Anthropogenic Influence on the Distribution of the Longlegged Ant (Hymenoptera: Formicidae)

The longlegged ant Anoplolepis gracilipes (Smith) is a highly invasive tramp ant species known for its deleterious effects on native ecosystems. While tramp ants are associated with human activity, information on how different intensities of human activity affect their distribution is limited. This study investigated how anthropogenic activities affected the distribution of A. gracilipes in Penang, a tropical island in northern peninsular Malaysia. Three study sites (Youth Park, Sungai Ara, and Bukit Jambul/Relau) were selected, containing four sub-locations corresponding to different levels of human activity (low, moderate, high, and very high), determined by the average number of passersby observed over 30 min. Baited index cards were placed at each sub-location to evaluate ant abundance and distribution. The results demonstrated that A. gracilipes worker abundance was highest in areas of moderate human activity, as opposed to areas with low and higher human activity. The low abundance of A. gracilipes in comparatively undisturbed localities may be attributed to unsuitable microclimate, lack of propagule pressure, and diminished honeydew availability. In contrast, its exclusion from more urbanized localities could be explained by high interspecific competition with other tramp species and the absence of preferred nesting sites.

Genetic analysis reveals the putative native range and widespread double-clonal reproduction in the invasive longhorn crazy ant

Clonal reproduction can provide an advantage for invasive species to establish as it can circumvent inbreeding depression which often plagues introduced populations. The world's most widespread invasive ant, Paratrechina longicornis, was previously found to display a double-clonal reproduction system, whereby both males and queens are produced clonally, resulting in separate male and queen lineages, while workers are produced sexually. Under this unusual reproduction mode, inbreeding is avoided in workers as they carry hybrid interlineage genomes. Despite the ubiquitous distribution of P. longicornis, the significance of this reproductive system for the ant's remarkable success remains unclear, as its prevalence is still unknown. Further investigation into the controversial native origin of P. longicornis is also required to reconstruct the evolutionary histories of double-clonal lineages. Here, we examine genetic variation and characterize the reproduction mode of P. longicornis populations sampled worldwide using microsatellites and mitochondrial DNA sequences to infer the ant's putative native range and the distribution of the double-clonal reproductive system. Analyses of global genetic variations indicate that the Indian subcontinent is a genetic diversity hotspot of this species, suggesting that P. longicornis probably originates from this geographical area. Our analyses revealed that both the inferred native and introduced populations exhibit double-clonal reproduction, with queens and males around the globe belonging to two separate, nonrecombining clonal lineages. By contrast, workers are highly heterozygous because they are first-generation interlineage hybrids. Overall, these data indicate a worldwide prevalence of double clonality in P. longicornis and support the prediction that the unusual genetic system may have pre-adapted this ant for global colonization by maintaining heterozygosity in the worker force and alleviating genetic bottlenecks.

First Polycipivirus and Unmapped RNA Virus Diversity in the Yellow Crazy Ant, Anoplolepis gracilipes

The yellow crazy ant, Anoplolepis gracilipes is a widespread invasive ant that poses significant threats to local biodiversity. Yet, compared to other global invasive ant species such as the red imported fire ant (Solenopsis invicta) or the Argentine ant (Linepithema humile), little is known about the diversity of RNA viruses in the yellow crazy ant. In the current study, we generated a transcriptomic database for A. gracilipes using a high throughput sequencing approach to identify new RNA viruses and characterize their genomes. Four virus species assigned to Dicistroviridae, two to Iflaviridae, one to Polycipiviridae, and two unclassified Riboviria viruses were identified. Detailed genomic characterization was carried out on the polycipivirus and revealed that this virus comprises 11,644 nucleotides with six open reading frames. Phylogenetic analysis and pairwise amino acid identity comparison classified this virus into the genus Sopolycivirus under Polycipiviridae, which is tentatively named "Anoplolepis gracilipes virus 3 (AgrV-3)". Evolutionary analysis showed that AgrV-3 possesses a high level of genetic diversity and elevated mutation rate, combined with the common presence of multiple viral strains within single worker individuals, suggesting AgrV-3 likely evolves following the quasispecies model. A subsequent field survey placed the viral pathogen "hotspot" of A. gracilipes in the Southeast Asian region, a pattern consistent with the region being recognized as part of the ant's native range. Lastly, infection of multiple virus species seems prevalent across field colonies and may have been linked to the ant's social organization.

