The Argentine ant: challenges in managing an invasive unicolonial pest

Catch and release: controlling eastern yellowjacket Vespula maculifrons colonies using horizontal insecticide transfer

BACKGROUND

Horizontal insecticide transfer is thought to play an important role in controlling a wide range of urban pests including ants, bed bugs, cockroaches and termites. Trap-treat-release is an effective experimental approach that has been used to successfully manage populations of invasive ants in field applications. Trap-treat-release is based on the principles of horizontal transfer. Individuals are captured, treated with the toxicant and released back into the environment. The treated individuals then return to the colony and transfer the toxicant to other members of the population resulting in secondary mortality. The goal of the current study was to evaluate the efficacy of the trap-treat-release technique for controlling field populations of the eastern yellowjacket, Vespula maculifrons.

RESULTS

Laboratory experiments demonstrated that fipronil was highly toxic against V. maculifrons across a wide range of concentrations. Furthermore, fipronil was efficiently transferred from treated donors to untreated recipients and caused significant secondary mortality. A field experiment utilized trap-treat-release and demonstrated that fipronil was effectively transferred when foraging worker wasps are trapped, treated, released and allowed to return to their respective colonies.

CONCLUSION

The trap-treat-release method may be an effective alternative to direct nest treatments and could help alleviate problems such as insecticide runoff, environmental contamination, and non-target effects. This method has the potential to provide effective management of social wasps. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Do Dominant Native Ants Outcompete the Invasive Argentine Ant in Mediterranean Citrus Ecosystems? A Laboratory Test

The invasive Argentine ant (Linepithema humile) poses a significant threat to ecosystem stability worldwide. In Mediterranean citrus ecosystems, its spread may be limited by interactions with dominant native ant species. We conducted laboratory experiments to investigate the competitive dynamics between Argentine ants and two major native species, Tapinoma nigerrimum and Lasius grandis. At the individual level, both native species exhibited superior competitive performance, attributed to their larger body sizes and potential differences in chemical defences. At the colony level, T. nigerrimum demonstrated efficiency in interference competition, successfully defending food resources from Argentine ants. However, the Argentine ant exhibited higher recruitment capacity, albeit it was density-dependent. Our findings support the hypothesis that dominant native ants can serve as barriers against Argentine ant invasion in citrus ecosystems, highlighting the importance of interspecific competition in shaping community dynamics and invasive species management. This study underscores the potential role of native ant species in mitigating the impacts of invasive ants on ecosystem functioning and biodiversity conservation in agricultural landscapes, offering valuable insights for invasive species management strategies in Mediterranean citrus ecosystems.

Corpse-associated odours elicit avoidance in invasive ants

BACKGROUND

Invasive ants, such as Linepithema humile (the Argentine ant), pose a global threat, necessitating a better understanding of their behaviour in order to improve management strategies. Traditional eradication methods, including baiting, have had limited success, but the causes of control failure are not always clear. This study aims to investigate whether ants form associations between food odours and corpses, and subsequently avoid areas or food sources with food odours associated with corpses. We propose that ants may learn to avoid toxic baits in part because of their association with ant corpses, which could have implications for pest control strategies.

RESULTS

Ants were tested on a Y-maze after exposure to scented corpses or dummies. 69% (n = 64) of ants avoided branches bearing the scent of scented corpses. Colonies neglected food with corpse-associated odours, with only 42% (n = 273) of foragers feeding from such sources. However, if corpses were produced by feeding ants scented toxicant, focal ants encountering these corpses did not avoid the corpse-associated scent on a Y-maze (53%, n = 65). In dual-feeder tests, ants did not avoid feeding at food sources scented with odours associated with conspecific corpses.

CONCLUSION

Conspecific corpses act as a negative stimulus for ants in a foraging situation, potentially causing avoidance of toxic baits. This study suggests adding odours to baits and cycling them to disrupt the bait-corpse association may be helpful. Interestingly, although avoidance of baits was observed, feeding preferences were not significantly affected. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Invasive ant learning is not affected by seven potential neuroactive chemicals

Argentine ants Linepithema humile are one of the most damaging invasive alien species worldwide. Enhancing or disrupting cognitive abilities, such as learning, has the potential to improve management efforts, for example by increasing preference for a bait, or improving ants' ability to learn its characteristics or location. Nectar-feeding insects are often the victims of psychoactive manipulation, with plants lacing their nectar with secondary metabolites such as alkaloids and non-protein amino acids which often alter learning, foraging, or recruitment. However, the effect of neuroactive chemicals has seldomly been explored in ants. Here, we test the effects of seven potential neuroactive chemicals-two alkaloids: caffeine and nicotine; two biogenic amines: dopamine and octopamine, and three nonprotein amino acids: β-alanine, GABA and taurine-on the cognitive abilities of invasive L. humile using bifurcation mazes. Our results confirm that these ants are strong associative learners, requiring as little as one experience to develop an association. However, we show no short-term effect of any of the chemicals tested on spatial learning, and in addition no effect of caffeine on short-term olfactory learning. This lack of effect is surprising, given the extensive reports of the tested chemicals affecting learning and foraging in bees. This mismatch could be due to the heavy bias towards bees in the literature, a positive result publication bias, or differences in methodology.

Ants evade harmful food by active abandonment

Invasive ants, such as the Argentine ant, pose a severe economic and ecological threat. Despite advancements in baiting techniques, effectively managing established ant populations remains a daunting challenge, often ending in failure. Ant colonies employ behavioural immunity against pathogens, raising the question of whether ants can collectively respond to toxic baits. This study investigates whether ant colonies actively abandon palatable but harmful food sources. We provided two sucrose feeders, each generating a new foraging trail, with one transitioning to offering toxic food. Six hours later, ant activity on that path decreases, while activity on the non-toxic food and the trunk trail remains unaffected, excluding factors like population decline or satiation as reasons for the activity decline. Laboratory experiments confirmed that ants remained alive six hours after ingesting toxic food. Ant presence remains low on the toxic food path for days, gradually decreasing along the nearest section of the trunk trail. This abandonment behaviour minimises the entry of harmful food into the nest, acting as a protective social mechanism. The evasion of toxic bait-treated areas likely contributes considerably to control failures. Understanding the behavioural response to toxic baits is essential for developing effective strategies to combat invasive ant species.

