Comptes Rendus

Ethology / Éthologie
The raiding success of Pheidole megacephala on other ants in both its native and introduced ranges
Comptes Rendus. Biologies, Volume 331 (2008) no. 8, pp. 631-635.


We studied the behaviour of the invasive African myrmicine ant, Pheidole megacephala, when confronted with colonies of other common ant species in Cameroon, a part of its native range, and in Mexico, where it has been introduced. P. megacephala raided the nests of the other ants in both cases. Eleven species out of 12 put up a rather strong resistance to raiding P. megacephala workers in Cameroon compared to only three species out of 11 in Mexico, where only colonies of Solenopsis geminata, Dorymyrmex pyramicus and Dolichoderus bispinosus resisted these raids. We conclude that P. megacephala's heightened ability to successfully raid colonies of competing ants may help explain its success and the decline of native ants in areas where it has been introduced.

Nous avons étudié le comportement de la fourmi envahissante Pheidole megacephala confrontée avec des colonies d'autres fourmis du Cameroun, faisant partie de son aire d'origine, et du Mexique où elle a été introduite. Au Cameroun, 11 espèces sur 12 ont montré une résistance plutôt forte aux raids de P. megacephala en comparaison de trois espèces sur 11 au Mexique, où les colonies de Solenopsis geminata, Dorymyrmex pyramicus et Dolichoderus bispinosus ont bien résisté aux raids. Nous concluons que la capacité de P. megacephala de piller les nids des autres fourmis peut expliquer en partie son succès et le déclin des espèces natives dans les zones où elle a été introduite.

Published online:
DOI: 10.1016/j.crvi.2008.05.004
Keywords: Invasive ants, Predation, Raiding other ants, Recruitment
Mot clés : Fourmis envahissantes, Prédation, Pillage d'autres colonies, Recrutement

Alain Dejean 1; Corrie S. Moreau 2; Martin Kenne 3; Maurice Leponce 4

1 CNRS-Guyane (UPS 2561 and UMR-CNRS 5174), résidence “Le Relais ”, 16, avenue André-Aron, 97300 Cayenne, France
2 Integrative Biology Department, University of California, Berkeley, 3101 Valley Life Sciences, Berkeley, CA 94720, USA
3 Département de biologie des organismes animaux, faculté des sciences de l'université de Douala, BP 24157 Douala, Cameroon
4 Biological Evaluation Section, Royal Belgian Institute of Natural Sciences, 29, rue Vautier, 1000 Brussels, Belgium
     author = {Alain Dejean and Corrie S. Moreau and Martin Kenne and Maurice Leponce},
     title = {The raiding success of {\protect\emph{Pheidole} megacephala} on other ants in both its native and introduced ranges},
     journal = {Comptes Rendus. Biologies},
     pages = {631--635},
     publisher = {Elsevier},
     volume = {331},
     number = {8},
     year = {2008},
     doi = {10.1016/j.crvi.2008.05.004},
     language = {en},
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Alain Dejean; Corrie S. Moreau; Martin Kenne; Maurice Leponce. The raiding success of Pheidole megacephala on other ants in both its native and introduced ranges. Comptes Rendus. Biologies, Volume 331 (2008) no. 8, pp. 631-635. doi : 10.1016/j.crvi.2008.05.004. https://comptes-rendus.academie-sciences.fr/biologies/articles/10.1016/j.crvi.2008.05.004/

Version originale du texte intégral

1 Introduction

The success of invasive, introduced species depends on several characteristics, including their own biology, the recipient community, and the abiotic conditions that together are known as a ‘niche opportunity’ [1]. Consequently, it is a challenge to identify the factors that regulate these species in their native range and those factors that allow such species to become invasive once introduced into a new area.

