Invicta Populations Research Articles

Population genetic analyses reveal host association and genetically distinct populations of social parasite Solenopsis daguerrei (Hymenoptera: Formicidae)

Inquiline ant social parasites exploit other ant species for their reproductive benefit because they do not possess a worker caste. Due to their relative rarity in nature, the biology and natural history of inquilines are largely unknown. Likewise, not much research exists that details the close relationship between inquilines and their host(s), and how each organism influences the genetic structure of the other. Here, we conducted a comparative population genetics study to assess patterns of genetic structure within and among populations of inquiline Solenopsis daguerrei and its known fire ant hosts, which includes invasive Solenopsis invicta. Using nuclear and mitochondrial markers, we show that four genetically distinct groups of S. daguerrei likely exist, each with different degrees of host association. Consistent with previous inferences of the inquiline lifestyle, we find that inbreeding is common in S. daguerrei, presumably a result of intranidal mating and restricted dispersal. Results from this study, specifically host association patterns, may inform future biological control strategies to mitigate invasive S. invicta populations.

Transcriptomic Analysis Reveals the Impact of the Biopesticide Metarhizium anisopliae on the Immune System of Major Workers in Solenopsis invicta.

The red imported fire ant (Solenopsis invicta Buren, 1972) is a globally significant invasive species, causing extensive agricultural, human health, and biodiversity damage amounting to billions of dollars worldwide. The pathogenic fungus Metarhizium anisopliae (Metchnikoff) Sorokin (1883), widely distributed in natural environments, has been used to control S. invicta populations. However, the interaction between M. anisopliae and the immune system of the social insect S. invicta remains poorly understood. In this study, we employed RNA-seq to investigate the effects of M. anisopliae on the immune systems of S. invicta at different time points (0, 6, 24, and 48 h). A total of 1313 differentially expressed genes (DEGs) were identified and classified into 12 expression profiles using short time-series expression miner (STEM) for analysis. Weighted gene co-expression network analysis (WGCNA) was employed to partition all genes into 21 gene modules. Upon analyzing the statistically significant WGCNA model and conducting Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis on the modules, we identified key immune pathways, including the Toll and Imd signaling pathways, lysosomes, autophagy, and phagosomes, which may collectively contribute to S. invicta defense against M. anisopliae infection. Subsequently, we conducted a comprehensive scan of all differentially expressed genes and identified 33 immune-related genes, encompassing various aspects such as recognition, signal transduction, and effector gene expression. Furthermore, by integrating the significant gene modules derived from the WGCNA analysis, we constructed illustrative pathway diagrams depicting the Toll and Imd signaling pathways. Overall, our research findings demonstrated that M. anisopliae suppressed the immune response of S. invicta during the early stages while stimulating its immune response at later stages, making it a potential biopesticide for controlling S. invicta populations. These discoveries lay the foundation for further understanding the immune mechanisms of S. invicta and the molecular mechanisms underlying its response to M. anisopliae.

The Facet of Human Impact: Solenopsis invicta Buren, 1972 Spreading around the Atlantic Forest

The present investigation deals with some aspects of the diversity of fire ants (Hymenoptera: Formicidae) in their native range. The Red Imported Fire Ant Solenopsis invicta is native to the tropical and subtropical inland territories of South America. In Brazil, it mainly occurs around the Pantanal region and across the Paraguay river, a region composed of grasslands which are seasonally flooded. Recent studies have evidenced this fire ant species is gradually spreading to other regions of Brazil. In the present investigation, we surveyed the molecular diversity of S. invicta populations across fragments of Atlantic Forest in São Paulo, Brazil, using mtDNA COI haplotypes. Fire ant nests were sampled along the highways lining the northern and southern slope sides of the mountain range Serra do Mar, SP, Brazil. Four haplotypes were identified (H1–H4), which were assessed for similarity to deposited records by other authors, revealing that the haplotypes H1 and H2 are likely of foreign origin through recent reintroduction via a marine port to the south of the Serra do Mar mountain range. On the other hand, the haplotypes H3 and H4, predominating among the inland samples from the northern side of the mountain range, were most similar to previous records from more central regions of Brazil. Haplotypes clustered into distinct supergroups, further pointing to the occurrence of two separate expansion waves of S. invicta in the region. We suggest the obtained pattern indicates the mountain range may function as a geographical barrier deferring gene flow.