Undesirable dispersal via a river pathway of a single Argentine ant supercolony newly invading an inland urban area of Japan

Invasive ants pose a risk to human well-being and social/ecosystem stability. Linepithema humile Mayr is among the most damaging invasive ants worldwide. Most L. humile populations invade ports/wharfs isolated from surrounding landscapes, but unfortunately, a new population was discovered in an inland urban area (Nara Prefecture) of Japan in 2021. In this study, first, the supercolony type of the Nara L. humile population was identified via a hostility test, and then its distribution pattern was characterized. In aggression tests between L. humile from Nara and four supercolonies (haplotypes LH1, LH2, LH3, LH4), this ant showed extremely strong hostility against all supercolonies exept LH2, which was detected only in Japan in its introduced range. In Nara, L. humile was abundant in and around the urban river. Simulations revealed that using this environment for movement/dispersal increased the annual dispersal ability by 14 times compared with that achieved via ground (125 m), as mentioned in the literature. Therefore, river channels can serve as major pathways of long-distance dispersal for L. humile invading inland urban areas. Since applying chemical strategies around rivers is problematic, preventing L. humile from moving to rivers from initial invasion sites is crucial.

Laboratory evaluations of biodegradable boric acid hydrogel baits for the control of Argentine ant (Hymenoptera: Formicidae)

Due to their mutualistic relationship with plant pests, the Argentine ant is considered a major pest in subtropical fruit orchards and vineyards. Besides insecticide sprays, liquid baiting has been demonstrated as an effective method to suppress the Argentine ant populations. To improve the economic feasibility of liquid baiting, hydrogel materials have been recently tested as a carrier for liquid baits containing various insecticidal active ingredients. Here, we tested boric acid as a toxicant in the aqueous sugar bait delivered in a biodegradable calcium alginate hydrogel. Laboratory tests demonstrated that boric acid (1%) liquid bait incorporated in the calcium alginate hydrogel effectively killed Argentine ant workers. Potassium sorbate (0.25%) added to the liquid bait as a preservative did not impact the efficacy of boric acid even though it significantly reduced the degree of swelling of the hydrogel beads in the bait solution. Testing with 2-month-old bait suggested that long-term storage might impact bait efficacy even with potassium sorbate preservative.

Development of an effective hydrogel bait and an assessment of community-wide management targeting the invasive white-footed ant, Technomyrmex brunneus

BACKGROUND

Ants are one of the most serious household pests. White-footed ants in the genus Technomyrmex (Hymenoptera: Formicidae) are invasive species of increasing global importance as evidenced by recent range expansion, nuisance pest status in residential areas and control difficulties driven mainly by lack of effective bait products. The goal for the current study was to develop an effective hydrogel bait and community-wide management program for controlling the invasive T. brunneus in a residential area of Hachijo Island, Japan.

RESULTS

In laboratory insecticide screening, hydrogel baits containing thiamethoxam achieved higher T. brunneus mortality than those containing dinotefuran, imidacloprid, boric acid or fipronil at the concentrations tested. Hydrogel baits containing 0.01% and 0.001% thiamethoxam resulted in ≥90% mortality within seven days. However, the horizontal transfer effect was not strong with either concentration. Field experiments confirmed that hydrogel baits containing 10% and 30% sugar are highly attractive to T. brunneus. The community-wide treatment utilizing hydrogel bait containing 0.001% thiamethoxam and 30% sugar significantly suppressed T. brunneus. Town residents were given the opportunity to participate in the program by applying hydrogel baits around their homes, and a follow-up survey revealed that the residents regarded hydrogel baits as easy to apply and highly attractive and effective against T. brunneus.

CONCLUSION

The hydrogel bait and management program developed in this study can be used to suppress T. brunneus. Furthermore, our results demonstrate that a large network of highly motivated and properly trained members of the public can be highly effective in managing invasive ant populations. © 2022 Society of Chemical Industry.

Chemical identification of an active component and putative neural mechanism for repellent effect of a native ant’s odor on invasive species

The invasive Argentine ants (Linepithema humile) and the red imported fire ants (Solenopsis invicta) constitute a worldwide threat, causing severe disruption to ecological systems and harming human welfare. In view of the limited success of current pest control measures, we propose here to employ repellents as means to mitigate the effect of these species. We demonstrate that cuticular hydrocarbons (CHCs) used as nestmate-recognition pheromone in the Japanese carpenter ant (Camponotus japonicus), and particularly its (Z)-9-tricosene component, induced vigorous olfactory response and intense aversion in these invasive species. (Z)-9-Tricosene, when given to their antennae, caused indiscriminate glomerular activation of antennal lobe (AL) regions, creating neural disarray and leading to aversive behavior. Considering the putative massive central neural effect, we suggest that the appropriate use of certain CHCs of native ants can facilitate aversive withdrawal of invasive ants.

Urban forest invertebrates: how they shape and respond to the urban environment

Invertebrates comprise the most diversified animal group on Earth. Due to their long evolutionary history and small size, invertebrates occupy a remarkable range of ecological niches, and play an important role as “ecosystem engineers” by structuring networks of mutualistic and antagonistic ecological interactions in almost all terrestrial ecosystems. Urban forests provide critical ecosystem services to humans, and, as in other systems, invertebrates are central to structuring and maintaining the functioning of urban forests. Identifying the role of invertebrates in urban forests can help elucidate their importance to practitioners and the public, not only to preserve biodiversity in urban environments, but also to make the public aware of their functional importance in maintaining healthy greenspaces. In this review, we examine the multiple functional roles that invertebrates play in urban forests that contribute to ecosystem service provisioning, including pollination, predation, herbivory, seed and microorganism dispersal and organic matter decomposition, but also those that lead to disservices, primarily from a public health perspective, e.g., transmission of invertebrate-borne diseases. We then identify a number of ecological filters that structure urban forest invertebrate communities, such as changes in habitat structure, increased landscape imperviousness, microclimatic changes and pollution. We also discuss the complexity of ways that forest invertebrates respond to urbanisation, including acclimation, local extinction and evolution. Finally, we present management recommendations to support and conserve viable and diverse urban forest invertebrate populations into the future.

Development and Demonstration of Low-Impact IPM Strategy to Control Argentine Ants (Hymenoptera: Formicidae) in Urban Residential Settings

Argentine ants are one of the most common nuisance pest ants treated by pest management professionals (PMPs) in southern and western urban residential areas of the United States. Two new technologies (spraying with a pheromone adjuvant and using a biodegradable hydrogel bait delivery method) were used to develop a unique low-impact integrated pest management (IPM) protocol for Argentine ants in urban residential settings. The IPM protocol included a one-time perimeter spray treatment with 0.03% fipronil (mixed with a pheromone adjuvant) at the beginning of the ant season to achieve a quick knockdown. The initial spray application was followed by a biodegradable hydrogel baiting with 1% boric acid as a maintenance treatment. This low-impact IPM protocol was compared with two other conventional methods: (1) one initial fipronil application and one pyrethroid spray application for maintenance, or (2) one initial fipronil application and one essential oil insecticide spray application for maintenance. Based on Argentine ant foraging activity, the protocols were compared for their control efficacy. Insecticide use information and treatment time were also recorded and compared among different treatment protocols. Our results provided empirical data to support the effectiveness and economic feasibility of the low-impact IPM protocol for managing Argentine ants in urban residential settings.