Demographic Characteristics and Population Simulation of Newly Invasive Fall Armyworm on Arachis hypogaea (Fabales: Fabaceae) and Dominant Green Manure Plant in Taiwan

Spodoptera frugiperda (J.E. Smith) is a severe and fast-spreading pest of numerous agro-economic crops, including miscellaneous, vegetables, and green manure crops. Understanding pest ecology represents a core component in integrated pest management decision-making. In Taiwan, peanut (Arachis hypogaea L.) is an important miscellaneous crop, whereas sesbania (Sesbania roxburghii Merr.) is the most frequently used green manure crop. To improve the S. frugiperda management in Taiwan, the demographic characteristics and population simulation of this pest reared on peanut and sesbania leaves were analyzed using the age-stage, two-sex life table theory. The intrinsic rate of increase, finite rate of increase, and net reproductive rate of S. frugiperda were higher when reared on peanut (0.1625 d-1, 1.1764 d-1, 264.9 offspring) than on sesbania (0.0951 d-1, 1.0997 d-1, and 30.3 offspring). Population projection of S. frugiperda on peanut demonstrated that this crop is a more suitable host plant than sesbania. Yet, this suboptimal host still assures an increasing trend of more than 357-fold individuals in 75 d, from the initial cohort of 10 eggs. Our data suggest that green manure plants in fallowing fields may support the pest's survival all year round, and may be responsible for a successful establishment and unexpected outbreaks of this invasive pest on the neighboring crops in Taiwan. Our study thus highlights the importance of assessing the population dynamics and areawide pest management of an invasive polyphagous pest on a noneconomic crop to mitigate the potential risk of reinfestation and thus outbreaks.

Laboratory and Field Evaluations of Two Bait Formulations Against the Invasive Fire Ant, Solenopsis invicta (Hymenoptera: Formicidae)

The red imported fire ant, Solenopsis invicta Buren, was first discovered in Japan in 2017. As this ant remains at the early invasion stage, efforts in establishing a rapid-response framework, such as evaluating available control methods, are urgently needed. Despite the presence of numerous household insecticides against invasive/household pest ants in Japan, the effects of these products on fire ants remain poorly understood. This study assessed the efficacy of two bait products designed to target common household ant pests in Japan on S. invicta through under laboratory and field conditions in Taiwan. The two baits are Arino-su-korori (AK), a granule-formulated bait product with hydramethylnon as an active ingredient (A.I.), and Hyper Arino-su-korori (HAK), a paste-formulated bait with fipronil as A.I., respectively. We showed that both AK and HAK resulted in more than 99% mortalities of fire ant within 8 wk under laboratory conditions and significantly reduce fire ants' foraging activities in the field. AK generally performed slightly better than HAK in terms of the time required to achieve total laboratory colony elimination and also long-term suppression of field fire ants. Such differences most likely are attributable to the active ingredient's mode of action and/or formulation and their interactions with fire ant biology. This study demonstrates the feasibility of the two bait products in effectively controlling laboratory and field fire ants, thus representing a promising candidate pest management tool to cope with ongoing/future fire ant invasions in Japan.

Biology, Ecology, and Management of the Invasive Longlegged Ant, Anoplolepis gracilipes

The longlegged ant (Anoplolepis gracilipes) is one of the most damaging invasive tramp ants globally. It is generally found between latitudes 27°N and 27°S in Asia, although it has been introduced to other continents. Its native range remains debatable, but it is believed to be in Southeast Asia. Anoplolepis gracilipes invasion has many serious ecological consequences, especially for native invertebrate, vertebrate, and plant communities, altering ecosystem dynamics and functions. We examine and synthesize the literature about this species' origin and distribution, impacts on biodiversity and ecosystems, biology and ecology, chemical control, and potential biocontrol agents. We highlight emerging research needs on the origin and invasion history of this species, its reproductive mode, its relationship with myrmecophiles, and its host-microbial interactions, and we discuss future research directions.

Improving invasive species management using predictive phenology models: an example from brown marmorated stink bug (Halyomorpha halys) in Japan

BACKGROUND

In order to better understand the population dynamics of invasive species in their native range, we developed two predictive phenological models using the ubiquitous invasive insect pest, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), as the model organism. Our work establishes a zero-inflated negative binomial regression (ZINB) model, and a general additive mixed model (GAMM) based on 11 years of black light trap monitoring of H. halys at three locations in Japan.

