Editorial Board

Six insecticides (carbosulfan, 20% SC; fenobucarb, 50% EC; etofenprox, 20% EC; dinotefuran, 10% WP; imidacloprid, 10% WP; thiamethoxam, 10% WG) registered for SBPH control in Korea were purchased commercially, and fipronil (5% EC) was experimentally provided by Sungbo Chemical Co. Ltd (Korea) instead of the registered 0.5% FG formulation (Table 1). The susceptible SBPH (Sus-SBPH) strain maintained without exposure to any insecticide for ten years in an insectarium (Division of Crop protection, National Institute of Agricultural Sciences) in acryl cage (30 × 22 × 30 cm³) by serving seedlings of Chucheong rice variety conditioned at 25 ± 2^oC, 50-60% relative humidity, and a 16:8 (L:D) photoperiod was used (Jeong et al., 2017). Egg-laid rice seedling were divided in five groups and hatched in different temperature condition (15, 20, 25, 30 and 35^oC). The 3^rd nymphs were introduced into bioassay.

Bioassay was conducted by rice-seedling dipping (Jeong et al., 2017). Rice (Chucheong) was seeded on the seedling pot, and young seedling stem was used for assay after 3-4 weeks. The seedlings were picked up from the pot, washed, and the seedling stems (7 cm long) were cut without damaging the whole root after being dried in the shade. Five stems were packed to one batch and soaked in designated concentration of insecticides for 30 seconds and dried in the shade. The roots were then rolled with sanitary cotton and put into the glass test tube (φ27 × h200 mm²) and 15 nymphs of 3 days old were introduced into the glass tube and dead insects were counted at 72 h after treatment. Every treatment was performed in triplicate. For calculating median lethal concentration (LC₅₀), statistical program of SPSS 13.0 (IBM Analytics, Armonk, NY) was used.

Two insecticide-resistant SBPH strains were previously established in our published paper (Jeong et al., 2021). Briefly, SBPHs collected from the rice paddies in 2018 were divided into two groups and then treated with either imidacloprid (10% WP) or etofenprox (20% EC) with LC30 concentrations (5 ppm and 25 ppm, respectively) and total 7-9 selections were conducted. The resistance ratio (RR) of the imidacloprid-resistant SBPH strain (ImR-SBPH) and the etofenprox-resistant SBPH one (EtR-SBPH) was 88.9 and 48.1, respectively (Table 2).

The 3^rd instar nymphs of one susceptible (Sus-SBPH) and two resistance strains (ImR-SBPH and EtR-SBPH) were introduced to bioassay with rice-stem dipping method previously described. The exposure temperature was only different with 20 and 30^oC after 72 h insecticide treatment. These two temperatures were selected in consideration of the range (18-32^oC) of average monthly temperature from April to September. Two insecticides, imidacloprid WP and etofenprox EC were treated to the imidacloprid strains and etofenprox strains, respectively. For calculating median lethal concentration (LC₅₀), statistical probit analysis of SPSS 13.0 (IBM Analytics, Armonk, NY) was used and the resistance ratios of the populations were compared.

The 14 populations of SBPH were collected in the early winter of 2018 (before-overwintering population) and the spring of 2019 (after-overwintering population) at seven local areas, Chungju (CB_CJ), Sangju (GB_SJ), Yeongcheon (GB_YC), Taean (CN_TA), Buan (GB_BA), Shinan (JN_SA) and Jindo (JN_JD). The 3^rd nymphs of populations collected were reared two generations in laboratory.

Five insecticides (carbosulfan, 20% SC; etofenprox, 20% EC; dinotefuran, 10% WP; imidacloprid, 10% WP; fipronil, 5% EC) were used to study insecticide responses (Table 1) and each diagnostic concentration (DC) (40 ppm, 10 ppm, 50 ppm, 300 ppm and 3 ppm, respectively) was deduced from the two times concentration of LC₉₀ value to susceptible SBPH. Bioassay was conducted by the rice-seedling dipping method (Jeong et al., 2017) with the same materials and procedures. After 72 h of insecticide treatment, dead insects were counted for the calculation of mortality based on Abbott’s formula (Abbott, 1925). The differences between mortality of the before- and after-overwintering populations in each area were compared and analyzed by statistical t-test using SPSS 13.0 (IBM Analytics, Armonk, NY).

To explore the influence of temperature on the toxicity of insecticide, the toxicities of five insecticides to SBPH under different temperature conditions were determined (Table 3). Phenylpyrazole and neonicotinoids demonstrated positive temperature coefficients. Based on LC₅₀ values, the toxicity of fipronil increased 3.7, 12.5, 40.5 and 345.2 folds at each sequential temperature (20, 25, 30 and 35^oC) when compared with the toxicity at 15^oC. Similarly, the toxicities of three neonicotinoids showed a significant increase (1.1, 5.9, 24.8 and 494.3 folds for dinotefuran, 1.6, 6.1, 44.8 and 446.6 folds for imidacloprid, 2.2, 4.9, 10.2 and 41.3 folds for thiamethoxam) when compared with their toxicity at 15^oC. These results were similar to the results shown in other studies involving other pest insects (Boina et al., 2009; Mao et al., 2019). Therefore, the higher the temperature, the better the control efficiency. In contrast, the toxicity of pyrethroid, etofenprox showed negative temperature coefficient and its toxicity of decreased by 2.2, 6.5, 8.9 and 3.8 folds at each sequential temperature when compared with the toxicity at 15^oC. This is in agreement with published findings from studies involving other pyrethroid insecticides (Mao et al., 2019) and pyrethroid insecticides will perform better efficacy at low temperatures in the field. Carbamates (carbosulfan and fenobucarb) showed no relations with temperature increase.

