The urban fire ant paradox: native fire ants persist in an urban refuge while invasive fire ants dominate natural habitats

In contrast to the widespread extirpation of native fire ants (Solenopsis geminata) across southern US following the invasion by imported red fire ants (S. invicta), some residential areas of Austin form unexpected refuges for native fire ants. Ironically, these urban environments provide refuges for the native fire ants while adjacent natural habitats have been overrun by invasive fire ants. Resistance to invasive fire ants in these urban areas occurs mainly in older residential properties constructed prior to the S. invicta invasion, while more recent construction has allowed establishment by S. invicta. The invasive ability of S. invicta is often attributed to escape from parasitoids and efficient dispersal of polygyne multiple queen colonies. Here we also show the importance of landscape parameters in the invasion process, where low levels of disturbance and continuous plant cover in older residential areas form possible barriers to colonization. Dense leaf cover (high NDVI) was also found to be associated with native ant refuges. Long term residential land ownership may have resulted in lower recent disturbance levels and increased plant cover that support refuges of native fire ants.     

Successful establishment of the invasive fire ant Solenopsis invicta in Taiwan: insights into interactions of alternate social forms

Aim Understanding the factors underlying the successful establishment of invasive ant species is critical for developing quarantine strategies to prevent additional invasions as well as for determining how such species overcome the selective pressures in invaded areas. Although several studies have revealed differences in the social organization and population genetics of invasive ants in their native and introduced ranges, few studies have considered the potential interactions between alternate social forms within newly‐invaded areas simply because many invasive ants are characterized as polygyne or unicolonial. Location Taoyuan and Chiayi County, Taiwan. Methods Both social forms (polygyne and monogyne) of the red imported fire ant, Solenopsis invicta, occur in two separately invaded areas in Taiwan (Taoyuan and Chiayi). We employed intensive sampling methods and diagnostic polymerase chain reaction assays to determine the distribution of these two social forms in both infested areas in Taiwan. Results The distribution of social forms differs dramatically between the two infested areas, consistent with different invasion histories regarding the original make‐up of individuals comprising the initial founder group. The Taoyuan population likely was colonized initially by ants of both social forms, with the subsequent spread characterized by continuous outward movement of the two forms, particularly the monogyne form. In contrast, the initial founders of the Chiayi population likely were of the polygyne social form only, suggesting the monogyne social form in this population appeared only recently and likely arose directly from polygyne queens converting to the alternate social form. Main conclusions Our results provide detailed insights into the invasion history of S. invicta in Taiwan and suggest that the distinct reproductive biology of the two social forms may have shaped the current distribution of S. invicta in these infested areas and that the dynamics of two forms may affect the long‐term persistence and potential for spread of this pest ant species.     

Are red imported fire ants facilitators of native seed dispersal?

Invasive ants threaten native communities, in part, through their potential to disrupt mutualisms, yet invasive species may also facilitate native species. The red imported fire ant (Solenopsis invicta) is one of the most conspicuous invasive ants in North America and its high densities, combined with its potential to displace native ants, have led to concerns that it may disrupt ant-plant seed dispersal mutualisms. We examined the potential of fire ants to disperse seeds in the longleaf pine ecosystem by comparing the removal of elaiosome-bearing seeds by fire ants versus native ants. A total of 14 ant species were observed removing seeds, with fire ants responsible for more than half of all removals. While fire ants were the dominant seed remover in this system, they did not remove significantly more seeds than would be expected based on their population density (46% of ground-dwelling ants). Moreover, red imported fire ants were similar to native ants with respect to distance of seed movement and frequency of moving seeds back to the nest. Areas of higher fire ant densities were found to have greater rates of seed removal by ants without a subsequent drop in seed dispersal by native ants, suggesting that fire ant-invaded areas may experience overall higher levels of seed dispersal. Thus, fire ants may actually facilitate dispersal of elaiosome-bearing plant species in the longleaf pine ecosystem.     

