New records of P. megacephala in St. John's, US Virgin Islands

Pheidole megacephala has been reported from the US Virgin islands since the late 1800s. It shares the island group with various other exotic and invasive species, such as Solenopsis geminata, Solenopsis invicta, Wasmannia auropunctata, and Paratrechina longicornis.

In St. John's, Previous records of the species include:

Location: 18.324,-64.788
Location description: Great Cruz Bay, Hotel grounds
Collection date: 2005-11-13
Collector: Wetterer, J.

Location: 18.331, -64.795
Location description: Cruz Bay, downtown
Collection date: 2005-11-14
Collector: Wetterer, J.

Location: 18.348, -64.726
Location description: Carolina, 1 km up 20
Collection date: 2005-11-10
Collector: Wetterer, J.

Location: 18.352, -64.760
Location description: St John; Cinnamon Bay; moist beach
Collector: Pressick and Herbst 1973

Location: 18.317, -64.718
Location description: St John; Lamesaur; Croton-Acacia Scrub
Collector: Pressick and Herbst 1973

To the records above, the following records are being added:

Location: 18.35261, -64.76818
Location description: Trunk Bay Covered Picnic Area
Collection date: 2017-2

The ants formed trails along the sides of the covered cemented picnic area, as well as nearby areas, where their friable nest mounds are very easily noticed thrusting out from the ground.

Location: 18.32757, -64.79318
Location description: Car Ferry Parking Lot
Collection date: 2017-2

The ants formed trails along the sides of the parking lot leading up to the car ferry.

I surveyed parts of Cinnamon Bay, including the the beach itself and the grounds making up the Cinnamon Bay resort and Campground, and did not find P. megacephala in this area, although I did find another larger Pheidole species.

Author: A.R. Sunjian

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.

Butterfly effects: Acacias, Elephants, and the Ants that guide their relationship

Acacia drepanolobium and Crematogaster nigriceps.
Image courtesy of Pharaoh han

It is sometimes difficult for people to imagine how the activities of minute creatures like ants can have major effects at the meter scale world, but a paper I read recently demonstrates such a process clearly.

In Kenya there is a tree called the whistling thorn acacia (Vachellia drepanolobium – formerly Acacia drepanolobium), an iconic shrub of the East African savanna that grows to 6 meters tall and is covered with intimidating thorns, some of which have bulbous bases and are called domatia.

The bulbous bases are hollow, and house symbiotic ants that protect the tree from browsing by herbivores. The tree also provides the resident ants nutrition in the form of sugary secretions from glands at the base of their leaves.

The tree houses one of four different species of ants: Crematogaster sjostedti,  C. mimosae, C. nigriceps, and Tetraponera penzigi, all of which swarm out to attack browsing animals with varying degrees of effectiveness, though the overall efficacy of the ants as guardians is quite high. As shown in the youtube video above, acacias with no ants tend to do very badly when large herbivores are present.

This relationship between the acacias, herbivores like elephants, and the ants that guard the acacias has recently been disrupted by the appearance of Pheidole megacephala (BHA) in the Laikipia region of Kenya within the last 10-15 years. In a research paper in the journal Ecology, Corinna Riginos and her colleagues moved complete trees along with their resident ants from uninfested locations to areas infested with BHA.

Almost immediately, BHA discovered the new trees and started moving up the trunk, only to be met in force by the aggressive Crematogaster species, who streamed down to meet and repel the invaders. In all such cases, however, the resident ants were fairly quickly pushed up the trunk and soon vanquished, their nests looted and any remaining adult defenders killed if they could not escape. The only exception was in trees with the Tetraponera ants, who instead of meeting the invaders head on, instead retreated into their domatia where the BHA could not get to them.

Acacia seedpods. Image courtesy of Chr. Kooyman

The researchers found that even 30 days after the invasion, with BHA patrolling throughout the entire tree, the Tetraponera ants managed to eke out an existence. Foragers who came out lay still and did not fight BHA ants who came to them, and the BHA ants did not seem to consider them to be hostile. In fact, the density of Tetraponera colonies significantly increased in invaded areas, due to the removal of their Crematogaster competitors.

