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On this page (which is divided into two parts to speed up downloading) I will discuss in turn:
Because C. opuntiae is the primary user of extrafloral nectaries on my field site, they are also the species that suffers most when S. aurea colonies begin invading EFN-bearing cacti in the late summer and early fall.
A typical S. aurea invasion begins when a few subterranean workers make their way to the surface. They usually do this just after sunset and generally near the base of the contested plant. On plants with few C. opuntiae workers, the host colony may simply abandon the EFN-bearing cactus when S. aurea workers begin invading. But, when there are larger numbers of workers, the host colony normally resists the initial invasion. In fact, the first indication I often had of an invasion was the presence of a large number C. opuntiae workers running around beneath the contested plant. Close examination invariably revealed soil-covered S. aurea workers being assaulted by frenzied workers from the resident C. opuntiae colony.
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But almost always this initial advantage in numbers waned as more and more S. aurea workers came to the surface; in over a dozen cases, I only observed possibly two instances where an invasion was repelled. Most of the time the invaders had established trails to the EFN-bearing cactus they were after by daybreak. The following evening normally found S. aurea workers in complete control of the contested plant and visiting its extrafloral nectaries. Typically the only C. opuntiae workers around were a few individuals in the trail leading to the plant. These workers would stop at some distance from the plant and would go no farther. Once, though, a C. opuntiae colony continued the battle for a second night and below is a photograph of two isolated ants struggling in this contest.
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There are probably a couple of reasons for why C. opuntiae workers lose in fights with S. aurea workers. First, while C. opuntiae workers have dull mandibles and lack a functional sting, S. aurea workers are well-equipped in both of these departments. Any advantages C. opuntiae has in the use of chemical warfare (Davidson and McKey 1993) may be offset by the fact that S. aurea also engages in it by flagging poison from their sting gland. They accomplish this by turning their gasters up and shaking them violently. In the process small droplets of poison are turned into a noxious aerosol.
A second reason why C. opuntiae colonies usually fail in their resistance is that while the workers of a single C. opuntiae colony may be dispersed among a number of EFN-bearing cacti in their territory, S. aurea workers generally invade a single plant en force. It is interesting to note in this regard that one of the instances where a C. opuntiae colony repelled an S. aurea invasion involved a chainfruit cholla that regularly had the highest number of C. opuntiae workers per plant on my site. Not only this, but the main nest site of this C. opuntiae colony was very close to the cholla.
The above reasons for why C. opuntiae workers generally lose in fights are also possible reasons for why they generally avoid continued fighting. As if these weren't enough reasons there are two additional ones. First, as shown in the photograph below, S. aurea workers can and will kill large numbers of C. opuntiae workers given the opportunity. On the left side of this photograph is a clump of ants consisting primarily of dead C. opuntiae workers.
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A second reason for avoiding fights is that active C. opuntiae recruitment trails may serve as conduits for further S. aurea invasions. As an example, theC. opuntiae colony mentioned above that continued fighting for a second night ended up losing not only the contested cholla but also two of its nests. Apparently invadingS. aurea workers used incoming C. opuntiea workers or the trails laid down by them as a guide to the nests. The invaders only stayed in the nests for about a week and then abandoned them. Whether they were in search of other EFN-bearing plants, C. opuntiae brood, or just trying to eliminate an overly combative enemy once and for all isn't clear. It is interesting, though, that their forward progress stopped when the C. opuntiae colony broke off contact with them and kept their distance. Perhaps this explains why most C. opuntiae workers don't get very close to plants that have been taken over recently by S. aurea colonies. It may also explain why C. opuntiae workers sometimes remove brood from nearby nests.