Isozymes of two fire ant species and their hybrid

Electrophoretic banding patterns of eight enzyme systems in Solenopsis xyloni, S. germinata and their hybrid were studied. Repression of paternal alleles was found in aldehyde oxidase, isocitrate dehydrogenase, leucine amino peptidase and malic enzyme.     

Biogeographic effects of red fire ant invasion

The red imported fire ant, Solenopsis invicta , was accidentally introduced to North America over 60 years ago and has spread throughout the southeastern United States. We document the biogeographic consequences of this invasion. We censused ground-foraging ant communities on a 2000 km transect from Florida through New York that passed through invaded and intact biotas. Native ant species density peaks at mid-latitudes in the eastern United States, and the location of this peak corresponds to the range limit of S. invicta . In uninvaded sites, ant species co-occur less often than expected by chance. In the presence of S. invicta , community structure converges to a random pattern. Our results suggest that the effects of S. invicta on native ant communities are pervasive: not only does the presence of S. invicta reduce species density at local scales, it alters the co-occurrence patterns of surviving species at a biogeographic scale.     

红火蚁及其重要近似种的鉴别

介绍了红火蚁及其重要近似种的分类特征和分布,以及火蚁属中国已知种类,还提供了上述种类的鉴别检索表。     

三种植物源药剂对红火蚁的防治效果评估及其对本地蚂蚁群落影响

红火蚁Solenopsis invicta Buren是一种危险性入侵害虫,可对人类健康、农林业生产、公共安全和生态系统产生严重的负面影响。目前,化学农药是防治红火蚁的主要方法。然而,大多数化学农药并不适用于有机农场、水产养殖区等生态敏感区。为筛选出用于生态敏感区红火蚁防治的药剂,本文结合室内和田间试验评估了植物源杀虫剂苦参碱Matrine、鱼藤酮Rotenone和除虫菊素Pyrethrin对红火蚁的防治效果。室内试验结果表明,苦参碱、鱼藤酮和除虫菊素对红火蚁均具有较高的毒性。田间试验表明,25.00 mg/L和12.50 mg/L苦参碱、250.00 mg/L 和125 mg/L鱼藤酮、75.00 mg/L和37.50 mg/L除虫菊素灌巢处理,红火蚁蚁巢减退率分别达到100%、81.25%、87.50%、79.17%、71.73%和66.67%。然而,除25.00 mg/L苦参碱外,其他处理均导致在被灌巢的蚁巢周围短期内出现新蚁巢。灌巢处理后,处理区的蚂蚁多样性指数和均匀度指数升高,优势度指数下降。综上所述,苦参碱、鱼藤酮和除虫菊素适合灌巢处理防治红火蚁,其中0.5%苦参碱可溶性液剂能有效降低红火蚁种群数量,使得蚁巢减退率达100%,同时对本地蚂蚁群落没有负面影响,因此该商品制剂在红火蚁防治方面具有较好的开发潜力。     

Major gene effects on phenotype and fitness: the relative roles of Pgm-3 and Gp-9 in introduced populations of the fire ant Solenopsis invicta

The objective of this study was to disentangle the relative effects of Pgm-3 and Gp-9 and/or other closely linked genes on the phenotypes and reproductive success of queens in introduced (USA) populations of S. invicta. Gp-9 or a closely linked gene(s) was found to have major effects on queen weight, the likelihood that queens shed their wings (a behaviour associated with the onset of reproduction), and the probability that queens are accepted in polygyne (multiple-queen) colonies. Our analyses show that once the effect of Gp-9 genotype is taken into account, Pgm-3 genotype no longer is significantly associated with differences in queen phenotype or the probability of queens being accepted in polygyne colonies. This suggests that the associations of Pgm-3 genotype with weight, wing shedding rate and probability of acceptance by polygyne colonies previously reported in studies that did not control for the effects of Gp-9 are due to the strong linkage disequilibrium that exists between Pgm-3 and Gp-9, or to linkage disequilibria between these and other genes affecting queen phenotype and fitness. Several lines of evidence, including data from the native South American range, suggest that additional cryptic alleles at Gp-9, or additional genes in the same linkage group as Gp-9, must be involved in controlling queen phenotype and the large suite of traits important in determining social organization of S. invicta colonies.     