Patterns of abundance of fire ants and native ants in a native ecosystem

Abstract 1. This correlational study examines the relationship between the red imported fire ant (Solenopsis invicta) and native ants in a longleaf pine savanna. Fire ants are frequently associated with a decline in native ants throughout the invaded range, but fire ant invasion is often coupled with habitat disturbance. Invasion of fire ants into the longleaf pine savanna provides an opportunity to examine the structure of the ant community in the absence of habitat disturbance. 2. Pitfall trapping was conducted within the longleaf pine savanna as well as across a naturally occurring soil moisture gradient, in plots that had been artificially watered. 3. Species richness did not vary as a function of fire ant density. There was an inverse relationship between native ant density and fire ant density, but this abundance pattern does not necessarily imply a causal link between fire ant invasion and native ant decline. For individual species, fire ant densities were negatively correlated with the densities of only two native ant species, including Solenopsis carolinensis, a native species that potentially limits the invasion of fire ants. Additionally, fire ants and native ants respond differently to soil moisture, with native ants favouring drier conditions than fire ants. 4. The possible exclusion of fire ants by some native ants, as well as differences in habitat preferences, provide alternative explanations for the frequently observed negative correlation between fire ants and native ants.     

Forest edges and fire ants alter the seed shadow of an ant-dispersed plant

Exotic species invade fragmented, edge-rich habitats readily, yet the distinct impacts of habitat edges and invaders on native biota are rarely distinguished. Both appear detrimental to ant-dispersed plants such as bloodroot, Sanguinaria canadensis. Working in northeastern Georgia (USA), an area characterized by a rich ant-dispersed flora, fragmented forests, and invasions by the red imported fire ant, Solenopsis invicta , I monitored the interactions between ants and S. canadensis seeds in uninvaded forest interiors, uninvaded forest edges, invaded forest interiors, and invaded forest edges. I observed 95% of the seed dispersal events that occurred within the 60-min observation intervals. Seed collection rates were similar among all four (habitat × invasion) groups. The presence of invasive ants had a strong effect on seed dispersal distance: S. invicta collected most seeds in invaded sites, but was a poorer disperser than four of five native ant taxa. Habitat type (interior versus edge) had no effect on seed dispersal distance, but it had a strong effect on seed dispersal direction. Dispersal towards the edge was disproportionately rare in uninvaded forest edges, and ants in those habitats moved the average dispersed seed approximately 70 cm away from that edge. Dispersal direction was also skewed away from the edge in uninvaded forest interiors and invaded forest edges, albeit non-significantly. This biased dispersal may help explain the rarity of myrmecochorous plants in younger forests and edges, and their poor ability to disperse between fragments. This is the first demonstration that forest edges and S. invicta invasion influence seed dispersal destination and distance, respectively. These forces act independently.     

红火蚁入侵对棉花粉蚧近距离扩散的促进作用

[背景]红火蚁与棉花粉蚧入侵到同一地区,因侵入生境重叠而相遇,进而产生互惠关系。这种互惠关系对红火蚁、棉花粉蚧的生存、扩散传播、入侵的意义和作用规律、机制等是需要解释的生态学问题。[方法]采用田问试验生态学的方法,通过迁移红火蚁蚁巢、向扶桑上接粉蚧等观察研究了红火蚁入侵对棉花粉蚧近距离扩散的影响。[结果]随着离蚁巢距离的增大,扶桑上工蚁数量逐渐减少,距离为1．0、2．0m时数量较多;发生该蚁区域距蚁巢2．0、3．0m扶桑感染粉蚧比率（75％、45％）显著高于无红火蚁区（25％、10％）,其感染比率与工蚁数量呈显著正相关,符合方程Y=0．0042X＋0．1992。[结论与意义]红火蚁入侵促进了棉花粉蚧的近距离扩散,扩散范围在2—3m。研究结果可为深入了解红火蚁与棉花粉蚧协同入侵规律等提供支持。     

Population genetic structure of the red imported fire ant, <Emphasis Type="Italic">Solenopsis invicta</Emphasis>, in Taiwan

