Introduction

The Argentine ant, Linepithema humile, is native to South America but commonly occurs in a Mediterranean climate, mainly in temperate and subtropical regions (Orr & Seike, 1998). It is a polygynous species, with dozens to hundreds of queen ants gathering in the colony and worker ants are brown and uniform in size (Suarez et al., 2001). This species can inhabit various environments, such as piles of stones, under trees, construction materials, and old trees (Hartley & Lester, 2003). They are recognized as one of the world 100 worst invasive alien species by the International Union for Conservation of Nature (2020), and as a dangerous species in Europe, New Zealand, and the United States (Wetterer et al., 2009). Cargo transportation among countries has been reported as the main cause of the invasion by this species, with records of their dissemination by ship to locations such as Hawaii and New Orleans (Suarez et al., 2001). Although damage to humans has not been reported, there are reports of damage to the ants and arthropods living in invaded areas because of the great fertility and adaptability of L. humile (Bolger et al., 2000). The species negatively impacts biodiversity by damaging the seeds and fruits of plants and by having a symbiotic relationship with aphids and scale insects that damage plants, ingesting their secretions while protecting them from their natural enemies (Ness & Bronstein, 2004; Newell & Barber, 1913). In areas where L. humile has invaded and settled, the species competes with native ants and effectively drives them out because of its high adaptability and population density (Cammell et al., 1996; Carpintero et al., 2005; Cole et al., 1992; Ward, 1987). Further, it has also been reported, along with other species, to have actively disseminated Phytophthora citricola, a pathogen responsible for stem cankers in avocado groves in California, USA (El-Hamalawi & Menge, 1996). Additionally, this species act as hosts for various pathogens that cause diseases in bees (Gruber et al., 2017).

L. humile was first discovered in Korea in October 2019, during the inspection of red imported fire ants, and was identified as an ecosystem disturbance organism after undergoing an ecosystem risk assessment by the Ministry of Environment and the National Institute of Ecology (Ministry of Environment, 2020). To date, research on L. humile in Korea includes the first discovery in South Korea reported by Lee et al. (2020). Subsequently its molecular characteristics were studied by Park et al. (2021). However, detailed studies on the habitats and bionomics of this species in South Korea are lacking. Therefore, in this study, we identified the distribution status, host plants, symbiotic species, habitat status, bait preference, and cytochrome c oxidase subunit I (COI) sequence of L. humile collected near the Busan Station in Busan. The results of this study shall be used as a basis to minimize the damage to the Korean ecosystem caused by this invasive species.

Materials and Methods

Results

Discussion

L. humile has been reported as having dispersed from several to several thousand kilometers by human transport over the last 100 years, with records of dissemination by ship transport from invaded areas such as Hawaii and New Orleans (Carpintero et al., 2005; Harris, 2002). As the species does not perform nuptial flights, its mobility is low and, consequently, its worldwide spread occurs indirectly in association with movement owing to trade among countries (Sanders et al., 2001). Indeed, it is more than likely that L. humile was unintentionally introduced in Korea by shipping containers. As a highly adaptable and invasive species, it settles in urban and suburban areas with low vegetation density and succeeds at exploitation and competition by expelling native ants (Holway, 1999; Human & Gordon, 1996). Therefore, an ecosystem imbalance in Korea is expected.

L. humile prefers temperate climate zones with high humidity, and the optimum spawning temperature is 28°C; thus, it is most active during spring and autumn (Abril et al., 2008). In 2016-2021, the average monthly maximum temperatures and average monthly minimum temperatures in Busan were approximately 28°C and 22°C in summer, respectively (AKorea Meteorological Administration, 2020), offering suitable conditions for spawning and feeding activities of L. humile.

The COI sequences from L. humile individuals collected in the Busan Station area were 99% identical to those in the NCBI database. The Korean populations were found to be genetically close to individuals from invaded areas such as China, Japan, California, and New Zealand. Previous studies have reported the absence of attacks among newly introduced L. humile individuals that do not inhabit the same habitat (Giraud et al., 2002). Therefore, the genetic similarity among individuals introduced to other areas is probably high.

A total of nine species within families of plants fed by L. humile were identified, and among them, Chrysanthemum morifolium, Fatsia japonica, and Camellia japonica had a symbiotic relationship with four species of aphids. Damage to crops has been reported because L. humile ingests secretions while protecting aphids and scale insects, which are agricultural pests (Carroll & Dale, 2021). Moreover, the increasing number of aphids causes damage to host plants (Ness & Bronstein, 2004; Newell & Barber, 1913). Further, in South Africa, native ants that aid in the reproduction of the shrub plant Fynbos have reportedly been displaced, and the migration of seeds of Dendromecon rigida has been reduced, compared to regions inhabited by Pogonomyrmex subnitidus (Carney et al., 2003).

L. humile has also invaded Doñana National Park in Spain and occupied 115 of 182 cork trees (Quercus suber), reducing the population of the native Camponotus scutelaris (Carpintero et al., 2005). Therefore, it is only reasonable to expect that, if the population of L. humile increases, damage will be caused to Korean food plants and crops by protecting aphids, causing an imbalance in the ecosystem by reducing native ants related to seed dispersion.

There are several reports of L. humile competing with most of the native ant species, driving them out with its high adaptability and population density, rapidly dominating the areas it invades (Carpintero et al., 2005). Moreover, the displacement of Tetramorium tsushimae, Nylanderia flavipes, and Lasius hayashi from areas in Korea by L. humile has been reported. Recycled waste, wood planks, and piles of factory materials are commonly moved from the areas inhabited by L. humile to treatment plants throughout the country, and trees planted in the soil of these areas are moved or transplanted to other areas. As reported by Sanders et al. (2001), the dissemination of L. humile is highly likely due to unintentional moving by humans, and such spread of this species may lead to a future imbalance in the native ecosystem.

L. humile is highly reproductive and adaptable. Therefore, unless there is proper control after initial detection, effective control will become increasingly difficult, as reported in Japan and California (Xu et al., 2022). Therefore, continuous monitoring of areas suitable for habitat, such as ports and container logistics, is necessary, including areas already invaded by L. humile. Furthermore, this challenge demands a concerted response by all related organizations and institutions.