Efficacy of different insecticides against the yellow stem borer (Scirpophaga incertulus Walker) (Lepidoptera: Crambidae) in spring rice cultivation

Abstract The Yellow Stem Borer (YSB), Scirpophaga incertulas Walker, (Lepidoptera: Crambidae) is a devastating pest that significantly affects the yield of spring rice crops. Insecticides have been the primary method used to control YSB, but comparative efficacy studies of different insecticides against YSB in Nepalese rice production systems are limited. This study aimed to investigate the efficacy of seven insecticide treatments against YSB on the Hardinath-1 variety of rice during the period of February to June 2022. A Randomized Complete Block Design was employed, and the efficacy of the treatments against YSB was evaluated by monitoring the mean values of dead hearts and white heads, recorded at different intervals of post-treatment. The results showed that Chlorantraniliprole 18.5SC (0.4 ml/l) and Chlorpyriphos 20EC (2.0 ml/l) were the most effective treatments, as they outperformed all other treatments, not only in terms of controlling dead heart and white head but also in their effect on yield. The yield of rice plants treated with Chlorantraniliprole 18.5 SC and Chlorpyriphos 20 EC was 3.31 t/ha and 3.24 t/ha, respectively. These results suggest that the application of these two insecticides can provide effective control against YSB and protect rice plants from damage caused by dead heart and white head. However, despite the positive results obtained in this study, it is important to note that the efficacy of the tested insecticides needs to be tested on a larger scale, as well as in different ecological regions.


Introduction
The Rice Yellow Stem Borer (YSB), Scirpophaga incertulas is a destructive monophagous insect pest of the order Lepidoptera and family Crambidae.It is a key pest that inflicts damage to rice crops from seedlings to maturity, causing significant crop losses of around 25-30% (Bhagat et al., 2022).Damage during the early growth phase of the plant (50%) has a greater effect on reducing crop yield than damage during later stages such as reproductive (30%) or ripening (20%) phases (Kinjale et al., 2021).
The Yellow Stem Borer undergoes a complete metamorphosis consisting of four distinct stages: eggs, larvae, pupae, and adults.During the reproductive phase, the female moth lays approximately 50-250 disc-shaped eggs on the underside of leaves, which are then covered with webbing material to prevent parasitism.The larvae begin by feeding on the upper portion of the leaves and later move on to the stem, where they consume nutrients (Devi & Sowndarya, 2022).The newly hatched larvae are pale yellow with a dark brown head, hanging down by a silken thread, falling on the water and swimming freely.The larval stage is responsible for the maximum infestation.Before pupation, the mature larvae cover the exit hole with webbing and then form a white silken cocoon.The pupae are found at the base of the plant, and the adult emerges in 6-10 days (Devi & Sowndarya, 2022;Rath et al., 2020).
Yellow Stem Borer is seen during the vegetative phase and reproductive phase, which is called "dead heart" and "white head" or "white ear," respectively.The damage is seen in the young or vegetative state, in which the central leaf, i.e., the tillers, are damaged and turn brown, which is called "dead heart."During the reproductive stage, after the formation of the spikelet, the panicle turns white, and no grain filling occurs, which represents "white heads" or "white ears."In both phases, the central tiller and panicle can be easily pulled out by hand and show feeding near the base (Devi & Sowndarya, 2022).The presence of YSB in rice cultivation leads to a rise in production costs.Farmers are compelled to invest in pesticides and labor to control the pest, incurring additional expenses.According to a study by Li et al. (2018), the cost of insecticide application for controlling YSB infestation in rice fields in China ranges from US$ 17.9 to 30.8 per hectare.Similarly, a study by Sogawa et al. (2002) estimated that YSB control measures account for 9-18% of the total production cost in rice fields in Japan.YSB infestation significantly reduces rice yield.A study by Kumar et al. (2017) in India reported that YSB infestation caused a yield loss of 42.7%.Similarly, a study by Haq et al. (2017) in Bangladesh found that YSB infestation caused a yield loss of 46.4%.These yield losses translate to significant economic losses for farmers and the rice industry.
According to a study by Ali et al. (2022), there is a correlation between insecticide use and rice production, indicating that using insecticides has a positive impact on rice production.It was noted that in order to manage YSB effectively, it is necessary to study the efficacy of different insecticides against YSB.In addition, comparative efficacy studies of different insecticides against YSB in Nepalese rice production systems are limited.Therefore, the main focus of the study was to assess the effectiveness of various insecticides against the Yellow Stem Borer (YSB).Specifically, the study evaluated the efficacy of six different insecticides, namely Chlorantraniliprole (18.5 SC), Chlorpyriphos (20 EC), Lambda cyhalothrin (5 EC), Fipronil (5 EC), Emamectin benzoate (5 SG), and Cartap hydrochloride (50 SP), which are commonly used by farmers in Nepal to control YSB.Among them, Chlorantraniliprole is a novel insecticide that acts as a potent ryanodine receptor modulator, causing muscle paralysis in insects.A study by Zhang et al. (2019) in China reported that Chlorantraniliprole was effective against YSB infestation, with a mortality rate of 87.5% after seven days of application.Further, Chlorpyriphos is an organophosphate insecticide widely used to control a range of pests in various crops.A study by Patel et al. (2015) in India reported that Chlorpyriphos was effective against YSB infestation, with a mortality rate of 86.7% after three days of application.Thus, by analyzing the efficacy of these insecticides, the study aimed to provide valuable information to farmers and agricultural authorities on the most effective insecticides to use for controlling YSB, a major pest that could cause substantial crop damage and yield losses.The results of the study could be utilized to improve pest management strategies and reduce the impact of YSB on agricultural production.

