Invicta Colonies Research Articles

Group defense by colony-founding queens in the fire ant Solenopsis invicta

Mutualistic associations among nonkin can form when animals in groups have a greater chance of overcoming challenges presented by the environment than do solitary animals. Colony founding by small groups of unrelated queens, a habit documented in several species of ants, is often interpreted as a mutualistic interaction selected by intense competition among incipient colonies. However, many new colonies in these species are founded in areas where their chief enemies are mature ant colonies, rather than other newly founded colonies. In this study, we tested whether group nest-founding in the fire ant Solenopsis invicta improved the ability of queens to survive attacks by mature colonies. In the laboratory, queens in groups of three were more likely than solitary queens to survive attacks by workers of the native fire ant Solenopsis geminata. When newly mated queens were established experimentally in the field, workers from mature S. invicta colonies caused the majority of queen deaths. Queens in groups of two, but not in groups of four, had higher survival rates than did solitary queens during the period between colony establishment and the appearance of the first workers. The advantage of cooperative defense approximately counterbalanced the disadvantages caused by competition within foundress associations of two to three queens. Previous studies have shown that colonies founded by multiple queens produce larger worker populations than colonies founded by solitary queens; however, experimentally increasing worker number in incipient colonies had no effect on colony survival in the field.

Lack of detectable nepotism in multiple-queen colonies of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

Multiple-queen (polygyne) colonies of the introduced fire ant Solenopsis invicta present a paradox for kin selection theory. Egg-laying queens within these societies are, on average, unrelated to one another, and the numbers of queens per colony are high, so that workers appear to raise new sexuals that are no more closely related to them than are random individuals in the population. This paradox could be resolved if workers discriminate between related and unrelated nestmate sexuals in important fitness-related contexts. This study examines the possibility of such nepotism using methods that combine the following features: (1) multiple relevant behavioral assays, (2) colonies with an unmanipulated family structure, (3) multiple genetic markers with no known phenotypic effects, and (4) a statistical technique for distinguishing between nepotism and potentially confounding phenomena. We estimated relatedness between interactants in polygyne S. invicta colonies in two situations, workers tending egg-laying queens and workers feeding maturing winged queens. In neither case did we detect a significant positive value of relatedness that would implicate nepotism. We argue that the non-nepotistic strategies displayed by these ants reflect historical selection pressures experienced by native populations, in which nestmate queens are highly related to one another. The markedly different genetic structure in native populations may favor the operation of stronger higher-level selection that effectively opposes weaker individual-level selection for nepotistic interactions within nests.

Nestmate discrimination and competition in incipient colonies of fire ants

Workers from incipient colonies of the fire ant, Solenopsis invicta, are less aggressive towards conspecific non-nestmates than are workers from reproductively mature colonies. This often leads to brood raiding between incipient colonies, where workers take brood from other incipient colonies without opposition. The development of aggressive reactions by incipient colony workers and queens towards non-nestmate workers and queens was examined. Both workers and queens show nestmate discrimination from the earliest stages of colony activity. Workers from the colony’s first cohort vigorously attacked workers from mature colonies. These same workers rarely attacked non-nestmate workers from incipient colonies, although they were more aggressive towards them than towards nestmates. Workers and queens often vigorously attacked non-nestmate queens, and were more aggressive towards unfamiliar than familiar queens. Queens bitten by workers were likely to be expelled or killed within 3 days of introduction. The highly aggressive response of incipient colony workers towards unfamiliar queens suggests that brood-raiding colonies do not consolidate for mutual benefit. Rather, they struggle for possession of brood while attempting to prevent the entry of additional reproductive females into the colony. In addition, heritable cues were neither necessary nor sufficient for worker acceptance of queens. Finally, although incipient colony workers from colonies with four foundresses were slightly less aggressive towards non-nestmates than workers in colonies with single foundresses, they still discriminated against non-nestmate queens. This result suggests that cooperative nest founding alone cannot explain the development of non-territorial, polygyne S. invicta colonies.

