Defensiveness of the fire ant, Solenopsis invicta, is increased during colony rafting

Colonies of the fire ant, Solenopsis invicta, can survive flood conditions by forming a raft of ants that floats on the water’s surface until the flood recedes or higher ground is found. Having been forced from the protection of their subterranean nests, rafting colonies are totally exposed and are without retreat. I tested the hypothesis that rafting S. invicta colonies would compensate for their elevated vulnerability by increasing their defensiveness. I measured defensiveness using the amount of venom workers delivered per sting (venom dose), since the repellent effects (i.e., pain and tissue damage) of fire-ant venom are dose-dependent. In the laboratory I assayed colony defensiveness before and after flooding colonies from their nests with water. Colonies were consistently and significantly more defensive while rafting (i.e., each colony’s workers delivered higher venom doses when their colony was rafting than they did when it was assayed pre-flood). The larger venom doses of rafting colonies may reduce their chances of being damaged by encounters with other animals by reducing the duration of such encounters through increased repellency. Encounters with S. invicta during flood conditions have the potential to be unusually dangerous; large concentrations of workers are exposed and available for defense, and they deliver significantly larger venom doses when they sting. Received 29 March 2005; revised 20 June 2005; accepted 24 June 2005.     

Modelling the potential spread of Solenopsis invicta Buren (Hymenoptera: Formicidae) (red imported fire ant) in Australia

Abstract  Quantifying the potential spread and density of an invading organism enables decision-makers to determine the most appropriate response to incursions. We present two linked models that estimate the spread of Solenopsis invicta Buren (red imported fire ant) in Australia based on limited data gathered after its discovery in Brisbane in 2001. A stochastic cellular automaton determines spread within a location (100 km by 100 km) and this is coupled with a model that simulates human-mediated movement of S. invicta to new locations. In the absence of any control measures, the models predict that S. invicta could cover 763 000–4 066 000 km² by the year 2035 and be found at 200 separate locations around Australia by 2017–2027, depending on the rate of spread. These estimated rates of expansion (assuming no control efforts were in place) are higher than those experienced in the USA in the 1940s during the early invasion phases in that country. Active control efforts and quarantine controls in the USA (including a concerted eradication attempt in the 1960s) may have slowed spread. Further, milder winters, the presence of the polygynous social form, increased trade and human mobility in Australia in 2000s compared with the USA in 1940s could contribute to faster range expansion.     

Development of Pseudacteon nocens (Diptera: Phoridae) on Solenopsis invicta and Solenopsis richteri fire ants (Hymenoptera: Formicidae)

We studied the developmental performance of the large morph of Pseudacteon nocens Borgmeier (Diptera: Phoridae), a prospective biological control agent of imported fire ants (Hymenoptera: Formicidae). We measured selected life history traits of this parasitoid as a function of 1) host species (Solenopsis invicta Buren versus Solenopsis richteri Forel), 2) temperature (22 versus 28 degrees C), 3) source population of the fly (Corrientes and Santiago del Estero, Argentina), and 4) varied size distributions of offered host ants. Developmental periods were influenced by host species, although the populations responded in opposing manners. Developmental times, however, were most strongly influenced by temperature with total developmental periods lengthened by 17-32% at 22 degrees C. Pupal mortality was also significantly lower at this temperature. Although numbers of progeny per female were significantly higher for the Corrientes population, we found no significant differences in progeny per female according to host species. Interestingly, we found that females were larger than males, and flies from Corrientes were larger than those from Santiago del Estero, even after statistical adjustments for host size. The modal frequency of host size elected in all treatment combinations tested was identical (0.6 mm), a size that represented the apparent threshold for producing female progeny. These laboratory and additional field observations demonstrate considerable interpopulational variation in P. nocens and lend further support to the applied approach focusing at the population, as opposed to the species level, with respect to both source and target areas for classical biological control introductions of Pseudacteon flies.     

