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.

Inter-species horizontal toxicity: Experimental control of Argentine ants and their citrophilus mealybug mutualists in a viticultural system

<p>The invasive Argentine ant, Linepithema humile, is known to form a trophobiotic association with honeydew excreting homopterans Pseudococcus sp. providing protection from natural enemies in exchange for the honeydew they excrete. The vine mealybug Pseudococcus calceolariae, can transmit Grapevine leafroll- associated virus 3 (GLRaV-3) between vines as it travels and feeds with the ensuing leafroll disease negatively impacting on vine health and wine quality. Therefore, if an effective chemical control method targeting incursions of Argentine ants in vineyards contributes to the dissociation of this invasive ant species with its citrophilus mealybug mutualist, then in theory the spread of GLRaV-3 in vineyards by its mealybug vector can be stemmed. Three insecticidal treatments targeting Argentine ants in the canopy of potted Pinot Noir grapevines inoculated with citrophilus mealybugs were trialled at a field site established in Nelson during the summer of 2016/2017. Bifenthrin (1200ppm) was sprayed on vine trunks and the low- toxicity baits, thiamethoxam (0.0006%) or boric acid (0.5%) carried in polyacrylamide gel with 25% sucrose and 0.15% citric acid solution, were placed at the base of vines. A significant decline in ant activity (p < 0.001) and citrophilus mealybugs was observed for the bifenthrin treatment. A follow-on bioassay was conducted at Mt. Albert Plant and Food Research, in the absence of P. calceolariae’s natural enemies to test the hypothesis that the decline in citrophilus mealybugs in response to vines treated with bifenthrin, could in fact be due to inter-species horizontal toxicity because of Argentine ants transferring the toxicant bifenthrin to citrophilus mealybugs while tending them or contaminating the substrate that they fed on. The significant decrease in average citrophilus mealybug activity rate (p < 0.001) for bifenthrin treatments compared with the controls provides evidence for inter-species horizontal toxicity. Bifenthrin sprayed on grapevine trunks may be suitable to control Argentine ants in the vine canopy and indirectly control P. calceolariae, a known vector of GLRaV-3 between grapevine hosts. The concept of inter-species horizontal toxicity could become a model for targeted pest management by exploiting different insect mutualisms in various horticultural cropping systems.</p>

Inter-species horizontal toxicity: Experimental control of Argentine ants and their citrophilus mealybug mutualists in a viticultural system

<p>The invasive Argentine ant, Linepithema humile, is known to form a trophobiotic association with honeydew excreting homopterans Pseudococcus sp. providing protection from natural enemies in exchange for the honeydew they excrete. The vine mealybug Pseudococcus calceolariae, can transmit Grapevine leafroll- associated virus 3 (GLRaV-3) between vines as it travels and feeds with the ensuing leafroll disease negatively impacting on vine health and wine quality. Therefore, if an effective chemical control method targeting incursions of Argentine ants in vineyards contributes to the dissociation of this invasive ant species with its citrophilus mealybug mutualist, then in theory the spread of GLRaV-3 in vineyards by its mealybug vector can be stemmed. Three insecticidal treatments targeting Argentine ants in the canopy of potted Pinot Noir grapevines inoculated with citrophilus mealybugs were trialled at a field site established in Nelson during the summer of 2016/2017. Bifenthrin (1200ppm) was sprayed on vine trunks and the low- toxicity baits, thiamethoxam (0.0006%) or boric acid (0.5%) carried in polyacrylamide gel with 25% sucrose and 0.15% citric acid solution, were placed at the base of vines. A significant decline in ant activity (p < 0.001) and citrophilus mealybugs was observed for the bifenthrin treatment. A follow-on bioassay was conducted at Mt. Albert Plant and Food Research, in the absence of P. calceolariae’s natural enemies to test the hypothesis that the decline in citrophilus mealybugs in response to vines treated with bifenthrin, could in fact be due to inter-species horizontal toxicity because of Argentine ants transferring the toxicant bifenthrin to citrophilus mealybugs while tending them or contaminating the substrate that they fed on. The significant decrease in average citrophilus mealybug activity rate (p < 0.001) for bifenthrin treatments compared with the controls provides evidence for inter-species horizontal toxicity. Bifenthrin sprayed on grapevine trunks may be suitable to control Argentine ants in the vine canopy and indirectly control P. calceolariae, a known vector of GLRaV-3 between grapevine hosts. The concept of inter-species horizontal toxicity could become a model for targeted pest management by exploiting different insect mutualisms in various horticultural cropping systems.</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.

