Sanford D. Porter

Invasion of Polygyne Fire Ants Decimates Native Ants and Disrupts Arthropod Community

The fire ant Solenopsis invicta Buren invaded southeastern United States from South America >50 yr ago. Urban and agricultural consequences of this invasion are well documented; however, ecological effects are still poorly understood. Increasing frequencies of polygyne or multiple—queen fire ants in Texas and other areas of the Southeast are disturbing because nest densities of this new form are often ten times as great as those of the more familiar monogyne form. We studied the ecological impacts of a polygyne fire ant invasion on ants and other surface—active arthropods at a field station in central Texas. Arthropod abundance and species richness were assessed using a combination of baits, pitfall traps, and litter samples. This invasion decimated the indigenous ant fauna. Competitive replacement appears to be the primary mechanism behind this effect. Species richness of ants in infested areas dropped by 70%, while the total number of native individuals dropped by 90%. Of 35 species of ants collected in this study, 23 were either significantly less common or absent from infested sites; only S. invicta was more common at infested sites. The most dramatic effect of the invasion was a 10—30 fold increase in the total number of ants at infested sites–of which >99% were the imported fire ant S. invicta. The impact of this invasion on other surface—active arthropods was less severe, but still substantial. The abundance of isopods, erythraeid mites, and tumblebug scarabs declined significantly, while the abundance of ground crickets, a brachypterous roach, and a symbiotic scarab increased significantly. Overall, the species richness of non—ant arthropods was 30% lower in infested sites, and individual numbers were 75% lower. Total arthropod species richness (including ants) was 40% less at infested sites. These data indicate that polygyne fire ants pose a substantial threat to the biodiversity of native arthropod communities.

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.

Potential global range expansion of the invasive fire ant, Solenopsis invicta

The red imported fire ant, Solenopsis invicta Buren, is an invasive pest that has become widespread in the southern United States and Caribbean after introduction from South America in the 1930s. This species, which has diverse detrimental impacts on recipient communities, was recently discovered in Australia and New Zealand and has the potential to colonize numerous other regions. We used a dynamic, ecophysiological model of colony growth to predict the potential global range expansion of this invasive species. Based on minimum and maximum daily temperatures, the model estimates colony alate production and predicts future geographic range limits. Because S. invicta populations are limited by arid conditions as well as cold temperatures, we superimposed precipitation data upon temperature-based predictions, to identify regions that do not receive enough rainfall to support this species across the landscape. Many areas around the globe, including large portions of Europe, Asia, Africa, Australia, and numerous island nations, are at risk for S. invicta infestation. Quarantine officials should be vigilant for any accidental introductions of this pest in susceptible regions. Costs of eradication increase dramatically as the area of infestation grows, and large infestations may be impossible to eradicate. Other South American Solenopsis fire ants (e.g., S. richteri Forel) may become invasive if the opportunity arises, and our predictions for S. invicta may approximate the potential range limits for these species as well.

Impact of temperature on colony growth and developmental rates of the ant, Solenopsis invicta

Abstract Growth rates of the fire ant, Solenopsis invicta, were studied across a range of constant temperatures. Growth in established colonies only occurred at rather warm temperatures: between 24 and 36°C, with maximal growth around 32°C. Colony growth ceased below 24°C even though 17°C was the theoretical minimum for brood development. Total developmental time (egg to adult) for minor worker brood decreased from 55 days at 24°C to 23 days at 35°C. The proportion of time required by each developmental stage (i.e. eggs, larvae, pupae) was independent of temperature. Nanitic brood developed about 35% faster than did minor worker brood, an important benefit for founding colonies. Data are still fragmentary, but it appears that temperature-adjusted development rates of ants and other social Hymenoptera are quite similar.

BIOLOGY AND BEHAVIOR OF PSEUDACTEON DECAPITATING FLIES (DIPTERA: PHORIDAE) THAT PARASITIZE SOLENOPSIS FIRE ANTS (HYMENOPTERA: FORMICIDAE)

Larvae of phorid flies in the genus Pseudacteon have the unusual habit of decapitating fire ant workers and pupating inside the empty head capsule which they use as a pupal case. Flies in this genus are the subject of considerable interest because they have the potential of being used as classical biological control agents against imported fire ants in North America. This paper details what is known and not known about their interesting life history, attack behavior, mating behavior, host specificity, and impacts on fire ant behavior. The biogeography, community structure, and possible impacts on fire ant populations are also considered.