Unfortunately for the acacia trees, the Tetraponera is the least effective defender against encroaching herbivores, and BHA itself does not attack large mammals who decide to browse on the plant. This resulted in significant damage to the host acacias, with the researchers finding up to a seven fold increase in the number of trees catastrophically damaged by elephants in invaded areas versus those trees in uninvaded locations. This damage to the trees due to the disruption of the mutualism between plants and ants has the potential to cause significant changes in the dynamics of the savanna ecosystems where this acacia is dominant.

The relevant paper is:

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

The abstract of the paper:

Invasive species can indirectly affect ecosystem processes via the disruption of mutualisms. The mutualism between the whistling thorn acacia (Acacia drepanolobium) and four species of symbiotic ants is an ecologically important one; ants strongly defend trees against elephants, which can otherwise have dramatic impacts on tree cover. In Laikipia, Kenya, the invasive big headed ant (Pheidole megacephala) has established itself at numerous locations within the last 10-15 years. In invaded areas on five properties, we found that three species of symbiotic Crematogaster ants were virtually extirpated, whereas Tetraponera penzigi co-occurred with P. megacephala. Tetraponera penzigi appears to persist because of its non-aggressive behavior; in a whole-tree translocation experiment, Crematogaster defended host trees against P. megacephala but were extirpated from trees within hours. In contrast, T. penzigi retreated into domatia and withstood invading ants for >30 days. In the field, the loss of defensive Crematogaster ants in invaded areas led to a five- to seven-fold increase in the number of trees catastrophically damaged by elephants compared to un-invaded areas. In savannas, tree cover drives many ecosystem processes and provides essential forage for many large mammal species; thus, the invasion of big-headed ants may strongly alter the dynamics and diversity of East Africa’s whistling thorn savannas by disrupting this system’s keystone acacia-ant mutualism.

My first encounter

Photograph by R.H. Scheffrahn, University of Florida

I had known about this ant of course.

Ever since I was perhaps sixteen I had read about its status as an exotic species, one which had profoundly negative effects on native populations of animals and plants.

And yet I did not get to see them first hand until the late 1980s, when I visited the island of Kauai in Hawaii. I wrote this in a journal during my stay there:

This Pheidole species dominates the local ant fauna and seems to thrive almost anywhere: from the sandy beaches near our resort, to the rugged, stony sides of the island's mountains.

The major workers are extremely pugnacious, and the minors only a bit less so. The majors do not feed at mayonnaise baits, but rather move randomly around them, occasionally rushing threateningly at passing minor workers.

The introduction of a formicine ant into a congregation of minor workers at a bait elicited an immediate response. The intruding ant was quickly immobilized by the minor workers, who bit and held onto its legs and antennae. Some two minutes later, a relatively large number of major workers rushed into the scene and started moving all around the place.

This Pheidole is one of the most aggressive extirpators that I have seen. However, due to their relatively small size, they are sometimes not very efficient at acquiring moving "baits".

For example, the black formicines were very quickly and efficiently dragging a large dead wasp to their nest. I took the wasp by the wings and dropped it near some P. megacephala ants, and the formicines immediately began dragging the wasp away again, even before the Pheidole could react. I did this a couple of times more, and at last some minors managed to grab good holds on the wasp legs and prevented the formicines from dragging it away.

The minors were incredibly aggressive, given the fact that they were only a third the size of the very much faster formicine. They grabbed onto the appendages of the black ants whenever the opportunity arose, and let go only when squirted with formic acid. And since the P. megacephala ants are very efficient mass recruiters, the flood of workers that soon followed effectively drove the formicines away.

In order to do justice to the black ants, I placed gobs of mayonnaise near their home nest, at the base of a large pot, about 1 meter away from the scene of the first tussle. The bait was soon covered with black ants.

Just before I left, I noticed a single P. megacephala scout near the black ants' home, and when I came back 20 minutes later, the Pheidole ants had claimed all the mayonnaise gobs near the black ants' nest. Major workers were moving all around the baits as well.