The tiny little rover ants, Brachymyrmex patagonicus, made the news a few weeks ago in an article in the Arizona Daily Star. Actually, it was about Javier Miguelena’s doctoral research on rover ants. During his research, Miguelena found that rover ants need added moisture to do well in the deserts of Arizona. By adding water to the landscape, people are encouraging survival of rover ants.
When my son found a colony of rover ants in a potted plant we had just purchased at a plant sale this weekend, we decided to do some research ourselves. We separated the rover ants from the soil. There were 312 rover ants and 9 cocoons. We never found a queen, but we didn’t remove all the soil from the plant’s roots either.
Although I set an ambitious goal of Chapters 13-16, as you could probably gather, it took more time then I expected to digest the information in Chapters 13-15. Let’s take a look at those chapters and save Chapter 16 for next week.
(For those of you jumping in late, we are discussing The Fire Ants by Dr. Walter Tschinkel by going over a few chapters per week. Click “The Fire Ants Book Discussion” category for related posts.)
Chapter 13. Another way for a queen to take over a colony
In addition to the drama of incipient colony foundation by independent queens discussed in the last post, Solenopsis invicta colonies have yet another way to gain reproductive success. Mature colonies produce a second set of reproductives in the fall, which overwinter in the nest. The overwintered females fly in early spring, under much different conditions than reproductives reared in the spring. The overwintered females try to take over established colonies that have lost their own queen, or have become orphaned. If they are not successful, the overwintered females are incapable of starting a colony of their own and simply perish. Tschinkel calls this type of behavior “dependent colony founding,” apparently because the queen depends on the presence of orphaned mature colonies to be successful (?).
The overwintering females may also take over their own natal nest if their mother dies. In this case they do not mate, and produce only males. In orphaned colonies with overwintering females, 86% had unmated sisters for their queen, whereas 14% had queens that had stolen in from other colonies. When no overwintering females were present in the orphaned colonies, 62% had unrelated queens and 38% percent had sisters (top of page 189). I did wonder where those sisters came from if there were no overwintering females. Any thoughts?
Tschinkel reaches the conclusion that Solenopsis invicta is a “weedy species” because independently founding queens are much more successful in sites lacking competing mature colonies. Only 3% of colonies showed evidence of dependent founding. In contrast, Solenopsis geminata had a much higher success rate of dependent queen founding, with up to a third of colonies showing evidence of that type of founding.
What do you think of the technique of using match-mated females to determine the success of dependent founding queens?
Chapter 14. Colony Growth
How do you keep track of how an ant colony grows? In the case of Solenopsis invicta, you can start with even-aged cohorts by opening a new area to colonization. To get a count of number of workers, Markin drove a large cylinder into the soil around a fire ant colony and then slowly flooded out the workers. Tschinkel developed a method for sampling soil and ants from a colony during excavation. It is tedious and difficult work, which explains why so few of these types of studies have been undertaken.
The results suggest a colony on rough average reaches maturity around four years (and lives up to eight years), with around 200,000 workers.
As with individual organisms, temperature and the amount/quality of food available determined how fast a colony grew in laboratory conditions.
Did you find any points particularly interesting for this chapter? Anything I skipped over that you would like to discuss?
Chapter 15. Another aspect of colony growth: What types of ants are produced and when?
When a colony grows, it changes in composition as well as in worker number. The incipient colony starts out with tiny minims. Over time, workers of a range of sizes are added. Eventually the colony begins to produce reproductives.
Fire ants, in this case S. xyloni, have a range of worker sizes.
How much energy does a colony allocate to each type of ant? Tschinkel estimates, based on worker mortality, that a average fire ant queen produces 2.74 million workers during her lifetime. The colony’s reproductive success depends on optimally allocating resources between workers and reproductives. Too many workers, and the colony loses opportunities to reproduce. If there are too many alates, there won’t be enough workers to feed them all.
As a colony grows and becomes mature, a higher percentage of the workers are the large or major workers. He only briefly mentions juvenile hormone as a possible mechanism in the switch to larger workers, as well as level of food.
Do you know of any recent research that sheds more light on this topic?
Tschinkel also disscusses how the nest grows with colony size and how the size of the colony’s territory changes as well. He ends with a summary of how the colony ends, which he describes as resulting from the queen likely running out of sperm and thus being no longer able to produce replacement workers or female alates.
Do you have any comments on Chapter 15?
Interlude: The Porter Wedge Micrometer
A device for measuring 200 head widths of ants in one hour? Sounds like a dream come true. What do you think of Tschinkel’s idea that it has been slow to catch on due to “cultural viscosity?” I admit I did it the old-fashioned way, but I pinned all my specimens first. Now that was tedious!
Have you tried a wedge micrometer? How do you measure your ant head widths?
Let’s try to cover Chapters 16 and 17 this week, with the Interludes (up to page 271). Is anyone further along than that? Let us know, and I’ll try to move ahead more quickly.
