2022 年 47 巻 4 号 p. 208-212
Slow-acting fipronil is one of the best components for controlling invasive ants. However, its efficacy against invasive Lepisiota frauenfeldi, which recently invaded Japan, remains unclear. Here, its acute toxicity to Le. frauenfeldi was assessed, and its lethal concentrations were compared with those against other invasive ants (Linepithema humile and Solenopsis invicta). The LC10 and LC50 values of fipronil for Le. frauenfeldi were significantly lower than the previously reported values for Li. humile and/or S. invicta, and its LC90 value against Le. frauenfeldi was in the same range as that required for Li. humile extermination. Additionally, Le. frauenfeldi can be more sensitive to fipronil than non-target arthropods. Therefore, recent fipronil-based Li. humile and S. invicta eradication/control programs may be effective against Le. frauenfeldi as well. Moreover, applying fipronil at dosages appropriate for Le. frauenfeldi would lead to effective Le. frauenfeldi extermination/control with low damage to other native species/ants.
The frequency of biological invasion events has been increased over the years due to increased international trade and tourism.1,2) Among biological invaders, the number of species that are introduced unintentionally is proportional to the increased rate of importation of commodities.3,4) In particular, invasive ants have serious impacts on native communities and ecosystems.5,6) For example, Linepithema humile Mayr, 1868 (Argentine ant), Solenopsis invicta Buren, 1972 (red imported fire ant), and Wasmannia auropunctata Roger, 1863 (little fire ant) (Hymenoptera: Formicidae) are considered the world’s most damaging invasive species7); thus, their eradication/control is proactively conducted in introduced areas.8–10)
Chemical toxicity measures constitute the best tested methods for exterminating nuisance associated with pests, including ants,11–13) even though they cause harm to non-target organisms.14,15) Further, during the initial stage of invasion, when the distribution ranges of invasive species are narrow, such measures can lead to achievement of rapid eradication at a relatively low cost. Among the frequently used chemical toxicity measures, such as insecticidal spraying and fumigation, toxic baits with slow-acting chemicals, particularly fipronil, are among the best eradication/control agents against invasive ground-dwelling arthropods,14,16,17) and have been recommended by the Japanese administrative organization for the control of invasive ants, including Li. humile and S. invicta.18,19) However, it is thought that the quick-acting toxicants may only kill workers outside the nest; thus, are unlikely to lead to the collapse of the entire ant colony.18)
Fipronil, a phenylpyrazole insecticide that disrupts neurotransmission in various insects via γ-aminobutyric acid (GABA) receptor inhibition,20,21) is often preferentially used for exterminating invasive ants.9,22) For example, Sakamoto and Goka12) reported that fipronil has high insecticidal effects on S. invicta (48-hr LD50: 0.6 ng/ant) at doses 2–3 orders of magnitude lower than those of other insecticides, such as pyrethroids and neonicotinoids.
Lepisiota frauenfeldi Mayr, 1855 (browsing ant) (Hymenoptera: Formicidae) is native to southern Europe, and has been unintentionally introduced in India, Southeast Asia, Guam, Australia, and Middle Eastern countries via the importation of commodities.23) For example, in Japan, it was first detected in Aichi Prefecture and Tokyo in 2017, and by 2022 it was detected in Osaka, Fukuoka, Kagoshima, and Okinawa Prefectures.24,25) Further, this species, which forms a super colony,26) mainly inhabits bare lands and developed areas, such as wharfs, airports, and urban spaces, and reportedly, it has negative ecological and agricultural impacts,24) and can diminish biodiversity in the introduced regions. In fact, in Australia, Le. frauenfeldi is a notorious and serious environmental pest, which eats, kills, and displaces not only native ant species, but also various other invertebrates in the infested area.26,27) Additionally, it can negatively impact agricultural and horticultural activities via “ant-Sternorrhyncha (particularly, scale insects which are one of the major agricultural pests) mutualisms”.26) This mutualism has been found to exert a wide range of effects on arthropods, and it also eventually affects plant health.28,29) Based on previously reported findings, Le. frauenfeldi has been listed as an invasive alien species (IAS) under the Invasive Alien Species Act of Japan since 2020,30) and its eradication/control using chemical agents has been initiated in several areas, including Aichi and Okinawa prefectures. Although it is important to initiate a prompt response to eradicate invasive Le. frauenfeldi, it is unclear if the most effective control strategies (e.g., exposure concentration) against this ant have been fully explored.