Laboratory and field insights into the dynamics and behavior of Argentine ants, Linepithema humile, feeding from hydrogels

BACKGROUND

Hydrogels that have absorbed a liquid containing a toxicant are a novel form of bait-delivery for ant control. Here, we study the abilities of Argentine ant (Linepithema humile) workers to imbibe liquid from hydrogels. We quantified feeding behavior with: (i) hydrogels containing different sucrose concentrations (20, 30, 40 and 50% w/w); (ii) hydrogels versus liquid droplets; and (iii) hydrogel age (air exposure time). We also performed a field assay to assess visits by L. humile and other ant species to hydrogels.

RESULTS

Ingested volume and feeding time decreased with increasing sucrose concentrations, but the number and duration of pauses were similar. Feeding from hydrogels was slower than from a liquid droplet and ants imbibed less liquid and fed for shorter times from hydrogels. Feeding time increased with hydrogel age, whereas ingested volume decreased and approached zero after 120 min under laboratory conditions. In the field, ants attended the hydrogels during the full 120-min study period. When L. humile workers found a hydrogel, they monopolized it to the exclusion of other ant species. L. humile occupied and dominated hydrogels predominantly in shaded locations.

CONCLUSION

Hydrogels with sucrose concentrations no greater than 30% appear best for liquid uptake by L. humile. Hydrogels not in direct sunlight will have greater attendance by L. humile and, therefore, less attendance by non-target ant species. Shady and humid places may prolong the longevity of hydrogels, which would imply higher intakes. © 2021 Society of Chemical Industry.

A comparison of insecticide susceptibility levels in 12 species of urban pest ants with special focus on the odorous house ant, Tapinoma sessile

BACKGROUND

Many ant species are pests in urban, agricultural, and natural habitats around the world. The primary means of reducing or eliminating ant infestations utilizes chemical control, mainly applications of residual insecticides. Control failures with residual insecticides are common, driven in part by a lack of understanding of basic biological and life history characteristics, including interspecific variation in susceptibility to insecticides. The current study evaluated the susceptibility of 12 species of urban pest ants to three classes of insecticides.

RESULTS

Results show significant variation in susceptibility across species. Contrary to the hypothesis of proportionality, no significant relationship was detected between body mass and median lethal time (LT₅₀ ) or time to 100% mortality. The odorous house ant (Tapinoma sessile) was consistently the least susceptible to all insecticides, as indicated by the highest LT₅₀ values and the greatest amount of time required to reach 100% mortality. Comparatively low susceptibility to commonly used spray insecticides may explain why T. sessile is such a persistent pest. Broadcast applications of spray insecticides may kill off the most susceptible species, leaving behind T. sessile. Lack of competition from other ant species, combined with increased access to nesting and feeding resources may allow T. sessile to fill a vacant ecological niche and expand its range.

CONCLUSION

Considering T. sessile's relatively low susceptibility to insecticides, its ability to become established in areas colonized by other invasive ants, and its highly invasive behaviors, it should be watched for by biosecurity programs as it has high potential to become a globally invasive pest. © 2021 Society of Chemical Industry.

Controlling invasive Argentine ants, Linepithema humile, in conservation areas using horizontal insecticide transfer

Invasive ants are major agricultural and urban pests and a significant concern in conservation areas. Despite long history of control and eradication efforts, invasive ants continue to spread around the globe driven by a multitude of synergistic factors. Lack of effective management tools is one of the biggest challenges in controlling invasive ants. The goal of the current study was to improve the efficacy and safety of ant management and to develop effective control strategies for sensitive conservation areas. We utilized the Argentine ant (Linepithema humile) as a model system to evaluate a target-specific pesticide delivery system that exploits the interconnected nature of social insect colonies to distribute a toxicant effectively within the colony. The approach, based entirely on horizontal transfer, takes advantage of various levels of social interactions in ant colonies to disseminate a toxicant throughout the colony. Results of laboratory studies coupled with LC/MS/MS analysis demonstrate that fipronil is toxic to Argentine ants in extremely small (nanogram) quantities and is efficiently transferred from a single treated donor to multiple recipients, causing significant secondary mortality. A field study was conducted in native fynbos plots invaded by Argentine ants. The study consisted of collecting naïve workers, treating them with fipronil, and releasing them within invaded plots. Results show that the release of fipronil-treated ants reduced Argentine ant abundance by >90% within 24 h. The horizontal transfer approach offers environmental benefits with regard to pesticide use in ecologically sensitive environments and appears ideally suited for ant management in conservation areas.

Trap-treat-release: horizontal transfer of fipronil in field colonies of black carpenter ants, Camponotus pennsylvanicus

BACKGROUND

Horizontal insecticide transfer is thought to play an important role in controlling a wide range of urban pests including ants, bed bugs, cockroaches, and termites. Despite decades of research and numerous laboratory studies, horizontal transfer has never been demonstrated in the field. As a result, the importance of horizontal transfer (and the resulting secondary kill) for practical pest management remains unknown. The goal of this study was to provide the first experimental examination of horizontal transfer under field conditions. The specific objective was to investigate horizontal transfer of fipronil in field colonies of black carpenter ants, Camponotus pennsylvanicus.

RESULTS

Laboratory experiments demonstrated that fipronil is effectively transferred from treated donors to untreated recipients and causes significant secondary mortality. Fipronil was effectively vectored to untreated ants from donors exposed via residual and direct spray applications, and 100% mortality was achieved with both exposure routes. Furthermore, horizontal transfer continued beyond secondary mortality and resulted in significant tertiary mortality, which has not been previously demonstrated in ants. Field experiments utilized a novel, three-step control method consisting of trap-treat-release and demonstrated that fipronil is effectively transferred when foraging workers are trapped, treated, and subsequently released back into their colonies.

CONCLUSION

The current study is the first field demonstration of the importance of horizontal transfer for the control of pest ants. The trap-treat-release method may be an effective alternative to broadcast spray applications and could help alleviate problems such as insecticide run-off, environmental contamination, and non-target effects. This method has the potential to provide effective management of invasive and pest ants and should be further tested across a wider range of ant species, habitats, and active ingredients. © 2019 Society of Chemical Industry.

