Resistance Monitoring of Chilo suppressalis (Lepidoptera: Crambidae) to Chlorantraniliprole in Five Field Populations from Hunan China 2013–2020¹

Rice stem borers, Chilo suppressalis Walker (Lepidoptera: Crambidae), have the characteristics of rapid reproduction, large food intake, ability to bore into plant structures, cold resistance, and pesticide resistance. It is a widely distributed and destructive insect pest of rice, Oryza sativa L., in tropical and subtropical Asia (Huang et al. 2011, Jiang and Cheng 2003, Meng et al. 2019, 2022). Chilo suppressalis is an oligophagous pest, mainly damaging rice, but also damaging water bamboo, sugarcane, corn, wheat, broad bean, rape, barnyard grass, and other plants. The larvae of C. suppressalis bore into the leaf sheaths and stems of rice, and then feed on these tissues, forming dead sheaths, dead hearts, and white heads, eventually causing plant lodging and unfilled grains (Jiang and Cheng 2003, Lu et al. 2017, Wei et al. 2019). Over the past 10 years, more than 14 million ha of rice in China have been destroyed annually (National Agricultural Technology Extension and Service Center [NATESC] 2022).

Chemical control remains an important tactic to manage C. suppressalis in the long term (Zhao 2019). Unfortunately, overuse of insecticides has resulted in the development of resistance in field populations of C. suppressalis, which is an important factor contributing to pest outbreaks (Cheng et al. 2010, Hu et al. 2010, Su et al. 2014, Zibaee et al. 2009). Since the 1980s, a variety of insecticides, including organophosphates, chlorpyrifos, and abamectin, have been used to control field populations of C. suppressalis (He et al. 2008, Lahm et al. 2009, Qu et al. 2003, Su et al. 2014, Shuijin et al. 2017). However, due to abundant and unnecessary use of insecticides, their efficacy against C. suppressalis has been greatly reduced with resistance development (He et al. 2008, Sun et al. 2018).

Chlorantraniliprole belongs to the first generation of anthranilic diamide and was officially registered for use in China in 2008. Chlorantraniliprole acts on a novel target, the insect ryanodine receptor, which has not been reported with other pesticides, resulting in the uncontrolled release of calcium stores from muscle cells, leading to feeding cessation, lethargy, paralysis, and ultimately death (Jeanguenat 2013, Lahm et al. 2005, Sial et al. 2011). On the other hand, as a selectively targeted insecticide, chlorantraniliprole is reportedly safe for human health and our environment and is efficacious against a variety of lepidopteron and other pests (Claudio et al. 2009, Teixeira et al. 2009). Therefore, chlorantraniliprole has become a key product for the control of C. suppressalis in the field in most areas of China.

Yet, due to the strong pressure exerted by insecticide selection caused by its long-term use, chlorantraniliprole has declined as an effective control for C. suppressalis, and failure of control in some areas have been reported in China (Hu et al. 2010; Sun et al. 2018, 2021; Wei et al. 2019; Wu 2013; Xu et al. 2018; Zhang et al. 2011; Zhang 2012; Zhao 2019). Therefore, monitoring the status of resistance of C. suppressalis field populations to chlorantraniliprole is an important prerequisite for developing sustainable pest management strategies for this important pest. The present study monitored insecticide resistance in five field populations of C. suppressalis to chlorantraniliprole from 2013 to 2020 in Hunan to evaluate the levels of resistance to insecticides by these field populations. The results of this investigation could provide a scientific basis for the rational selection of insecticides and delay the development of resistance by C. suppressalis field populations.

Samples of five populations of C. suppressalis were collected from different locations in Hunan (Table 1; Fig. 1). Overwintering pupae from the five populations (100 to 2,000 pupae) were collected and transported to the laboratory where the populations were reared in an insectary at 28 ± 1°C, 70–80% relative humidity (RH), and on a 16:8 h (L:D) photoperiod. Larvae were fed on an artificial diet (Ma et al. 2015), and the adults were fed with 10% (v/v) honey. Third and fourth instars from the next generation (F1) were used for the bioassays.

Chlorantraniliprole (97.3% purity) was provided by Bayer (China) Co., Ltd. Acetone, dimethylformamide (DMSF), and Triton X-100 were purchased from Sigma–Aldrich (St. Louis, MO).

Chlorantraniliprole resistance in the five C. suppressalis populations was assayed using a topical application seedling dip method (Gao et al. 2013, Su et al. 2014). Briefly, rice seeds were planted in 5-cm diameter pots with 30 seeds/pot. Seedlings were allowed to grow in a greenhouse for 3 wk to approximately 25 cm in height. The seedlings were not exposed to insecticides during this time.