We generated and analyzed microsatellite genotypic data and mtDNA sequence data from the fire ant Solenopsis invicta collected from two separate infested areas (Taoyuan and Chiayi) in Taiwan to infer the population and colony structure of these recently established populations. These genetic analyses revealed the following patterns: 1) Relatedness among worker nestmates was significantly greater than zero for both social forms from both populations; 2) No significant isolation by distance was found among nests within each social form from either population; 3) Significant mtDNA but no nuclear differentiation occurs between sympatric social forms in Taoyuan; 4) Molecular signatures of genetic bottlenecks associated with recent introductions are evident in both populations; and 5) The two sampled populations, Taoyuan and Chiayi, are highly genetically differentiated at both the nuclear or mtDNA genomes and most likely derive from two separate introductions into Taiwan. While results from these analyses generally were consistent with predictions based on the known biology of these ants and similar studies of S. invicta in the U.S.A. and South America, some patterns likely reflect the recent introduction and human-mediated inadvertent transport of ants in Taiwan. This is the first study to investigate the population and colony structure of fire ants in Taiwan and results from our study represent an important contribution to the ongoing efforts aimed at eradicating this invasive pest in Taiwan. Received 18 June 2007; revised 11 October 2007; accepted 17 October 2007.     

Experimental evidence for weak effects of fire ants in a naturally invaded pine‐savanna ecosystem in north Florida

1. Fire ants naturally invade some undisturbed ecosystems of high conservation value and may negatively impact co‐occurring ants. 2. Over 3 years, fire ants were added and removed from a longleaf pine savanna ecosystem that naturally supports a low density of fire ants. Impacts on co‐occurring ants were monitored using pitfall traps. 3. Treatments resulted in significant differences in average fire ant abundance across all plots only in the first year of the experiment. Fire ants had little discernible impact. The abundance and species richness of co‐occurring ants in removal plots never differed from unmanipulated control plots. The abundance of co‐occurring ants was very slightly lower and ant species richness was slightly higher where Solenopsis invicta Buren colonies were added, but neither contrast was significant. 4. The poor conditions in this habitat for many native ants may explain this outcome. More broadly, the impact of fire ants on ant assemblages still appears to be secondary and largely a consequence of human impacts on the environment.     

Invasion history of Solenopsis invicta （Hymenoptera： Formicidae） in Fujian,China based on mitochondrial DNA and its implications in development of a control strategy

One of the most invasive species worldwide, Solenopsis invicta Buren, has been described in China since 2003. Recent studies have suggested that China populations are the result of introductions from the USA; however, detailed molecular studies need to be performed in order to understand the expansion and potential multiple introductions from other countries into China. As there were populations of red imported fire ant, S. invicta in different areas and with different methods of introduction, mitochondrial cytochrome oxidase Ⅰ gene was used as a marker from 12 populations in four cities in Fujian Province, China, to determine the relationship of invasion among these populations. The three most common haplotypes previously describe in invasive populations of S. invicta： H5, H22 and H36, were found in Fujian. However, frequencies in each city were different. For instance, three populations from Longyan city which invaded with waste plastics, shared haplotype H5. Populations from Xiamen city and Jinjiang city which dispersed with nursery stock, sward and scrap leather, shared haplotype H22. The population from Nanyan village of Xinluo district, Longyan city, bore haplotype H36. Mitochondrial data reveals that the invasion history of S. invicta in Fujian Province is complex, including multiple invasions probably from other provinces within China. Security measures to prevent S. invicta spreading within China are as important as from overseas.     

The Role of Habitat in the Persistence of Fire Ant Populations

The association of the exotic fire ant, Solenopsis invicta with man-modified habitats has been amply demonstrated, but the fate of such populations if ecological succession proceeds has rarely been investigated. Resurvey of a fire ant population in a longleaf pine plantation after 25 years showed that the recovery of the site from habitat disturbance was associated with a large fire ant population decline. Most of the persisting colonies were associated with the disturbance caused by vehicle tracks. In a second study, mature monogyne fire ant colonies that had been planted in experimental plots in native groundcover of the north Florida longleaf pine forest had mostly vanished six years later. These observations and experiments show that S. invicta colonies rarely persist in the native habitat of these pine forests, probably because they are not replaced when they die. A single site harbored a modest population of polygyne fire ants whose persistence was probably facilitated by reproduction through colony fission.     