Effects of Wolbachia on mtDNA variation in two fire ant species

Wolbachia are endosymbiotic bacteria that infect arthropods. As they are maternally transmitted, the spread of Wolbachia variants within host populations may affect host mtDNA evolution. We sequenced a portion of the mitochondrial cytochrome oxidase I gene from numerous individuals of two Wolbachia-infected fire ant species, Solenopsis invicta and S. richteri, to determine how these bacteria influence patterns of mtDNA variation. As predicted, there was a strong association between Wolbachia strain and host mtDNA lineage within and between these fire ant species. However, there was no consistent association between the presence of Wolbachia and a reduction in mtDNA diversity. Moreover, patterns of mtDNA variation within Wolbachia-infected populations did not differ consistently from neutral expectations, despite our prediction that strong positive selection acting on Wolbachia influences the evolutionary dynamics of other cytoplasmic genomes. Specifically, while values of Tajima's D consistently were less than zero for all six samples of fire ants harbouring Wolbachia, MacDonald-Kreitman tests suggested that the patterns of variation were different from those expected under neutrality in only two of the samples. We conclude that these neutrality tests do not unambiguously reveal a clear effect of Wolbachia infection on patterns of mtDNA variation and substitution in fire ants. Finally, consistent with an earlier study, our data revealed the presence of two divergent mtDNA haplotype lineages and Wolbachia strains within S. invicta. Recognition of these two lineages has important consequences for interpreting patterns of mtDNA evolution and genetic differentiation between conspecific social forms of this species.     

The effects of fire on ant trophic assemblage and sex allocation

Fire plays a key role in ecosystem dynamics worldwide, altering energy flows and species community structure and composition. However, the functional mechanisms underlying these effects are not well understood. Many ground‐dwelling animal species can shelter themselves from exposure to heat and therefore rarely suffer direct mortality. However, fire‐induced alterations to the environment may change a species' relative trophic level within a food web and its mode of foraging. We assessed how fire could affect ant resource utilization at different scales in a Mediterranean forest. First, we conducted isotopic analyses on entire ant species assemblages and their potential food resources, which included plants and other arthropods, in burned and unburned plots 1 year postfire. Second, we measured the production of males and females by nests of a fire‐resilient species, Aphaenogaster gibbosa, and analyzed the differences in isotopic values among workers, males, and females to test whether fire constrained resource allocation. We found that, in spite of major modifications in biotic and abiotic conditions, fire had little impact on the relative trophic position of ant species. The studied assemblage was composed of species with a wide array of diets. They ranged from being mostly herbivorous to completely omnivorous, and a given species' trophic level was the same in burned and unburned plots. In A. gibbosa nests, sexuals had greater δ¹⁵N values than workers in both burned and unburned plots, which suggests that the former had a more protein‐rich diet than the latter. Fire also appeared to have a major effect on A. gibbosa sex allocation: The proportion of nests that produced male brood was greater on burned zones, as was the mean number of males produced per nest with the same reproductive investment . Our results show that generalist ants with relatively broad diets maintained a constant trophic position, even following a major disturbance like fire. However, the dramatically reduced production of females on burned zones compared to unburned zones 1 year postfire may result in considerably reduced recruitment of new colonies in the mid to long term, which could yield genetic bottlenecks and founder effects. Our study paves the way for future functional analyses of fire‐induced modifications in ant populations and communities.     

