,869 E. burchelli workers were killed by their own colony members, while an average of 1,380 E. hamatum army ants were killed by their fellow nest mates. These casualties make up a not insubstantial 0.4 % and 0.55% of the total estimated population of typical E. burchelli and E. hamatum colonies, and the loss of outgoing raiders stifled the invasion of the P. megacephala nests.

The researchers noted that it seems as if an aqueous compound(s) was transferred to the attacking army ants as they battled P. megacephala, which caused their colony mates to attack them instead, and thus indirectly abort the raid.

Given the high mortality suffered by the army ants, and since P. megacephala is not native to the neotropics and thus a "novelty" to the native ants there, it would be interesting to speculate on whether the army ants might later evolve mechanisms to avoid raiding such nests, just as they avoid other heavily protected species like Atta leafcutters.

For more information on this interesting study, consult the paper:

Dejean Alain, Azémar Frédéric, Roux Olivier. (2014) "An invasive ant species able to counterattack marabunta raids." Comptes rendus biologies 337 (7-8): 474-9.

Abstract:

In the Neotropics where it was introduced, the invasive ant Pheidole megacephala counterattacked raids by the army ants Eciton burchellii or E. hamatum. The Eciton workers that returned to their bivouac were attacked and spread-eagled and most of them killed by their outgoing colony mates. Little by little the zone where returning and outgoing Eciton workers encountered one another moved away from the Pheidole nest which was no longer
attacked, so that most of the colony was spared. Using a water-based technique rounded out by bioassays, we show that Pheidole compounds were transferred onto the Eciton cuticle during the counterattacks, so that outgoing workers do not recognize returning colony mates, likely perceived as potential prey. Because P. megacephala is an introduced African species, this kind of protection, which cannot be the result of coevolutive processes,
corresponds to a kind of by-product due to its aggressiveness during colony defence.

Dejean Alain, Azémar Frédéric, Roux Olivier. (2014) "An invasive ant species able to counterattack marabunta raids." Comptes rendus biologies 337 (7-8): 474-9.

J. Le Breton, A. Dejean, G. Snelling, J. Orivel.  (2007) Specialized predation on Wasmannia auropunctata by the army ant species Neivamyrmex compressinodis. J Appl Entomol, 131, pp. 740–743

PERFECTO I. Observations of a Labidus coecus (Latreille) underground raid in the central highlands of Costa Rica. Psyche. 1992;99:214-22

Swartz, M. (1994) Predation on an Atta cephalotes colony by an army ant, Nomamyrmex esenbeckii. Biotropica 30(4):682-684

Wilson EO. (1976) The organization of colony defenses in the ant Pheidole dentata Mayr (Hymenoptera: Formicidae). Behavioral Ecology and Sociobiology  1: 63–81.