Distribution of the Red Imported Fire Ant Solenopsis invicta (Hymenoptera: Formicidae) in Central Florida Pastures.

Habitat disturbance has been found to facilitate the introduction of a wide range of species, including the red imported fire ant Solenopsis invicta Buren (Hymenoptera: Formicidae: Myrmicinae). Despite the link between S. invicta colonization and disturbance, little is known about how different intensities or types of disturbance might impact S. invicta populations. In this study, we used S. invicta populations in cattle pastures to understand how variation in disturbance type and frequency correlates with the density of S. invicta mounds. In total, 56 plots were surveyed for mound abundance during both the wet and dry seasons on a subtropical south Florida ranch. Explanatory variables were grouped into five categories based on disturbance type: 1) historic pasture conversion; 2) modern pasture management (mowing, dragging, chopping, or aerating); 3) grazing intensity (a measure vegetation height and dung pat abundance); 4) distance to human-made and natural localized disturbance (roads, ditches, and wetlands); and 5) abiotic conditions (soil temperature, soil moisture). Overall, the average number of mounds per plot was not significantly different between seasons, but was significantly higher in intensive pastures, which are converted to nonnative forage grasses than in seminative pastures during the dry season. Time since soil disturbance (aeration and chopping of pasture) was a significant predictor of S. invicta densities in both dry and wet seasons, with an increase in time since disturbance being associated with higher mound densities. Other forms of pasture management that did not disturb the soil, such as dragging and mowing, as well as distance to localized disturbances (wetlands, roads, and ditches) were not found to have a significant correlation in either season.

Speciation and hybridization in invasive fire ants

BackgroundA major focus of evolutionary biology is the formation of reproductive barriers leading to divergence and ultimately, speciation. Often, it is not clear whether the separation of populations is complete or if there still is ongoing gene flow in the form of rare cases of admixture, known as isolation with migration. Here, we studied the speciation of two fire ant species, Solenopsis invicta and Solenopsis richteri, both native to South America, both inadvertently introduced to North America in the early twentieth century. While the two species are known to admix in the introduced range, in the native range no hybrids were found.ResultsWe conducted a population genomic survey of native and introduced populations of the two species using reduced representation genomic sequencing of 337 samples. Using maximum likelihood analysis over native range samples, we found no evidence of any gene flow between the species since they diverged. We estimated their time of divergence to 190,000 (100,000–350,000) generations ago. Modelling the demographic history of native and introduced S. invicta populations, we evaluated their divergence times and historic and contemporary population sizes, including the original founder population in North America, which was estimated at 26 (10–93) unrelated singly-mated queens.ConclusionsWe provide evidence for complete genetic isolation maintained between two invasive species in their natïve range, based, for the first time, on large scale genomic data analysis. The results lay the foundations for further studies into different stages in the formation of genetic barriers in dynamic, invasive populations.

Positive selection on sociobiological traits in invasive fire ants.

The fire ant Solenopsis invicta and its close relatives are highly invasive. Enhanced social cooperation may facilitate invasiveness in these and other invasive ant species. We investigated whether invasiveness in Solenopsis fire ants was accompanied by positive selection on sociobiological traits by applying a phylogenomics approach to infer ancient selection, and a population genomics approach to infer recent and ongoing selection in both native and introduced S.invicta populations. A combination of whole-genome sequencing of 40 haploid males and reduced-representation genomic sequencing of 112 diploid workers identified 1,758,116 and 169,682 polymorphic markers, respectively. The resulting high-resolution maps of genomic polymorphism provide high inference power to test for positive selection. Our analyses provide evidence of positive selection on putative ion channel genes, which are implicated in neurological functions, and on vitellogenin, which is a key regulator of development and caste determination. Furthermore, molecular functions implicated in pheromonal signalling have experienced recent positive selection. Genes with signatures of positive selection were significantly more often those overexpressed in workers compared with queens and males, suggesting that worker traits are under stronger selection than queen and male traits. These results provide insights into selection pressures and ongoing adaptation in an invasive social insect and support the hypothesis that sociobiological traits are under more positive selection than nonsocial traits in such invasive species.

