Invasive ants reduce abundance of small rainforest skinks

Invasive ants are among the world’s most damaging invasive species, often directly or indirectly affecting native fauna. Insecticidal baits are the main method for suppressing or eradicating invasive ant populations, but their use must be considered against potential for unintended effects on native organisms. The invasive yellow crazy ant (Anoplolepis gracillipes) is widespread in the tropics, particularly on islands, where they have displaced a range of invertebrates. Effects of this ant on vertebrates, and in continental ecosystems generally, are less studied. We investigated the effects of yellow crazy ants and bait application on rainforest skinks and their invertebrate prey. We compared skink and skink prey abundance across four replicated rainforest site categories: high and low yellow crazy ant sites had both been baited but differed in yellow crazy ant activity; control sites had never had yellow crazy ants or been baited; and buffer sites had never had yellow crazy ants but had been baited. We recorded significantly lower abundance of two small skink species (Lygisaurus laevis and Saproscincus tetradactylus) in high yellow crazy ant sites compared to all other site categories. The differences persisted even after baiting reduced yellow crazy ant activity by 97.8% ± 0.04% (mean ± SD). A larger rainforest skink species (Carlia rubrigularis) was not negatively affected by yellow crazy ant invasion. Skink prey abundance was significantly lower in high yellow crazy ant sites compared to control sites and low yellow crazy ant sites, but not compared to buffer sites. These differences did not persist following baiting. We found no evidence that baiting negatively affects skinks or their invertebrate prey. Our data suggest that yellow crazy ants, but not the bait used to treat them, pose a direct threat to small rainforest skinks.

Diversity and Distribution of Forest Ants (Hymenoptera: Formicidae) in Nepal: Implications for Sustainable Forest Management

The information available on the diversity of ant species and their distribution and interaction with forest health in Nepal remains limited. As part of a nationwide project on forest health, we conducted inventories to assess the diversity and distribution of forest ants and their role in forest management in Nepal. Ants were collected from 187 plots of 10 m × 10 m size along the north–south belt transects in eastern, central, and western Nepal. We used vegetation beating, sweeping, and hand collection methods in selected forest types. In each transect, we designed six plots in each major forest type (Sal, Schima–Castanopsis, and broadleaf mixed forests) and three plots each in deodar, Alnus, riverine, and Cryptomeria forests. We recorded 70 ant species from 36 genera and six subfamilies. This includes five genera and nine species new for the country, as well as eight tramp species, four of which are major ecological, agricultural, and/or household pests. Our study indicates that forest ant species richness is high in western Nepal and the Siwaliks, and it decreases as elevation increases. The high diversity of ant species in the forests of Nepal needs to be assessed with further exploration using multiple sampling methods covering all seasons and forest types. Ants can be useful indicators for ecosystem management and human impacts on forests. Reports of invasive ants in Nepalese forests indicate the relevance of urgent interventions through sustainable forest management initiatives to prevent future incursions.

Variation in behavioural traits and their molecular basis in the globally invasive Argentine ant

<p>A salient question in invasion biology is to elucidate what traits make some species invasive and not others. Recently, the introduction process has been hypothesised to function as a filter, selecting for traits that promote invasiveness in introduced populations. Behaviour has been increasingly recognised as a significant driver of animal invasions, but few studies focus on how the introduction process may affect behavioural variation across an invader’s range. This thesis aims at investigating the consequences of the introduction process on behavioural variation along an introduction pathway in the globally widespread Argentine ant. In Chapter I, I investigated transcriptome-wide variations associated with morphological caste as well as worker foraging propensity. I found a number of transcripts showed caste and behaviour-specific expression profiles, and that foraging workers exhibit higher expression of genes related to chemosensory receptors and neurological pathways known to be associated with foraging behaviour and aggression. In chapter II, I investigated octopamine-mediated modulation of foraging behaviour and aggression using pharmacological manipulations using synthetic octopamine (OA) and one of its antagonists. I found that OA was associated with colony-level foraging activity, but did not appear to similarly drive changes in individual behaviour. However, my results suggest that OA is associated with individual aggressiveness, and I also found an association between OA and foraging propensity. In chapter III, I tested the hypothesis that the introduction process selected for increased risk-taking behaviour along the Argentine ant introduction pathway. I used behavioural assays combined with OA diet supplementation in four regions along the introduction pathway, and showed that both behavioural variation and plasticity were consistent across the range, not in support of our hypothesis. Finally, in chapter IV I investigated transcriptome-wide expression profiles in workers along the introduction pathway and discovered distinct transcriptomic signatures associated with range. I found that specific immune and neural pathways were differentially regulated in the introduced range compared to the native range. Overall, my results contribute to a better understanding of caste differentiation and neuromodulation of key behaviours in invasive ants. While I did not find behavioural differentiation between native and invasive Argentine ant populations, I show that they are characterised by specific transcriptomic signatures perhaps underlying the ant’s invasion success. This is one of the first global and integrative studies to focus on changes associated with the introduction of invasive species, and may have important implications for the understanding of biological invasions.</p>

