Walter R. Tschinkel

Fire ant polymorphism: the ergonomics of brood production

SummarySocial organization is generally assumed to increase colony efficiency and survival; however, little quantitative information is available to support this assumption. Polymorphism is an important aspect of labor division in colonies of the fire ant, Solenopsis invicta. Our objective was to investigate the effect of fire ant polymorphism on brood production efficiency. We set up standardized polymorphic colonies with a full range of worker sizes and artificial monomorphic colonies that contained only small, medium or large workers respectively. Polymorphic colonies produced brood at about the same rate as colonies composed of only small workers (Fig. 2A). Colonies composed of only medium workers produced about 30% less brood, and colonies composed of only large workers produced little or no brood at all. This pattern was independent of colony size; however, smaller colonies (0.75 g, live weight) produced almost twice as much brood per gram of workers as larger colonies (3.0g). Additional experiments revealed that the size of workers in the artificial monomorphic colonies affected all stages of brood rearing. Large workers not only inhibited the development of early and late instar larvae (Fig 4), but also reduced the queens oviposition rate (Fig. 3). Brood production efficiency on an energetic basis was determined by dividing the grams of brood produced per unit time by the energetic costs expended for the maintenance and production of each worker size class. Worker maintenance costs were estimated from respiration while production costs were determined from the caloric content of worker tissue divided by their average longevity. Worker respiration per milligram body weight decreased about 40% as body size increased (Fig. 5). Large workers lived about 50% longer than small workers (Fig. 6) and contained 9% more energy per milligram of tissue (Fig. 7). Energetic efficiency in polymorphic colonies was approximately 10% higher than in colonies composed of only small workers (Fig. 9). In other words, when food supplies are limiting, polymorphism may offer a slight advantage in brood production.

Colony growth and the ontogeny of worker polymorphism in the fire ant, Solenopsis invicta

SummaryColony size and worker polymorphism (headwidth) were determined for fire ant colonies ranging from incipient to 12 years of age. Colonies grew approximately logistically, reaching half size between 21/2 and 31/2 yr and reaching their maximum size of about 220000 workers after 4 to 6 yr. Colony size showed strong seasonal variation. There was some evidence that growth rate may vary with food density. Incipient colonies are monomorphic and consist of small workers only, but as colonies grow, production of larger workers causes the size-frequency distributions to become strongly skewed. These skewed distributions were shown to consist of two slightly overlapping normal distributions, a narrow one defined as the minor workers, and a much broader one defined as the major workers. Major workers differ from minor workers in having been subjected to a discrete, additional stimulation of body growth, resulting in a second normal subpopulation. The category of “media” is seen to be developmentally undefined. The mean headwidth of the workers in both of these subpopulations increased during the first 6 mo. of colony life, until colonies averaged about 4000 workers. Headwidth of minors declined somewhat in colonies older than about 5 yr, but that of majors remained stable. When the first majors appear, their weight averages about twice that of minors. This increases to about 4 times at 6 mo. and remains stable thereafter. The range of weights of majors is up to 20 times that of minors. Growth of the subpopulation of major workers is also logistic, but more rapid than the colony as a whole, causing the proportion of major workers to increase with colony size. In full sized colonies, about 35% of the workers are majors. Total biomass investment in majors increases as long as colonies grow, beginning at about 10% at 2 months and reaching about 70% in mature colonies. This suggests that major workers play an important role in colony success. The total dry biomass of workers peaked at about 106 g, that of majors at about 72 g. These values then fluctuate seasonally in parallel to number of workers. When colony growth ceases, the proportion of majors remains approximately stable. Colony size explained 98% of the variation in the number of major workers.

Colony founding by pleometrosis in the fire ant, Solenopsis invicta

SummaryNewly mated queens of the fire ant, Solenopsis invicta, found colonies either alone (haplometrosis) or by joining with other newly mated queens (pleometrosis). Surveys after mating flights showed that nests and queens were usually aggregated in space, that queens were aggregated among occupied nest chambers, and that the occurrence and degree of pleometrosis was related to the mean queen density. Queens and nests were strongly associated with slightly higher ground, away from rainwash areas and puddles.The effects of queen density and microtopography (small hills) on pleometrosis were tested in a two-factor factorial experiment. A 64-fold increase in applied queen density resulted in a 2.19-fold increase in mean queens/nest (pleometrosis). Variation in queen density accounted for 70% of the variation in the mean queens per nest, as well as 78% of the aggregation of queens among the available nest chambers. Queen density also accounted for 86% of the aggregation of queens in area. Thus, at all densities, queens are moving into areas and nests of higher density, increasing both the local mean densities and the level of aggregation. Microtopography had no significant effect. Lab experiments suggest that the interactions leading to association take place on the surface.A mechanism is proposed in which the central causal factor regulating pleometrosis is local queen density, local being one to a few square meters, and a variety of factors affect pleometrosis by their action upon the local queen density.In the laboratory, groups of 5 foundresses produce more workers than do groups of 10 or 15, or single queens. Nests founded by groups begin the growth period with about 3 times as many workers as do those founded by single queens, and the former remain about three times as large for at least the first 100 days of growth and probably more. Higher worker production rate probably confers an advantage in survival and competition throughout colony growth. These differences between haplo- and pleometrotically founded nests may be among the factors favoring foundress associations.