When compared with the toxicity at the temperature of 20^oC (about average temperature ranged from 16 to 26^oC in May to June in Korea), fipronil and neonicotinoid insecticides would be more recommended for SBPH management than pyrethroid insecticides (Fig. 1). Many researchers have suggested that the toxicity change of insecticides on the different temperature is related with the penetration and biotransformation of insecticides (Harwood et al., 2009), nerve impulse movement (Song and Narahasi, 1996), feeding consumption rate (Mao et al., 2019), detoxification and metabolic mechanism of insects (Cui et al., 2012; Shao et al., 2013). However, information on the toxicity dynamics of insecticides with temperature change is scarce. The assessment of temperature effects on the toxicity of different insecticides against a target insect is important to establish the chemical control strategies in given environmental conditions.

Fig. 1. 
Comparison of insecticide toxicities on different temperature. ^a) RL is the ratio of LC₅₀ value at each temperature compared by 20^oC.

To determine the influence of temperature on the toxicity of insecticide to the resistance SBPH strains, the toxicities of two strains, imidacloprid-resistant and etofenprox-resistant SBPH were assessed under different temperatures of 20 and 30^oC (Table 4). The toxicity ratio (TR) between the two temperatures of imidacloprid-resistant strain was higher (204.4) than that (1.0) of etofenprox-resistant strains. The resistance ratio (RR) of two strains decreased from 94.4 to 5.3 times in imidacloprid-resistant strain and from 38.3 to 16.9 times in etofenprox-resistant strain as temperature rises, respectively. The toxicity of the imidacloprid-resistant strain increased highly with temperature rise, which is similar to that of the susceptible strain, but the etofenprox-resistant strain showed no change in toxicity with temperature increase. The resistance ratio (RR) of the two resistant strains decreased with temperature increase, which indicates the toxic activity of the resistant strain is more sensitive to temperature change than that of the susceptible strain. Previously we reported that there was no strong correlation between the activities of the detoxifying enzymes (total esterase, acetylcholinesterase and glutathione S-transferase) and temperature changes in the various SBPHs. In contrast, the amounts of ROS, which were more abundant in the two resistant SBPH strains than in the Sus-SBPH strain, showed a strong correlation with temperature changes in all the strains, suggesting that ROS can play an important role in the temperature-related resistance change of SBPH (Jeong et al., 2021). This finding that resistance intensity can change with temperature has implications for the dynamics of resistance management. More researches on the biochemical, physiological, and genetic changes in a wide range of insects are needed to understand the insecticide resistance associated with temperature.

To compare the changes of insecticide toxicity with overwintering, the mortality effects of five insecticides on 14 populations in seven location collected before and after overwintering seasons were evaluated (Table 5). The etofenprox showed the highest mortality in Taean population (89.8 %) and low in other populations (29.6 - 60.5%) in the before-overwintering season. After overwintering, its mortality increased in Chungju, Sangju, Yeongcheon; maintained in Buan, and decreased in Taean, Shinan and Jindo. The mortality rate of carbosulfan showed a more increasing change in all location from a range of 10.0 - 67.9 % to 50.0 -79.2%. The mortality rates of dinotefuran and imidacloprid have large increased in all locations after overwintering. Dinotefuran and imidacloprid showed low mortality range of 25.6 - 66.7% and 3.5 - 60.5% in the before-overwintering season and high mortality range of 61.2 - 95.9 % and 60.4 - 94.9 % in the after-overwintering season, respectively. The mortality of fipronil increased in Chungju, Sangju, Yeongcheon, maintained in Taean, and decreased in Buan, Shinan and Jindo. Although the efficacy of five pesticides has not increased on all the local populations, the mean mortality (66.3%) of after-overwintering populations were significantly higher than that (35.1%) of the before ones (Fig. 2).

Fig. 2. 
Mean mortality of seven local SBPH populations against five insecticides before- and after-overwintering.

Overwintering is the crucial factor for the survival of insect in the subtropical and temperate climate zones with stressful environmental condition, e.g., unavailable food resources, low temperatures or water deficits (Williams et al., 2015). The low temperature during winter can affect the mortality of insects due to severe tissue damages or accumulated chill injuries (Turnock and Fields, 2005). The overwintering mortality rates are influenced by many factors besides the low winter temperature.

Fitness costs associated with insecticide resistance are accompanied by high energetic cost or significant disadvantage that diminishes the insect’s fitness compared with its susceptible counterparts in the population. This fitness costs are known to affect differences with resistant and susceptible strains in some properties, such as developmental time, fecundity and fertility, sensitivity to alarm pheromone, and overwintering success (Talebi et al., 2015). In this study, the efficacy of five insecticides on SBPH showed the tendency to rise after overwintering, which suggested that the resistant SBPH would be more susceptible to overwintering (Fig. 2). Apparently, when fitness costs are high at low temperatures, relaxation of selection pressure is likely to favor a population’s reversion to susceptibility in regions of a relatively cold winter. Understanding the relationship of fitness cost and insecticidal resistance could be useful to design effective insecticide resistance management strategies. Overwintering would be helpful to lower the severity of resistance problem in domestic population, most of all, monitoring the migratory SBPH populations which move from China to Korea and mix with domestic populations every year would be especially emphasized.