RESULTS

The ZINB model indicated that degree days (DD) have a significant effect on the trap catch of adult H. halys, and that precipitation has no effect. A dataset generated by 1000 simulations from the ZINB suggested that higher predicted trap catches equated to a lower probability of encountering a zero-count. The GAMM produced a cubic regression smooth curve which forecasts the seasonal phenology of H. halys as following a bell-shaped trend in Japan. Critical DD points during the field season in Japan included 261 DD for first H. halys adult detection and 1091 DD for peak activity.

CONCLUSIONS

This study establishes the first models capable of forecasting native H. halys population dynamics based on DD. These robust models practically improve population forecasting of H. halys in the future and help fill gaps in knowledge pertaining to its native phenology, thus ultimately contributing to the progression of efficient management of this globally invasive species. © 2021 Society of Chemical Industry.

The complete mitochondrial genome of a parthenogenetic ant Monomorium triviale (Hymenoptera: Formicidae)

Monomorium is one of the most species-rich yet taxonomically problematic ant genus. An East Asian species, M. triviale Wheeler, W.M., 1906, is reproduced by obligate thelytokous parthenogenesis and performs strict reproductive division of labor. We sequenced the M. triviale mitogenome using next-generation sequencing methods. The circular mitogenome of M. triviale was 16,290 bp in length, consisting of 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNAs, and a single non-coding region of 568 bp. The base composition was AT-biased (82%). Gene order rearrangements were detected and likely to be unique to the genus Monomorium. We announce the M. triviale mitogenome as additional genomic resources for elucidating phylogenetic and taxonomic problems of Monomorium and comparative genomics of parthenogenetic ant species.

Random or handedness? Use of laterally paired penises in Nala earwigs (Insecta: Dermaptera: Labiduridae)

Animals can show bias in their use of laterally paired organs that do not have any conspicuous anatomical differentiation between the right and left organs. Like right handedness in humans, males of the giant earwig Labidura riparia (Labiduridae: Labidurinae) preferentially (~90%) use the right one of their laterally paired penises for copulation. To elucidate the evolutionary origin of this lateralization, patterns of penis use were examined for the related species of the genus Nala (Labiduridae: Nalinae). In multiple populations and broods of both Nala lividipes and Nala nepalensis, males that were ready to use the right or left penis were equally frequent, providing a striking contrast to Labidura. Surgical ablation of one of the two penises revealed that both penises are functionally competent in N. lividipes. Nevertheless, each male almost consistently used only one of the paired penises, either the right or the left one. Changes in penis use were estimated to occur only once per 64–143 days per male. The present study is the first report of individual-level lateralization for animal genitalia that do not show any conspicuous anatomical differentiation between the right and left organs. Possible advantages of lateralization are discussed in relationship to co-evolution of the genitalia between the sexes.

Comparison of Artificial Diets and Natural Prey for Mass Rearing of Orius strigicollis (Hemiptera: Anthocoridae) Using Demographic Characteristics to Optimize Cost-Efficiency

Orius strigicollis (Poppius) has been commonly released to control minute pests due to its remarkable foraging and predation ability. Despite decades long history of mass-rearing using eggs of Cadra cautella Walker (Lepidoptera: Pyralidae) as food, the cost of this food impeded the predator's marketing potential. Finding cost-effective artificial diets for mass rearing programs is, therefore, the key to promote the use of this generalist predator in practice. The aim of this study was to explore suitability of alternative diets for mass rearing of this predator, using the age-stage, two-sex life table. Eight recipes of artificial diets were initially screened, and the candidate diets were subsequently evaluated using the population parameters of O. strigicollis. The highest intrinsic rate of increase was found in the O. strigicollis fed on the meridic combined Diet 1, where nymphs and adults were fed different diets. However, the intensive period of oviposition by females reared on oligidic Diet O3 contributed to the shorter rearing period (42 d) and lower rearing cost than that on combined Diet 1 (60 d). Besides, with the harvest rate of 0.919, the minimal population size of 10,774 individuals with a stable age-stage distribution was needed for daily harvesting 1,000 third instars of O. strigicollis reared on Diet O3 at the rearing cost of 0.295 NTD (new Taiwanese dollar) per nymph. Thus, Diet O3 was determined to be the most cost-effective recipe for the mass-rearing of O. strigicollis among those used in this study.

