Rice is the staple food in India (Prasad et al., 2017) and it is attacked by >800 insect pests, of which planthoppers are serious causing significant economic loss. Planthoppers constitute a large group of phytophagous insects (Catindig et al., 2009). Brown planthopper (BPH) Nilaparvata lugens (Stal) and white backed planthopper (WBPH) Sogatella furcifera (Horvath) are the major pests considered as green revolution induced pests (Gunathilagaraj and Ganesh Kumar, 1997). Besides, the direct damage caused by the feeding, planthoppers serve as a vector for virus disease such as southern rice black streak dwarf virus (SRBSDV) (Zhang et al., 2008; Zhou et al., 2008). Chemical control remains the most efficient means to manage planthoppers. However, due to the large scale and intensive use of these, resistance to insecticides has been reported in rice planthoppers. Neonicotinoid and phenyl pyrazole insecticides were used in mid-1990’s in many East Asian countries. At present neonicotinoid insecticides including imidacloprid, thiamethoxam, and nitenpyram are most frequently used in China (Su et al., 2013b; Zhang et al., 2014). Currently, S. furcifera has developed resistance to 12 compounds as reported in the Arthropod Pesticide Resistance Database (APRD), and this involve various detoxification enzymes (Denholm and Rowland, 1992; Whalon et al., 2008). Systemic neonicotinoids like imidacloprid, thiamethoxam and their analogues sulfoxaflor, triflumezopyrim and flupyradifurone have similar mode of action (Casida, 2018; Taillebois et al., 2018). Imidacloprid and thiamethoxam exhibit more resistance in planthoppers. Resistance monitoring is necessary to understand the current status of susceptibility of field populations of S. furcifera. Early detection of changes can prompt adoption of alternative measures. The use of insecticides alternatively with same mode of action helps the farmer overcome resistance (Liao et al., 2019). For newer molecules, baseline susceptibility helps in determining the resistance level and enables location specific management practices. Hence, the present study to assess the current status of resistance levels of field populations of S. furcifera to thiamethoxam 25WG and sulfoxaflor 21.8SC in different locations of Tamil Nadu.

The laboratory reared TNAU susceptible S. furcifera population and populations collected from major rice growing locations of Tamil Nadu viz., Coimbatore (11^°0’11”N; 76^°55’26”E), Bhavani (11^°36’15”N, 77^°43’8”E) and Nagapattinam (10^°47’43”N, 79^°44’5”E) during December 2019 were mass cultured separately and maintained as stock culture in glasshouse (25± 1°C, 70-80% RH). Susceptible variety TN1 was used for rearing S. furcifera. in aluminium cages (45x 45x 60 cm) on 10 days old seedlings transplanted in plastic pots (10 cm dia). The potted plants maintained in the glass house were covered with the mylar film cages. Different aged rice seedlings were maintained separately for nymphs and adult rearing. Initially five pairs of S. furcifera were released on 35 days old seedlings and allowed for oviposition (Heinrichs, 1985). The seedlings with eggs were placed in separate cages for the nymphal emergence and further emerged nymphs were released on 7-10 days old seedlings. These were maintained without exposure of insecticides up to F₂ generation and used for the bioassay studies. For baseline susceptibility studies, standard insects (1 to 2 days old female adult) were collected from the rearing cages daily. Dried and wilted plants together with S. furcifera were removed from the cages and plants were tapped gently to dislodge the hoppers and the insects were transferred to fresh rice seedlings (Heong et al., 2011).

Technical grade thiamethoxam 25WG and sulfoxaflor 21.8SC were purchased from UPL India Private Ltd., Mumbai for the bioassay; and the TNAU susceptible S. furcifera population (maintained for more than 20 generation without the exposure of insecticide) was taken as a baseline and preliminary range test was conducted to fix dose that gives 50% mortality (LC₅₀). Initially the dose was fixed from wider to narrow range. The range which gave 20 to 80% mortality was taken finally for the study. Mortality data was taken at 24 hr interval, with each concentration replicated thrice by releasing 15 adult female insects/ mylar film cage. F₁ generation of the susceptible population was taken for the preliminary range test and the x dose from the preliminary range test was selected. Dose for the other field populations (resistant population) were fixed at different levels and baseline susceptibility test was undertaken. For the resistant population, F₂ generation insects were taken for the baseline susceptibility study. Dose response bioassay for the selected insecticides was performed using seedling dip method (IRAC, 2012; Tsujimoto et al., 2016; Murtiati et al., 2021).

