Fig. 3 Trends in Argentine ant capture numbers in Ota Ward (total of Jonan-jima and Tokai area). By the end of the third year, zero captures were achieved, but the challenge remains of determining the duration for which bait placement and monitoring should be continued thereafter. 121 Chemical control of Argentine ant Institute imported fire ant (Solenopsis invicta) first arrived in Japan in 2017, a the red that a minimal number of colonies may still be concealed within the area. Consequently, it remains necessary to continue the placement of bait and ongoing surveillance—but this raises critical questions: How long should these control measures be maintained, and at what point should an “eradication declaration” be issued? A scientific basis is required to make such a determination regarding the termination of control efforts. If control is prematurely halted despite the presence of remaining Argentine ants, it could lead to a resurgence and possible spread to other further control efforts and regions, necessitating increasing associated costs. Conversely, if baiting continues after true eradication has been achieved, the potent toxicity of the agents used could negatively impact native ant species and ground-dwelling fauna in the area. routine control measures—namely, In response, the National As a result, the estimated persistence probability of Argentine ants in Ota Ward was shown to significantly decrease after the 20th month of control efforts. The persistence probability was calculated to fall below 1 percent at month 38 in the Tokai area and at month 42 in the Jonan-jima area (Fig. 1). Based on this criterion, Argentine ants were deemed eradicated in May 2014 in the Tokai area and in August 2015 in the Jonan-jima area, leading to the termination of control activities (Sakamoto et al., 2017). Subsequently, when ground survey was done in the area as part of the red imported fire ant surveillance efforts (actually, for an exposé article by a weekly magazine). No alien ants were found, only native ant species, further strongly supporting the conclusion that the Argentine ants were successfully eradicated through our planned control efforts. To date, control efforts for invasive alien ants have been undertaken worldwide, yet no clear criteria have been established to determine eradication success. Indeed, cases have been reported both domestically and internationally where residual populations were detected after control was deemed successful and subsequently discontinued. In our studies mentioned above, for the first time globally, we were able to declare the eradication of Argentine ants based on statistical evidence. 3.3 Ecological Risk Assessment for the Insecticide Fipronil The control agent fipronil, used in this eradication project, has a prodrug structure in which its degradation products exhibit higher physiological activity than the parent compound (e.g., Weston & Lydy, 2014). It is anticipated these degradation products, which accumulate over time within ant colonies, will yield enhanced insecticidal effects. This prolonged residual property is considered one of the factors that has allowed fipronil to achieve notable effectiveness against large colonies, such as those of Argentine ants. This characteristic, however, also poses a risk of prolonged adverse effects on non-target organisms. Therefore, careful attention is required regarding both the dosage and method of applying fipronil. Over the three-year control period, the amount of fipronil applied was estimated to be 0.4 to 0.9 g/ha per year, based on the active ingredient, which is significantly lower than the single-use application rate of 100 to 279 g/ha in typical agricultural uses. The photodegradation rate of fipronil in water (half-life) is only one to several hours under natural light (Hirashima et al., 2023), and it is expected that any remaining fipronil on the surface of the ground will rapidly degrade due to moisture and UV exposure. However, fipronil adsorbed in soil may have a half-life exceeding seven months (e.g., Tingle et al., 2003), indicating the necessity of avoiding prolonged application. Thus, it is essential to adhere to a planned and appropriate dosage when using this agent. for Environmental Studies devised a method of estimating the probability of Argentine ant persistence using monthly monitoring data, aiming to provide a scientific basis for an eradication declaration. A multinomial-mixture model was developed to estimate eradication probability based on 59 months of monitoring data and the history of the detection pesticide applications. By calculating probability from traps and the mortality probability from fipronil, this model illustrated the trend of Argentine ant persistence probability within the area. In our control study, we monitored the dynamics of native ant species and other arthropods using sticky traps and conducted ecological risk assessments throughout the control period. Previous studies assessed non-target effects only in terms of the increase or decrease of abundance or species richness after pesticide applications. Positive effects of the release from pressure caused by invasive ant species have not been considered so far. To assess pesticide effects more accurately in the field, the non-target effects of pesticides should be considered that
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