The influence of semiochemicals on the co-occurrence patterns of a global invader and native species

<p>The success of invasive species in their introduced range is often influenced by interactions with resident species communities. Chemical communication is one the factors which contributes to a variety of aspects of a species life cycle, ranging from mating, to food localization and interactions with members of the same and other species. In my thesis, I investigate the effects of venoms and semiochemicals on interactions between the invasive Argentine ant (Linepethima humile) with other resident ant species and demonstrate how pheromones can potentially be utilized as an area wide control mechanism of this species, by disrupting their foraging success. I studied the effects of venom composition, their toxicity and utilization on the outcome of aggressive interactions between the Argentine ant and the four Monomorium species in New Zealand occurring. The toxicity of the venom of the two species co-occurring with Argentine ants was significantly higher than the toxicity of the species which do not. However, no correlation between venom toxicity and Monomorium survival was found. For M. antipodum a significant relationship between venom utilization and its mortality was found, indicating that the way venom is used might be an important aspect of these interactions. Physical Aggression between Monomorium and Argentine ants also had strong effects on Monomorium worker mortality, which provided evidence that a variety of factors and strategies contribute to the ability of interacting organisms to withstand the pressure of a dominant invader at high abundance. I conducted bioassays with food sources and synthetic trail pheromones, providing a proof of concept on disrupting the foraging ability of Argentine ants. Other resident species benefited from the reduced success of Argentine ants, but to a varying degree. Behavioural variations between the resident species provided an explanation for observed differences in foraging success and how much each of these individual competitors was able to increase their foraging. The mechanism for the observed increase in resource acquisition of resident species appeared to be a decrease in aggressive behaviour displayed by Argentine ants. I expanded the usage of the synthetic pheromone to a commercial vineyard, were Argentine ants can have negative effects on crop development by dispersing and tending to homopteran pest species. Argentine ants’ access to the crop canopy could be significantly reduced by placing pheromone dispensers at the base of the vine plant, while dispensers in the plant canopy had little effect on Argentine ant numbers. Doubling the amount of pheromone did not result in an additional reduction of ant activity. Lastly incorporating the knowledge gained in the previous chapter, I extended the application of the pheromone to a large field trial over a three month period. Argentine ant activity and foraging success was significantly supressed compared to untreated control plots, providing evidence that this form of large scale application might be a possible way to control large invasive ant populations by disrupting their trail following and foraging behaviour for a prolonged period of time. While initial calculations have suggested that the treatment is currently not feasible (13.3 US$/mg/ha), I found a significant reduction in body fat in workers collected from treated plots compared with untreated plots, suggesting adverse effects on nest fitness. My findings provide new insights into chemical communication between invasive and resident species, support existing dominance hierarchy models in ant populations, and help to establish a target specific potential management technique of wide-spread invasive ant species.</p>

Population dynamics and pathogens of the invasive yellow crazy ant (Anoplolepis gracilipes) in Arnhem Land, Australia