Contrasting exotic Solenopsis invicta and native Forelius pruinosus ants as mutualists with Catalpa bignonioides, a native plant

Abstract. 1. The suitability of the red imported fire ant Solenopsis invicta Buren and a native ant Forelius pruinosus (Roger) as participants in a food-for-protection mutualism with a native nectaried tree Catalpa bignonioides Walter was compared.
2. The mean mortality of folivore larvae of Ceratomia catalpae Boisduval was similar for S. invicta and F. pruinosus although S. invicta attacked fewer caterpillar aggregations and was a devastating pupal predator. Solenopsis invicta also differed from the native ant in that it attacked the parasitoid Cotesia congregata Say, another plant mutualist, and visited extrafloral nectaries less frequently.
3. Habitats invaded by S. invicta are characterised by a scarcity of both herbivores and of beneficial insects that visit extrafloral nectaries. The plants do not require protection, and extrafloral nectaries are visited rarely. Although plants are defended incidentally by S. invicta , the insect-plant mutualism therein is greatly simplified or defunct.     

Characterization of 24 microsatellite markers in 11 species of fire ants in the genus Solenopsis (Hymenoptera: Formicidae)

The social parasite ant Solenopsis daguerrei infests colonies of several mound-building fire ant species. Twenty-four microsatellite markers were isolated from a repeat-enriched genomic library of S. daguerrei. Eleven loci were polymorphic in this ant with two to six alleles per locus. Expected heterozygosity ranged from 0.0222 to 0.7940 among loci. Most microsatellites amplified successfully across the 11 Solenopsis species tested and will be useful for evolutionary genetic studies in this diverse ant group.     

红火蚁复眼的扫描电镜观察

了解红火蚁Solenopsis invicta Buren复眼形态结构及其与不同性别、品级的关系,为探索其基于视觉行为习性的、有效的非化学防控措施提供新思路和依据。采用扫描电镜技术,比较研究红火蚁工蚁、有翅雌蚁、雄蚁的复眼形态差异。结果表明:(1)工蚁复眼圆形,略外凸,小眼数约110个;雌蚁复眼长椭圆形,外凸,小眼数约510个;雄蚁复眼近半球形,小眼数约805个;(2)工、雌和雄蚁复眼中心区域小眼排列较紧密,多为较规则的五、六边形,边缘区域小眼排列不紧密,多为不规则的四至六边形,且少量相邻小眼的间距较大。工蚁、雌蚁和雄蚁复眼小眼面积大小依次为500,360,348.61μm2,同品级内小眼面大小相差不大;(3)雌、雄蚁复眼中心区域近背区小眼间着生少量感觉毛,感觉毛长度和直径依次为:雌蚁17.5~90.2,2.16~4.29μm,雄蚁17.5~27.9,1.41~2.52μm。表明雌蚁、雄蚁复眼及视力较发达,工蚁则较弱,不同性别或品级个体复眼的形状、小眼数目和形状、表面被物均有较大差异和区域性分化。     

Response of Solenopsis invicta (Hymenoptera: Formicidae) to potassium oleate water solution

Abstract In contrast to floating on the surface of distilled water, ants were immediately submerged after being placed in the potassium oleate (PO) water solution, which led to immobilization within minutes. However, some workers survived after being immersed in 0.03% PO water solution at 25°C for up to 640 min. Elevated temperature of the PO water solution is needed to kill ants within a shorter time frame. At 50°C and 0.13% PO concentration, total mortality was achieved with 10-min immersion for all ants, including brood and adult ants. Soil has a negative effect on the effectiveness of potassium oleate; however, such negative effect can be overcome by increasing either treatment temperature or duration of the treatment. In addition to immobilizing and lethal effect, PO repels ants. PO may have the potential to be incorporated into immersion treatment for the quarantine of imported fire ants to reduce the use of synthetic contact insecticides.     