Prospecting for viral natural enemies of the fire ant Solenopsis invicta in Argentina.

Metagenomics and next generation sequencing were employed to discover new virus natural enemies of the fire ant, Solenopsis invicta Buren in its native range (i.e., Formosa, Argentina) with the ultimate goal of testing and releasing new viral pathogens into U.S. S. invicta populations to provide natural, sustainable control of this ant. RNA was purified from worker ants from 182 S. invicta colonies, which was pooled into 4 groups according to location. A library was created from each group and sequenced using Illumina Miseq technology. After a series of winnowing methods to remove S. invicta genes, known S. invicta virus genes, and all other non-virus gene sequences, 61,944 unique singletons were identified with virus identity. These were assembled de novo yielding 171 contiguous sequences with significant identity to non-plant virus genes. Fifteen contiguous sequences exhibited very high expression rates and were detected in all four gene libraries. One contig (Contig_29) exhibited the highest expression level overall and across all four gene libraries. Random amplification of cDNA ends analyses expanded this contiguous sequence yielding a complete virus genome, which we have provisionally named Solenopsis invicta virus 5 (SINV-5). SINV-5 is a positive-sense, single-stranded RNA virus with genome characteristics consistent with insect-infecting viruses from the family Dicistroviridae. Moreover, the replicative genome strand of SINV-5 was detected in worker ants indicating that S. invicta serves as host for the virus. Many additional sequences were identified that are likely of viral origin. These sequences await further investigation to determine their origins and relationship with S. invicta. This study expands knowledge of the RNA virome diversity found within S. invicta populations.

Imported fire ants near the edge of their range: disturbance and moisture determine prevalence and impact of an invasive social insect

1. Habitat disturbance and species invasions interact in natural systems, making it difficult to isolate the primary cause of ecosystem degradation. A general understanding requires case studies of how disturbance and invasion interact across a variety of ecosystem - invasive species combinations. 2. Dramatic losses in ant diversity followed the invasion of central Texas by red imported fire ants (Solenopsis invicta). However, recent manipulative studies in Florida revealed no effect on ant diversity following the removal of S.invicta from a disturbed pasture habitat, but moderate loss of diversity associated with their introduction into undisturbed habitat and no invasion occurred without disturbance. Thus, the importance of S.invicta in driving diversity loss and its ability to invade undisturbed systems is unresolved. 3. We examine the distribution and abundance of a large monogyne S.invicta population and its association with the co-occurring ant assemblage at a site in south Texas close to the aridity tolerance limit of S.invicta. 4. We document that moisture modulates S.invicta densities. Further, soil disturbing habitat manipulations greatly increase S.invicta population densities. However, S.invicta penetrates all habitats regardless of soil disturbance history. In contrast, controlled burns depress S.invicta densities. 5. In habitats where S.invicta is prevalent, it completely replaces native fire ants. However, S.invicta impacts native ants as a whole less strongly. Intriguingly, native ants responded distinctly to S.invicta in different environments. In wet, undisturbed environments, high S.invicta abundance disrupts the spatial structure of the ant assemblage by increasing clumping and is associated with reduced species density, while in dry-disturbed habitats, sites with high S.invicta abundance possess high numbers of native species. Analyses of co-occurrence indicate that reduced species density in wet-undisturbed sites arises from negative species interactions between native ants and S.invicta. However, these same data suggest that the high native species density of abundant S.invicta sites in dry-disturbed environments does not result from facilitation. 6. Monogyne S.invicta populations play different roles in different environments, driving ant diversity loss in some, but being largely symptomatic of habitat disturbance in others.