Using DMSF as solvent, the original insecticide, a stock solution of the chlorantraniliprole was prepared containing 10% Triton X-100. This stock solutions were serially diluted to establish different insecticide concentrations for the bioassays. Water with the same amount of DMSF and Triton X-100 served as the control in each bioassay. Rice seedlings were then dipped in each solution for 10 s and air dried. The rice stems were excised from the roots and cut into 5.5- to 6-cm sections with no leaves. Fifteen stem sections were placed in a 9.0-cm diameter Petri dish containing four filter paper disks moistened with 3-mL distilled water. Ten F1 generation 3rd–4th instar larvae were transferred to each Petri dish with a brush, and the Petri dish was covered with two layers of black cotton cloth to prevent C. suppressalis larvae from escaping. Each treatment was replicated four times for a total of 80 larvae per concentration per population. The Petri dish assay arenas were placed in an incubator maintained at 25 ± 1°C, 70% RH, and on a photoperiod 16:8 h L:D. Mortality was recorded 6 d after treatment. Larvae were considered dead if stimulation from a brush did not result in movement. Mortality in the controls was <10%.

Mortality data were corrected using Abbott’s (1925) formula and subjected to probit analysis using the IBM SPSS Statistics 22 software package to calculate the median lethal dose (LC₅₀) and other parameters associated with the dose-mortality response of each population to chlorantraniliprole. The resistance ratio (RR) was determined by dividing the LC₅₀ value of a field population by the corresponding LC₅₀ value of the susceptible baseline (1.393 mg ai/L) according Gao et al. (2013). The degree of resistance was classified as demonstrated by Ministry of Agriculture of the People’s Republic of China (NY/T 2058-2014, 2014): resistance with RR ≤ 5-fold was classified as susceptible, RR = 5 − 10 fold as a low resistance level, RR = 10 − 100 fold as a moderate resistance level, and RR > 100-fold as a high resistance level.

The rice seedling dipping assays showed that rice stem borer larvae collected from the five field populations exhibited different levels of resistance to chlorantraniliprole in 8 years, with LC₅₀ values ranging from 14.799 to 103.587 mg a.i/L (Table 2). The population from Hengyang showed the highest resistance to chlorantraniliprole, while the population from Daoxian showed the lowest resistance to the insecticide (Table 2). These results indicate that resistance of C. suppressalis to chlorantraniliprole is increasingly serious in Hunan during those 8 years (Fig. 2). The level of resistance ratio of C. suppressalis to chlorantraniliprole in Changsha (CS), Daoxian (DX), Lingxiang (LX), Xiangtan (XT) and Hengyang (HY) ranged from 11.1-fold to 32.9-fold, 12.9-fold to 30.9-fold, 14.2-fold to 53.9-fold, 11.6-fold to 52.7-fold, and 28.5-fold to 74.4-fold, respectively. During the 10 years, the resistance level of C. suppressalis to chlorantraniliprole at five monitoring points showed a fluctuating upward trend (Fig. 1A) with a decline in resistance in CS, DX, and HY in 2018 (Fig. 1A).

The field populations of C. suppressalis in Hunan developed moderate resistance to chlorantraniliprole from 2013 to 2020 (Table 2; Fig. 1). While the initial resistance of the five monitoring sites had reached a moderate level, the highest level of resistance occurred in Hengyang in 2013 (Table 2). Previous studies showed that a low level of resistance of C. suppressalis against chlorantraniliprole had developed in seven provinces in China by 2011–2012 (Gao et al. 2013), only 3 to 4 yr after approval to use the insecticide. Resistance levels had increased to moderate and high levels in many provinces in the following 5 yr (Huang et al. 2017, Lu et al. 2017, Yao et al. 2017). The resistance of C. suppressalis to chlorantraniliprole in Hengyang is highest in each year (Fig. 1A). Specifically, the field populations of C. suppressalis showed a significant increase from 2014 to 2015. This is likely related to the farming model of monitoring sites in Hengyang which are double-cropped rice where the C. suppressalis populations are exposed to greater levels of insecticide pressure than where the rice is produced in one crop per year. The resistance levels at XT and LX (the single cropping rice model area) were moderate, and CS and DX (the mix cropping rice model area) were lowest during the 8 yr (Fig. 1B). These results indicate that rice cropping model might have a certain impact on resistance development.

Our results in combinations with others indicate that there is relatively high risk for resistance development to chlorantraniliprole among populations of C. suppressalis in Hunan. Moreover, the resistance status of each rice cropping area will provide specific information for designing the resistance management program suitable for its own region. Such a program will help to maintain the effectiveness of chlorantraniliprole against rice stem borer and to ensure sustainable management. Therefore, continuous monitoring of resistance development and rational use of insecticides with different mode of action and different resistance mechanisms are necessary for delaying the evolution of resistance to chlorantraniliprole in C. suppressalis, especially in Hengyang where this insecticide is intensively used.

This research was supported by Agricultural Science and Technology Innovation Program of Hunan Province (2022CX06 and 2022CX65) and The Open Project Program (20210302) of State Key Laboratory of Rice Biology in China.