Invasive ants offer a good model for exploring this issue. Out of the 12,431 ant species known [2], about 150 so-called ‘tramp species’ have been transported and introduced into many parts of the world through human activity. Although the impact of many introduced ant species is limited to human settlements, some species have also become invasive, penetrating into many habitats in introduced regions where they seriously affect agricultural production and native biodiversity in part due to their ability to form huge supercolonies [3,4]. Invasive species share four characteristics generally noted as making this possible. First, the species must possess the intrinsic ability to shift from a multi-colonial social structure (i.e., several independent colonies) in their native range to unicoloniality (i.e., populations do not have colonial boundaries resulting in the absence of intraspecific territoriality over extended areas) in the introduced range. This genetically-based feature has been demonstrated in several species [5–8]. Second, separation from co-evolved parasites, predators and competitors (known as the ‘enemy release’ hypothesis) allows invasive species to allocate more energy and workers to tasks other than colony protection [4]. Third, the ability to have a mutualistic association with hemipterans permits invasive ants to tend native as well as introduced hemipterans [4,9,10]. Fourth, the success of most invasive ants is associated with the monopolization of resources in part due to mass recruitment and a high level of aggressiveness towards native ants that are either displaced or eliminated through competition [4,9,11,12].

Once their colonies have reached a relatively high density, invasive ants lower native ant abundance and diversity by eliminating native species and by altering the community organization among those species that survive their invasion [4,13,14]. Thus, invasive ants directly or indirectly affect all other organisms, so that they have become known as ‘pests’ and are considered to be one of the greatest causes of lowering biodiversity in the world [1,4].

Although invasive ants have been the focus of numerous studies in areas where they have been introduced, little is known about their behaviour in their native range [4]. We felt, then, that the best way to understand the mechanisms leading to these ants' invasive success was to see whether, in their native range, these ants already possessed certain characteristics that may allow some of them to become invasive once introduced into other areas [1,15]. To that end, we focused this study on the activity of one species, P. megacephala, when confronted with other ants both in Cameroon, a part of its native range, and in Mexico where the species has been introduced.

2 Materials and methods

This study was conducted on four P. megacephala colonies in Cameroon (on the campus of Yaoundé University, 3°50′N, 11°30′E), and in Mexico (Puerto Morelos, Quintana Roo; 20°N, 86°W) on a huge colony whose territory has spread over several kilometres along the Caribbean coast [16]. The colonies we studied in Yaoundé very likely correspond to the P. megacephala colonies referred to as ‘Cameroon 109’ in Taylor [17] as they were found in the same areas (although these colonies extended for hundreds of meters along the road side, different colony recognition was observed, as workers from different regions will fight each other during confrontation tests; A.D. pers. obs.).

Pheidole megacephala is native to tropical Africa and has become pantropical [18] after having been unwittingly dispersed by human commerce throughout the tropics and sub-tropics. Because it is able to produce dominant supercolonies in introduced areas [3,4,17], it is one of the invasive ants that has most successfully and severely reduced native arthropod abundance both in disturbed habitats and in tropical rain forests [9,19].

As it is the case for most species in this genus, the worker caste is dimorphic, with no intermediary body size between the small minors (approximately 2 mm long) and the big-headed majors (or soldiers; 3–4 mm long). Also, workers have an atrophied sting that they use to lay scent trails, but not to subdue prey or competitors [20]. This species nests in the ground, in termite mounds or in the crevices of tree bark and, like most other invasive ants, is omnivorous. It has always been assumed that P. megacephala is a good predator, because arthropod abundance and diversity decline in areas where it has been introduced [3,17,21,22]; however, this has also recently been demonstrated in its native range [23,24].

To examine P. megacephala's response to potential competing ant species, we reared colonies of the most frequently encountered species in Cameroon, and in a non-invaded area in Mexico adjacent to the invasion zone. The ants tested were reared in the laboratory in 22×2.5 cm test tubes supplied with a water source and opening into hunting arenas composed of wooden boxes (50×30×7 cm) covered with a plate of glass (see Table 1 for the list of species reared). Depending on the size of the workers from the tested species, one to three test tubes were connected to the hunting area. The mouths of the tubes were stopped with a cork pierced with a hole permitting the reared ants to pass through. One side of each hunting arena was pierced with small holes permitting the P. megacephala workers and soldiers to enter (these holes were plugged with cotton prior to the field experiments). Each time 200–250 workers plus brood (or entire colonies for certain species) were reared for three days prior to the experiments.