Corpse management of the invasive Argentine ant inhibits growth of pathogenic fungi

A dead conspecific poses a potential pathogen risk for social animals. We have discovered that Argentine ants (Linepithema humile) prevent spread of pathogenic fungi from corpses by depositing the dead to combined toilet and refuse areas and applying pygidial gland secretion on them. The presence of a corpse in a nest increases this secretion behaviour. We identified three fungi growing on Argentine ant corpses. Growth of the Argentine ant pathogen Aspergillus nomius and the plant pathogen Fusarium solani on corpses was inhibited as long as the ants were constantly attending them as the ant anal secretion only delayed germination of their spores. In contrast, the effect of the ant anal secretion on the human pathogen Aspergillus fumigatus was much stronger: it prevented spore germination and, accordingly, the fungus no longer grew on the treated corpses. The Argentine ants are one of the world's worst invasive alien species as they cause ecological and economical damage in their new habitats. Our discovery points at a novel method to limit Argentine ant colonies through their natural fungal pathogens.

Impacts of the Invasive European Red Ant (Myrmica rubra (L.): Hymenoptera; Formicidae) on a Myrmecochorous System in the Northeastern United States

We investigated the impact of an invasive ant species from Europe, Myrmica rubra (L.), on a myrmecochorous system (seeds dispersed by ants) in its invaded range in North America. We assessed: 1) how M. rubra process the myrmecochorous diapsores (seeds and elaiosome as a single dispersal unit transported by ants) in comparison with native ants; 2) its preference for common native and invasive diaspore species relative to native ants; 3) how far they disperse diaspores in the field; and 4) the diaspore removal rate by invertebrates and vertebrates in infested areas compared to noninvaded sites. Field experiments demonstrated higher diaspore removal rates over a 10-min and 24-h period by M. rubra compared to native ants. M. rubra's diaspore dispersal distance was 40% greater compared to native ants. In two of three laboratory studies and one field study, there was no significant difference between the seed species which M. rubra and native ants selected. Our data suggest no long-term deleterious effects of M. rubra's invasion on diaspore dispersal in the Maine plant community that is comprised of both native and invasive species. This implies that M. rubra benefits from the myrmechorous plant species' diaspores by increasing their dispersal range away from the parent plant and potentially reducing seed predation. However, it is not known whether the fact that the native ant fauna and M. rubra are attracted to the same plant species' diaspores creates a high level of competition between the ants with deleterious effects on the native ant community.

Foraging Distance of the Argentine Ant in California Vineyards

Argentine ants, Linepithema humile (Mayr) (Hymenoptera: Formicidae), form mutualisms with hemipteran pests in crop systems. In vineyards, they feed on honeydew produced by mealybugs and soft scales, which they tend and protect from natural enemies. Few options for controlling Argentine ants are available; one of the more effective approaches is to use liquid baits containing a low dose of an insecticide. Knowledge of ant foraging patterns is required to estimate how many bait stations to deploy per unit area. To measure how far ants move liquid bait in vineyards, we placed bait stations containing sugar water and a protein marker in plots for 6 d, and then collected ants along transects extending away from bait stations. The ants moved an average of 16.08 m and 12.21 m from bait stations in the first and second years of the study, respectively. Marked ants were found up to 63 m from bait stations; however, proportions of marked ants decreased exponentially as distance from the bait station increased. Results indicate that Argentine ants generally forage at distances <36 m in California vineyards, thus suggesting that insecticide bait stations must be deployed at intervals of 36 m or less to control ants. We found no effect of insecticide on distances that ants moved the liquid bait, but this may have been because bait station densities were too low to affect the high numbers of Argentine ants that were present at the study sites.

Colony size and brood investment of Myrmica rubra ant colonies in habitats invaded by goldenrods

Ant richness and abundance are negatively affected by the invasion of alien goldenrods (Solidago sp.). However, little is known about the mechanisms standing behind the impact of the invaders on ant life history, such as colony investments in growth and reproduction. We examined this problem of the investments of Myrmica rubra ant colonies living in different grasslands invaded and non-invaded by goldenrods. Altogether, 47 colonies were analysed; and for each colony, we calculated the number of queens, workers and the production of young workers, gynes, and males. We found that colonies from invaded meadows are smaller in size, but have a similar number of adult queens compared to colonies from non-invaded sites. We also found different brood investments among colonies from invaded and non-invaded meadows-colonies from non-invaded meadows produce more young workers and invest more in growth, whereas colonies from invaded meadows invest more in reproduction through higher gyne production. Male production was at a similar level in colonies from both habitat types. The observed patterns may be explained by the effect of various environmental factors occurring in both grassland types, such as stress in changed habitats, higher competition among gynes in non-invaded grasslands, or finally, by the adaptive colony-level response of ants to stress. The higher production of gynes observed in the invaded grasslands may support dispersal and enhance the probability of establishing a colony in a more favourable location.

Host-Tree Selection by the Invasive Argentine Ant (Hymenoptera: Formicidae) in Relation to Honeydew-Producing Insects

The Argentine ant, Linepithema humile (Mayr; Hymenoptera: Formicidae), is one of the world's most hazardous invasive species, and thus its eradication from Japan is important. Physical and chemical controls can be expensive and cause strong adverse effects on local terrestrial ecosystems regardless of their high efficacy. Here, presence/absence of host-tree selection by Argentine ants was investigated to understand the ant-honeydew-producing insects interactions in order to develop new cultural controls compatible with biodiversity conservation. Abundance of Argentine ants and their tree utilization ratio was measured among dominant roadside trees (Cinnamomum camphora, Myrica rubra, Nerium indicum, Rhaphiolepis indica var. umbellata, Juniperus chinensis var. kaizuka) in two areas around Kobe, Japan. Almost all ants collected were Argentine ants suggesting that native ants would have been competitively excluded. Tree utilization of Argentine ants clearly differed among host trees. Abundance of both Argentine ants and honeydew-producing insects and tree utilization rate of the ants were significantly lower in especially C. camphora and J. chinensis. Few Argentine ants were observed trailing on C. camphora, J. Chinensis, and N. indicum, most probably due to low abundance of honeydew-producing insects on these trees with the toxic and repellent chemical components. On the other hand, high abundance of both Argentine ants and homopterans were found in M. rubra and especially R. indica. We suggest that reductions of R. indica and M. rubra would lead to a decrease in abundance of honeydew-producing insects, and thus effectively control populations of Argentine ants. At the same time, planting of C. camphora, J. Chinensis, and N. indicum may also play a role in restraint efficacy against invasion of the invasive ants.

Development of an alginate hydrogel to deliver aqueous bait for pest ant management

BACKGROUND

Insecticide sprays used for ant control cause environmental contamination. Liquid bait is a safe and effective alternative, but it requires bait stations to dispense the toxicant. We developed a biodegradable hydrogel to deliver liquid bait obviating the need for bait stations.