Herein, the acute toxicity of fipronil to invasive Le. frauenfeldi was assessed under laboratory conditions, and the fipronil sensitivity of Le. frauenfeldi was compared with those of two other known invasive ants (Li. humile and S. invicta).14,31)
According to Sunamura et al.,13) to ensure the effective and rapid eradication of Le. frauenfeldi, workers, which are indispensable for sustaining ant colony organization, should be primarily targeted. In this study, Le. frauenfeldi workers were sampled from the Tobishima wharf (30°02′03.7″N, 136°49′20.4″E), Nagoya City, Aichi Prefecture in October 2017, before their designation as IAS. The collected individuals were kept in plastic cases (300-mm length×155-mm width×155-mm depth) containing dry cotton filled with 25% sucrose solution for three days to allow for acclimation to the environment. This was then followed by the acute toxicity tests. It is also worth noting that the tested individuals were reared in an incubator (LH-30-8CT, Nippon Medical & Chemical Instruments Co., Ltd., Osaka) maintained at a constant temperature of 22±1°C.
2. Oral acute toxicity bioassays of fipronil to Lepisiota frauenfeldi workersOral acute toxicity bioassays of fipronil in Le. frauenfeldi workers were performed as previously described by Hayasaka et al.14) In brief, commercial fipronil [Prince Flowable, fipronil/water, and surfactant (5 : 95, v/v)] (BASF Japan Ltd., Tokyo), which was used in this study, was dissolved in dechlorinated tap water to prepare test solutions.
Thereafter, the individuals tested were fed via a melamine sponge (1 cm3) soaked with either 0.5 mL of 25% sucrose water that was spiked with 0.5 mL of fipronil solution (treatment group) or with 1.0 mL of 25% sucrose water (control). Furthermore, in this study, the nominal concentration of fipronil varied in the ranged 98–6250 µg/L (no. for a fipronil concentration of 7), and the concentration ratio between successive solutions was 2.0. Further, the fipronil solutions were prepared via serial dilution of the stock solution using aerated tap water. Five Le. frauenfeldi individuals were placed on a melamine sponge filled with test solution at the center of a 90-mm petri dish, and each dish was sealed to prevent the escape of the specimens. Four replicates were used for each concentration (treatment). Mortality, which was defined as the absence of movement after the cover of the dish was gently flicked, was checked at 48 hr before the commencement of the experiments; this was to enable the calculation of different lethal concentrations (LC10, LC50, and LC90). The survival rate of the test species in the control group throughout the experiments was 100% (20/20 individuals).
3. Statistical analysesAll statistical analyses were conducted using R software version 4.2.1.32) The lethal concentration (LC10, LC50, and LC90) values of fipronil with respect to Le. frauenfeldi workers were calculated based on data from the 48-hr acute toxicity bioassays. Acute toxicity values were determined using the Probit method. Further, each fipronil toxicity (LC10, LC50, and LC90) value towards Le. frauenfeldi was compared with those towards Li. humile14) and S. invicta,31) which were calculated using oral acute toxicity data (48 hr) obtained based on the same and/or similar bioassays as was performed in this study. Specifically, given that the lethal concentration values corresponding to four Li. humile populations with different genetic structures had been previously reported by Hayasaka et al.,14) all the acute toxicity values for this species were used as comparative data. The statistical differences between these species with respect to fipronil sensitivity were determined using the CompParm function of the ‘drc’ package in R.33) This function implements t-test for parameter differences (LC50 population X–LC50 population Y) that were compared relative to 0. However, t-test for S. invicta was performed using the same CompParm function, but with summary statistics owing to the absence of raw data on acute toxicity.