The role of anthropogenic disturbance and invasion of yellow crazy ant in a recent decline of land crab population

The yellow crazy ant, Anoplolepis gracilipes was first reported in Kenting National Park, Taiwan, in the 2000s, raising the concern of this invasive ant's potential effects on the native land crab populations. We investigated the distribution and abundance of A. gracilipes and its adverse effect on the land crab populations at four land crab hotspots within the park. Our survey results indicated that A. gracilipes was widespread and abundant at three of the study sites (Hsiangchiaowan, Shadao, and Natural Spring), while the distribution was much more confined at the fourth site (Houwan). Land crab populations experienced a notable decline at all the study sites except Houwan, indicating an inverse relationship between the population of land crabs and distribution/abundance of A. gracilipes. Combining the results of visual observations, the decline of land crab populations can be attributed, at least partially, to direct attacks by A. gracilipes on land crabs in their natural habitat and during the breeding migration season. Generalized additive model showed that worker abundance of this ant is associated with human disturbance levels, suggesting that anthropogenic disturbance may have contributed to the decline in land crab populations via promoting the abundance of A. gracilipes.

Deformed Wing Virus in Two Widespread Invasive Ants: Geographical Distribution, Prevalence, and Phylogeny

Spillover of honey bee viruses have posed a significant threat to pollination services, triggering substantial effort in determining the host range of the viruses as an attempt to understand the transmission dynamics. Previous studies have reported infection of honey bee viruses in ants, raising the concern of ants serving as a reservoir host. Most of these studies, however, are restricted to a single, local ant population. We assessed the status (geographical distribution/prevalence/viral replication) and phylogenetic relationships of honey bee viruses in ants across the Asia–Pacific region, using deformed wing virus (DWV) and two widespread invasive ants, Paratrechina longicornis and Anoplolepis gracilipes, as the study system. DWV was detected in both ant species, with differential geographical distribution patterns and prevenance levels between them. These metrics, however, are consistent across the geographical range of the same ant species. Active replication was only evident in P. longicornis. We also showed that ant-associated DWV is genetically similar to that isolated from Asian populations of honey bees, suggesting that local acquisition of DWV by the invasive ants may have been common at least in some of our sampled regions. Transmission efficiency of DWV to local arthropods mediated by ant, however, may vary across ant species.

Ongoing Coevolution of Wolbachia and a Widespread Invasive Ant, Anoplolepis gracilipes

While Wolbachia are commonly found among arthropods, intraspecific infection rates can vary substantially across the geographic populations. Here we report nearly 100% prevalence of Wolbachia in the global populations of the yellow crazy ant, Anoplolepis gracilipes. To understand coevolutionary history between Wolbachia and A. gracilipes, we identified single nucleotide polymorphisms (SNPs) in Wolbachia from the ant across 12 geographical regions and compared the phylogeny of SNP-based Wolbachia to patterns of the ant's mitochondrial DNA (mtDNA) variation. Our results revealed a strong concordance between phylogenies of Wolbachia and host mtDNA, providing immediate evidence of co-divergence. Among eight identified SNP loci separating the genetic clusters of Wolbachia, seven loci are located in potential protein-coding genes, three of which being non-synonymous SNPs that may influence gene functions. We found a Wolbachia hypothetical protein gene with signature of positive selection. These findings jointly allow us to characterize Wolbachia-ant coevolution and also raise a question about mechanism(s) underlying maintenance of high prevalence of Wolbachia during the colonization of this invasive ant.

Population Monitoring, Egg Parasitoids, and Genetic Structure of the Invasive Litchi Stink Bug, Tessaratoma papillosa in Taiwan

Simple Summary

The litchi stink bug (LSB) was inadvertently introduced to Taiwan recently and has since become a severe pest with substantial economic losses. The aim of this study is therefore to improve our knowledge of this invasive pest through multiple approaches including population monitoring, surveillance of natural enemies, and population genetic analysis. Major findings include: (1) a population fluctuation trend that is largely similar to most native LSB populations, (2) a total of seven egg parasitoid species were discovered, two of which (Anastatusdexingensis and A. fulloi) being most abundant throughout the LSB infestation in Taiwan, and (3) the occurrence of multiple introductions of LSB to Taiwan. All these data represent a preliminary yet necessary step for the design of future integrated pest management strategies and would help mitigate negative impacts of this invasive pest in Taiwan.