Sources of spores for the possible horizontal transmission of Thelohania solenopsae (Microspora: Thelohaniidae) in the red imported fire ants, Solenopsis invicta

We screened adult and larval secretions and midden piles for the presence of Thelohania solenopsae spores to decipher potential sources for the horizontal transmission of the pathogen in fire ants. Hemolymph samples from both adult and larvae were also screened to rule out hemolymph contamination of samples. In adults, Thelohania spores were found in the crop and the fecal fluids, although only free spores were found in the fecal fluids of adults. In fourth instar larvae, both free and octospores were seen in midgut and the meconium samples. All of the midden pile samples had T. solenopsae spores of both types. Based on these results, we theorize that the pathogen may be horizontally transmitted within a colony by the removal and sharing of meconium of prepupating fourth instar larvae by adult workers and by the adult fecal droppings, and intercolonially by contamination of midden piles or brood raiding.     

Molecular evolutionary analyses of the odorant-binding protein gene Gp-9 in fire ants and other Solenopsis species

The fire ant Solenopsis invicta exists in two social forms, one with colonies headed by a single reproductive queen (monogyne form) and the other with colonies containing multiple queens (polygyne form). This variation in social organization is associated with variation at the gene Gp-9, with monogyne colonies harboring only the B allelic variant and polygyne colonies containing b-like variants as well. We generated new Gp-9 sequences from 15 Solenopsis species and combined these with previously published sequences to conduct a comprehensive, phylogenetically based study of the molecular evolution of this important gene. The exon/intron structure and the respective lengths of the five exons of Gp-9 are identical across all species examined, and we detected no evidence for intragenic recombination. These data conform to a previous suggestion that Gp-9 lies in a genomic region with low recombination, and they indicate that evolution of the coding region in Solenopsis has involved point substitutions only. Our results confirm a link between the presence of b-like alleles and the expression of polygyny in all South American fire ant species known to possess colonies of both social forms. Moreover, phylogenetic analyses show that b-like alleles comprise a derived clade of Gp-9 sequences within the socially polymorphic species, lending further support to the hypothesis that monogyny preceded polygyny in this group of fire ants. Site-specific maximum likelihood tests identified several amino acids that have experienced positive selection, two of which are adjacent to the inferred binding-pocket residues in the GP-9 protein. Four other binding-pocket residues are variable among fire ant species, although selection is not implicated in this variation. Branch-specific tests revealed strong positive selection on the stem lineage of the b-like allele clade, as expected if selection drove the amino acid replacements crucial to the expression of polygyne social organization. Such selection may have operated via the ligand-binding properties of GP-9, as one of the two amino acids uniquely shared by all b-like alleles is predicted to be a binding-pocket residue.     

Diversity and virulence of soil-dwelling fungi Isaria spp. and Paecilomyces spp. against Solenopsis invicta (Hymenoptera: Formicidae)

To comprehend the diversity and potential control of soil-dwelling fungi, Isaria and Paecilomyces, against the red imported fire ant Solenopsis invicta (Hymenoptera, Formicidae), an investigation was carried out between 2004 and 2008. From 258 soil samples collected from 16 central and southern provinces and cities in China, a total of 171 isolates of the genra Isaria and Paecilomyce were isolated, and the species I. javanicus, P. marquandii and I. fumosoroseus were found more abundant than I. cateniobliquus, P. carneus, P. inflatus and P. lilacinus. Geographic differences of isolating rates were observed as well. Samples from the southern areas had higher fungal isolating rates than those from the central areas. Subsequently, 47 isolates were further tested for pathogencity against the red imported fire ant. All isolates except P115 showed certain pathogenic potential (the mean is 52.3% at 4000 conidiospores/mL) to the ant. I. javanicus was the most effective species with a mean pathogenicity of 80.6%, while pathogenicities of P. marquandii, P. gunni and I. fumosoroseus were 44, 21 and 49%, respectively. Furthermore, the more effective isolates P028 of I. javanicus and P003 of I. fumosoroseus were tested in a virulence experiment. The LD₅₀ values of P028 and P003 against major and miner workers were determined as 412,280 and 854,451 conidiospores/cm², respectively. Meanwhile, the LT₅₀ values at 1000 conidiospores/cm² were 7.1 and 6.6 d in isolate P003 and 6.8 and 6.6 d in isolate P028.     