Infection characteristics of Solenopsis invicta virus 2 in the red imported fire ant, Solenopsis invicta

Solenopsis invicta virus 2 (SINV-2) is the second virus identified from the fire ant, S. invicta Buren. SINV-2 is unique among positive-strand RNA viruses from insects by possessing four cistrons in a monopartite genome. Fire ant colonies testing positive for SINV-2 by RT-PCR did not exhibit any discernable symptoms. RT-PCR-based surveys for SINV-2 among 688 fire ant mounds in Alachua County, Florida, sampled during the period January, 2006 through December, 2007 showed that the prevalence of SINV-2 among nests ranged from 1.6% to 16.4%. Unlike S. invicta virus 1, no seasonal-associated prevalence was observed with regard to SINV-2 infection among fire ant colonies. No social form specificity was evident; SINV-2 was found in both monogyne and polygyne S. invicta ants. Real-time quantitative PCR experiments showed that SINV-2 genome equivalents per individual ant ranged from 1.9 × 10⁷ in. pupae to 4.3 × 10¹¹ in. inseminated queens. The SINV-2 infection was detected in all ant stages examined (eggs, larvae, pupae, workers, and queens). Tissue tropism studies indicated that the alimentary canal (specifically the midgut) is most likely the susceptible tissue. SINV-2 was successfully transmitted to uninfected S. invicta ants by feeding a partially purified homogenate of SINV-2-infected ants. The SINV-2 transmission rate ranged from 30% to 80%, and both positive (genomic) and negative (replicative) SINV-2 RNA strands accumulated in recipient ants over the course of the experiment. These results indicated that SINV-2 replicates within S. invicta.     

Arthropod prey of imported fire ants (Hymenoptera: Formicidae) in Mississippi sweetpotato fields

The red imported fire ants, Solenopsis invicta (Buren), are generally considered pests. They have also been viewed as beneficial predators feeding on other insect pests of various agroecosystems. This study documents the foraging habits of fire ants in a sweetpotato field in Mississippi. Fire ant foraging trails connecting outside colonies to a sweetpotato field were exposed and foraging ants moving out of the field toward the direction of the colony were collected along with the solid food particles they were carrying. The food material was classified as arthropod or plant in origin. The arthropod particles were identified to orders. Fire ant foragers carried more arthropods than plant material. Coleoptera and Homoptera were the most abundant groups preyed upon. These insect orders contain various economically important pests of sweetpotato. Other major hexapod groups included the orders Hemiptera, Diptera and Collembola. The quantity of foraged material varied over the season. No damage to sweetpotato roots could be attributed to fire ant feeding. Imported fire ant foraging may reduce the number of insect pests in sweetpotato fields.     

红火蚁对新入侵龙眼园和荒草地蚂蚁类群多样性的影响

于2007年7月至2008年2月期间在增城龙眼园及周边的荒草地人工移殖1蚁巢及10蚁巢红火蚁,并采用陷阱法和诱饵诱集法调查研究红火蚁对新入侵龙眼园区和荒草地区蚂蚁类群多样性的影响。结果表明,在荫蔽、少干扰的龙眼园和杂草茂密、少干扰的未割草区,红火蚁难以成功定殖,对这两种生境蚂蚁类群的多样性几乎没有影响;在红火蚁成功定殖的1蚁巢割草区和10蚁巢割草区,红火蚁以这两种密度入侵对蚂蚁类群多样性的影响程度不同。在1蚁巢割草区,红火蚁入侵后5个月期间,由于其数量占蚂蚁个体总数的比例很少,对蚂蚁类群多样性的影响很小;在红火蚁成功定殖5个月后,第6—7个月其工蚁所占比例增加,蚂蚁类群个体数和优势度下降,多样性和均匀度增加;第8—9个月其工蚁所占比例继续增加,而蚂蚁个体数、多样性和均匀度下降,优势度增加。在10蚁巢割草区,红火蚁入侵后,蚂蚁类群个体数、多样性和均匀度下降,优势度增加。红火蚁入侵对1蚁巢割草区和10蚁巢割草区蚂蚁物种数的影响较小。     