Table 1

Ant species tested against Pheidole megacephala workers foraging under natural conditions (the tested ant colonies were in captivity). A: all individuals were killed in less than one hour; B: all individuals were killed in over one hour; C: only some individuals were killed; D: none of the individuals were killed. We defined four scores of species' resistance to P. megacephala raids as follow: (1) low resistance to P. megacephala (A, B or AB), (2) average resistance (medium: ABC or BC), (3) rather high resistance (BCD), and (4) high resistance (C, CD or D). Statistical comparison based on scores (frequency at which species resisted P. megacephala raids) between Cameroon and Mexico: Chi-square test for trends, χ2=5.59; 1df; P=0.018

Native range: Cameroon (12 species) No of cases A B C D Score
Ponerinae Leptogenys sp.A 4 2 2 Low
Odontomachus troglodytes Santschi 4 1 3 Medium
Plectroctena minor Emery 11 1 6 4 Rather high
Pachycondyla analis (Latreille) 3 3 High
Pachycondyla soror (Emery) 12 11 1 High
Pachycondyla tarsata (Fabricius) 8 5 3 High
Cerapachyinae Cerapachys sp.A 3 3 High
Myrmicinae Crematogaster striatula Emery 5 1 2 2 Rather high
Myrmicaria opaciventris Emery 6 2 4 High
Tetramorium bicolor Viehmeyer 6 1 4 1 Rather high
Formicinae Anoplolepis tenella Sanstchi 2 2 High
Camponotus brutus Forel 5 5 High
Total 69 2 6 45 15
Invaded area: Mexico (11 species)
Ponerinae Leptogenys mexicana (Mayr) 3 3 Low
Odontomachus brunneus (Patton) 5 2 3 Low
Pachycondyla harpax Fabricius 8 3 5 Low
Pachycondyla obscuricornis Emery 4 3 1 Low
Pachycondyla villosa (Fabricius) 4 4 Low
Ectatomminae Ectatomma tuberculatum (Olivier) 4 1 2 1 Medium
Myrmicinae Crematogaster sp.A 5 2 3 Medium
Solenopsis geminata (Fabricius) 2 2 High
Dolichoderinae Dolichoderus bispinosus (Olivier) 5 5 High
Dorymyrmex pyramicus (Roger) 8 8 High
Formicinae Camponotus planatus (Roger) 6 1 3 2 Medium
Total 54 13 20 13 8

For each experiment, we placed one box plus the connected test tubes in the field, on the natural territory of a P. megacephala colony, 3–4 m from a nest entrance (several nest entrance sites were tested in both Cameroon and Mexico). We then monitored P. megacephala workers for two hours following the installation of the introduced ant colonies, or less if the time required for P. megacephala to find and completely destroy the ant colonies was under two hours. The encounters between P. megacephala and workers from colonies of the tested species therefore occurred in the hunting arenas. We observed and scored the four following situations. A: All of the individuals of the tested colony were killed in less than one hour following the discovery of the introduced nest by a P. megacephala scout; B: the same result in between the first and second hour; C: only some of the individuals were killed during the total duration of the experiment (monitoring of each introduced colony for two hours); or D: none of the individuals were killed. Then, a statistical comparison between the results found in Cameroon and Mexico was conducted on the frequency at which species resisted P. megacephala raids from both localities using a contingency table and the Chi-square test for trends (GraphPad Prism 4.0 software).

3 Results

In all tested cases, the experimental device/colony was discovered in less than 20 min by P. megacephala scout workers (only minor workers) in both study sites. In Cameroon, when these scouts discovered the experimental devices, they entered the hunting arenas and then immediately returned to their nest to recruit nestmates in all cases. Recruited P. megacephala individuals gathered and retrieved refuse from the competing species' nests (prey debris and dead workers deposited in a corner of the foraging arena). In Cameroon, seven species out of twelve resisted the raids without losing a worker (High in Table 1); three others resisted well overall, but in some instances their colonies were killed in more than an hour (Rather high in Table 1). Leptogenys sp. A was the most vulnerable species tested and the only species in the native range with all of its colonies killed, sometimes in less than one hour.

In Mexico, the colonies of eight species out of 11 were always (Low in Table 1; n=5) or often killed (Medium in Table 1; n=3) by P. megacephala. In contrast, only Dorymyrmex pyramicus, Dolichoderus bispinosus and Solenopsis geminata always resisted the raid with little or no loss of life.

