# Comparative Efficacy of Carbonyl Sulfide Against Phosphine-Resistant and Phosphine-Susceptible Strains of the Rice Weevil Sitophilus oryzae

## TL;DR

This study shows that carbonyl sulfide (COS) is equally effective against rice weevils that are resistant to phosphine and those that are not, suggesting COS could be a useful alternative fumigant.

## Contribution

The study demonstrates that COS maintains efficacy against phosphine-resistant rice weevils, offering a potential solution to phosphine resistance in pest control.

## Key findings

- Phosphine-resistant rice weevils showed over 20-fold resistance to phosphine compared to susceptible ones.
- Carbonyl sulfide (COS) had nearly identical resistance ratios of 1 for both resistant and susceptible weevils.
- COS LCt50 values were similar across all life stages for both resistant and susceptible rice weevils.

## Abstract

Phosphine resistance has become a serious problem worldwide. This study was conducted to evaluate the feasibility of using a new fumigant, carbonyl sulfide (COS), to control the rice weevil (Sitophilus oryzae), a pest known to be resistant to phosphine. Tests were performed to treat phosphine-resistant and phosphine-susceptible rice weevils with either phosphine or COS to derive concentration time (Ct) values, analyze the control effects, and compare the resistance ratios. The test results revealed that compared with phosphine-susceptible rice weevils, phosphine-resistant rice weevils exhibited more than 20-fold greater resistance to phosphine. However, the resistance ratio to COS was close to 1, indicating no difference in efficacy between resistant and susceptible pests. These results suggest that COS may be applicable to control phosphine-resistant pests. However, further research, such as combined treatment, is needed to increase its field applicability.

Recently, the occurrence of phosphine-resistant pests has been increasingly reported in many countries. In this study, the efficacy of carbonyl sulfide (COS) on phosphine-resistant and phosphine-susceptible strains of the rice weevil, Sitophilus oryzae, was evaluated to determine the applicability of COS as a fumigant to control phosphine resistance. S. oryzae at the egg, larval and adult stages was treated with phosphine and COS to determine the 50 and 99% lethal concentration time (LCt50 and LCt99, respectively) values. The LCt50 values of phosphine for phosphine-susceptible S. oryzae at the egg, larval and adult stages were 1.44, 0.63, and 0.66 mg h/L, respectively, and those for phosphine-resistant S. oryzae were 30.65, 17.60, and 8.37 mg h/L, respectively. In contrast, the LCt50 values of COS for phosphine-susceptible S. oryzae at the egg, larval, and adult stages were 284.19, 171.11 and 212.55 mg h/L, respectively, and those for phosphine-resistant S. oryzae were 289.78, 149.87 and 229.06 mg h/L, respectively. The COS-resistance ratios were 1.02, 0.88, and 1.08 for S. oryzae at the egg, larval, and adult stages, respectively. These results indicate that the efficacy of COS is similar for phosphine-susceptible and phosphine-resistant pests, suggesting that COS can be used to control phosphine-resistant grain pests.