Traits allowing some ant species to nest syntopically with the fire ant Solenopsis saevissima in its native range

Supercolonies of the red fire ant Solenopsis saevissima (Smith) develop in disturbed environments and likely alter the ant community in the native range of the species. For example, in French Guiana only 8 ant species were repeatedly noted as nesting in close vicinity to its mounds. Here, we verified if a shared set of biological, ecological, and behavioral traits might explain how these 8 species are able to nest in the presence of S. saevissima. We did not find this to be the case. We did find, however, that all of them are able to live in disturbed habitats. It is likely that over the course of evolution each of these species acquired the capacity to live syntopically with S. saevissima through its own set of traits, where colony size (4 species develop large colonies), cuticular compounds which do not trigger aggressiveness (6 species) and submissive behaviors (4 species) complement each other.     

红火蚁觅食行为及其在防治中的应用

红火蚁Solenopsis invicta Buren是极具危害性的入侵生物,已在我国多个省份地区扩散成灾。近年来,国内外在红火蚁觅食行为领域取得了一系列研究进展,为红火蚁防控提供了新的思路与方法。本文回顾了红火蚁觅食行为的相关研究,综述了红火蚁觅食的过程、行为机制及影响因素,并分析了觅食行为与防治的联系。红火蚁可广泛地采集和利用不同类型、大小和成分的食物。其觅食过程分为食物搜寻、搬运与处理等阶段,在这一过程中也可能伴随着食物埋藏、铺路越障和工具利用等行为。目前已报道多种跟踪信息素和关键基因可以调控红火蚁的觅食行为。此外,多种生物和非生物因素也会影响红火蚁的觅食行为和策略。基于红火蚁特殊的觅食行为模式,有望开发出专一性更强、对非靶标生物安全的监测和防控技术。建议在今后的研究中进一步探索红火蚁觅食行为的模式及其分子和化学机制,更好地将理论研究与防治实践相结合,控制红火蚁疫情蔓延危害。     

红火蚁与两种本地蚂蚁间的干扰竞争

为了探讨重大入侵生物红火蚁Solenopsis invicta Buren与本地蚂蚁的竞争机制以及红火蚁成功入侵的机理, 本研究以红火蚁和2种本地蚂蚁黑头酸臭蚁Tapinoma melanocephalum (Fabricius)及亮红大头蚁Pheidole fervida Smith为材料, 室内测定了红火蚁分别与黑头酸臭蚁、亮红大头蚁之间在个体水平和种群水平上的攻击性和攻击强度。一对一攻击试验结果表明：红火蚁和黑头酸臭蚁之间攻击级别多集中在Ⅲ级, 即竞争优势明显的红火蚁对黑头酸臭蚁缺乏激烈的攻击, 只是在相互攻击时多摆出威胁姿势; 红火蚁与亮红大头蚁之间攻击性较强, 尤其是红火蚁中型和小型工蚁与亮红大头蚁兵蚁之间的攻击性（3.35和3.30）显著强于红火蚁大型工蚁与亮红大头蚁兵蚁的攻击性（2.70）。群体攻击试验结果表明：与黑头酸臭蚁群体攻击的红火蚁各处理组合中, 无死亡红火蚁出现, 而黑头酸臭蚁死亡率为31.80%; 而与亮红大头蚁群体攻击中, 红火蚁死亡率为0.20%~12.00%, 而亮红大头蚁平均死亡率为49.91%。可见, 红火蚁的群体攻击能力强于黑头酸臭蚁和亮红大头蚁, 其中红火蚁与亮红大头蚁间的相互攻击程度激烈, 死亡率较高, 而与黑头酸臭蚁间的攻击程度较弱, 可能由于黑头酸臭蚁化学防御对本身起到一定的保护作用, 这为进一步加强红火蚁发生区本地蚂蚁优势种--黑头酸臭蚁的保护利用提供理论基础。     

The possible advantage of myrmecochory in diplochorous species: A test on two Mediterranean Euphorbia species