Initially, series of insecticide concentrations were prepared and the seedlings were dipped in these for about 10-30 sec and allowed to dry for 15 min. Suitable hoppers were collected from the rearing cage using a suction device. It was ensured that, only one target life stage was used, with care to ensure that life stages of short/ long winged forms do not get mixed in one test (Zhuang et al., 2000). Forty-five adult female insects were used against one insecticide concentration with three replications. Number of live and dead insects at 24 hr interval was counted and recorded. Moribund insect was counted as dead. Mortality in the control treatment was also recorded. Results were expressed as % mortality and corrected using Abbott’s formula (Abbott, 1925). Concentration mortality results were subjected to probit analysis (Finney, 1971). Regression equation for dosage-mortality responses were computed to determine the LC₅₀ value. Log-concentration-probit mortality lines were computed by Finney’s probit analysis (Regupathy and Dhamu, 1990). The resistance ratio (RR) was calculated by dividing the LC₅₀ value of field population by susceptible one. Resistance levels were classified on the basis of the standard (Su et al., 2013a; Zhang et al., 2014) as susceptible (RR < 3-folds), minor resistance (RR = 3-5 folds), low resistance (RR = 5-10 fold), medium resistance (RR = 10-40 folds) and high resistance (RR = 40-160 folds).

Baseline toxicity to thiamethoxam 25WG of S. furcifera from different regions was relatively high with narrow variation in LC₅₀ (0.3523 to 1.9861 ppm) and LC₉₅ values (1.0993 to 6.3840 ppm). Among the resistant ones, Nagapattinam population showed 5.6 folds resistance followed by the one from Bhavani (3.7 folds) and Coimbatore (2.5 folds) (Figs. 1, 2; Table 1); the Coimbatore population was susceptible to thiamethoxam while Nagapattinam one was found comparatively more resistant. Application of insecticides at frequent intervals to manage rice planthoppers lead to development of resistance in rice growing areas of Nagapattinam. The results on lower resistance level to thiamethoxam 25WG (2.5- 5.6 folds) are in conformity with those of Su et al. (2013b) where 1.5 to 8.0 folds resistance was seen. The present findings agree with Zhang et al. (2014) that S. furcifera is showing less resistance of 0.6 to 3.1 folds with LC₅₀ ranging from 0.06 to 0.28 ppm. Zhang et al. (2017) reported that the field population of S. furcifera from China is exhibiting lower to moderate resistance (0.8 to 14.9 folds) to thiamethoxam with LC₅₀ value of 0.1 to 1.4 ppm. These results are in agreement with present ones. Nagapattinam population showed more resistance (6.2 folds) to imidacloprid 17.8SL followed by Bhavani (4.1 folds) and Coimbatore populations (2.8 folds) with LC₅₀ values ranging from 0.4 to 2.5 ppm (Surya Raj et al., 2020).

The LC₅₀ of sulfoxaflor 21.8SC to field populations of S. furcifera ranged from 1.3121 to 3.8392 ppm Nagapattinam population recorded higher LC₅₀ (3.8392 ppm) followed by Bhavani (3.2116 ppm) and Coimbatore populations (2.3782 ppm). LC₉₅ values followed similar trend and varied from 3.1507 to 5.2754 ppm. The resistant ratio was 2.9, 2.4 and 1.8 folds as compared to the susceptible one (1.3121 ppm) (Table 1). These findings derive support from Kapasi et al. (2017) from northeastern Karnataka. Priyadharshini et al. (2020) observed that N. lugens sulfoxaflor 21.8SC shows resistance of 1.9 to 3.35 folds. The present results contradict with those of Ghosh et al. (2013) showing comparatively lower LC₅₀ values for sulfoxaflor (0.382 to 2.986 ppm) to selected N. lugens populations of West Bengal.

Thus, S.furcifera populations from different regions of Tamil Nadu differed significantly in their response to sulfoxaflor. The regional variation in susceptibility of different S. furcifera populations had been already reported in India (Basanth et al., 2013). Elevated activity of glutathione-S-transferase, peroxidase and mixed function oxidase (MFO) might be responsible for this (Vontas et al., 2002). The planthoppers showing resistance to imidacloprid was due to the increased P450 monooxygenase activity (Nakao, 2017).

References