Experimental evidence that human impacts drive fire ant invasions and ecological change

Biological invasions are often closely associated with human impacts and it is difficult to determine whether either or both are responsible for the negative impacts on native communities. Here, we show that human activity, not biological invasion, is the primary driver of negative effects on native communities and of the process of invasion itself. In a large-scale experiment, we combined additions of the exotic fire ant, Solenopsis invicta, with 2 disturbance treatments, mowing and plowing, in a fully crossed factorial design. Results indicate that plowing, in the absence of fire ants, greatly diminished total native ant abundance and diversity, whereas fire ants, even in the absence of disturbance, diminished some, but not all, native ant abundance and diversity. Transplanted fire ant colonies were favored by disturbance. In the absence of disturbance and on their own, fire ants do not invade the forest habitats of native ants. Our results demonstrate that fire ants are “passengers” rather than “drivers” of ecological change. We propose that fire ants may be representative of other invasive species that would be better described as disturbance specialists. Current pest management and conservation strategies should be reassessed to better account for the central role of human impacts in the process of biological invasion.

A comparative study of the chemical defensive system of tenebrionid beetles: Chemistry of the secretions

Abstract The chemical compositions of the defensive secretions of 147 species of tenebrionid beetles from 55 genera and 16 tribes were analysed by gas-liquid chromatography on three different stationary phases. All species contained toluquinone and ethylquinone, but benzoquinone was relatively rare. A great many species contained various 1-alkenes in addition to quinones. Species of Pedinini were distinguished by the secretion of n -propylquinone, whereas 2,3-methylmethoxyquinone set apart the subgenus Blapylis (of Eleodes ) and possibly Amphidora . Large amounts of octanoic acid were restricted to the genera Embaphion and Neobaphion (both Eleodini), and small amounts of 1-nonadecene were restricted to Lariversius (Eleodini) and Cratidus (Amphidorini). Most species of Eleodes had complex secretions with toluquinone, ethylquinone, 1-nonene, 1-undecene, and 1-tridecene in various ratios. 1-Pentadecene and/or 1-heptadene occurred in substantial amounts in Tribolium, Uloma, Phaleria, Pyanisia, Melanopterus, Gonopus, Schelodontes, Psorodes, Neatus , and Merinus . The genus Argoporis (Scaurini) is unique in its synthesis of 6-alkyl-1,4-naphthoquinones, and other members of the tribe also contain characteristic, as yet unidentified compounds. Many groups not distinguished by unique compounds still show characteristic combinations and ratios of compounds, and these are discussed along with problems of variation at the individual, geographic, specific, and generic levels.

Desiccation resistance in arboreal and terrestrial ants

ABSTRACT Arboreal and terrestrial ants were exposed to 0, 25, 50, 75 and 100 (control)% r.h., at 30oC. Desiccation resistance increased with body size (as dry weight0.55), but not as quickly as expected from the consequences of the surface area and volume relationship (as dry weight0.67). Arboreal ants took 8 times longer to die than terrestrial ants of comparable size. Even after size effects were removed, desiccation resistance differed between various terrestrial species and showed a correlation with foraging patterns.