Myrmecochory commonly complements the advantages of ballistic dispersal in diplochorous species. We studied the role of the elaiosome in two populations of the two diplochorous Mediterranean spurges Euphorbia boetica and E. nicaeensis, which share an efficient ballistic dispersal mechanism followed by secondary removal by ants. They differ in elaiosome persistence, as most E. boetica seeds lose the elaiosome during explosive dispersal. Self-assessed dietary preferences with seeds with and without elaiosomes of each species showed differences in behaviour among and within ant species. In general, the absence of elaiosome entailed a decrease in the number of disperser ant species interacting with the seeds, whereas the number of predatory ants remains invariable. However, in one population of E. nicaeensis, experimental elimination of the elaiosome did not affect seed removal by mutualistic ants. On the other hand, analysis of refuse piles of the granivorous Messor marocanus and M. bouvieri suggests that they act as seed predators in E. boetica, whereas unintentional dispersal can be important in E. nicaeensis. We suggest, therefore, that the presence of the elaiosome in the seeds of the studied spurges increases the interaction with disperser ant species, but the possible dispersal advantage is not apparent and is spatially variable.     

Testing myrmecochory from the ant’s perspective: The effects of Datura wrightii and D. discolor on queen survival and brood production in Pogonomyrmex californicus

Interspecific interactions are often assumed to be mutualistic if one species appears to benefit. However, most studies do not test whether both participants benefit. Myrmecochory, or seed dispersal by ants, is characterized by a lipid-rich appendage, or elaiosome, on a seed. Typically, ants gather the diaspores (i.e., seeds with elaiosomes), carry them to the nest, consume the elaiosome, and discard the seed unharmed either inside the nest or on a refuse pile. The benefit to the ants is presumably the nutritional content of the elaiosome, whereas benefits to the plant include dispersal from the parent plant, protection from predators, reduced seedling competition, protection from fire, or transportation to nutrient-rich microsites. Most studies of myrmecochory focus on potential benefits to the plants and simply assume that ants receive a benefit from consuming elaiosomes. I tested whether Pogonomyrmex californicus benefits from consuming Datura wrightii and D. discolor elaiosomes by raising newly-mated queens (i.e., foundresses) on different diets and measuring their survival and brood production. Foundresses reared solely on D. wrightii or D. discolor had similar probabilities of surviving and producing brood as foundresses fed a standard diet, but the number and developmental stage of the brood produced was severely reduced. Because the initial number of brood produced is critical for successful colony establishment, the future fitness of foundresses consuming only Datura is likely reduced. In addition, adding Datura to a standard diet did not increase queen survival or brood production. Although it is possible that Datura may help sustain a colony through periods of scarcity, P. californicus do not appear to receive nutritional benefits from myrmecochorous interactions with Datura in the northern Sonoran Desert. Received 4 July 2005; revised 17 April 2006; accepted 9 May 2006.     

Phylogeographic structure of the fire ant Solenopsis invicta in its native South American range: roles of natural barriers and habitat connectivity

We generated mitochondrial DNA (mtDNA) sequence data from 402 individuals of the fire ant Solenopsis invicta collected from 11 native populations and analyzed these data using a combination of demographic, phylogenetic, and phylogeographic methods to infer features of the evolutionary history of this species. Prior expectations regarding high levels of genetic structure and isolation by distance among populations were supported by the data, but we also discovered several unanticipated patterns. Our analyses revealed a major genetic break between S. invicta mtDNA haplotypes that coincides with the Mesopotamia wetlands region of South America, resulting in two higher level nested clade groupings. In addition, we identified contrasting patterns of genetic differentiation within these two major groups, which may reflect differences in connectivity of suitable habitat in different parts of the native range of S. invicta. Our study represents the first attempt to understand the phylogeographic history of S. invicta across its native range.     