It so happens that in both Cameroon and Mexico, the guards of the colonies of some species were ineffectual when faced with small P. megacephala workers, and hardly reacted when the latter entered the hunting arena or even the mouths of the test tubes (i.e., their nest entrance). On the contrary, species that resisted well first counter-attacked, and then stayed just behind the holes pierced on one side of the hunting arena, ready to bite any intruder. Even, D. pyramicus workers, which are relatively small, sometimes exited the hunting arena to attack the intruders outside of the nest.

The overall result is a significant difference between Cameroon and Mexico concerning the frequency at which species resisted P. megacephala raids (Table 1).

4 Discussion

The negative impact on native species by invasive ants in areas where they have been introduced is hypothesized to be related to a high level of aggressiveness, because the invaders come from species-rich environments where heightened aggressiveness is adaptive [4,9,11,12]. Indeed, in central Africa, P. megacephala colonies can compete with territorially dominant arboreal ants such as Crematogaster africana and Oecophylla longinoda in the low canopies of cocoa tree plantations [10,17]. In addition, the success of invasive species is facilitated when they are able to take advantage of a ‘niche opportunity’, or the combination of a so-called ‘escape opportunity’ and a ‘resource opportunity’ [1]. An ‘escape opportunity’ arises when native species do not abound or are not effective in keeping out introduced species, a situation that is generally true for islands, but not for the Mexican study site, which is species-rich, at least in ants, and has one of the highest densities of army ants ever noted [16]. A ‘resource opportunity’ arises when the resources that a species needs are highly available, a situation difficult to apply to the Mexican study site as numerous food sources and nesting sites, if not all, were exploited by native ants before P. megacephala was introduced, as shown by a study conducted in an adjacent non-invaded area [16]. It is therefore probable that the P. megacephala colony first successfully invaded the disturbed habitat of the small harbour of Puerto Morelos, far from competition with native ants or predation by army ants, before spreading into the village of Puerto Morelos, and then over a wider, natural, undisturbed area.

The present results highlight the capacity of P. megacephala to raid other ant colonies in both its native and introduced ranges, a behaviour also noted in Linepithema humile and Solenopsis invicta in their introduced range [4,25]. It is likely that, in addition to its ability to achieve unicoloniality, raiding other ants constitutes a prerequisite for P. megacephala for becoming an ‘ecologically dominant’ species in areas where it is introduced. This is particularly true on islands where organisms (including ants) offer only little resistance to invasive species due to their high taxonomic endemism [1,4,7,26,27]. Furthermore, invasive species not only eliminate native species, but also disassemble their communities where each species occupies and maintains its position after a long co-evolutionary process [28]. Comparing these situations for P. megacephala's in both the native and introduced range has given us new insight into ‘how’ disassembling ant communities can initially occur: in its native range, the ant community is structured in large part due to numerous species that are able to resist raids, a trait that we have now shown is reduced or absent in a number of species in the introduced range.

During successful P. megacephala raids in the native and introduced range, the guards of the attacked colonies seem stunned, even when the P. megacephala workers do not come directly into contact with them. Since the same thing has been noted for non-ant prey, it has been suggested that the workers release a secretion from their mandibular glands [24], but secretions from the pygidial gland could also be involved [29]. The size of the attacked workers does not play a role, as relatively large Poneromorphs workers are inefficient guards when confronted with P. megacephala raiders.

Moreover, the efficacy of P. megacephala scout workers is enhanced by the fact that they recruit nestmates when they discover the landmarks of competing ant colonies, avoiding actually having to come into contact with alien workers, and potentially being attacked and even killed; the same is also true when they perceive termite scents [23,24].

In conclusion, this study suggests that P. megacephala's heightened ability to raid the colonies of most of the ant species it encounters in its introduced range permits it to very rapidly destabilize the native ant communities that constitute a first bulwark of defence against invasive ants.


We are grateful to Barry Bolton and the late Roy R. Snelling for the identification of the ants from Cameroon and Mexico, respectively, and to Andrea Dejean for proofreading early versions of the manuscript. CSM would like to thank Stefan P. Cover and Edward O. Wilson for many helpful discussions regarding Pheidole and the Miller Institute for Basic Research in Science, University of California Berkeley. This research was supported by a project from the French ‘Ministère des Affaires étrangères’ (CORUS program, research agreement No. 02 412 062).


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