Fire ant thermal preferences: behavioral control of growth and metabolism

SummaryThermal preferences of well-fed and food-limited fire ant colonies (Solenopsis invicta) were studied in relation to colony growth and metabolic costs. The growth curve for well-fed colonies was strongly skewed toward warmer temperatures with maximal growth occurring near 32° C (Fig. 2A). The growth curve for food-limited colonies was skewed toward cooler temperatures with maximal colony size occurring around 25° C (Fig. 2B). Food-limited colonies apparently grew larger at cooler temperatures because metabolic costs of workers were reduced. A series of binary choice tests confirmed three predictions concerning fire ant thermal preferences (Figs. 3–4). First, well-fed colonies preferred brood temperatures very near the optimum for colony growth (31° C versus 32° C). Colonies were also able to select appropriate suboptimal growth temperatures when the optimal range was unavailable. Secondly, as predicted, a large percentage of colony workers (∼ 30% in well-fed colonies) consistently chose cooler temperatures than those selected for the brood. This strategy probably increases longevity of workers not directly associated with brood care. Thirdly, food-limited colonies preferred cooler temperatures than well-fed colonies. Metabolic costs of food-limited colonies were reduced by approximately 7% because of (1) slightly cooler brood temperatures (30° C versus 31° C) and because (2) an additional 20–30% of the workers selected cooler temperatures. The addition of excess food reversed food-limited thermal preferences within 12 h for the brood (Fig. 5) and several days for the workers. Contrary to expectations, thermal preferences for brood in food-limited colonies did not match the food-limited growth curve, perhaps because fire ant colonies can choose to rear brood at warm temperatures while maintaining accumulated colony biomass at cooler temperatures.

Insect sociometry, a field in search of data

SummaryThe study of social insects has proceeded without adequate descriptive data on social insect attributes. The term “sociometry” is proposed for the collection and analysis of the physical and numerical attributes of social insect colonies and their inhabitants. Sociometry can be seen as having 3 levels — the compilation of data, the distribution patterns of the attributes among species (comparative studies), and the detection of relationships of the attributes to each other. Many sociometric attributes may be linked, hence evolve under constraint from other attributes. The study of social insects would benefit greatly through the organized collection of sociometric data.

Sociometry and sociogenesis of colonies of the harvester ant, Pogonomyrmex badius : worker characteristics in relation to colony size and season

Summary: As colonies of all monogyne ants grow from a single, colony-founding queen to a mature colony with many workers, they develop the species-typical characteristics of the mature colony. This ontogeny, and these species-typical characteristics and their seasonal changes were studied in the Florida harvester ant by excavating 31 colonies of the full range of sizes, on 4 dates representing major phases of the annual cycle. Worker characteristics varied strongly with colony size, location in the nest, and season. All but incipient colonies contained both major and minor workers. The proportion of the colony which was major workers was unaffected by colony size, averaging about 7 %, but showed a small increase in mid-summer. Minor workers increased in size as colonies grew larger, but major workers did not. Most of the changes in minor worker weight were caused by changes in fat stores, which varied by 85 % or more. The patterns of weight variation supported the following life and seasonal history of minor workers. The young, lighter-colored workers (callows), occurred near the bottom of the nest between June and October, confirming a strong age-stratification in the nest. For a given headwidth, callow minors were fatter than their older, darker sisters. In the course of the season, young workers gained lean weight, but lost fat as they moved to nest chambers ever closer to the surface. Finally, when their fat content was less than 10 %, they became foragers on the surface. Along with these age-related changes, minor workers were lightest and least fat in July after the colony had produced its annual crop of sexual alates, and gained an average of 24 % by winter. Workers were heavier and fatter if they came from a larger colony, and heavier and younger deeper in the nest. Seasonal variation in fat weight and dry weight was greater in smaller colonies than large. Although less variable, lean weight was lower in July and in the bottom of the nest, and higher in a larger colony. These patterns were similar, though less precise, for major workers. ¶Altogether, in the life history of harvester ant colonies, the large pulse of early-spring sexuals probably cannot be produced solely from current foraging intake. The metabolic and labor resources needed for sexual production are stored in the bodies of the young workers, whose fat content reaches the annual minimum after producing sexuals. After these sexuals have flown, the colony once again switches to producing workers and storing excess foraging intake as worker fat for over-wintering, and for producing the next years sexuals. As colonies get larger, this proportional excess increases, giving rise to fatter and larger workers.

Nest architecture of the ant Formica pallidefulva: structure, costs and rules of excavation

SummaryThe architecture of underground ant nests was studied in the ant Formica pallidefulva. These ants build shallow (30–45 cm deep) nests, which consist of more or less vertical shafts that bear chambers. Shafts are modular units of nest growth; nests are enlarged by adding more shafts or extending previously existing ones. The nests are top-heavy, their volume declining exponentially with depth. The total volume of the nest is strongly correlated with the number of worker occupying the nest (R2 = 0.87). Some of the rules and templates used by workers for nest construction were determined: (a) chambers are formed in the direction of the tunnels leading up to them, (b) the amount of soil excavated per unit time increases with soil temperature and moisture content. The amount of time and energy required to construct a typical nest were approximated using digging ability parameters determined in the lab. We estimate that if a colony was to move twice a year, it would expend around 20% of its energy intake and at least 6% of its worker time on nest excavation.