RESULTS

Alginate hydrogel beads with preferred rigidity and maximum hydration in 25% sucrose solution were engineered by optimizing a crosslinking process. The moisture content of the substrate on which the beads were placed and the relative atmospheric humidity significantly influenced water loss dynamics of the hydrated hydrogel beads. Laboratory choice studies indicated that hydrated hydrogel beads had reduced palatability to foraging ants when they lost ≥50% water. An enzyme-linked immunosorbent assay (ELISA) indicated that the insecticide thiamethoxam added to sucrose solution was absorbed into the hydrogel beads. Hydrogel beads conditioned in sucrose solution with 1 mg L^-1 thiamethoxam provided complete control of all castes of Argentine ant Linepithema humile (Mayr) colony by 14 days post treatment in the laboratory trial and provided a 79% reduction in ant activity after 8 weeks in the field trial.

CONCLUSION

Alginate hydrogel beads provided an effective delivery system for liquid baits laced with low concentrations of insecticide to control Argentine ants. © 2017 Society of Chemical Industry.

Insecticides for Suppression of Nylanderia fulva

Nylanderia fulva (Mayr) is an invasive ant that is a serious pest in the southern United States. Pest control operators and homeowners are challenged to manage pest populations below acceptable thresholds. Contact and bait insecticides are key components of an Integrated Pest Management (IPM) strategy, however, little is known about their efficacy. In repellency and efficacy bioassays, N. fulva were not completely repelled by any insecticide tested, although fewer ants crossed a surface treated with Temprid^®. Few insecticides provided rapid control. Termidor^® and Temprid^® were the best performing with mean mortality of 100% in 13.4 and 19.0 days, respectively. In no-choice bait acceptance studies, it was shown that N. fulva generally had greater acceptance of carbohydrate-based ant baits (Advion^®, InTice^TM (gel), and InTice^TM (granular)). However, mortality was low for the InTice^TM baits in a 7-day bioassay. Maxforce^® Ant Killer Bait Gel and Advance^® 375A in the spring and Maxforce^® Complete in the summer and fall required the fewest days to reach 100% mortality. Bait active ingredients that resulted in the highest mortality were hydramethylnon and fipronil. These data on the efficacy of commercially available contact and bait insecticides provide valuable information to manage this invasive pest.

Development of Liquid Bait With Unique Bait Station for Control of Dolichoderus thoracicus (Hymenoptera: Formicidae)

The Dolichoderus thoracicus (Smith) has seriously infested urban, village, and agricultural environments in Taiwan. To develop optimal bait for the effective control of D. thoracicus, we investigated the feeding preferences of this ant at different sugar and amino acid concentrations in a laboratory setting. The number of feeding workers was significantly higher for 20% sucrose compared with that for other sugars at 10% and 20% concentrations. Furthermore, among all tested concentrations of monosodium glutamate (MSG; 1%, 3%, 5%, and 10%), the highest number of ants were attracted by 1% MSG with 20% sucrose. The consumption of each bait solution was also measured, and the results were similar to those for ant number. Regarding toxicants, the toxicity and repellency of 3% boric acid and borax bait were evaluated individually. Both 3% boric acid and borax demonstrated no repellency against D. thoracicus workers. The LT50 of the 3% boric acid bait was 4.87 d, whereas that of 3% borax bait was only 1.56 d. Thus, 3% boric acid showed further delayed action, facilitating workers spreading the toxicant to their nestmates through trophallaxis. In combination with our unique bait station, we propose an optimal bait formulation comprising 20% sucrose, 1% MSG, and 3% boric acid as a promising candidate for controlling D. thoracicus.

Declaration of local chemical eradication of the Argentine ant: Bayesian estimation with a multinomial-mixture model

Determining the success of eradication of an invasive species requires a way to decide when its risk of reoccurrence has become acceptably low. In Japan, the area populated by the Argentine ant, Linepithema humile (Mayr), is expanding, and eradication via chemical treatment is ongoing at various locations. One such program in Tokyo was apparently successful, because the ant population decreased to undetectable levels within a short time. However, construction of a population model for management purposes was difficult because the probability of detecting ants decreases rapidly as the population collapses. To predict the time when the ant was eradicated, we developed a multinomial-mixture model for chemical eradication based on monthly trapping data and the history of pesticide applications. We decided when to declare that eradication had been successful by considering both ‘eradication’ times, which we associated with eradication probabilities of 95% and 99%, and an optimal stopping time based on a ‘minimum expected economic cost’ that considered the possibility that surveys were stopped too soon. By applying these criteria, we retroactively declared that Argentine ants had been eradicated 38–42 months after the start of treatments (16–17 months after the last sighting).

Using spatially explicit surveillance models to provide confidence in the eradication of an invasive ant

Effective detection plays an important role in the surveillance and management of invasive species. Invasive ants are very difficult to eradicate and are prone to imperfect detection because of their small size and cryptic nature. Here we demonstrate the use of spatially explicit surveillance models to estimate the probability that Argentine ants (Linepithema humile) have been eradicated from an offshore island site, given their absence across four surveys and three surveillance methods, conducted since ant control was applied. The probability of eradication increased sharply as each survey was conducted. Using all surveys and surveillance methods combined, the overall median probability of eradication of Argentine ants was 0.96. There was a high level of confidence in this result, with a high Credible Interval Value of 0.87. Our results demonstrate the value of spatially explicit surveillance models for the likelihood of eradication of Argentine ants. We argue that such models are vital to give confidence in eradication programs, especially from highly valued conservation areas such as offshore islands.

Feeding behavior and social interactions of the Argentine ant Linepithema humile change with sucrose concentration

Liquid sugar baits are well accepted by the Argentine ant Linepithema humile and are suitable for the chemical control of this invasive species. We evaluated how sugar concentrations affect the foraging behavior of L. humile individuals. We quantified feeding variables for individual foragers (ingested load, feeding time and solution intake rate) when feeding on sucrose solutions of different concentrations, as well as post-feeding interactions with nestmates. Solutions of intermediate sucrose concentrations (10-30%) were the most consumed and had the highest intake rates, whereas solutions of high sucrose concentrations (60 and 70%) resulted in extended feeding times, low intake rates and ants having smaller crop loads. In terms of post-feeding interactions, individuals fed solutions of intermediate sucrose concentrations (20%) had the highest probability of conducting trophallaxis and the smallest latency to drop exposure (i.e. lowest time delay). Trophallaxis duration increased with increasing sucrose concentrations. Behavioral motor displays, including contacts with head jerking and walking with a gaster waggle, were lowest for individuals that ingested the more dilute sucrose solution (5%). These behaviors have been previously suggested to act as a communication channel for the activation and/or recruitment of nestmates. We show here that sucrose concentration affects feeding dynamics and modulates decision making related to individual behavior and social interactions of foragers. Our results indicate that intermediate sucrose concentrations (ca. 20%), appear to be most appropriate for toxic baits because they promote rapid foraging cycles, a high crop load per individual, and a high degree of stimulation for recruitment.