The 48-hr acute toxicity values (LC10, LC50, and LC90) of fipronil to Le. frauenfeldi workers were 35.96, 457.72, 5825.56 µg/L, respectively (Fig. 1, Table 1). Further, its lethal median concentration (LC50) towards Le. frauenfeldi workers was significantly lower than that towards Li. humile (271–2782 µg/L)14) and S. invicta (2510 µg/L)31) (p<0.001). In particular, its toxicity towards the main Li. humile population in Japan, which is the most invasive, but have the highest fipronil sensitivity14) and towards Le. frauenfeldi were statistically similar (p>0.05) (Fig. 1b). Similarly, Le. frauenfeldi showed the lowest LC10 value with respect to fipronil the different among species (p<0.001) (Fig. 1a). Conversely, no significant differences were observed between Le. frauenfeldi and Li. humile populations with respect to their LC90 values (p>0.05) (Fig. 1c).
| Species | Range of conc. tested (µg/L) | No. of conc. | 48-hr LCx values [µg/L (95% CI)] | Reference | ||
|---|---|---|---|---|---|---|
| LC10 | LC50 | LC90 | ||||
| Lepisiota frauenfeldi (Le. frauenfeldi) | 97.65–6250 | 7 | 35.96(−4.84–76.76) | 457.72(247.04–668.39) | 5825.56(311.40–11339.71) | This study |
| Linepithema humile populationsa) (Li. humile) | Hayasaka et al.14) | |||||
| Japanese main (Jm) | 62.5–1000 | 5 | 57(17–96) | 271(180–362) | 1295(362–2229) | |
| Kobe A (KA) | 156.25–10000 | 7 | 718(364–1071) | 2782(1947–3617) | 10776(4674–16877) | |
| Kobe B (KB) | 156.25–1000 | 7 | 537(342–751) | 1437(1085–1790) | 3844(2307–5381) | |
| Kobe C (KC) | 156.25–5000 | 6 | 343(177–509) | 1183(849–1516) | 4081(1985–6177) | |
| Solenopsis invicta (S. invicta) | 1000–10000 | 6 | — | 2510(2150–2870) | — | Xiong et al.31) |
a) Indicates populations with different genetic structures39)
In this study, the LC10 and LC50 values of fipronil against Le. frauenfeldi were almost the same as those against the main Li. humile population in Japan, which is the most invasive, but shows the highest fipronil susceptibility.14) Further, its LC50 was five times lower than that of S. invicta31) (Fig. 1a, b, Table 1). However, the LC90 value of Le. frauenfeldi was approximately the same as that corresponding to the Li. humile population (Fig. 1c). Incidentally, the sensitivity of invasive Li. humile populations to fipronil was relatively higher and/or similar to those of other arthropods, including other ants, cockroaches, and isopods.14) This means that Le. frauenfeldi workers can be more sensitive to the insecticide than non-target organisms as well. Therefore, recent eradication/control programs using slow-acting fipronil against Li. humile and S. invicta can also be effective enough against Le. frauenfeldi. The relatively small differences between the fipronil sensitivities (within one order of magnitude in LC50, Table 1) of these three species of ants may be due to their similar body sizes (Le. frauenfeldi, 2.5–4.0 mm24); Li. humile, 2.56–2.72 mm14); S. invicta, 2.5–6.0 mm19)). Body size/mass dependence of toxicant sensitivity has been reported although there are exceptions.34–36) A significant correlation between fipronil toxicity and body length has also been reported for aquatic arthropods.37)
Again, slow-acting toxic baits are likely to efficacious against invasive ants after their administration because the baits can be brought back to the nest by workers and then shared with nest mates, such as queens and broods.38) However, given that studies on the horizontal transfer process of slow-acting insecticides in ant nests13,17) are limited, a further understanding of the efficacy of fipronil is required.
Despite the aforementioned high insecticidal effect of fipronil on invasive Le. frauenfeldi, its adverse ecological impacts on native organisms and the environment still need to be considered; thus, environmentally friendly eradication/control strategies for Le. frauenfeldi are preferred. Indiscriminatory fipronil susceptibility has been observed regardless of inter- and intra-species.14) Conversely, given the high sensitivity of invasive ants, including Le. frauenfeldi to fipronil compared to non-target organisms,14) it would be better to design baits of specific fipronil dosages targeting only Le. frauenfeldi. With this strategy, Le. frauenfeldi would be controlled and/or exterminated effectively with causing low ecological damage to other native species.
We thank Dr. Takuo Sawahata for valuable technical advice. We also express our gratitude to Kota Kohara for helping with field sampling. Furthermore, the authors thank Editage (www.editage.com) for English language editing. Part of this study was supported by the Japan Society for the Promotion of Science (JSPS), KAKENHI (grant numbers JP16K15080, JP19K06098, and JP20J12743). The paper benefitted from the constructive comments of two anonymous reviewers.