Are you ready for ant season to begin? I sure am! We’ve been having some unusually cold and rainy weather, so the ants haven’t been very active. One warmer afternoon last week, however, I did spot a few ants other than rover ants. They were Solenopsis amblychila workers. Solenopsis amblychila workers are pale, golden yellow. Solenopsis amblychila can tolerate dry conditions. Colonies of this species are found mostly in the Sonoran Desert, that is southern Arizona, southern California, northern Mexico and Baja California, although they do extend further east into Texas as well.
These workers are nesting at one of our local parks, along a sidewalk. The surrounding area is compacted, dry Bermuda grass trampled by thousands of feet. The attraction may be a Solenopsis xyloni colony about a foot away, or may be a nearby ramada full of messy, snacking children.
I found a queen last year, but it met with an accident (also child-related as it turns out) and never had a chance to produce eggs.
Do you agree that 10-12 are the best chapters yet?
(For those of you jumping in late, we are discussing The Fire Ants by Dr. Walter Tschinkel by going over a few chapters per week. Click “The Fire Ants Book Discussion” category for related posts.)
Chapter 10 covers mating swarms and colony foundation by new queens.
In many ways Solenopsis invicta follows the classic ant colony foundation script, at least at first. Males and unmated queens are produced in spring and hang around in the nest until conditions are right. Mating flights are triggered the day after there is a locally heavy rain. The workers start to mill around while the males and unmated queens fly from the nest. After mating, the queens fly some distance (from monogyne nests) and then land at a suitable location to start a new colony. For the males, it is a dead end trip. The mating swarms occur most frequently in May and June, but can take place in any month if conditions are right. Not all the reproductives leave in any one swarming event.
A fire ant queen prefers to land on a roadside or recently disturbed land, although she sometimes ends up in a parking lot (see Interlude below) or swimming pool. If the local conditions are suitable upon landing, the queen immediately removes her wings. She searches for a place to begin digging and prepares a hole by removing soil with her mandibles. She creates a chamber and crawls inside for a typical claustral, or closed inside a chamber, founding.
Here in Arizona, both ants and termites are sometimes induced to swarm by sprinkler irrigation. Tschinkel indicates that fire ants can also be induced to swarm artificially by applying water.
Once the newly mated queen loses her wings, the proteins from the wing muscles begin to deteriorate and, with the storage proteins and fats already residing in her body, are used to fuel the development of eggs. She lays 20 to 100 eggs in the first week, some which are trophic eggs that serve as food for the larvae when they hatch. The queen cares for the eggs, larvae and pupae until the first workers emerge. The first workers are always extra small workers called “minims.”
Now come the interesting part, where fire ants begin to diverge from the traditional ant colony founding script.
Upon landing, some of the queens form founding associations with other queens. Tschinkel was able to show that this behavior was density dependent, meaning the more queens in a given area, the more queens founding in groups. He was also able to show that a pre-formed hole of the proper depth and dimensions was highly attractive to fire ant queens. (Cool!)
Queens that found in groups tend to produce fewer minims per individual, but also weigh more at the end of the claustral period. Weighing more has an advantage during the next stage.
Interlude: What do you think about the essay “Sharon’s House of Beauty?” Why do you think the fire ants are attracted to such a site?
Chapter 12. Brood Raiding
Now the script becomes more like one for “Desperate Housewives.” Once the minims have emerged and the many incipient nests are opened to the world, the tiny colonies begin a process that results in only one nest with one queen.
As soon as the minim workers are active, the queens of nests with multiple queens start to fight. Over time one queen wins by dominating the brood pile. Often she’s the heaviest queen. Others are pushed away, where they are more likely to be attacked and killed by workers. The result is a nest with one queen.
At the same time, the minim workers may wander from incipient nest to incipient nest. The minims do not fight with minims of other colonies, as would be the case for larger workers from more established colonies. Eventually workers from colonies with a larger number of minims act like very rude guests, pick up brood from other nests they visit and bring it back to their own. The minims from the raided nest may go retrieve their sisters and take them back to their initial nest. The workers from the competing nests may go back and forth for a time. but eventually one colony wins and all the minims and brood end up in the winning nest. Then the raiding begins in another nest, until all in a given area are combined into one. Tschinkel recalls one raiding series where 80 incipient colonies consolidated into just two.
As Tschinkel points out, the behavior of minims of abandoning their own mother to join another unrelated queen seems to fly in the face of evolutionary theory. His suggestion is that because the abandoned queen tends to move to the winning colony where, if she is able, she joins the colony and may eventually take over, the minims have a small chance of having their mother be the winning queen even if they leave their natal nest.
What do you think of this period in the colony founding process? What about the idea that the minims are moving because of the small chance their mother might win? Wouldn’t it make sense that the minims would stay with their sister brood rather than their mother because they are more closely related to their sisters than their mother?