Control of Linepithema micans (Hymenoptera: Formicidae) and Eurhizococcus brasiliensis (Hemiptera: Margarodidae) in Vineyards Using Toxic Baits

Linepithema micans (Forel) (Hymenoptera: Formicidae) is the main ant species responsible for dispersal of Eurhizococcus brasiliensis (Wille) (Hemiptera: Margarodidae), a root scale that damages grapevines in southern Brazil. The effects of different formulations of toxic baits based on boric acid and hydramethylnon to control L. micans and E. brasiliensis were evaluated. Toxic baits with boric acid (1.0%) mixed in different concentrations of inverted sugar (20%, 30%, and 40%), and hydramethylnon, mixed with sardines (paste), cassava flour and peanut, brown sugar (sucrose), or sardine oil-based gel, were evaluated in a greenhouse and in the field. In the greenhouse experiment, the number of foraging ants was significantly reduced in the pots where the hydramethylnon in sardine paste (Solid S), sardine oil-brown sugar-based gel (GEL SAM), and peanut oil-brown-sugar gel (GEL AM) formulations were applied. The GEL SAM toxic bait effectively reduced the infestation of L. micans, and could be used for indirect control of E. brasiliensis on young grapevines.

Development of a Pheromone-Assisted Baiting Technique for Argentine Ants (Hymenoptera: Formicidae)

Current control measures for Argentine ants, Linepithema humile (Mayr), in urban settings typically include perimeter applications of insecticides around structures, resulting in potential problems with insecticide runoff and environmental contamination. Insecticidal baits can be an effective alternative to perimeter spray applications and are largely considered target-specific with minimal nontarget impact and environmental contamination. We report a "pheromone-assisted baiting technique" as an economically viable approach to maximize the efficacy of conventional baits targeting Argentine ants. Laboratory experiments with a commercially available gel bait indicated that foraging activity and final mortality of Argentine ants were significantly improved by incorporating (Z)-9-hexadecenal in the bait. The field study demonstrated that the pheromone-treated gel bait achieved a 74% reduction in Argentine ant activity by the end of 4 wk when it was compared with its own pretreatment value. This was a significant improvement over the untreated gel bait that provided a 42% reduction over the same period of time. The pheromone-assisted baiting technique has the potential in providing effective ant control with reduced amount of insecticides applied in the environment.

Eradication of Invading Insect Populations: From Concepts to Applications

Eradication is the deliberate elimination of a species from an area. Given that international quarantine measures can never be 100% effective, surveillance for newly arrived populations of nonnative species coupled with their eradication represents an important strategy for excluding potentially damaging insect species. Historically, eradication efforts have not always been successful and have sometimes been met with public opposition. But new developments in our understanding of the dynamics of low-density populations, the availability of highly effective treatment tactics, and bioeconomic analyses of eradication strategies offer new opportunities for developing more effective surveillance and eradication programs. A key component that connects these new developments is the harnessing of Allee effects, which naturally promote localized species extinction. Here we review these developments and suggest how research might enhance eradication strategies.

Different acute toxicity of fipronil baits on invasive Linepithema humile supercolonies and some non-target ground arthropods

Fipronil is one of the most effective insecticides to control the invasive ant Linepithema humile, but its effectiveness has been assessed without considering the genetic differences among L. humile supercolonies. We hypothesized that the susceptibility of the ant to fipronil might differ among supercolonies. If so, dosage and concentration of fipronil may need to be adjusted for effective eradication of each supercolony. The relative sensitivities of four L. humile supercolonies established in Hyogo (Japan) to fipronil baits were examined based on their acute toxicity (48-h LC(50)). Toxicities of fipronil to seven ground arthropods, including four native ant species, one native isopoda, and two cockroaches were also determined and compared to that of L. humile supercolonies using species sensitivity distributions. Marked differences in susceptibility of fipronil were apparent among the supercolonies (P < 0.008), with the 'Japanese main supercolony' (271 μg L(-1)) being five to ten times more sensitive to fipronil than other colonies (1183-2782 μg L(-1)). Toxicities to non-target species (330-2327 μg L(-1)) were in the same range as that of L. humile, and SSDs between the two species groups were not significantly different (t = -1.389, P = 0.180), suggesting that fipronil's insecticidal activity is practically the same for L. humile as for non-target arthropods. Therefore, if the invasive ant is to be controlled using fipronil, this would also affect the local arthropod biodiversity. Only the 'Japanese main supercolony' can be controlled with appropriate bait dosages of fipronil that would have little impact on the other species.

Laboratory and Field Evaluations of Polyacrylamide Hydrogel Baits Against Argentine Ants (Hymenoptera: Formicidae)

The development of effective baits to control the Argentine ant, Linepithema humile (Mayr), has been problematic because foragers prefer sweet liquids, while many toxicants are insoluble in water and liquid baits are generally difficult to deliver. The incorporation of thiamethoxam and sucrose solutions into a water-absorbing polyacrylamide hydrogel provides a unique and novel carrier and method of application for liquid baits. Formulations of thiamethoxam affected the size of the hydrogels, and sucrose solutions containing 0.0003% technical thiamethoxam provided hydrogels as large as those made with 25% sucrose solution or deionized water. Concentrations of thiamethoxam as low as 0.000075% in the hydrogels provided 50% kill of workers within 3 d in a laboratory setting. In small colony studies, baiting with 0.00015 and 0.000075% thiamethoxam hydrogels provided 100% mortality of workers and queens within 8 d. An enzyme-linked immunosorbent assay indicated that thiamethoxam was absorbed into the interior of the polyacrylamide matrix. The water loss rates of the hydrogels were dependent upon the relative humidity. Polyacrylamide hydrogels with >50% water loss were less attractive to ants. Field studies in highly infested areas indicated that concentrations of 0.0006 or 0.0018% thiamethoxam were more effective than 0.00015%. Hydrogels may provide a cost-effective alternative to providing aqueous baits to control Argentine ants.