Abstract

Here we assessed population dynamics, natural enemy fauna (with emphasis on egg parasitoid), and population genetic structure (based on mitochondrial DNA) of the invasive litchi stink bug (LSB), Tessaratoma papillosa in Taiwan. Our major findings include: (1) fluctuations of LSB in numbers of adults, mating pairs, and egg masses over a 2-year period in Taiwan generally resemble those in the native populations; (2) Anastatusdexingensis and A. fulloi are among the most dominant LSB egg parasitoids, with the former consistently outnumbering the latter throughout Taiwan; (3) the presence of two genetically distinct clades suggests LSB in Taiwan most likely derived from multiple invasions. All these data practically improve our understanding of this invasive insect pest, particularly its ecological and genetic characteristics in the introduced area, which represents critical baseline information for the design of future integrated pest management strategies.

Correction to: Complete genome sequences of two novel dicistroviruses detected in yellow crazy ants (Anoplolepis gracilipes)

Authors would like to correct the error in Fig. 1 which was incorrectly updated in the original publication.

Complete genome sequences of two novel dicistroviruses detected in yellow crazy ants (Anoplolepis gracilipes)

We report two novel RNA viruses from yellow crazy ants, (Anoplolepis gracilipes) detected using next-generation sequencing. The complete genome sequences of the two viruses were 10,662 and 8,238 nucleotides in length, respectively, with both possessing two open reading frames with three conserved protein domains. The genome organization is characteristic of members of the genus Triatovirus in the family Dicistroviridae. The two novel viruses were tentatively named "Anoplolepis gracilipes virus 1" and "Anoplolepis gracilipes virus 2" (AgrV-1 and AgrV-2). Phylogenetic analyses based on amino acid sequences of the non-structural polyprotein (ORF1) suggest that the two viruses are triatovirus-like viruses. This is the first report on the discovery of novel triatovirus-like viruses in yellow crazy ants with a description of their genome structure (two ORFs and conserved domains of RNA helicase, RNA-dependent RNA polymerase, and capsid protein), complete sequences, and viral prevalence across the Asia-Pacific region.

The Association between Virus Prevalence and Intercolonial Aggression Levels in the Yellow Crazy Ant, Anoplolepis Gracilipes (Jerdon)

The recent discovery of multiple viruses in ants, along with the widespread infection of their hosts across geographic ranges, provides an excellent opportunity to test whether viral prevalence in the field is associated with the complexity of social interactions in the ant population. In this study, we examined whether the association exists between the field prevalence of a virus and the intercolonial aggression of its ant host, using the yellow crazy ant (Anoplolepis gracilipes) and its natural viral pathogen (TR44839 virus) as a model system. We delimitated the colony boundary and composition of A. gracilipes in a total of 12 study sites in Japan (Okinawa), Taiwan, and Malaysia (Penang), through intercolonial aggression assay. The spatial distribution and prevalence level of the virus was then mapped for each site. The virus occurred at a high prevalence in the surveyed colonies of Okinawa and Taiwan (100% infection rate across all sites), whereas virus prevalence was variable (30%–100%) or none (0%) at the sites in Penang. Coincidentally, colonies in Okinawa and Taiwan displayed a weak intercolonial boundary, as aggression between colonies is generally low or moderate. Contrastingly, sites in Penang were found to harbor a high proportion of mutually aggressive colonies, a pattern potentially indicative of complex colony composition. Our statistical analyses further confirmed the observed correlation, implying that intercolonial interactions likely contribute as one of the effective facilitators of/barriers to virus prevalence in the field population of this ant species.

Genetic Diversity and Wolbachia Infection Patterns in a Globally Distributed Invasive Ant

Understanding the phylogeographic history of an invasive species may facilitate reconstructing the history and routes of its invasion. The longhorn crazy ant, Paratrechina longicornis, is a ubiquitous agricultural and household pest throughout much of the tropics and subtropics, but little is known about the history of its spread. Here, we examine worldwide genetic variation in P. longicornis and its associated Wolbachia bacterial symbionts. Analyses of mtDNA sequences of 248 P. longicornis workers (one per colony) from 13 geographic regions reveal two highly diverged mtDNA clades that co-occur in most of the geographic regions. These two mtDNA clades are associated with different Wolbachia infection patterns, but are not congruent with patterns of nDNA (microsatellite) variation. Multilocus sequence typing reveals two distinct Wolbachia strains in P. longicornis, namely, wLonA and wLonF. The evolutionary histories of these two strains differ; wLonA appears to be primarily transmitted maternally, and patterns of mtDNA and nDNA variation and wLonA infection status are consistent with a relatively recent Wolbachia-induced selective sweep. In contrast, the observed patterns of mtDNA variation and wLonF infections suggest frequent horizontal transfer and losses of wLonF infections. The lack of nDNA structure among sampled geographic regions coupled with the finding that numerous mtDNA haplotypes are shared among regions implies that inadvertent long-distance movement through human commerce is common in P. longicornis and has shaped the genetic structure of this invasive ant worldwide.