<p>Though many populations of introduced species have been observed to collapse, the reasons behind these declines are seldom investigated. Anoplolepis gracilipes is considered among one of the top six most economically and ecologically damaging invasive ant species in the world. However, introduced populations of A. gracilipes have been observed to decline. My overall aims in this thesis were to document A. gracilipes population declines, to investigate the possibility that pathogens were playing a role in the observed population declines, and to identify putative pathogens infecting A. gracilipes as potential candidates for biocontrol agents. I documented the observed A. gracilipes population declines that were the driving force for this project. I detailed large-scale reductions in the spatial extent of four populations with before and after survey data. I also presented data on three populations that were recorded as present, but disappeared before they could be spatially delimited. I speculated on the possible reasons for these declines and explained why I do not think other explanations are likely. I then investigated the hypothesis that a pathogen or parasite is affecting A. gracilipes queens in declining Arnhem Land populations. I did this in three ways: 1) based on preliminary findings, I looked at the effect of an artificial fungal infection on A. gracilipes reproduction. I compared reproductive output between control colonies and those treated with either a fungal entomopathogen (Metarhizium anisopliae) or fungicidal antibiotics. There was no correlation between either treatment and the number of eggs, larvae, pupae or males a colony produced after 70 days. I found queen number had no effect on colony reproductive output, suggesting that queens are able to adjust their egg-laying rate in the presence of other queens. I found no evidence that M. anisopliae affected reproductive output at the tested concentrations; 2) I explored the hypothesis that a pathogen that kills or affects the reproductive output of A. gracilipes queens is the mechanism or reason behind the population declines. I measured queen number per nest, egg-laying rate, fecundity and fat content and compared them between sites in different stages of decline or expansion (population types, consisting of low, medium and high-density populations). I discovered that 23% of queens had melanized nodules, a cellular immune response in insects, in their ovaries or fat bodies. The presence of nodules was correlated with a 22% decrease in the number of oocytes per ovary; however, nodule presence was not associated with population type, suggesting that though there are clearly pathogens or parasites capable of penetrating the cuticle of A. gracilipes, they are unlikely to be responsible for the observed population declines; 3) I compared microbial communities (bacteria and viruses) between queens from different population types. I found viral sequences that match to the Dicistroviridae family of viruses in low and medium-density populations. I found no differences in bacterial community structure between population types. The presence of sequences similar to the entomopathogens Rhabdochlamydia and Serratia marcescens, as well as the reproductive parasite Cardinium in A. gracilipes, deserves further investigation. Though introduced species’ populations have been observed to decline, this is one of the first studies to quantitatively examine, document, and investigate a mechanism behind such a decline. Understanding the mechanisms by which an invader declines may have important implications for invasive ant management worldwide.</p>

Population dynamics and pathogens of the invasive yellow crazy ant (Anoplolepis gracilipes) in Arnhem Land, Australia

<p>Though many populations of introduced species have been observed to collapse, the reasons behind these declines are seldom investigated. Anoplolepis gracilipes is considered among one of the top six most economically and ecologically damaging invasive ant species in the world. However, introduced populations of A. gracilipes have been observed to decline. My overall aims in this thesis were to document A. gracilipes population declines, to investigate the possibility that pathogens were playing a role in the observed population declines, and to identify putative pathogens infecting A. gracilipes as potential candidates for biocontrol agents. I documented the observed A. gracilipes population declines that were the driving force for this project. I detailed large-scale reductions in the spatial extent of four populations with before and after survey data. I also presented data on three populations that were recorded as present, but disappeared before they could be spatially delimited. I speculated on the possible reasons for these declines and explained why I do not think other explanations are likely. I then investigated the hypothesis that a pathogen or parasite is affecting A. gracilipes queens in declining Arnhem Land populations. I did this in three ways: 1) based on preliminary findings, I looked at the effect of an artificial fungal infection on A. gracilipes reproduction. I compared reproductive output between control colonies and those treated with either a fungal entomopathogen (Metarhizium anisopliae) or fungicidal antibiotics. There was no correlation between either treatment and the number of eggs, larvae, pupae or males a colony produced after 70 days. I found queen number had no effect on colony reproductive output, suggesting that queens are able to adjust their egg-laying rate in the presence of other queens. I found no evidence that M. anisopliae affected reproductive output at the tested concentrations; 2) I explored the hypothesis that a pathogen that kills or affects the reproductive output of A. gracilipes queens is the mechanism or reason behind the population declines. I measured queen number per nest, egg-laying rate, fecundity and fat content and compared them between sites in different stages of decline or expansion (population types, consisting of low, medium and high-density populations). I discovered that 23% of queens had melanized nodules, a cellular immune response in insects, in their ovaries or fat bodies. The presence of nodules was correlated with a 22% decrease in the number of oocytes per ovary; however, nodule presence was not associated with population type, suggesting that though there are clearly pathogens or parasites capable of penetrating the cuticle of A. gracilipes, they are unlikely to be responsible for the observed population declines; 3) I compared microbial communities (bacteria and viruses) between queens from different population types. I found viral sequences that match to the Dicistroviridae family of viruses in low and medium-density populations. I found no differences in bacterial community structure between population types. The presence of sequences similar to the entomopathogens Rhabdochlamydia and Serratia marcescens, as well as the reproductive parasite Cardinium in A. gracilipes, deserves further investigation. Though introduced species’ populations have been observed to decline, this is one of the first studies to quantitatively examine, document, and investigate a mechanism behind such a decline. Understanding the mechanisms by which an invader declines may have important implications for invasive ant management worldwide.</p>