A comparison of queen oviposition rates from monogyne and polygyne fire ant, Solenopsis invicta, colonies

Abstract. The oviposition rate of individual queens of Solenopsis invicta Buren (Hymenoptera: Formicidae) in relation to their weight and number of queens present in the colony was investigated by direct 2 h observations. There is a strong positive correlation between the weight of a queen and its oviposition rate in both monogyne and polygyne colonies. However, the number of eggs laid per mg queen is higher for moonogyne queens than for polygyne queens. This difference is more evident when the total weight of queens present in a colony is considered. The individual queen oviposition rate is negatively correlated with the number of queens in the colony. In addition, the weight loss per egg laid is significantly greater for polygyne than for monogyne queens, probably due to differences in egg size. These data suggest that oviposition is more efficient in monogyne than in polygyne queens at the individual level; however, at the colony level, polygyne colonies produce significantly more eggs. Comparison of colony level efficiency predicts that polygyne colonies must have at least nine queens to compete reproductively with a mature monogyne queen. Therefore, oligogyny does not appear to be a viable strategy for S.invicata.     

Fire ant myrmecophiles: feeding relationships ofMartinezia dutertrei andEuparia castanea (Coleoptera: Scarabaeidae) with their host ants,Solenopsis spp. (Hymenoptera: Formicidae)

Summary Feeding relationships of adultEuparia castanea Serville andMartinezia dutertrei Chalumeau with their ant hosts were studied in the laboratory using the radioactive tracer³²P.Euparia castanea was tested withSolenopsis geminata (F.),Martinezia dutertrei Chalumeau was tested withS. invicta Buren,S. richteri Forel, andS. geminata. Unlabeled beetles were exposed to various radioisotope labeled conditions for 24 hr and then checked for acquired radioactivity. In whole colony tests, both species of beetles acquired radioactivity.M. dutertrei obtained food from live ants, butE. castanea did not. Both species of beetles ate ant larvae.E. castanea also obtained food from ant larvae by strigilation. Neither species of beetle fed on ant feces or other secretions on the substrate. Both species of beetles obtained food by strigilation from fresh and decomposed worker ant cadavers.M. dutertrei also ate both kinds of ant cadavers. Both species of beetles also ate dead house flies, indicative of scavenging or feeding on ant booty.Martinezia dutertrei showed no preference for any particular ant species. Ants did not obtain food by trophallaxis or glandular secretion from either species of beetle. Martinezia dutertrei Chalumeau, 1983 (=Myrmecaphodius excavaticollis Auct.,nec Blanchard 1843).This article reports the results of research only. Mention of a proprietary product does not constitute an endorsement or a recommendation for its use by the U.S. Department of Agriculture.     

Detection of Thelohania solenopsae (Microsporidia: Thelohaniidae) in Solenopsis invicta (Hymenoptera: Formicidae) by multiplex PCR

Oligonucleotide primer pairs were designed to unique areas of the small subunit (16S) rRNA gene of Thelohania solenopsae and a region of the Gp-9 gene of Solenopsis invicta. Multiplex PCR resulted in sensitive and specific detection of T. solenopsae infection of S. invicta. The T. solenopsae-specific primer pair only amplified DNA from T. solenopsae and T. solenopsae-infected S. invicta. This primer pair did not produce any amplification products from DNA preparations from uninfected S. invicta, seven additional species of microsporidia (including Vairimorpha invictae), or Mattesia spp. The Gp-9-specific primers recognized and amplified DNA from Solenopsis xyloni, Solenopsis richteri, Solenopsis geminata, the invicta/richteri hybrid, and monogyne and polygyne S. invicta, but not from T. solenopsae, and, as such, served as a positive control verifying successful DNA preparation. Multiplex PCR detected T. solenopsae in worker fire ants infected with as few as 5000 spores. Furthermore, multiplex PCR detected T. solenopsae in all developmental stages of S. invicta. However, detection could be made more sensitive by using only the T. solenopsae-specific primer pair; ants infected with as few as 10 spores were able to be discerned. Multiplex PCR detection of T. solenopsae offers the advantages of a positive control, a single PCR amplification, detection of all developmental stages, and increased sensitivity and specificity compared with microscopy.     