Red Imported Fire Ant Impacts on Northern Bobwhite Populations

The stability of Northern Bobwhite (Colinus virginianus) populations in Texas, where high density polygyne red imported fire ants (Solenopsis invicta) account for >50% of all (S. invicta) colonies, has been cited as a reason to repudiate impacts by this exotic species on Northern Bobwhite. We used two approaches to investigate the relationship between red imported fire ants and Northern Bobwhite. In the first approach, we used correlation analysis to compare Northern Bobwhite abundance trends, determined from Christmas Bird Count data in 15 Texas counties, before and after fire ant infestation. Before red imported fire ant infestation, no significant trend in Bobwhite abundance existed (r = ‐0.355, P = 0.314). After fire ant infestation, Northern Bobwhite abundance declined and was highly negatively correlated with years of infestation (r = ‐0.867, P < 0.001). Bobwhite populations from 16 uninfested counties in Texas revealed no trend over a 27‐yr (1966‐1992) period (r = ‐0.081, P = 0.688). In the second approach, red imported fire ant populations were reduced on five 202‐ha study areas in the Texas Coastal Bend; autumn Northern Bobwhite densities were monitored for 2 yr on those reduced areas and five untreated areas. By the 2nd yr, Bobwhite autum density was higher (P = 0.028) on areas where red imported fire ants were suppressed. We concluded that polygyne red imported fire ants were negatively impacting Northern Bobwhite in this region of Texas.

A colony-growth model for the imported fire ant: potential geographic range of an invading species

We present a colony-growth model for the red imported fire ant ( Solenopsis invicta), compare predicted colony dynamics to field data, and use the model to examine the potential range of S. invicta in the United States. The model incorporates temperature-driven development rates of S. invicta life-stages to simulate the number of workers in a colony. Even though driving factors such as precipitation and competition were not included in the model, it tracks seasonal dynamics reasonably well, though the number of workers in the early stages of colony development is overestimated. The overwintering ability of the simulated S. invicta colony supports the −17.8°C January isotherm as the northern limit for range expansion. Simulations suggest that the ratio of minor to major workers is an important determinant of overwintering ability and S. invicta colony dynamics. The model suggests that other factors, such as alate production, food allocation within the colony, and competition with other ant colonies should be considered in future models.

Sociometry and Sociogenesis of Colonies of the Fire Ant Solenopsis Invicta During One Annual Cycle

In social insects, colonies as well as individuals have evolving life histories. Identification of the life history tactics of a social insect requires data on colony attributes and their development. To this end a full range of fire ant (Solenopsis invicta) colony sizes was sampled and censused on seven dates throughout 1 yr. Data included: mound volume; the number, dry masses, and fat contents of sexual and worker adults and immatures; stratified nest temperatures; worker distribution within the nest throughout the year; duration of the pupal stages; and respiration rates. Analysis showed: 1. Colonies reached their annual maximum population size in midwinter and their maximum biomass in spring. During the spring sexual production period they declined to a midsummer minimum. Calculations showed that the magnitude of this decline increased with colony size. During January to July, worker mortality exceeded natality, causing colony decline, while from July to December, natality predominated, causing growth. 2. Mound volume was closely related to the total mass of ants in the colony, and varied with season paralleling the mass of ants. 3. The mean size and variability of workers, and the percent major workers, increased with colony size and changed over the year. 4. The fat content (percent fat) of workers increased with worker size and colony size. Worker percent fat was lowest in summer after sexual production, climbed immediately to the annual maximum and then declined gradually through winter and spring. 5. Although sexual male and female pupae were close in mean dry mass (2.55 mg and 3.10 mg, respectively), males gained only 6% during adult maturation while females gained 290%. Females gained fat more rapidly than lean tissue causing their percent fat to increase from 31% to 49%. Mean mass of male and female sexual adults did not change with colony size. 6. The cost of worker maintenance declined from nearly 100% of total colony cost in winter to 46% in late spring when brood production peaked. 7. Production rates peaked in spring, with colonies investing 50% of their daily production in sexuals. This peak production was not sustained through the summer, and was probably fueled by stored worker fat. Worker production dominated in the latter part of the summer. All measures of production rate as well as total annual production increased with colony size, but most did so less rapidly than colony size, resulting in a declining efficiency of production and a declining natality rate. 8. The percent of annual production invested in sexuals increased sharply in colonies of between 20 000 and 50 000 workers, then remained at ≈33% for the remainder of colony growth, showing that the transition from the ergonomic to the reproductive stages is sharp, and that colonies must grow in order to produce more sexuals. 9. Many quantitative colony attributes were related to one another by differential growth, and can thus be seen as isometric or allometric measures. Rules of relative growth may thus constrain the possible combinations of attributes and their evolution. The methods of morphometric size and shape analysis are discussed as tools for understanding suites of colony attributes, and comparing them among species. 10. The sociometric/sociogenic method is discussed as a way to compile, analyze and compare data on social insect colony attributes and their growth and development.