Assessing the effects of sodium on fire ant foraging in the field and colony growth in the laboratory

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A case study of human exacerbation of the invasive species problem: transport and establishment of polygyne fire ants in Tallahassee, Florida, USA

Understanding how exotic invasive species are spread is fundamental for ecology and conservation biology. Human transport has become one of the primary modes of dispersal for exotic species. We examined how the multiple queen, or polygyne, social form of the fire ant Solenopsis invicta is spread along roadsides in Tallahassee, Florida, USA. We then determined the likely source of this expanding population, which was a central soil depot. A survey of road maintenance practices in counties of several southeastern states and Texas revealed that the use of a central soil depot is a common practice. Road maintenance therefore may be the primary source for the establishment of new polygyne fire ant populations in this region and elsewhere. Control efforts focused on the soil depots will help to limit further spread of polygyne fire ants and perhaps other invasive organisms, particularly invasive weeds.     

The Effect of Diet and Opponent Size on Aggressive Interactions Involving Caribbean Crazy Ants (Nylanderia fulva)

Biotic interactions are often important in the establishment and spread of invasive species. In particular, competition between introduced and native species can strongly influence the distribution and spread of exotic species and in some cases competition among introduced species can be important. The Caribbean crazy ant, Nylanderia fulva, was recently introduced to the Gulf Coast of Texas, and appears to be spreading inland. It has been hypothesized that competition with the red imported fire ant, Solenopsis invicta, may be an important factor in the spread of crazy ants. We investigated the potential of interspecific competition among these two introduced ants by measuring interspecific aggression between Caribbean crazy ant workers and workers of Solenopsis invicta. Specifically, we examined the effect of body size and diet on individual-level aggressive interactions among crazy ant workers and fire ants. We found that differences in diet did not alter interactions between crazy ant workers from different nests, but carbohydrate level did play an important role in antagonistic interactions with fire ants: crazy ants on low sugar diets were more aggressive and less likely to be killed in aggressive encounters with fire ants. We found that large fire ants engaged in fewer fights with crazy ants than small fire ants, but fire ant size affected neither fire ant nor crazy ant mortality. Overall, crazy ants experienced higher mortality than fire ants after aggressive encounters. Our findings suggest that fire ant workers might outcompete crazy ant workers on an individual level, providing some biotic resistance to crazy ant range expansion. However, this resistance may be overcome by crazy ants that have a restricted sugar intake, which may occur when crazy ants are excluded from resources by fire ants.     

Zombie fire ant workers: behavior controlled by decapitating fly parasitoids

Laboratory observations were conducted on four separate red imported fire ant, Solenopsis invicta, colonies that contained workers parasitized by the decapitating fly, Pseudacteon tricuspis. Parasitized S. invicta workers remained inside the nest during parasitoid larval development and left the nest approximately 8 – 10 hours before decapitation by the parasitoid. When parasitized ants left the nest, they were highly mobile, were responsive to tactile stimuli, and showed minimal defensive behavior. Ants ultimately entered into a grass thatch layer, where they were decapitated and the fly maggots pupariated. This study reveals that parasitized ants exhibit behaviors that are consistent with host manipulation to benefit survival of the parasitoid. Received 9 November 2006; revised 26 January 2007; accepted 7 February.     

COMPARATIVE BIOCHEMICAL GENETICS OF THREE FIRE ANT SPECIES IN NORTH AMERICA,WITH SPECIAL REFERENCE TO THE TWO SOCIAL FORMS OF SOLENOPSIS INVICTA (HYMENOPTERA: FORMICIDAE)