Propagule pressure and colony social organization are associated with the successful invasion and rapid range expansion of fire ants in China

We characterized patterns of genetic variation in populations of the fire ant Solenopsis invicta in China using mitochondrial DNA sequences and nuclear microsatellite loci to test predictions as to how propagule pressure and subsequent dispersal following establishment jointly shape the invasion success of this ant in this recently invaded area. Fire ants in Wuchuan (Guangdong Province) are genetically differentiated from those found in other large infested areas of China. The immediate source of ants in Wuchuan appears to be somewhere near Texas, which ranks first among the southern USA infested states in the exportation of goods to China. Most colonies from spatially distant, outlying areas in China are genetically similar to one another and appear to share a common source (Wuchuan, Guangdong Province), suggesting that long-distance jump dispersal has been a prevalent means of recent spread of fire ants in China. Furthermore, most colonies at outlier sites are of the polygyne social form (featuring multiple egg-laying queens per nest), reinforcing the important role of this social form in the successful invasion of new areas and subsequent range expansion following invasion. Several analyses consistently revealed characteristic signatures of genetic bottlenecks for S.invicta populations in China. The results of this study highlight the invasive potential of this pest ant, suggest that the magnitude of international trade may serve as a predictor of propagule pressure and indicate that rates and patterns of subsequent range expansion are partly determined by the interplay between species traits and the trade and transportation networks.

WolbachiawSinvictaA Infections in Natural Populations of the Fire AntSolenopsis invicta: Testing for Phenotypic Effects

Wolbachia are intracellular bacteria that commonly infect many arthropods and some nematodes. In arthropods, these maternally transmitted bacteria often induce a variety of phenotypic effects to enhance their own spread within host populations. Wolbachia phenotypic effects generally either provide benefits to infected host females (cytoplasmic incompatibility, positive fitness effects) or bias host sex ratio in favor of females (male-killing, parthenogenesis, feminization), all of which increase the relative production of infected females in host populations. Wolbachia surveys have found infections to be exceedingly common in ants, but little is known at this juncture as to what phenotypic effects, if any, they induce in this group. Previous studies have demonstrated that individuals from native populations of the invasive fire ant Solenopsis invicta commonly harbor one or more of three Wolbachia variants. One of the variants, wSinvictaA, typically occurs at low prevalence in S. invicta populations, appears to have been transmitted horizontally into S. invicta three or more times, and has been lost repeatedly from host lineages over time. In order to determine the phenotypic effects and likely population dynamics of wSinvictaA infections in these ants, brood production patterns of newly mated fire ant queens were studied during simulated claustral founding and measured wSinvictaA transmission fidelity within mature single-queen families. No clear evidence was found for Wolbachia-mduced cytoplasmic incompatibility, significant fitness effects, or male-killing. Maternal transmission was perfect to both virgin queens and males. Possible mechanisms for how this variant could be maintained in host populations are discussed.

Phorid Flies,Pseudacteonspp. (Diptera: Phoridae), Affect Forager Size Ratios of Red Imported Fire AntsSolenopsis invicta(Hymenoptera: Formicidae) in Texas

Multiple species of Pseudacteon phorid flies (Diptera: Phoridae) are currently being released throughout the southern United States to aid biological control of red imported fire ants, Solenopsis invicta Buren (Hymenoptera: Formicidae). It is anticipated that these flies will interfere with S. invicta foraging, allowing native ant assemblages to outcompete S. invicta for available resources. Numerous studies have shown a decrease in S. invicta foraging intensity when exposed to phorids. This study documents a behavioral change in phorid-exposed S. invicta colonies at a phorid release site in central Texas. Significant differences in forager size ratios were detected between phorid-exposed and phorid-absent colonies. A similar phenomenon was recently documented in the native range of these insects in South America as well. Experimental manipulation of ratios of S. invicta worker sizes has been shown to have important effects on colony success. This newly documented phorid-mediated S. invicta colony-level effect represents a significant shift in S. invicta foraging dynamics and may provide an additional mechanism by which phorids can influence S. invicta populations in their United States range.

Fire Ants (Hymenoptera: Formicidae) Along an Important Sea Turtle Nesting Beach on St. Croix, USVI