Variation in behavioural traits and their molecular basis in the globally invasive Argentine ant

<p>A salient question in invasion biology is to elucidate what traits make some species invasive and not others. Recently, the introduction process has been hypothesised to function as a filter, selecting for traits that promote invasiveness in introduced populations. Behaviour has been increasingly recognised as a significant driver of animal invasions, but few studies focus on how the introduction process may affect behavioural variation across an invader’s range. This thesis aims at investigating the consequences of the introduction process on behavioural variation along an introduction pathway in the globally widespread Argentine ant. In Chapter I, I investigated transcriptome-wide variations associated with morphological caste as well as worker foraging propensity. I found a number of transcripts showed caste and behaviour-specific expression profiles, and that foraging workers exhibit higher expression of genes related to chemosensory receptors and neurological pathways known to be associated with foraging behaviour and aggression. In chapter II, I investigated octopamine-mediated modulation of foraging behaviour and aggression using pharmacological manipulations using synthetic octopamine (OA) and one of its antagonists. I found that OA was associated with colony-level foraging activity, but did not appear to similarly drive changes in individual behaviour. However, my results suggest that OA is associated with individual aggressiveness, and I also found an association between OA and foraging propensity. In chapter III, I tested the hypothesis that the introduction process selected for increased risk-taking behaviour along the Argentine ant introduction pathway. I used behavioural assays combined with OA diet supplementation in four regions along the introduction pathway, and showed that both behavioural variation and plasticity were consistent across the range, not in support of our hypothesis. Finally, in chapter IV I investigated transcriptome-wide expression profiles in workers along the introduction pathway and discovered distinct transcriptomic signatures associated with range. I found that specific immune and neural pathways were differentially regulated in the introduced range compared to the native range. Overall, my results contribute to a better understanding of caste differentiation and neuromodulation of key behaviours in invasive ants. While I did not find behavioural differentiation between native and invasive Argentine ant populations, I show that they are characterised by specific transcriptomic signatures perhaps underlying the ant’s invasion success. This is one of the first global and integrative studies to focus on changes associated with the introduction of invasive species, and may have important implications for the understanding of biological invasions.</p>

Direct and Indirect Effects of Invasive vs. Native Ant-Hemipteran Mutualism: A Meta-Analysis That Supports the Mutualism Intensity Hypothesis

Mutualism can facilitate the colonization, establishment, and spread of invasive species. By modifying interactions with third parties, mutualisms can have cascading community-wide effects. Both native and invasive ants are capable of forming mutualisms with hemipteran insects, preying on non-hemipteran herbivores and indirectly affecting primary production. Comparative research on the effects of both native and invasive ant exclusions on multitrophic interactions is therefore crucial for understanding the invasive potential of ants, along with any ecological consequences that invasions may have. We performed a quantitative review of the multitrophic effects of invasive and native ants on insect–plant food webs. Herbivorous insects are the most common food source for both invasive (comprising 56% of prey species caught) and native ants (55% of the prey species caught), followed by predators (31% for invasive ants, 45% for native ants). Excluding both invasive and native ants significantly reduced hemipteran abundance, and excluding invasive ants had a greater negative impact on hemipteran abundance than native ants. Native ant predation significantly reduced herbivore abundance, but excluding invasive ants had no effect. Cascading effects of native ants on plant fitness were significantly positive, but there was no significant impact of invasive ants. These findings suggest a weak relationship between the presence of invasive ants and non-hemipteran herbivore abundance. We suggest that the hemipteran–ant mutualism could represent a ‘symbiotic invasion’. The ecological dominance of invasive ants is often facilitated by hemipteran insects. This association requires invasive ant control strategies to expand beyond ants to consider mutualists.