An electrophoretic study of the genetics of three fire ant species in North America was undertaken with the primary objective of further clarifying the genetic relationship between two social forms of Solenopsis invicta. Such social forms are common in many groups of ants and may, in some cases, represent significant intermediate stages in the speciation process. The monogyne and polygyne forms of S. invicta, while differing in a number of important biological traits, are genetically indistinguishable, in contrast to the substantial genetic differentiation observed between S. invicta and a second, closely related, introduced species, S. richteri. The native fire ant, S. geminata, is genetically the most distinct of the three species studied, in accord with its taxonomic placement in a different species complex. Hypotheses concerning the derivation of the polygyne form of S. invicta from the monogyne form which invoke their long-term reproductive isolation in South America and separate introductions to North America appear unfounded. Although S. invicta and S. richteri are known to hybridize in North America, our study provided no evidence of gene introgression between S. invicta and the native species, S. geminata, in areas where our samples were collected. Analyses of population structure in S. invicta failed to reveal significant differentiation of populations or local inbreeding. Levels of genetic diversity in the three species studied, although not significantly different, were in the order predicted from knowledge of the population biology and recent history of the taxa, with S. richteri exhibiting the least diversity, S. geminata the greatest, and S. invicta having an intermediate level.     

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

Mutualistic associations among nonkin can form when animalsin groups have a greater chance of overcoming challenges presentedby the environment than do solitary animals. Colony foundingby small groups of unrelated queens, a habit documented in severalspecies of ants, is often interpreted as a mutualistic interactionselected by intense competition among incipient colonies. However,many new colonies in these species are founded in areas wheretheir chief enemies are mature ant colonies, rather than othernewly founded colonies. In this study, we tested whether groupnest-founding in the fire ant Solenopsis invicta improved theability of queens to survive attacks by mature colonies. Inthe laboratory, queens in groups of three were more likely thansolitary queens to survive attacks by workers of the nativefire ant Solenopsis geminata. When newly mated queens were establishedexperimentally in the field, workers from mature S. invictacolonies caused the majority of queen deaths. Queens in groupsof two, but not in groups of four, had higher survival ratesthan did solitary queens during the period between colony establishmentand the appearance of the first workers. The advantage of cooperativedefense approximately counterbalanced the disadvantages causedby competition within foundress associations of two to threequeens. Previous studies have shown that colonies founded bymultiple queens produce larger worker populations than coloniesfounded by solitary queens; however, experimentally increasingworker number in incipient colonies had no effect on colonysurvival in the field.     

红火蚁入侵强度对本地蚂蚁群落物种的发现-支配权衡的影响

【目的】发现-支配权衡(discovery-dominance trade off)在物种间的竞争起着重要作用。本研究旨在了解不同强度的红火蚁Solenopsis invicta入侵对本地蚂蚁群落物种的发现-支配权衡的影响,从而探讨红火蚁与本地蚂蚁的竞争机制。【方法】于2017年10-11月,在云南省楚雄州牟定县的一滇橄榄种植基地利用诱饵法和陷阱法结合的方式引诱和收集蚂蚁,观察并记录常见蚂蚁种类的发现能力、招募能力、支配能力、最早发现诱饵的时间段及个体数。【结果】红火蚁的入侵显著影响了本地蚂蚁群落物种的发现-支配权衡。强入侵区(5.6个活动蚁巢/100 m~2)蚂蚁群落中物种的相对发现能力和相对支配能力不存在相关性;弱入侵区(0.14个活动蚁巢/100 m~2)蚂蚁群落中两者存在较弱的负相关性,但不显著;对照区(无蚁巢)中两者存在显著负相关性(强入侵区:r=-0.01,P=0.97;弱入侵区:r=-0.60,P=0.21;对照区:r=-0.81,P=0.04)。强入侵区的红火蚁种群普遍在5 min之内就能发现诱饵,而弱入侵区的种群需要10～60 min才能发现诱饵(χ~2_((5))=33.37,P0.001);强入侵区的红火蚁种群在发现诱饵频率和支配行为频率上均显著高于弱入侵区(发现诱饵频率:χ_((1))~2=17.27,P0.001;支配行为频率:χ_((1))~2=6.25,P=0.03),而招募频率在两种入侵区间无显著差异(χ_((1))~2=1.75,P=0.55)。【结论】红火蚁入侵打破了本地蚂蚁群落物种的发现-支配权衡,而且随着红火蚁种群规模的增大,从能够影响到完全打破这种发现-支配权衡。研究结果可为了解红火蚁入侵机制提供证据。