Several predatory ant species, notably Solenopsis geminata and Solenopsis invicta, are known to attack and kill hatchlings of ground-nesting birds and reptiles. Here, we surveyed ants along a sea turtle nesting beach on St. Croix, USVI to evaluate the prevalence of predatory ants. Solenopsis geminata, a New World native, has a broad native distribution through South, Central, and North America. It also is found throughout the West Indies, where it may or may not be native (Wetterer 2010). Solenopsis invicta, a South American species, arrived in North America in the mid-20th century and has spread through much of the southeast US and to California. Recently, S. invicta has spread to the West Indies, where it was first found in Puerto Rico in 1981 and St. Croix in 1988 (Wetterer & Snelling 2006). Solenopsis invicta is now known from >20 West Indian islands (Wetterer & Davis 2010). Sea turtle hatchlings are especially vulnerable to fire ant attack because hatchlings can take up to several days after pipping before emerging from their nests (Parris et al. 2002). During this time, fire ants may invade the nests by the thousands and attack hatchlings. Ants also sting hatchlings as they exit the nest. Even a single sting can greatly reduce the probability of survival. Hatchlings may die as a result of the ant stings or increased predation and misorientation due to impairment caused by stings, particularly stings to their eyes (Krahe 2005). In Costa Rica, where S. invicta has not yet been recorded, Wetterer (2006) surveyed ants along an important sea turtle nesting beach at Tortuguero National Park, where ants have attacked sea turtle nests. Wetterer (2006) found S. geminata at 32 of 42 bait stations (76%), including very high numbers at all disturbed sites near Tortuguero village. McCurdy & Wetterer (2008) surveyed ants along a less-disturbed sea turtle nesting beach in Carate, Costa Rica and found S. geminata was less common, occurring at only 8 of 54 bait stations (15%). Solenopsis invicta has become the dominant ant species along many beaches of the southeastern U.S. Wetterer et al. (2007) surveyed ants on sea turtle nests along a beach in Palm Beach County, Florida. Solenopsis invicta was by far the most common species (on 431 nests). In contrast, S. geminata occurred on only 3 nests. We investigated the prevalence of S. geminata and S. invicta on the most important sea turtle nesting beach on St. Croix. In Mar 2006, we surveyed a 1.7-km stretch of beach in Sandy Point National Wildlife Refuge (17.68°N, 64.90°W), from marker 37 in the east to marker 210 in the northwest. We set out white note cards (7.6 × 12.7 cm), each with about 1 g of canned tuna at 50-m intervals (except for 1 pair 30 m apart) next to 36 permanent marker stakes along the vegetation line adjacent to the beach. We collected the cards 2 h later and put them into individual zip-lock bags. Stefan Cover (Harvard University) identified the ants. At 33 bait cards, we found ants representing 8 species (Table 1; one or 2 species per bait); 3 cards with no ants present all had the tuna removed, probably by mongooses, which commonly took bait from bait cards elsewhere on St. Croix (JKW, personal observation). Fire ants were present at 16 of 33 bait stations (48%: S. geminata occurred at 11 sites (markers 37, 47, 52, 57, 67, 77, 107, 112, 127, 132, and 205); S. invicta occurred at 5 sites (markers 92, 97, 147, 150, and 165). Solenopsis geminata was the more common fire ant at Sandy Point and could be an important threat to sea turtle nestlings. Solenopsis invicta, however, may pose a greater long-term threat to sea turtles at Sandy Point and elsewhere in the West Indies if S. invicta populations continue to increase and displace S. geminata. Our present

Prevalence, Spread, and Effects of the Microsporidium <I>Thelohania solenopsae</I> Released into Populations with Different Social Forms of the Red Imported Fire Ant (Hymenoptera: Formicidae)

The microsporidium Thelohania solenopsae Knell, Allen, and Hazard was released into colonies of red imported fire ants, Solenopsis invicta Buren, at four field sites in Louisiana. Social form of the ant affected the establishment of the microsporidium; long-term epizootics developed at two sites with predominantly polygyne populations, whereas the pathogen produced a low disease prevalence before it disappeared or did not infect ants at all at two monogyne sites. This study is the first report of artificial infection of monogyne S. invicta field colonies. When the microsporidium became established in mixed monogyne/polygyne ant populations, prevalence rates peaked at >75% in both social forms. In mixed ant populations, the social form suffering higher prevalence of the disease decreased proportionally to the other form, possibly indicating a competitive disadvantage. Host population density or site characteristics may have influenced spread of the disease; the rate of spread was 0.9 m/mo at one epizootic site and reached 9.4 m/mo at the other. There was little seasonal variation in prevalence, which averaged 47% in February, 51% in April/May, and 57% in October/November at the two epizootic sites. The strongest evidence of microsporidian impact on S. invicta populations was a negative correlation between colony numbers versus percentage infection and a sporadic decrease in the number of foragers. There was some evidence of a decrease in the size and number of colonies at one epizootic site and a decrease in brood at the other. This sporadic weakening of the S. invicta populations did not lead to significant immigration of other ant species.

