## 4 Discussion  
Various management methods (physical, biological, chemical, and behavioral) have been used to control \(D\). _suzukii_[36; 37; 38]. In this study, the controlling effects of combined low-temperature treatment and PH\({}_{3}\) fumigation were investigated. Some physical methods for the control of \(D\). _suzukii_, a quarantine pest, including the induction of sterilization through irradiation (such as X-ray and gamma irradiation), and the utilization of the low-temperature thermal tolerance of each developmental stage has been studied [39; 40; 41; 42; 43; 44; 45]. In previous studies, \(D\). _suzukii_ adults were observed to have the highest cold tolerance, and 8 d of treatment at 1 \({}^{\circ}\)C was required to control eggs, larvae, and pupae [39; 41; 43; 44; 45]. In this study, the adult stage was not completely controlled (98.0%), even after 7 d at 1 \({}^{\circ}\)C; however, 100% mortality was shown in the other developmental stages (egg, larva, pupa) of \(D\). _suzukii_ when treated at 1 \({}^{\circ}\)C for 7 d.  
As a chemical control method of pest control, fumigation with MB, ethyl formate (EF), and PH\({}_{3}\) is widely used [36]. However, there is a difference in fumigant susceptibility among insect species and developmental stages within the same species [46; 47; 48]. Previous studies showed that \(D\). _suzukii_ larvae had the highest tolerance to MB, while eggs had the highest tolerance to EF [38; 48; 49; 50]. In the current study using PH\({}_{3}\), \(D\). _suzukii_ pupae showed the highest tolerance. This experiment was conducted to improve \(D\). _suzukii_ control by combining low-temperature treatment and PH\({}_{3}\) fumigation to compensate for the disadvantages associated with single control methods. Previously, it has been shown that the combination of 0 \({}^{\circ}\)C treatment and MB fumigation has a synergistic effect on eggs, larvae, and pupae of \(D\). _suzukii_, and the combination of EF fumigation and low-temperature treatment (1 \({}^{\circ}\)C and 5 \({}^{\circ}\)C) also showed positive control effects on all developmental stages of \(D\). _suzukii_[38; 48; 49]. In this study, the combination of low-temperature treatment and PH\({}_{3}\) fumigation had a weak synergistic effect compared to the effects of a single treatment. Regarding the combined treatment method, a relatively slightly higher control effect was observed when PH\({}_{3}\) was applied after low-temperature treatment than when low-temperature treatment was applied after fumigation. However, the synergistic effect of the combined treatment on the young egg stage was significant. This is possibly because the egg stage of \(D\). _suzukii_ had a higher susceptibility to low temperatures than the other developmental stages, and older eggs (1-2 d old) had a higher cold tolerance than younger eggs (immediately following infestation) [51; 52]. Therefore, it seems that in recently laid eggs (less than 6 h), PH\({}_{3}\) fumigation after low-temperature treatment has a synergistic effect due to the influence of low temperature. Conversely, in the group exposed to low-temperature treatment after fumigation, eggs over 10 h old were not affected by the low-temperature treatment. Susceptibility to fumigants and temperatures is different among different insect species and developmental stages, so research on these factors should be continued. The sorption rate of fumigants is related to the product, the loading ratio, and the fumigant exposure time; in particular, the higher the loading ratio, the higher the sorption rate [53; 54]. PH\({}_{3}\) has shown a relatively low sorption rate: the concentration decreased by 13% compared to the control in an asparagus experiment with a 20% loading ratio (\(w/v\)) at 2 \({}^{\circ}\)C [55]. Additionally, when various imported foliage nursery plants were exposed to PH\({}_{3}\) at a 2% loading ratio (\(w/v\)) for 24 h in an experiment, _Heteropanax fragrans_ showed the highest sorption rate, at 41.5%, whereas _Schefflera arboricola_ showed the lowest sorption rate, at 12.2% [56].