Aim The Argentine ant, Linepithema humile, has been spreading via human activities from its native range in South America across much of the globe for more than a century. This invasive ant was first detected in Japan in 1993. Its successful world‐wide expansion is attributed to a social structure, namely supercoloniality, whereby individuals from separate nests cooperate. Here, we examined the genetic structure of L. humile populations to understand its invasion history. Location Japan. Methods We analysed mitochondrial DNA of Linepithema humile workers from native and other introduced populations and then integrated previously registered sequences. Results Sequencing revealed six haplotypes distributed across its introduced ranges, of which five were present in Japan. The first haplotype was shared by the dominant Japanese, European, North American, Australian and New Zealand supercolonies; the second by the Kobe C supercolony and a Florida population; and the third by the Kobe B and secondary Californian supercolonies and North Carolina colonies. The remaining three haplotypes were each restricted to the Kobe A, Tokyo and Catalonian supercolonies, respectively. Each of the five mutually antagonistic supercolonies was fixed for one of the five haplotypes, and multiple supercolonies were found within a small area. Main conclusions The large number of haplotypes found in Japan likely reflects the strong propagule pressure of L. humile resulting from the fact that the country is one of the top five importers of trade commodities world‐wide. The short invasion history of L. humile in Japan could explain the maintenance of genetic diversity of each independent introduction. In addition, our sampling mostly occurred at major international shipping ports that are likely to be primary sites of introduction. The several recently established L. humile populations within a small area in Japan provide an opportunity to identify the sources of introduction and the local patterns of spread.

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.

Many invasive ants, including the Argentine ant Linepithema humile, form expansive supercolonies, within which intraspecific aggression is absent. The behavioral relationships among introduced Argentine ant populations at within-country or within-continent scales have been studied previously, but the behavioral relationships

In recent years, highly invasive ant species successively invaded warm regions of Asia. In Japan, the Argentine ant, Linepithema humile, has become established in several coastal regions. This species forms unusual social organizations called supercolonies consisting of numerous mutually nonaggressive nests. We studied the behavioral relationships, similarity of cuticular hydrocarbon profiles (nestmate recognition cue), and genetic relationships among the introduced Argentine ant populations of Japan. The Japanese populations were divided into four behaviorally, chemically, and genetically distinct supercolonies, which may have derived from independent source populations. The result represents the recent trend of increasing invasions of invasive ants to Asia. The discontinuous distribution of one supercolony throughout most of the Japanese range suggests rapid expansion of the supercolony via human-mediated jump dispersal. Meanwhile, localization of the other three supercolonies in Kobe Port provides the first evidence for multiple invasions of distinct supercolonies into a base for international trade.

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.

Some invasive ants form large networks of mutually non-aggressive nests, i.e., supercolonies. The Argentine ant Linepithema humile forms much larger supercolonies in introduced ranges than in its native range. In both cases, it has been shown that little gene flow occurs between supercolonies of this species, though the mechanism of gene flow restriction is unknown. In this species, queens do not undertake nuptial flight, and males have to travel to foreign nests and cope with workers before gaining access to alien queens. In this study, we hypothesized that male Argentine ants receive interference from workers of alien supercolonies. To test this hypothesis, we conducted behavioral and chemical experiments using ants from two supercolonies in Japan. Workers attacked males from alien supercolonies but not those from their own supercolonies. The level of aggression against alien males was similar to that against alien workers. The frequency of severe aggression against alien males increased as the number of recipient workers increased. Cuticular hydrocarbon profiles, which serve as cues for nestmate recognition, of workers and males from the same supercolony were very similar. Workers are likely to distinguish alien males from males of their own supercolony using the profiles. It is predicted that males are subject to considerable aggression from workers when they intrude into the nests of alien supercolonies. This may be a mechanism underlying the restricted gene flow between supercolonies of Argentine ants. The Argentine ant may possess a distinctive reproductive system, where workers participate in selecting mates for their queens. We argue that the aggression of workers against alien males is a novel form of reproductive interference.

The white-footed ant, Technomyrmex brunneus, was newly introduced and established in a remote island of Japan and has caused unacceptable damage to the daily life of residents. To establish proper control measures, the present study investigated whether T. brunneus is effectively attracted to commercially available poison baits used to exterminate common household pest ants and the Argentine ant in Japan. Cafeteria experiments using three types of nontoxic baits and eight types of commercial poison baits for ants were conducted in the field, and the attractiveness was compared among the baits. The liquid poison bait “Arimetsu,” which consists of 42.6% water, 55.4% sugar, and 2.0% borate, and nontoxic 10% (w/v) sucrose water showed the highest attractiveness. On the other hand, other commercial poison baits were not as attractive. Therefore, sucrose liquid is the most effective attractive component to use in poison baits for T. brunneus.

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