Bacterial diversity and viral discovery in the invasive Argentine ant (Linepithema humile)

<p>Invasive species can lead to major economic and ecological issues. For this reason, biological controls are being developed in order to help with invasive species population management. Pathogenic bacteria and viruses offer good biological control opportunities as both micro-organisms have played a role in natural population declines. However, beneficial bacteria and viruses associated with the targeted invasive species may interfere with biological controls, by protecting their hosts from infections. Previous knowledge on both pathogenic and beneficial bacteria and viruses present in invasive species may then support the development of an active and efficient biological control. The Argentine ant, Linepithema humile, is a South American invasive ant species that has successfully spread over five continents. The ants were introduced to New Zealand after a complex invasion path, from Argentina their home range to Europe, then to Australia and finally to New Zealand. In their new environments, invasive Argentine ants affect species diversity and can cause agricultural losses. In the absence of any biological controls, the Argentine ant population is controlled by chemical sprays and poison baits. Management of these invasive ants in New Zealand is estimated to cost NZ$ 60 million a year. The Argentine ant population in New Zealand was reported to have unexpectedly declined. It was hypothesised that pathogens were the cause of this population collapse. In this study, bacteria and viruses present in the invasive ants were investigated using 454 sequencing and Illumina sequencing for future developments of possible biological controls for the Argentine ants, and a better understanding of the observed population decline in New Zealand. Bacterial diversity present in Argentine ants either declined or diminished along the invasion pathway. At the same time, the invasive ants maintained a core of nine bacteria genera, including Lactobacillus and Gluconobacter, two bacterial genera with members known for their beneficial associations with honey bees. The presence of these core bacteria may have participated in the success of Argentine ants in their new environments. In the laboratory, the use of ampicillin and gentamicin antibiotics on the ants increased bacterial diversity present in the ants. Furthermore, ampicillin, kanamycin and spectinomycin antibiotic treatments increased ant survival but did not affect the ant fitness or intra-species aggressiveness. Only spectinomycin treated ants presented a higher level of inter-species aggressiveness. Bacterial diversity may play an important role in the ant health and at length population dynamics. Finally, Argentine ants are the hosts of two viruses: the Deformed wing virus (DWV) involved in colony collapse disorder in honey bees, and Linepithema humile virus 1 (LHUV-1), a new virus related to DWV. Both viruses actively replicate within the ants, indicating a possible reservoir role of the ants. However, the effects of the viruses on the ants are not yet known. Further viral infection in the laboratory under different stress conditions and / or antibiotic treatment will give an insight in the role played by these viruses in the observed population collapse of Argentine ants in New Zealand. LHUV-1 may offer a possibility in the development of the first biological control for Argentine ants, depending on its specificity and its effects. This dissertation provides a first insight in the core bacteria as well as potential harmful viruses present in Argentine ants. These bacteria and viruses may play a role in the ant population dynamics. Invasive species may co-introduce harmful pathogens with them, and participate to the spread of local ones. The pathogens may affect both invasive ants and native species population dynamics.</p>

Bacterial diversity and viral discovery in the invasive Argentine ant (Linepithema humile)

<p>Invasive species can lead to major economic and ecological issues. For this reason, biological controls are being developed in order to help with invasive species population management. Pathogenic bacteria and viruses offer good biological control opportunities as both micro-organisms have played a role in natural population declines. However, beneficial bacteria and viruses associated with the targeted invasive species may interfere with biological controls, by protecting their hosts from infections. Previous knowledge on both pathogenic and beneficial bacteria and viruses present in invasive species may then support the development of an active and efficient biological control. The Argentine ant, Linepithema humile, is a South American invasive ant species that has successfully spread over five continents. The ants were introduced to New Zealand after a complex invasion path, from Argentina their home range to Europe, then to Australia and finally to New Zealand. In their new environments, invasive Argentine ants affect species diversity and can cause agricultural losses. In the absence of any biological controls, the Argentine ant population is controlled by chemical sprays and poison baits. Management of these invasive ants in New Zealand is estimated to cost NZ$ 60 million a year. The Argentine ant population in New Zealand was reported to have unexpectedly declined. It was hypothesised that pathogens were the cause of this population collapse. In this study, bacteria and viruses present in the invasive ants were investigated using 454 sequencing and Illumina sequencing for future developments of possible biological controls for the Argentine ants, and a better understanding of the observed population decline in New Zealand. Bacterial diversity present in Argentine ants either declined or diminished along the invasion pathway. At the same time, the invasive ants maintained a core of nine bacteria genera, including Lactobacillus and Gluconobacter, two bacterial genera with members known for their beneficial associations with honey bees. The presence of these core bacteria may have participated in the success of Argentine ants in their new environments. In the laboratory, the use of ampicillin and gentamicin antibiotics on the ants increased bacterial diversity present in the ants. Furthermore, ampicillin, kanamycin and spectinomycin antibiotic treatments increased ant survival but did not affect the ant fitness or intra-species aggressiveness. Only spectinomycin treated ants presented a higher level of inter-species aggressiveness. Bacterial diversity may play an important role in the ant health and at length population dynamics. Finally, Argentine ants are the hosts of two viruses: the Deformed wing virus (DWV) involved in colony collapse disorder in honey bees, and Linepithema humile virus 1 (LHUV-1), a new virus related to DWV. Both viruses actively replicate within the ants, indicating a possible reservoir role of the ants. However, the effects of the viruses on the ants are not yet known. Further viral infection in the laboratory under different stress conditions and / or antibiotic treatment will give an insight in the role played by these viruses in the observed population collapse of Argentine ants in New Zealand. LHUV-1 may offer a possibility in the development of the first biological control for Argentine ants, depending on its specificity and its effects. This dissertation provides a first insight in the core bacteria as well as potential harmful viruses present in Argentine ants. These bacteria and viruses may play a role in the ant population dynamics. Invasive species may co-introduce harmful pathogens with them, and participate to the spread of local ones. The pathogens may affect both invasive ants and native species population dynamics.</p>