Unique isozyme electromorphs in polygynous red imported fire ant populations

Alate females were collected from monogynous and polygynous Solenopsis invicta colonies in four Texas counties. The two forms were sympatric in two of the counties, and were separated by half a mile and two miles respectively in the other two counties. The alate females from each colony were subjected to electrophoresis in horizontal starch gels and stained for hexokinase and aminopeptidase. Both systems revealed electromorphs in the polygynous population which were not present in the monogynous population. A unique slow hexokinase electromorph was present in 83% of the polygynous alates, while a unique slow aminopeptidase electromorph was present in 46% of the polygynous female alates. No evidence of nest queen pheromonal control over production of the unique isozymes was found. Other non-genetic explanations for the presence of electromorphs unique to polygynous populations are addressed.

Comparison of Nestmate Recognition Between Monogyne and Polygyne Populations of Solenopsis invicta (Hymenoptera: Formicidae)

The occurrence of polygyne and monogyne Solenopsis invicta (Buren) populations offers a unique opportunity to test hypotheses related to nestmate recognition. Recognition bioassays were based on the aggressive behavior observed during intraspecific introductions of monogyne and polygyne S. invicta workers (intruders) into polygyne and monogyne S. invicta colonies (residents) and interspecific introductions into Solenopsis richteri Forel colonies. No aggression differences were observed between monogyne and polygyne colony workers that were removed from their colonies, then reintroduced into their parent colony (no aggressive behavior) or when S. richteri workers were introduced into monogyne or polygyne colonies (strong aggressive behavior). In all other cases, polygyne workers displayed significantly less aggressive behavior than monogyne workers, as residents and as intruders. Therefore, at the species level, polygyne workers are less able to distinguish nestmates from nonnestmates than monogyne workers. This result fits a theoretical prediction of diminished intraspecific aggression in polygyne populations. These lowered aggression levels are attributed to the polygyne population's exposure to a greater variety of environmental and endogenous stimuli. Although in monogyne S. invicta , the queen is not a major contributor to nestmate recognition cues, the role of queens in polygyne situations remains to be determined.

Studies of the territorial dynamics of the red imported fire ant ( Solenopsis invicta Buren, Hymenoptera: Formicidae)

The changes in foraging areas of two adjacent imported fire ant, Solenopsis invicta Buren, colonies were recorded for 20 consecutrive days. Through food, one colony was labeled with 65Zn and the other with 54Mn. Foraging ants were collected frm bait stations arranged in a formal grid matrix. Nest reloation and the close proximity of other S. invicta colonies were observed to be associated with territorial patterns that provided a unique perspective for the study of ant ethology. One radiolabeled colony was shown to maintain a foraging area that traversed a standing water barrier.

The Straw Itch Mite, Pyemotes tritici (Acari: Pyemotidae), as a Biological Control Agent of Red Imported Fire Ants, Solenopsis invicta (Hymenoptera: Formicidae)

The straw itch mite, Pyemotes tritici (Lagreze-Fossat and Montane), is a parasitic pyemotid mite that has generated considerable interest as a biological control agent of the red imported fire ant, Solenopsis invicta Buren. To evaluate the efficacy of this mite against S. invicta under the conditions of central Texas, a large-scale field trial was conducted during 1985. A total of 244 S. invicta colonies was mapped and colony size and activity ratings recorded. During May, two applications of mites cultured on Angoumois grain moth-infested wheat kernels were introduced into 103 colonies, and 141 control colonies were treated with Angoumois grain moth-infested wheat kernels not infested with mites. The colonies were also twice treated in October. Throughout the test period, colonies were periodically inspected and rated. Analysis of the data using t-tests did not reveal significant differences in ratings between mite-treated and control colonies. Therefore, the straw itch mite, P. tritici, was not found to be an effective biological control agent of the red imported fire ant.

A Survey for Pathogens of Fire Ants, Solenopsis spp., in the Southeastern United States

In a survey conducted in the Southeastern United States, one colony in a sample of 1,007 colonies of the red imported fire ant, Solenopsis invicta Buren, was infected with a microsporidium (Protozoa: Microsporida). The normal host of this parasite appears to be the tropical fire ant, Solenopsis geminata (F.). A benign or very mildly pathogenic yeast was associated with 93 (9.24%) of the S. Invicta colonies, and was most common in areas which have been infested with this ant for the longest periods. No pathogens were associated with 83 colonies of the black imported fire ant, Solenopsis richteri Forel. The apparent rarity of bona fide pathogens in imported fire ants in the United States is in marked contrast to the abundance of pathogens in these and other Solenopsis spp. In South America. Four species of microsporidia (possibly new genera) were detected in 22 (7.2%), 12 (3.9%), 6 (2.0%), and 4 (1.3%) of 307 colonies of the tropical fire ant, S. Geminata. One colony of this species was infected by a neogregarine (Sporozoa: Neogregarinida). No pathogens were found in a small sample (53 colonies) of the Southern fire ant, Solenopsis xyloni McCook.