The influence of semiochemicals on the co-occurrence patterns of a global invader and native species

<p>The success of invasive species in their introduced range is often influenced by interactions with resident species communities. Chemical communication is one the factors which contributes to a variety of aspects of a species life cycle, ranging from mating, to food localization and interactions with members of the same and other species. In my thesis, I investigate the effects of venoms and semiochemicals on interactions between the invasive Argentine ant (Linepethima humile) with other resident ant species and demonstrate how pheromones can potentially be utilized as an area wide control mechanism of this species, by disrupting their foraging success. I studied the effects of venom composition, their toxicity and utilization on the outcome of aggressive interactions between the Argentine ant and the four Monomorium species in New Zealand occurring. The toxicity of the venom of the two species co-occurring with Argentine ants was significantly higher than the toxicity of the species which do not. However, no correlation between venom toxicity and Monomorium survival was found. For M. antipodum a significant relationship between venom utilization and its mortality was found, indicating that the way venom is used might be an important aspect of these interactions. Physical Aggression between Monomorium and Argentine ants also had strong effects on Monomorium worker mortality, which provided evidence that a variety of factors and strategies contribute to the ability of interacting organisms to withstand the pressure of a dominant invader at high abundance. I conducted bioassays with food sources and synthetic trail pheromones, providing a proof of concept on disrupting the foraging ability of Argentine ants. Other resident species benefited from the reduced success of Argentine ants, but to a varying degree. Behavioural variations between the resident species provided an explanation for observed differences in foraging success and how much each of these individual competitors was able to increase their foraging. The mechanism for the observed increase in resource acquisition of resident species appeared to be a decrease in aggressive behaviour displayed by Argentine ants. I expanded the usage of the synthetic pheromone to a commercial vineyard, were Argentine ants can have negative effects on crop development by dispersing and tending to homopteran pest species. Argentine ants’ access to the crop canopy could be significantly reduced by placing pheromone dispensers at the base of the vine plant, while dispensers in the plant canopy had little effect on Argentine ant numbers. Doubling the amount of pheromone did not result in an additional reduction of ant activity. Lastly incorporating the knowledge gained in the previous chapter, I extended the application of the pheromone to a large field trial over a three month period. Argentine ant activity and foraging success was significantly supressed compared to untreated control plots, providing evidence that this form of large scale application might be a possible way to control large invasive ant populations by disrupting their trail following and foraging behaviour for a prolonged period of time. While initial calculations have suggested that the treatment is currently not feasible (13.3 US$/mg/ha), I found a significant reduction in body fat in workers collected from treated plots compared with untreated plots, suggesting adverse effects on nest fitness. My findings provide new insights into chemical communication between invasive and resident species, support existing dominance hierarchy models in ant populations, and help to establish a target specific potential management technique of wide-spread invasive ant species.</p>