Integrating biology into invasive species management is a key principle for eradication success: the case of yellow crazy ant Anoplolepis gracilipes in northern Australia

The lack of biological knowledge of many invasive species remains as one of the greatest impediments to their management. Here I detail targeted research into the biology of the yellow crazy ant Anoplolepis gracilipes within northern Australia and detail how such knowledge can be used to improve the management outcomes for this species. I quantified nest location and density in three habitats, worker activity over 24 h, infestation expansion rate, seasonal variation of worker abundance and the timing of production of sexuals. Nests were predominantly (up to 68%) located at the bases of large trees, indicating that search efforts should focus around tree bases. Nest density was one nest per 22, 7.1 and 6.3 m2 in the three habitats, respectively. These data form the baselines for quantifying treatment efficacy and set sampling densities for post-treatment assessments. Most (60%) nests were underground, predominantly (89%) occurring in an open area rather than underneath a rock or log. Some seasonality was evident for nests within leaf litter, with most (83%) occurring during the 'wet season' (October-March). Of the underground nests, most were shallow, with 44% being less than 10 cm deep, and 67% being less than 20 cm deep. Such nest location and density information serves many management purposes, for improving detection, mapping and post-treatment assessments, and also provided strong evidence that carbohydrate supply was a major driver of A. gracilipes populations. Just over half of the nests (56%) contained queens. Of the 62 underground nests containing queens, most queens (80%) were located at the deepest chamber. When queens were present, most often (38%) only one queen was present, the most being 16. Queen number per nest was the lowest in July and August just prior to the emergence of virgin queens in September, with queen numbers then remaining steadily high until April. Nothing is known for any ant species about how the queen number per nest/colony affects treatment efficacy, but further research would no doubt yield important breakthroughs for treating ants. Activity occurred predominantly nocturnally, ceasing during mid-day. These activity data determined the critical threshold above which work must be conducted to be considered reliable, and also suggests that treatments are best applied in the afternoon. Total brood production peaked in February and was the lowest around August and September. These abundance data form the baselines for quantifying treatment efficacy, and may have implications for treatment efficacy. Males were found every month, predominantly between August and November. Queen pupae were found in September. The reproductive timing of sexuals determines the treatment schedule. Targeted, site-specific research such as that described here should be an integral part of any eradication program for invasive species to design knowledge-based treatment protocols and determine assessment benchmarks.

A neurotoxic pesticide changes the outcome of aggressive interactions between native and invasive ants

Neurotoxic pesticides, such as neonicotinoids, negatively affect the cognitive capacity and fitness of non-target species, and could also modify interspecific interactions. We tested whether sublethal contamination with neonicotinoid could affect foraging, colony fitness and the outcome of behavioural interactions between a native (Monomorium antarcticum) and an invasive ant species (Linepithema humile). The foraging behaviour of both ants was not affected by neonicotinoid exposure. Colonies of the invasive species exposed to the neonicotinoid produced significantly fewer brood. In interspecific confrontations, individuals of the native species exposed to the neonicotinoid lowered their aggression towards the invasive species, although their survival probability was not affected. Exposed individuals of the invasive species interacting with non-exposed native ants displayed increased aggression and had their survival probability reduced. Non-exposed individuals of the invasive species were less aggressive but more likely to survive when interacting with exposed native ants. These results suggest that non-target exposure of invaders to neonicotinoids could either increase or decrease the probability of survival according to the exposure status of the native species. Given that, in any community, different species have different food preferences, and thus different exposure to pesticides, non-target exposure could potentially change the dynamics of communities and influence invasion success.

Effect of scattered and discrete hydramethylnon bait placement on the Asian needle ant

The Asian needle ant (Pachycondyla chinensis Emery) is invading natural and disturbed habitats across the eastern United States. While recent studies document the impact of P. chinensis on native ecosystems and human health, effective control measures remain unknown. Thus, we evaluated the field performance of a hydramethylnon granular bait, Maxforce Complete Granular Insect Bait, dispersed in clumps or scattered against P. chinensis. We also measured the effect of this bait on P. chinensis outside of the treatment zone. Surprisingly, unlike reports for other ant species, we achieved nearly complete P. chinensis population reductions 1 d after treatments were applied. Significant ant reductions were achieved until the end of our study at 28 d. No difference was recorded between clumped and scattered application methods. We found no overall difference in ant reductions from the edge out to 5 m beyond the treatment zone. Other local ant species appeared to be unaffected by the bait and foraging activity increased slightly after P. chinensis removal from treated areas. We suggest that Maxforce Complete Granular Insect Bait can be effective in an Asian needle ant treatment program.

Precision control of an invasive ant on an ecologically sensitive tropical island: a principle with wide applicability

Effective management of invasive ants is an important priority for many conservation programs but can be difficult to achieve, especially within ecologically sensitive habitats. This study assesses the efficacy and nontarget risk of a precision ant baiting method aiming to reduce a population of the invasive big-headed ant Pheidole megacephala on a tropical island of great conservation value. Area-wide application of a formicidal bait, delivered in bait stations, resulted in the rapid decline of 8 ha of P. megacephala. Effective suppression remained throughout the succeeding 11-month monitoring period. We detected no negative effects of baiting on nontarget arthropods. Indeed, species richness of nontarget ants and abundance of other soil-surface arthropods increased significantly after P. megacephala suppression. This bait station method minimized bait exposure to nontarget organisms and was cost effective and adaptable to target species density. However, it was only effective over short distances and required thorough bait placement. This method would therefore be most appropriate for localized P. megacephala infestations where the prevention of nontarget impacts is essential. The methodology used here would be applicable to other sensitive tropical environments.

Combined use of a synthetic trail pheromone and insecticidal bait provides effective control of an invasive ant

BACKGROUND

Except for sex pheromones, use of pheromones in pest management has been largely unexplored. A high concentration of trail pheromone disrupts ant trail following and foraging, and thus synthetic trail pheromone can be a novel control agent for pest ants. In this study, a year-long treatment of small areas (100 m(2) plots of urban house gardens) with synthetic trail pheromone, insecticidal bait or both was conducted to develop a novel control method of the invasive Argentine ant Linepithema humile (Mayr).

RESULTS

The ant population could be maintained lower than or similar to the initial level only by combined treatment with synthetic trail pheromone and insecticidal bait. Actually, the ant population was nearly always lowest in combination treatment plots. Throughout the study period, the ant population in plots treated with either one of synthetic trail pheromone or insecticidal bait remained similar to no-treatment plots.

CONCLUSIONS

Combination treatment may be a more effective and environmentally friendly method for controlling invasive ants than conventional methods. Extermination of ants by insecticidal bait and inhibition of re-infestation by pheromone may be the mechanism of the combination effect. This is the first study to show a significant effect of synthetic trail pheromone on ant population.