Multi-host viruses in Argentine ants and honey bees: Increased viral disease in honey bees is associated with Argentine ants

<p><b>Emerging infectious diseases threaten public health, livestock economies, and wildlife. Human-mediated species introductions can alter host and pathogen communities that shape the dynamics of infectious diseases. Several RNA viruses that have been linked to population declines in wild pollinators and losses of managed honey bees have been detected in multiple other species and are suspected to circulate within insect communities. Yet, we lack an understanding of how disease dynamics are affected by the introduction of novel species. These introduced species include invasive ants, which can disturb honey bees and become a pest in apiaries. The Argentine ant (Linepithema humile) is a globally successful invader that has been observed to attack bees and multiple bee-associated viruses have been detected in this ant species.</b></p> <p>Here, I studied interactions between Argentine ants and European honey bees (Apis mellifera) and how these interactions affect viral dynamics in beehives. I first tested a range of pollinators and associated insects for RNA viruses that are pathogenic to honey bees. Bee-associated viruses showed evidence for active viral replication in several pollinator species but also in species that cohabit in beehives such as ants, spiders, and cockroaches. Using phylogenetic analyses, I found that viral transmission within communities was shaped by geographic origin rather than being restricted by species barriers. Next, I used a longitudinal field study to test whether Argentine ant presence affected pathogen infections and survival in beehives. Argentine ants tested positive for three bee-associated viruses even before beehives were moved into ant-infested sites. Increased levels of deformed wing virus in beehives in autumn were associated with ant presence, although hive mortality was not affected by ants over the duration of this experiment. I used RNA sequencing on a subset of honey bee samples collected during autumn to study the RNA virome and identify transcriptomic responses associated with ant presence. Twelve RNA viruses were found in beehives, among those, three plant-associated viruses and an unclassified RNA virus that had not previously been observed in honey bees. Deformed wing virus showed the highest viral titres in most hives, but was only marginally affected by ant presence. Sacbrood virus and tomato ringspot virus levels were increased in hives with ants, however, both viruses are not known to infect Argentine ants and the plant-associated tomato ringspot virus seems unlikely to affect bee health.</p> <p>Lastly, I tested the feasibility of controlling Argentine ants in apiaries using a novel pest control strategy. RNA interference is a conserved cellular gene regulation mechanism that could be used to silence specific genes in ants. Using double-stranded RNA (dsRNA) to silence two immune-related genes in Argentine ants was expected to increase pathogen susceptibility, which could then lead to higher pathogen levels that reduce ant numbers. My results indicated that no consistent immune silencing could be achieved in the field. Immune gene expression changes were observed, but pathogen titres were not affected, and ant numbers stayed high. Argentine ant control using a conventional insecticide significantly increased bee survival, whereas many hives in the dsRNA and control group abandoned their hives due to ant attacks. Although population control was not successful using the two Argentine ant-specific dsRNAs, insights into ant immunity and ant-bee interactions could improve the development of novel control strategies.</p> <p>Bee-associated viruses have repeatedly been detected in ant species, yet, this is one of the first studies to investigate whether ants affect viral dynamics in honey bees. I showed that invasive Argentine ants are associated with increases in viral pathogens in honey bees. The mechanisms by which ants affect bee disease are unknown, although there is some evidence for ants transmitting viruses or causing stress responses in bees that affect immunity. The findings of this thesis highlight the risk of invasive ant species disrupting pollination services. New and environmentally-friendly methods to control invasive species are urgently needed to improve bee health and limit the spread of invasive ants, such as Argentine ants. The high prevalence of bee-associated viruses and viral diversity in ants suggests that pathogens that are suitable for population control might be present in ant populations, although risks of spillovers into other species need to be carefully considered.</p>