Relationships between the ant Brachymyrmex obscurior (Hymenoptera, Formicidae) and Acacia pennatula (Fabaceae)

Summary. In central Mexico, the ant Brachymyrmex obscurior Forel feeds on nectar produced by extrafloral nectaries of Acacia pennatula (Schlecht. & Cham.) Benth. However, no studies have determined whether the ants visitation is related to plant nectar availability and whether ants protect A. pennatula from herbivory. The objectives of this 2-yr study (2000–2001) were to assess whether seasonal changes in ant visitation coincide with extrafloral nectar productivity in A. pennatula and to determine whether ants protect the plant. At the end of the dry season (April–June) B. obscurior was the only ant species on A. pennatula and extrafloral nectar production is limited to this period. Exclusion experiments, performed at the end of the dry season showed that A. pennatula did not receive a protective benefit when visited by ants. Branches with ants and branches where ants are excluded had similar numbers of the nonmyrmecophile leafhopper Sibovia sp. which was the only herbivore observed under natural conditions.Received 24 March 2004; revised 4 September 2004; accepted 8 September 2004.     

研究红火蚁工蚁活动性的新方法—刻度图示法

应用一种新方法——刻度图示法研究红火蚁工蚁Solenopsis invicta Buren群体活动性规律,结果表明,标尺上红火蚁最高爬升高度、平均爬升高度、工蚁恢复平静时间、工蚁累计数量均随着蚁巢体积增大而增大,通过分析建立红火蚁工蚁活动性程度各指标值与蚁巢大小关系多个模型。     

Use of ribosomal DNA sequence data to characterize and detect a neogregarine pathogen of Solenopsis invicta (Hymenoptera: Formicidae)

A neogregarine parasite of the red imported fire ant, Solenopsis invicta, was discovered recently in Florida and tentatively placed in the Mattesia genus based on morphological characterization. S. invicta infected with this Mattesia species exhibited a characteristic yellowing of the cuticle which was designated Mattesia "yellow-head disease" (YHD). The 18S rRNA gene sequence from Mattesia YHD was elucidated and compared with the neogregarine pathogens, Mattesia geminata and Ophriocystis elektroscirrha. The sequence data support the previous conclusion that Mattesia YHD is a new species that infects S. invicta. Furthermore, high sequence identity between Mattesia YHD, M. geminata (95.7%), and O. elektroscirrha (86.2%) correctly place the YHD organism in the Mattesia genus and Neogregarinorida order. Oligonucleotide primer pairs were designed to unique areas of the 18S rRNA genes of Mattesia YHD and S. invicta. Multiplex PCR resulted in sensitive and specific detection of Mattesia YHD infection of S. invicta.     

Source of alate excitant pheromones in the red imported fire antSolenopsis invicta (Hymenoptera: Formicidae)

At the onset of mating flights inSolenopsis invicta, workers swarm excitedly over the mound as alates prepare to fly. Previous studies demonstrated that this excitement is stimulated by the male and female alates. We investigated the glandular source(s) of pheromones produced by the alates that cause excitement. The only common female and male alate body part that elicited excitement when crushed was the head. Within the head, excised mandibular glands were found to be responsible for worker excitement. Fire ant workers are very sensitive to external stimuli and some excitement was elicited by crushed female gasters and male thoraces, but the response was never as significant as with crushed heads. Tests with summer and winter alates revealed similar results, except that gasters of winter female alates had a greater excitant effect than did gasters of summer female alates. This may be due to the production of attractant pheromones by the poison glands of overwintering female alates. We conclude that the mandibular gland is the source of alate excitant pheromones.