Discontinuous gas exchange in the fire ant,Solenopsos invicta Buren: Caste differences and temperature effects

The discontinuous gas exchange cycle (DGC), the cyclic release of CO(2) and uptake of O(2), were investigated in workers and female and male alates of the red imported fire ant, Solenopsis invicta Buren, using real-time CO(2) emission measurement by flow-through respirometry. All S. invicta castes displayed discontinuous emission of CO(2) in the temperature range of 15-25 degrees C, but only male alates and workers exhibited a DGC at 30 degrees C. The closed (C) and flutter (F) periods of the DGC were distinguishable in alates of both sexes at the lowest temperature, but not clearly differentiated in females at temperatures above 15 degrees C, in males above 20 degrees C, or workers at any temperature. DGC frequency increased for all castes as temperature increased, ranging from a low of 0.9+/-0.05 mHz (male alates at 15 degrees C) to 18+/-0.79 mHz (workers at 30 degrees C). O period (or burst) volumes of all castes decreased as temperature increased, and increased with body mass - this mass effect was most pronounced at lower temperatures. Q(10) values for DGC frequency (4.27, 5.81, and 5.62 for workers, female and male alates, respectively) were high compared with Q(10)'s for standard Vdot;(CO(2)). Differences in the salient characteristics of the DGC between castes are presented and discussed, and S. invicta DGC patterns are compared to known values for some other ant species.     

Changes in reproductive roles are associated with changes in gene expression in fire ant queens

In species with social hierarchies, the death of dominant individuals typically upheaves the social hierarchy and provides an opportunity for subordinate individuals to become reproductives. Such a phenomenon occurs in the monogyne form of the fire ant, Solenopsis invicta, where colonies typically contain a single wingless reproductive queen, thousands of workers and hundreds of winged nonreproductive virgin queens. Upon the death of the mother queen, many virgin queens shed their wings and initiate reproductive development instead of departing on a mating flight. Workers progressively execute almost all of them over the following weeks. To identify the molecular changes that occur in virgin queens as they perceive the loss of their mother queen and begin to compete for reproductive dominance, we collected virgin queens before the loss of their mother queen, 6 h after orphaning and 24 h after orphaning. Their RNA was extracted and hybridized against microarrays to examine the expression levels of approximately 10 000 genes. We identified 297 genes that were consistently differentially expressed after orphaning. These include genes that are putatively involved in the signalling and onset of reproductive development, as well as genes underlying major physiological changes in the young queens.     

Sex and clonality in the little fire ant

Reproduction systems are controlling the creation of new genetic variants as well as how natural selection can operate on these variants. Therefore, they had historically been one of the main foci of evolutionary biology studies. The little fire ant, Wasmannia auropunctata, has been found to display an extraordinary reproduction system, in which both males and female queens are produced clonally. So far, native sexual populations of W. auropunctata have not been identified. Our goals were to identify such sexual populations and investigate the origins of female parthenogenesis and male clonality. Using mitochondrial DNA and microsatellite markers in 17 native populations, we found that traditional sexual populations occurred in W. auropunctata and are likely the recent source of neighboring clonal populations. Queen parthenogenesis has probably evolved several times through mutational events. Male clonality is tightly linked to queen parthenogenesis and thus appears to be female controlled. Its origin could be accounted for by 2 mutually exclusive hypotheses: either by the expected coevolution of the 2 sexes (i.e., a variant of the maternal genome elimination hypothesis) or by a shared mechanistic origin (i.e., by the production of anucleate ovules by parthenogenetic queens). Our results also show that W. auropunctata males and females do not form separate evolutionary units and are unlikely to be engaged in an all-out battle of sexes. This work opens up new perspectives for studies on the adaptive significance and evolutionary stability of mixed sexual and clonal reproduction systems in living organisms.     