Fitness advantages of the biased use of paired laterally symmetrical penises in an insect

The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population-level laterality. Our population studies revealed that in 10 populations, located at 2.23-43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left-handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left-handed males in copulation (the probability of establishing genital coupling during the 1-hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right-handed advantage may be due to the coiled morphology of the sperm storage organ with a right-facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.

Viral infections in fire ants lead to reduced foraging activity and dietary changes

Despite the presence of conserved innate immune function, many insects have evolved a variety of mechanical, chemical, and behavioral defensive responses to pathogens. Illness-induced anorexia and dietary changes are two behavioral defensive strategies found in some solitary insects, but little is known regarding the role of such behaviors in social insects, especially in ants. In the present study we examined if such reduced foraging activity exists for a social insect, the invasive fire ant Solenopsis invicta, and its viral pathogen, Solenopsis invicta virus 1 (SINV-1). Virus-free fire ant colonies were split into two colony fragments, one of which subsequently was inoculated with SINV-1. Four food resources with different macronutrient ratios were presented to both colony fragments. SINV-1-inoculated colony fragments consistently displayed reduced foraging performance (e.g., foraging intensity and recruitment efficiency), a decline in lipid intake, and a shift in dietary preference to carbohydrate-rich foods compared with virus-free fragments. These findings provide the first evidence for virus-induced behavioral responses and dietary shifts in shaping the host-pathogen interactions in fire ants. The findings also suggest a possible mechanism for how fire ant colonies respond to viral epidemics. Potential implications of these behavioral differences for current management strategies are discussed.

The complete mitochondrial genome of yellow crazy ant, Anoplolepis gracilipes (Hymenoptera: Formicidae)

The yellow crazy ant Anoplolepis gracilipes is an invasive species that threatens biodiversity in introduced ecosystems. We sequenced the A. gracilipes mitogenome using next-generation sequencing methods. The circular mitogenome of A. gracilipes was 16,943 bp included 13 protein-coding genes, two ribosomal RNA genes, 22 transfer RNAs, and a single large non-coding region of 893 bp. The base composition was AT-biased (72%). Three genomic rearrangements compared to ancestral insects were found. Phylogenetic analysis based on the concatenated nucleotide sequences of the 13 protein-coding genes supports A. gracilipes belonging to the Formicinae subfamily. We announce the A. gracilipes mitogenome as a DNA reference for further population genetic, phylogenetic, and evolutionary analyses.

Worker reproduction of the invasive yellow crazy ant Anoplolepis gracilipes

Background

Reproductive division of labor is one of the key features of social insects. Queens are adapted for reproduction while workers are adapted for foraging and colony maintenance. In many species, however, workers retain functional ovaries and can lay unfertilized male eggs or trophic eggs. Here we report for the first time on the occurrence of physogastric workers and apparent worker reproduction in the invasive yellow crazy ant Anoplolepis gracilipes (Fr. Smith). We further examined the reproductive potential and nutritional role of physogastric workers through multidisciplinary approaches including morphological characterization, laboratory manipulation, genetic analysis and behavioral observation.

Results

Egg production with two types of eggs, namely reproductive and trophic eggs, by physogastric workers was found. The reproductive egg was confirmed to be haploid and male-destined, suggesting that the workers produced males via arrhenotokous parthenogenesis as no spermatheca was discovered. Detailed observations suggested that larvae were mainly fed with trophic eggs. Along with consumption of trophic eggs by queens and other castes as part of their diet, the vital role of physogastric workers as “trophic specialist” is confirmed.

Conclusion

We propose that adaptive advantages derived from worker reproduction for A. gracilipes may include 1) trophic eggs provisioned by physogastric workers likely assist colonies of A. gracilipes in overcoming unfavorable conditions such as paucity of food during critical founding stage; 2) worker-produced males are fertile and thus might offer an inclusive fitness advantage for the doomed orphaned colony.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0210-4) contains supplementary material, which is available to authorized users.