The Argentine ant persists through unfavorable winters via a mutualism facilitated by a native tree

Mutualisms and facilitations can fundamentally change the relationship between an organism's realized and fundamental niche. Invasive species may prove particularly suitable models for investigating this relationship as many are dependent on finding new partners for successful establishment. We conducted field-based experiments testing whether a native tree facilitates the successful survival of the invasive Argentine ant, Linepithema humile (Mayr), through unfavorable winter conditions in the southeastern United States. We found Argentine ant nests aggregated around the native loblolly pine, Pinus taeda L., during the winter months. The bark of this tree absorbed enough radiant solar energy to reach temperatures suitable for Argentine ant foraging even when ambient temperatures should have curtailed all foraging. Conversely, foraging ceased when the trunk was shaded. The sun-warmed bark of this tree gave the Argentine ant access to a stable honeydew resource. Argentine ants were not found on or near deciduous trees even though bark temperatures were warm enough to permit Argentine ant foraging on cold winter days. Augmenting deciduous trees with sucrose water through the winter months lead to Argentine ant nests remaining at their base and Argentine ants foraging on the tree. The Argentine ant requires both foraging opportunity and a reliable winter food source to survive through unfavorable winter conditions in the southeastern United States. The loblolly pine provided both of these requirements extending the realized niche of Argentine ants beyond its fundamental niche.

Managing social insects of urban importance

Social insects have a tremendous economic and social impact on urban communities. The rapid urbanization of the world has dramatically increased the incidence of urban pests. Human commerce has resulted in the spread of urban invasive species worldwide such that various species are now common to many major urban centers. We aim to highlight those social behaviors that can be exploited to control these pests with the minimal use of pesticides. Their cryptic behavior often prohibits the direct treatment of colonies. However, foraging and recruitment are essential aspects of their social behavior and expose workers to traps, baits, and pesticide applications. The advent of new chemistries has revolutionized the pest management strategies used to control them. In recent years, there has been an increased environmental awareness, especially in the urban community. Advances in molecular and microbial agents promise additional tools in developing integrated pest management programs against social insects.

Combined effect of hemipteran control and liquid bait on Argentine ant populations

The invasive Argentine ant, Linepithema humile (Mayr), has become a worldwide problem capable of inflicting significant ecological and economic injury on urban, agricultural, and natural environments. The mobility of this pest ant has long been noted, rapidly moving nests to new food resources and then away as resources are depleted. This ant, like many pest ant species, has a special affinity for honeydew excreted by phloem-feeding Hemiptera. We investigated the effect of various hemipteran control strategies on terrapin scale densities and measured their indirect effect on local Argentine ant densities and foraging effort. We then determined whether this indirect treatment strategy improved the performance of an ant bait. We predicted that Argentine ants would move nests away from trees treated for Hemiptera and then move nests back when a liquid bait was offered, followed by a decline in ant numbers due to intake of the toxicant. A horticultural oil spray and soil application of the systemic insecticide, imidacloprid, had no effect on terrapin scale numbers. However, trunk-injected dicrotophos caused a reduction in scale and a decline in local Argentine ant nest density and canopy foraging effort. We also recorded a reduction in local Argentine ant ground foraging when large amounts of liquid bait were applied, and we found no evidence that combining dicrotophos with liquid ant bait performed better than each treatment alone. We suggest that a strategy of combined hemipteran control plus application of liquid ant bait can reduce local Argentine ant densities, when both components of this system are highly efficacious.

Northward expansion of the invasive Linepithema humile (Hymenoptera: Formicidae) in the eastern United States is constrained by winter soil temperatures

The invasive Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae) has been evident in the North Carolina Piedmont, United States for 90 yr but has failed to spread further north. We investigated the mechanisms preventing this expansion. The Argentine ant ceases foraging at temperatures below 5°C and we hypothesized that winter soil temperatures at higher latitudes restricted foraging long enough to cause colony starvation. We tested if the Argentine ant could successfully feed at temperatures below 5°C and found that colonies would starve. We subjected Argentine ant nests to a range of sub- and above-freezing temperatures and measured worker mortality at various time intervals. We found that Argentine ant colonies will collapse after 8.5 d at 5°C. Argentine ants can escape ambient cold temperatures by moving nests into the soil column. We tested how deeply into the soil Argentine ant queens and workers need to move to survive winter in North Carolina. Soil temperatures in the North Carolina Piedmont do not fall below 5°C for longer than nine consecutive days; therefore, Argentine ant colonies need only to retreat a few centimeters into the soil column to escape unsuitable temperatures. Winter soil temperature data from four climate stations situated from latitudes 35°, the current Eastern United States latitudinal limit for Argentine ant population expansion, to 39° were searched for periods where soil temperatures would have led to colony extirpation. North of their current distributions, extended periods of soil temperatures below 5°C regularly occur, preventing Argentine ant colonies from persisting.

Long-term field trial to control the invasive Argentine ant (Hymenoptera: Formicidae) with synthetic trail pheromone

Previous short-term experiments showed that trail following behavior of the Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), can be disrupted by a high concentration of synthetic trail pheromone component (Z)-9-hexadecenal. In this study, a long-term field trial was conducted in 100-m2 plots of house gardens in an urban area of Japan to see whether the control effect on Argentine ants can be obtained by permeating synthetic trail pheromone from dispensers. The dispensers were placed in the experimental plots during the ant's active season (April-November) for 2 yr with monthly renewal. To estimate Argentine ant population density, foraging activity of Argentine ants in the study plots was monitored by monthly bait surveys. Throughout the study period, Argentine ant foraging activity was suppressed in the presence of the dispensers, presumably via trail forming inhibition. In contrast, the level of foraging activity was not different between treatment and no-treatment plots when the dispensers were temporarily removed, suggesting that treatment with pheromone dispensers did not suppress Argentine ant density in the treatment plots. Population decline may be expected with larger-scale treatment that covers a significant portion of the ant colony or with improvement in the potency of the disruptant.

Development of virtual bait stations to control Argentine ants (Hymenoptera: Formicidae) in environmentally sensitive habitats

A novel bait station referred to as a virtual bait station was developed and tested against field populations of the invasive Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae), at White Beach, Camp Pendleton, in Oceanside, CA. White Beach is a nesting habitat for an endangered seabird, the California least tern (Sterna antillarum browni Mearns). The beach is heavily infested with Argentine ants, one of the threats for the California least tern chicks. Conventional pest control strategies are prohibited because of the existence of the protected bird species and the site's proximity to the ocean. The bait station consisted of a polyvinyl chloride pipe that was treated on the inside with fipronil insecticide at low concentrations to obtain delayed toxicity against ants. The pipe was provisioned with an inverted bottle of 25% sucrose solution, then capped, and buried in the sand. Foraging ants crossed the treated surface to consume the sucrose solution. The delayed toxicity of fipronil deposits allowed the ants to continue foraging on the sucrose solution and to interact with their nestmates, killing them within 3-5 d after exposure. Further modification of the bait station design minimized the accumulation of dead ants in the sucrose solution, significantly improving the longevity and efficacy of the bait station. The virtual bait station exploits the foraging behavior of the ants and provides a low impact approach to control ants in environmentally sensitive habitats. It excluded all insects except ants, required only milligram quantities of toxicant, and eliminated the problem of formulating toxicants into aqueous sugar baits.