Ant-seed interactions: combined effects of ant and plant species on seed removal patterns

Seed dispersal by ants (i.e. myrmecochory) is usually considered as a mutualism: ants feed on nutritive bodies, called elaiosomes, before rejecting and dispersing seeds in their nest surroundings. While mechanisms of plant dispersal in the field are well documented, the behaviour of the ant partner was rarely investigated in details. Here, we compared in laboratory conditions the foraging behaviour of two ant species, the omnivorous Lasius niger and the insectivorous Myrmica rubra to which seeds of two European myrmecochorous plants (Chelidonium majus and Viola odorata) were given. Ant colonies were simultaneously presented three types of items: entire seeds with elaiosome (SE), seeds without elaiosome (S) and detached elaiosomes (E). The presence of elaiosomes on seeds did not attract workers from a distance since ants first contact equally each type of items. Although ants are mass-recruiting species, we never observed any recruitment nor trail-laying behaviour towards seeds. For ants having contacted seed items, their antennation, manipulation and seed retrieval behaviour strongly varied depending on the species of each partner. Antennation behaviour, followed by a loss of contact, was the most frequent ant-seed interaction and can be considered as a “hesitation” clue. For both plant species, insectivorous Myrmica ants removed items in larger number and at higher speed than Lasius. This fits with the hypothesis of a convergence between odours of elaiosomes and insect preys. For both ant species, the small Chelidonium seeds were retrieved in higher proportion than Viola ones, confirming the hypothesis that ants prefer the higher elaiosome/diaspore-ratio. Thus, in these crossed experiments, the ant-plant pair Myrmica/Chelidonium was the most effective as ants removed quickly almost all items after a few antennations. The presence of an elaiosome body increased the seed removal by ants excepting for Myrmica which retrieved all Chelidonium seeds, even those deprived of their elaiosome. After 24 h, all the retrieved seeds were rejected out of the nest to the refuse piles. In at least half of these rejected items, the elaiosome was discarded by ants. Species-specific patterns and behavioural differences in the dynamics of myrmecochory are discussed at the light of ant ecology. Received 10 September 2007; revised 5 February 2008; accepted 5 March 2008.     

Associations of fire ant phorids and microhabitats

We examined flight activity patterns for a guild of fire ant parasitoids in western Argentina in relationship to their host's location (mound/foraging trail) and light condition (full sun/partial sun/full shade) at different scales, from the individually sampled mound to the full day's summation for each species. We asked first whether taxa showed preferences among these conditions, and second, whether certain species and sexes might be found together more frequently than expected to by chance. All species, except the P. obtusus species complex, were significantly more likely to be found attacking ants at disturbed mounds than at paired foraging trails. The P. nocens complex and P. litoralis were more likely to be in the shade when temperatures were above the overall mean of the study (28.3 degrees C), whereas others, such as the P. obtusus complex and P. tricuspis, were more likely to be in full sun under these same conditions. Our analyses indicated that a limited set of species, particularly P. nocens with P. litoralis, and males with female P. obtusus and P. tricuspis, were more likely to be found together than expected. We also found decreasing proportions of males with increasing time of analysis. We discuss the implications of host location, metereological conditions, and sex ratios in relationship to ongoing classical biological control efforts using species of these phorids.     

The effects of fire on ant communities in north-western Patagonia: the importance of habitat structure and regional context

Abstract. We investigated the effects of recent fires on the native ant communities in two habitats of north-west Patagonia that differ in vegetation structural complexity. Using bait traps, we sampled ants in replicated scrub and steppe areas including paired burned and unburned sites. Fires significantly reduced plant cover and ant diversity only in scrub sites. The drop in diversity was due to (a) a reduction in the abundance of rare species associated with woody vegetation, and (b) an increase in the abundance of the dominant species, which thrive in more xeric microclimatic conditions. Consequently, ant assemblage structure of burned scrub approaches that of steppe sites. Our findings suggest that the effects of disturbances on ant assemblages depends both on habitat characteristics, which in turn determine the extent of the changes induced by the disturbance, and on the regional context of the ant fauna, which in turn determines the ability of the ants to deal with the post-disturbance conditions.