Disentangling factors that assemble New Zealand's ant communities

<p>Several biotic and abiotic stressors can influence community assembly. The negative co-occurrence patterns observed within many communities, for example, may derive either from behavioural similarities (e.g. species displaying high aggression levels towards each other) or habitat preference. I evaluated the role of several stressors that may shape New Zealand’s ant communities. First, I investigated (in chapter 2) the co-occurrence patterns of two native ant communities located within transitional grassland-forest habitats. I also monitored the temperature variation in these habitats over a one-year period. I found that grasslands are exposed to higher temperature variation than forest habitats. I also found that some ants are mostly associated with forest habitats and others with grasslands. Using null models to examine these communities, I found evidence that two ant species (Monomorium antarcticum and Prolasius advenus) exhibit negative co-occurrence patterns. In the reminder of my thesis I developed a series of laboratory-based experiments to examine the processes that could explain the co-occurrence patterns that I observed in these ant communities. In chapter 3, I subjected heterospecific groups of ants to interactions in controlled conditions. I asked if interspecific aggression predict the survival probability and co-occurrence patterns described in chapter 2. My results demonstrated that aggression predicted the survival probability of interacting ant species and their co-occurrence patterns. I argued that aggressive behaviour might reflect the risks imposed by competitors. Differences in aggression may thus be a key factor influencing sympatric and allopatric co-occurrence patterns of these ant communities. In chapter 4, I tested the hypotheses that arrival sequence and diet influence the strength of interactions between colonies of two species that exhibited negative co-occurrence patterns (P. advenus and M. antarcticum). When arriving first, P. advenus displayed increased aggression and M. antarcticum a defensive reaction. The adoption of a defensive reaction by M. antarcticum increased their colony survival probability. Changes in carbohydrate and protein availability modulated colony activity rates of both species. These results indicate that arrival sequence can modulate the territorial behaviour displayed by interacting species in situations of conflict. Also, I showed that these ant species adjust their foraging activity rates in according to their diet, but different species do so differently. In chapter 5, I expanded the scope of chapter 4 and asked if aggression and foraging behaviour of P. advenus and M. antarcticum change in different conditions of temperature, diet and group size. For both ant species, changes in temperature had stronger effects on small than large colonies. Small groups of M. antarcticum displayed higher foraging activity at lower temperatures. Conversely, small groups of P. advenus displayed higher foraging activity at high temperatures. Also, small M. antarcticum colonies displayed increased aggression and significantly reduced the size of large P. advenus colonies, regardless of temperature and diet. These results suggest that P. advenus and M. antarcticum perform differently at different temperatures. Furthermore, I demonstrated that the persistence of these small colonies might be related to their ability to modulate foraging activities and interspecific aggression according to the environment. I also investigated (in chapter 6) the effects of a neurotoxic pesticide (neonicotinoid) on a native (M. antarcticum) and an invasive ant (Linepithema humile). I tested whether sublethal contamination with a neonicotinoid affects foraging, fitness and the outcome of interspecific interactions between these ants. Overall, pesticide exposure increased aggression of the invasive ant and reduced the aggression of the native species. Importantly, non-exposed individuals of the invasive species subjected to interactions against exposed natives were less aggressive, but more likely to survive. These results suggest that the modification of the physicochemical environment by pesticide contamination could change the dynamics of communities and influence invasion success. Overall, this thesis highlights that synergistic effects between several biotic and abiotic factors influence community assembly. My results suggest that non-random allopatric patterns of niche occupancy observed in these ant communities are better explained by high levels of aggression displayed between pairs of species that seldom co-occur, though I was unable to falsify the hypothesis that habitat preference also plays a role in determining their distribution and co-occurrence patterns. The modification of behaviour by external factors – either natural (e.g. temperature) or human mediated (e.g. pesticide exposure) – likely has broad effects on population and community dynamics and on patterns of species co-existence.</p>