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.

Worker longevity in the fire ant Solenopsis invicta: Ergonomic considerations of correlations between temperature, size and metabolic rates

Abstract We compared long-term economic costs of producing and maintaining different size fire ant workers ( Solenopsis invicta ) at two temperatures (24 and 30°C). Energetic costs for three sizes of workers were calculated from their biomass, longevity, respiration rates, and the energy content of worker tissue. Large workers lived considerably longer than small workers (50–140%, depending on temperature) and respired more slowly per mg of tissue (40%, independent of temperature). Overall, large workers cost about 30% less in cal/mo x mg than equal weights of small workers. However, on an individual basis, large workers were still four times more expensive than small workers because of a 6-fold weight difference. These data indicate that one large worker must provide services equivalent to at least four small workers, to justify the colonys energy investment.

Modeling Temperature-Dependent Range Limits for the Fire Ant Solenopsis invicta (Hymenoptera: Formicidae) in the United States

Abstract We predict the future range of the fire ant Solenopsis invicta Buren within the United States based on climate and its current extreme distributions. To reach that goal, a dynamic model of colony growth with two time steps per day was formulated that operates by colony area, S, and alate production, a. Colony growth rate depended on daily maximum and minimum soil temperatures. Temperature records at 4,537 meteorological stations within the current (near 1.5 million km2) and potential range of S. invicta were obtained from NOAA’s National Climatic Data Center. At each station, a colony was allowed to grow and lifetime female alate production was calculated. Estimated alate production was then examined at current extremes of the fire ant distribution at selected locations in Arkansas, Tennessee, and Oklahoma. Estimates from these locations were used to define four zones of colony proliferation success: certain, possible, undemonstrated, and improbable. An annual precipitation limit (510 mm) was selected to indicate regions where arid conditions may prohibit growth in areas without supplemental water sources. Results of the model predict that S. invicta will likely move 80–150 km north in Oklahoma and Arkansas. It will also likely continue expanding into portions of Virginia, Maryland, and Delaware in the east and New Mexico, Arizona, California, Oregon, Nevada, and maybe even Washington and Utah in the west.

Frequency and distribution of polygyne fire ants (Hymenoptera: Formicidae) in Florida.

In order to determine the frequency and distribution of polygyne and monogyne fire ants (Solenopsis invicta) in Florida, preselected sites were surveyed from Key West to Tallahassee. Polygyne colonies were found at 15% of infested sites--a frequency similar to other states in the southeastern United States, but much less than in Texas. Polygyny was most common in the region around Marion county, but smaller populations were also scattered across the state. The density of mounds at polygyne sites was more than twice that at monogyne sites (262 versus 115 mounds/ha), although mound diameters were about 20% smaller. Polygyne and monogyne queens averaged the same size (1.42 mm, head width), but monogyne queens were much heavier (24.3 mg versus 14.4 mg) due to their physogastry. As expected, workers in polygyne colonies were considerably smaller than those in monogyne colonies (0.28 mg versus 0.19 mg, dry fat-free).

Establishment and dispersal of the fire ant decapitating fly Pseudacteon tricuspis in North Florida

The decapitating fly Pseudacteon tricuspis Borgmeier was released at eight sites in North Florida between the summer of 1997 and the fall of 1999 as a self-sustaining biocontrol agent of the red imported fire ant, Solenopsis invicta Buren. Several releases used parasitized fire ant workers while most involved adult flies released over disturbed ant mounds. Establishment and dispersal of fly populations were monitored by disturbing about 10 fire ant mounds at each site and then inspecting them closely for hovering flies over a period of about 30 min. Overwintering populations of flies were successfully established at 6 of 8 release sites. Over several years, fly populations at these sites increased to levels as high or higher than those normally seen in their South American homeland. By the fall of 1999, flies had expanded out 1–6 km from five release sites and occupied about 125 km 2 . By the fall of 2000 the five initial release sites plus one new site had fused into one large area about 70 km in diameter. The flies had expanded out an additional 16–29 km and occupied about 3300 km 2 . By the fall of 2001 the flies had expanded out an additional 10–30 km and occupied approximately 8100 km 2 . Fly dispersal was not related to wind patterns in the Gainesville area. Based on the above rates of dispersal and an establishment rate of 66%, we estimate that a state the size of Florida would require 5–10 releases spaced over a 3-year period to cover the state in 6–9 years. 2003 Elsevier Inc. All rights reserved.