Evolutionary history of Wolbachia infections in the fire ant Solenopsis invicta

BackgroundWolbachia are endosymbiotic bacteria that commonly infect numerous arthropods. Despite their broad taxonomic distribution, the transmission patterns of these bacteria within and among host species are not well understood. We sequenced a portion of the wsp gene from the Wolbachia genome infecting 138 individuals from eleven geographically distributed native populations of the fire ant Solenopsis invicta. We then compared these wsp sequence data to patterns of mitochondrial DNA (mtDNA) variation of both infected and uninfected host individuals to infer the transmission patterns of Wolbachia in S. invicta.ResultsThree different Wolbachia (wsp) variants occur within S. invicta, all of which are identical to previously described strains in fire ants. A comparison of the distribution of Wolbachia variants within S. invicta to a phylogeny of mtDNA haplotypes suggests S. invicta has acquired Wolbachia infections on at least three independent occasions. One common Wolbachia variant in S. invicta (wSinvictaB) is associated with two divergent mtDNA haplotype clades. Further, within each of these clades, Wolbachia-infected and uninfected individuals possess virtually identical subsets of mtDNA haplotypes, including both putative derived and ancestral mtDNA haplotypes. The same pattern also holds for wSinvictaA, where at least one and as many as three invasions into S. invicta have occurred. These data suggest that the initial invasions of Wolbachia into host ant populations may be relatively ancient and have been followed by multiple secondary losses of Wolbachia in different infected lineages over time. Finally, our data also provide additional insights into the factors responsible for previously reported variation in Wolbachia prevalence among S. invicta populations.ConclusionThe history of Wolbachia infections in S. invicta is rather complex and involves multiple invasions or horizontal transmission events of Wolbachia into this species. Although these Wolbachia infections apparently have been present for relatively long time periods, these data clearly indicate that Wolbachia infections frequently have been secondarily lost within different lineages. Importantly, the uncoupled transmission of the Wolbachia and mtDNA genomes suggests that the presumed effects of Wolbachia on mtDNA evolution within S. invicta are less severe than originally predicted. Thus, the common concern that use of mtDNA markers for studying the evolutionary history of insects is confounded by maternally inherited endosymbionts such as Wolbachia may be somewhat unwarranted in the case of S. invicta.

Testing for population-level impacts of introduced Pseudacteon tricuspis flies, phorid parasitoids of Solenopsis invicta fire ants

Abstract Parasitoid phorid flies in the genus Pseudacteon are potential biocontrol agents of invasive Solenopsis fire ants in the US. One Pseudacteon species, P. tricuspis Borgmeier, has been released and is now established over a large region of north central Florida. We conducted a multi-year, large-scale field experiment to investigate potential population-level impacts on host Solenopsis invicta Buren fire ants and associated arthropods. We established and monitored 67 field sites over a 3.5 year period, documenting S. invicta density and activity by three complementary sampling methods: (1) mound counts and measurements, (2) attraction to baits, and (3) pitfall trap catches. P. tricuspis abundance was determined by censusing flies at disturbed S. invicta mounds. P. tricuspis populations increased in abundance over the first two years following colonization and then oscillated seasonally, reaching higher abundances in the fall than in the preceding spring. S. invicta populations varied greatly over time, and repeated-measures ANCOVA revealed no effect of phorid parasitism pressure on S. invicta density or activity. P. tricuspis likely has a relatively small effect on S. invicta populations, which may not be consistent over space and time and is difficult to document against the background of high variability in fire ant populations due to various environmental factors (i.e., rainfall, disturbance). P. tricuspis parasitism pressure (averaged over the course of the experiment) was positively associated with changes in S. invicta density. Additionally, P. tricuspis abundance was positively correlated with S. invicta density after P. tricuspis populations had been established for >1 year. Introduced phorid populations may require many years before a measurable impact is apparent. Alternatively, parasitism pressure of a single phorid species may not be a major factor regulating fire ant populations.