Multi-host viruses in Argentine ants and honey bees: Increased viral disease in honey bees is associated with Argentine ants

<p><b>Emerging infectious diseases threaten public health, livestock economies, and wildlife. Human-mediated species introductions can alter host and pathogen communities that shape the dynamics of infectious diseases. Several RNA viruses that have been linked to population declines in wild pollinators and losses of managed honey bees have been detected in multiple other species and are suspected to circulate within insect communities. Yet, we lack an understanding of how disease dynamics are affected by the introduction of novel species. These introduced species include invasive ants, which can disturb honey bees and become a pest in apiaries. The Argentine ant (Linepithema humile) is a globally successful invader that has been observed to attack bees and multiple bee-associated viruses have been detected in this ant species.</b></p> <p>Here, I studied interactions between Argentine ants and European honey bees (Apis mellifera) and how these interactions affect viral dynamics in beehives. I first tested a range of pollinators and associated insects for RNA viruses that are pathogenic to honey bees. Bee-associated viruses showed evidence for active viral replication in several pollinator species but also in species that cohabit in beehives such as ants, spiders, and cockroaches. Using phylogenetic analyses, I found that viral transmission within communities was shaped by geographic origin rather than being restricted by species barriers. Next, I used a longitudinal field study to test whether Argentine ant presence affected pathogen infections and survival in beehives. Argentine ants tested positive for three bee-associated viruses even before beehives were moved into ant-infested sites. Increased levels of deformed wing virus in beehives in autumn were associated with ant presence, although hive mortality was not affected by ants over the duration of this experiment. I used RNA sequencing on a subset of honey bee samples collected during autumn to study the RNA virome and identify transcriptomic responses associated with ant presence. Twelve RNA viruses were found in beehives, among those, three plant-associated viruses and an unclassified RNA virus that had not previously been observed in honey bees. Deformed wing virus showed the highest viral titres in most hives, but was only marginally affected by ant presence. Sacbrood virus and tomato ringspot virus levels were increased in hives with ants, however, both viruses are not known to infect Argentine ants and the plant-associated tomato ringspot virus seems unlikely to affect bee health.</p> <p>Lastly, I tested the feasibility of controlling Argentine ants in apiaries using a novel pest control strategy. RNA interference is a conserved cellular gene regulation mechanism that could be used to silence specific genes in ants. Using double-stranded RNA (dsRNA) to silence two immune-related genes in Argentine ants was expected to increase pathogen susceptibility, which could then lead to higher pathogen levels that reduce ant numbers. My results indicated that no consistent immune silencing could be achieved in the field. Immune gene expression changes were observed, but pathogen titres were not affected, and ant numbers stayed high. Argentine ant control using a conventional insecticide significantly increased bee survival, whereas many hives in the dsRNA and control group abandoned their hives due to ant attacks. Although population control was not successful using the two Argentine ant-specific dsRNAs, insights into ant immunity and ant-bee interactions could improve the development of novel control strategies.</p> <p>Bee-associated viruses have repeatedly been detected in ant species, yet, this is one of the first studies to investigate whether ants affect viral dynamics in honey bees. I showed that invasive Argentine ants are associated with increases in viral pathogens in honey bees. The mechanisms by which ants affect bee disease are unknown, although there is some evidence for ants transmitting viruses or causing stress responses in bees that affect immunity. The findings of this thesis highlight the risk of invasive ant species disrupting pollination services. New and environmentally-friendly methods to control invasive species are urgently needed to improve bee health and limit the spread of invasive ants, such as Argentine ants. The high prevalence of bee-associated viruses and viral diversity in ants suggests that pathogens that are suitable for population control might be present in ant populations, although risks of spillovers into other species need to be carefully considered.</p>

Exotic Ants of the Asia-Pacific: Invasion, National Response, and Ongoing Needs

Human activity has facilitated the introduction of many exotic species via global trade. Asia-Pacific countries comprise one of the most economically and trade-active regions in the world, which makes it one of the most vulnerable regions to invasive species, including ants. There are currently over 60 exotic ant species in the Asia-Pacific, with the red imported fire ant, Solenopsis invicta, among the most destructive. Exotic ants pose many economic and ecological problems for the region. Countries in the Asia-Pacific have dealt with the problem of exotic ants in very different ways, and there has been an overall lack of preparedness. To improve the management of risks associated with invasive ants, we recommend that countries take action across the biosecurity spectrum, spanning prevention, containment, and quarantine. The creation of an Asia-Pacific network for management of invasive ants should help prevent their introduction and mitigate their impacts. Expected final online publication date for the Annual Review of Entomology, Volume 67 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.