The red imported fire ant, Solenopsis invicta, is a relatively
new and particularly malevolent component of the entomofauna of North America.
It arrived in Mobile, AL, in 1943, with a load of dirt used for ballast
in a ship from Brazil. It has been spreading ever since, and arrived in
Grayson County, TX, within the last 15 years. Fire ants, as many of us
know, have a painful sting, and their colonies usually have large numbers
of workers (on average about 220,000!) with which to attack intruders.
They are not particular about the type of soil in which they build their
nests, but appear to avoid moist areas. The presence of fire ants in an
area appears to have drastic and detrimental effects on the native biota.
Fire ant workers are polymorphic, meaning that there are different sizes of workers. However, the polymorphism is a simple type, in that it is isometric and monophasic. Isometric means that body parts change size at the same rate as body length changes (in other words, big workers look pretty much like small workers). The alternative to isometry is allometry, in which some parts (i.e. head width, mandible size) get bigger faster (or slower) than the body does. Monophasic polymorphism means that there are not discrete size categories but that worker size varies in a continuous fashion. In fire ants, there are minor workers and major workers (the minors are about 1 or 2 mm in length, and the majors are about 5 or 6 mm), but there are all sizes of media workers between these extremes. Minors are the most prevalent, and mostly do brood care and nest construction, while media and major workers do foraging and defense, and serve as repletes. Worker longevity depends on body size and on temperature. Majors live about 65 weeks at 17o; minors live about 10 weeks at 30o.
Fire ant colonies reproduce during the summer and fall, although the timing and stimulatory cues are not well known. Colony reproduction (as opposed to production of workers by established queens) occurs through the production of alates, or new queens and drones. Theory predicts that a particular colony should produce alates in either a 1:1 ratio of queens and drones (if the established queens control reproductive output) or in a 3:1 ratio of queens and drones (if the workers control reproductive output). We will discuss these hypotheses in more detail in lecture. Details of the reproductive behavior of fire ants are sketchy. Alates of most ant species aggregate at landmarks in huge swarms, where the males and females copulate, after which the males die and the females disperse to try to establish new colonies. It appears that fire ant males form swarms at an altitude of 250 m or more, and that females fly up into the swarm, where pairs form. Following copulation, females alight and attempt to found new colonies. Each mature colony may produce thousands of new queens each year. Most alate queens fall victim to a plethora of predators; less than 1% successfully dig a new nest, and not all of those successfully rear a first brood of workers.
The literature indicates that polygyny (multiple queens within a colony) is common in fire ants in Georgia and north Texas, but that monogyny (each colony has a single queen) is more common in other areas. Some fire ant colonies appear to be founded by multiple queens (pleiometrosis). The genetic relationship between these multiple foundresses is poorly known. Allozyme studies indicate that the degree of genetic relatedness among workers is high in monogynous colonies (approximately 0.75, as predicted by theory), but is low in polygynous colonies (0.08, a considerable contrast to theory). The effect of these patterns of relatedness on social structure and reproductive behavior is not known.
Following rearing of the first brood (in a pleiometrotic colony), the
workers may execute all but one of the primary queens, which leads to monogynous
colonies. Other colonies may be haplometrotic (founded by single queens).
Secondary polygyny in either case may be established by adoption of newly
mated queens or by colony fusion. The timing of events following the establishment
of a new colony is poorly known, but colonies of 4-6 years of age are mature,
with large numbers of workers. Individual queens live about six years,
but colonies can live longer than this through replacement of senescent
queens.
The purpose of this laboratory is to try to determine the sex
ratio of alates or reproductive output within fire ant colonies. Our technique
is a simple one called "destructive sampling". Our sampling will be done
at the Sneed Environmental Research Area, where there are many fire ant
colonies. We will dig into several colonies, collecting all alates that
we find. These will be preserved in 70% ethanol for later processing in
the lab. Simple as this sounds, there are a number of difficulties with
this procedure. One is that we are disturbing a potentially harmful organism,
one that will try to defend itself. If you know that you are particularly
allergic to hymenopteran stings, please observe from a safe distance. We
will all take some precautions to try to avoid stings. You must wear long
pants that can be tucked into your socks. Heavy shoes or boots are mandatory;
sandals are definitely a no-no. We will be good allogrooming primates are
pick ants off one another that try to climb into our clothing. Insect repellent
is an appropriate precaution and I will provide some.
Our procedures and equipment will also be fairly simple. We will work in groups of four or five. Each group will have two or three shovels of various types as well as small hand trowels (you may want some work gloves for digging). The long-handled spade is best for actually digging out the colony. The sharp-shooter is good for digging into particularly ant-rich veins and for making the hole deeper. The trowels will be used to separate large chunks of soil and to scoop clumps of alates out of the hole. We will use some flat pieces of cardboard to make our dirt piles on, then fill in the holes when we are done. I have found that actually grabbing the ants with your fingers works best for collecting them. The queens and alates don’t sting fast enough to be dangerous. I will provide latex gloves for protection, and forceps for grabbing ants, for those who want them. Specimens from each colony should go into separate vials, which should be filled with preservative, and should be labeled on the outside with the date, some identifying number or other symbol, and the initials of the collectors. Another label should be written in pencil on bond paper, and should be placed directly into the vial with the specimens. I will provide vials, preservative, and labeling material.
Because it is often wise to collect any information in the field that might be important in subsequent analyses, we should measure the approximate diameter and height of each mound that we sample. Fire ants have a characteristic mound shape, which is more or less conical. The mound is constructed by the workers, and serves functions such as colony climate control and thermoregulation of the brood and of the alates. We will not attempt to quantify worker number or reproductive condition of the colonies (i.e. numbers of eggs and larvae), more because of time and energy constraints than because these phenomena are uninteresting. If any of you think of parameters that we should measure, please say so.
During a subsequent lab period we will process our catch and collect some quantitative data. The data that we will collect includes number and mean mass of queens and drones for each colony. We can then assess whether there is much or little variation among colonies in reproductive output. These data will be analyzed using either a t-test or analysis of variance, which is described on a separate handout.
If any student is interested in doing a project with fire ant queens,
workers, or larvae, we can collect some and not preserve them, but bring
them back alive for study in the lab. There are a variety of worthwhile
questions that could be addressed about fire ants, and anyone who is interested
should make arrangements for caring for ants in the lab before we go out.
Please see me about this.