Page 2 of A Natural History of Extrafloral Nectar Collecting Ants in the Sonoran Desert: Extrafloral nectar-collecting ant species: Solenopsis aurea

Pollen theft by S. aurea workers and their possible role as plant defenders

S. aurea collected not only nectar but also pollen at my field site. While workers from other ant species occasionally visited the flowers of EFN-bearing cacti on my field site, S. aurea workers were the only ones that regularly visited flowers and collected significant amounts of pollen from them. To collect this pollen S. aurea workers aren't particularly subtle. Using their sharp mandibles they cut the stamen filament and then carry away the anther. The effects of pollen theft are fairly inconspicuous on large, stamen-rich barrel cacti flowers. But this isn't the case when anthers are taken from chainfruit cholla flowers. As the photographs below show, S. aurea workers sometimes remove all of the anthers from these flowers. Typically they do this shortly after a flower opens.

 

		Click on either image to enlarge both (you will need to scroll right on the enlarged pictures to see flowers with anthers removed)
A chainfruit cholla flower in the process of having its anthers removed	Chainfruit cholla flowers with all their anthers removed

Just because barrel cactus pollen theft isn't conspicuous doesn't mean that it doesn't have fitness consequences for the plant. In fact, it may cost these plants far more than near total pollen removal does for chainfruit cholla. The reason for this is that while barrel cacti reproduce sexually chainfruit rarely if ever does (Johnson, 1918). Instead, chainfruit cholla reproduce vegetatively with branches and fruits serving as vegetative propagules. Unless successful cross-pollination is somehow required in order to produce fruits (fruits may or may not have viable seeds), the cost to chainfruit cholla of having its pollen stolen appears slight in comparison to the cost of having produced it in the first place.

But given that there may be costs to attracting S. aurea workers what are the possible benefits? To their credit, S. aurea workers are quite aggressive when disturbed. A pen raked across the crown of a barrel cactus or the top of chainfruit cholla would cause workers, especially smaller ones, to run up and down spines while releasing aerosolized poison from their sting gland ( a behavior known as flagging). Additional workers would even climb out onto the pen and attack it. Such behaviors suggest that these ants might be able to ward off vertebrate enemies of cacti. Also, I observed one instance where these ants drove off a large katydid.

Other factors are also probably important in evaluating what if any protection these ants may provide. First, their presence on cacti at certain times of the year and not others places limits on the potential protection they could provide. For instance, patterns of insect herbivory probably change throughout the year as does the vulnerability of these insects to ants as a function of their life stages. Second, because of the way S. aurea workers forage, their usefulness in plant defense may in part be a function of the hosting plant's size and architecture.

Scavenging by S. aurea workers

Snelling and George (1979) write of this ant: "Although dietary preferences are unknown this species is probably typical of the fire ants, a generally omnivorous scavenger-predator with preferences for oily or fatty meats and seeds." Since S. aurea spends a significant portion of its colony life cycle below ground it is difficult to determine with any certainly its entire dietary breadth. Nevertheless, in a rather extensive study of the ant community in and around Portal, Arizona, Andersen (1997) was unable to attract any of these ants to his tuna baits (he did capture a few in pitfall traps). Likewise, except for a single instance, I never observed S. aurea workers foraging for or feeding on anything other than nectar or pollen. Below is a picture of the one exception. I found the ground squirrel in this picture at the base of a chainfruit cholla (the base of the plant is in the upper left-hand corner) that was being used by a S. aurea colony. Workers from this colony already covered the animal when I arrived and these workers, especially the majors, seemed very proficient at carving it up. Given this and the other observations above, it suggests that these ants will scavenge but that they do not do it far from their nests -- at least on the desert surface.

S. aurea colony structure

In all of the above I have talked about S. aurea colonies as if they were readily identifiable entities; unfortunately they weren't. Most often an EFN-bearing cactus received workers from a single nest which was usually barely discernible and at the base of the plant or short distance from it. While I have not analyzed the data I have on this point, my subjective impression is that adjacent EFN-bearing cacti were often taken over at about the same time. This suggests that single colonies frequently visit more than one plant but multiple colonies responding to similar micoenvironmental cues could also explain this observation. Unfortunately, the easy although admittedly crude test of putting ants from different plants together and seeing how they behaved toward one another didn't work; even ants from very distant plants didn't attack each other. This obviously does not rule out the possibility of developing a more detailed behavioral assay. I have also collected individuals with the hopes of one day using DNA microsatellites to get at this question.

Literature Cited

Andersen, A.N. 1997. Functional groups and patterns of organization of North American ant communities: a comparison with Australia. Journal of Biogeography 24: 433-460.

Cokendolpher, J.C. and O.F. Francke. 1985. Temperature Preferences of four species of fire ants (Hymenoptera: Formicidae: Solenopsis). Psyche 92: 91-101.

Creighton, W.S. 1950. The ants of North America. Bulletin, Museum of Comparative Zoology. Harvard College, 104: 1-585. 57 pl.

Francke, O.F., L.R. Potts, and J.C. Cokendolpher. 1985. Heat tolerance of four species of fire ants (Hymenoptera: Formicidae: Solenopsis). Southwestern Naturalist 30: 59-68.

Heatwole, H. and R. Muir. 1989. Seasonal and daily activity of ants in the pre-Saharan steppe of Tunisia. Journal of Arid Environments 16: 49-67.

Jensen, T. F. 1978. An energy budget for a field population of Formica pratensis. Natura Jutlandica 20: 203-236.

Rissing, S.W. 1987. Annual cycles in worker size of the seed-harvester ant Veromessor pergandei. Behavioral Ecology and Sociobiology 20: 117-124.

Snelling, R.R. and C. George. 1979. The taxonomy, distribution, and ecology of California desert ants. Report to Bureau of Land Management, United States Department of Interior, Riverside California. 335 p.

Tschinkel, W. R. Sociometry and sociogenesis of colonies of the fire ant Solenopsis invicta during one annual cycle. 1993. Ecological Monographs 63: 425-457.

Copyright (c) 1998 Barry Sullender
Rice University
All rights reserved