Evaluating biological control of fire ants using phorid flies: effects on competitive interactions

The effects of the parasitic phorid fly, Pseudacteon tricuspis Borgmeier, on the competitive interactions between the red imported fire ant, Solenopsis invicta Buren, and a native North American ant, Forelius pruinosus (Roger), were investigated in the laboratory. P. tricuspis is a highly host-specific endoparasitoid of S. invicta workers that is currently being reared and released as a biological control agent of S. invicta in the US. We tested the effect of P. tricuspis on the colony growth rate of S. invicta when S. invicta was forced to compete with F. pruinosus for a protein resource (freeze-killed crickets) in laboratory competition arenas. In addition to colony growth rate, we quantified the effect of the phorid flies on the foraging rate of S. invicta. Though S. invicta significantly reduced its foraging rate in the presence of the phorid flies, we did not detect an effect of the flies on colony growth rate. Possible explanations for these results include behavioral compensation by S. invicta for the presence of the flies. We present these laboratory results in light of a literature search indicating that laboratory tests of biological control agent efficacy are good predictors of field efficacy. We conclude that P. tricuspis alone is unlikely to suppress S. invicta populations in the field by reducing their competitive ability.

Evaluating biological control of fire ants using phorid flies: effects on competitive interactions

The effects of the parasitic phorid fly, Pseudacteon tricuspis Borgmeier, on the competitive interactions between the red imported fire ant, Solenopsis invicta Buren, and a native North American ant, Forelius pruinosus (Roger), were investigated in the laboratory. P. tricuspis is a highly host-specific endoparasitoid of S. invicta workers that is currently being reared and released as a biological control agent of S. invicta in the US. We tested the effect of P. tricuspis on the colony growth rate of S. invicta when S. invicta was forced to compete with F. pruinosus for a protein resource (freeze-killed crickets) in laboratory competition arenas. In addition to colony growth rate, we quantified the effect of the phorid flies on the foraging rate of S. invicta . Though S. invicta significantly reduced its foraging rate in the presence of the phorid flies, we did not detect an effect of the flies on colony growth rate. Possible explanations for these results include behavioral compensation by S. invicta for the presence of the flies. We present these laboratory results in light of a literature search indicating that laboratory tests of biological control agent efficacy are good predictors of field efficacy. We conclude that P. tricuspis alone is unlikely to suppress S. invicta populations in the field by reducing their competitive ability.

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.

Conservation of biodiversity in an area impacted by the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae)

A conservation study was conducted at Camp Swift, Bastrop County, Texasto determine if the red imported fire ant, Solenopsisinvicta, could be selectively managed to protect endemic antdiversity. The study site was deemed to be an area where S.invicta had recently invaded, and was still in the process ofexpanding its population. The study was conducted between 1998 and 2001, andresulted in significantly higher ant diversity in the area where S.invicta was managed by prescription bait treatments. The controlsite, which was not conserved, lost four endemic ant species during the test,while the treatment site had an increase in one species. This provides evidencethat S. invicta populations can be managed, but may notneed to be eliminated, to preserve ant diversity. New county records are givenfor Brachymyrmex depilis, Foreliusmccooki, F. pruinosus, Gnamptogenyshartmani, Monomorium minimum,Pogonomyrmex comanche, and Tetramoriumspinosum.

Field Applications of Bait-FormulatedBeauveria bassianaAlginate Pellets for Biological Control of the Red Imported Fire Ant (Hymenoptera: Formicidae)

The red imported fire ant, Solenopsis invicta Buren, is an important pest in the southern United States, in part because few natural enemies exist. Beauveria bassiana is an entomopathogenic fungus that holds great promise as a biological control agent against S. invicta. In previous studies, B. bassiana was effective in laboratory trials, but field applications were less effective. In the current study, we compared the efficacy of different application methods of B. bassiana mycelia encapsulated in alginate pellets against S. invicta in heavily infested areas. Direct insertion of pellets into S. invicta mounds caused some reduction in activity rating. However, observations of ants removing pellets from mounds shortly after insertion led to the development of a bait system to encourage pellet retention in the mounds. Broadcast applications and individual mound treatments of B. bassiana alginate pellets coated with peanut oil reduced activity ratings of S. invicta populations.