Materials and methods
The study was conducted in a farmer's field located in Rupani rural municipality, Saptari District of Nepal (at latitude 26,063.12"N and longitude 86,071.24"E).The field experiment was carried out using the Hardinath-1 variety, which is a common spring rice variety used by most farmers in this area.Hardinath-1 rice is one of the most susceptible varieties to YSB, which is one of the most severe issues for rice producer in the region.

Layout and design of the experiment
The present study was designed as a Randomized Complete Block Design (RCBD) with seven treatments, including six insecticides and one untreated control (Table 1), and each treatment was replicated three times.The experimental units were spatially arranged with inter-unit distances of 1 m and 0.3 m, respectively, and the entire study area was divided into 21 blocks, with a total area of 255 m 2 .Each plot measured 12 m 2 (4 × 3 m 2 ).The application of insecticides was initiated upon reaching the Economic Threshold Level of 5%, which is the point where the level of pest-induced damage on the crop exceeded the economically acceptable limit.The insecticides were applied using a battery-operated knapsack sprayer equipped with a 2 mm nozzle, operated at 40 psi pressure, and with a tank capacity of 20 liters.
Prior to sowing, the rice seeds underwent a preventative treatment for fungal infection using SAAF fungicide (Carbendazim 12% + Mancozeb 63% WP) at a rate of 2.67 g/kg seeds.The seeds were immersed in a solution of SAAF and water for a duration of 12 hours, and then air-dried in the shade.The prepared nursery bed had dimensions of 4 m × 3 m (12 m 2 ) and was irrigated to create a layer of standing water.The recommended fertilizers of 160:60:45 gm of Urea, Di-Ammonium Phosphate (DAP), and Muriate of Potash (MoP) were added to the nursery bed.The treated seeds were sown on a film of water layer above the wet bed in mid-February 2022.Prior to transplantation, the main field was plowed three times and prepared four days in advance, and then irrigated.The 35-day-old seedlings were transplanted with two seedlings per hill and spaced at 20 × 20 cm row to row and plant to plant distance.Half the recommended dose of nitrogen and the full dose of phosphorus and potash were applied one day before transplantation of seedlings, with 2 kg of urea (first split dose), 3 kg of DAP, and 2 kg of MoP used as the basal dose according to general recommendations (100:50:50 kg NPK/ha).
The second and third doses of urea were applied at the early tillering and panicle initiation stage, respectively.
Harvesting was done manually using a traditional sickle when the plants had reached 80 % maturity, and panicles from five randomly selected plants from each plot were taken for data collection on filled and unfilled grains.The center 1 m 2 of each plot was marked and harvested separately.The harvested rice was sun-dried for a day and manually threshed a week later.The grain was cleaned and weighed for each plot using an electric weighing balance machine, and care was taken during threshing, drying, and cleaning to ensure accurate measurements.

Damage assessment
Dead heart: From each plot, total number of tillers and infected tiller (dead heart) were counted, and dead heart percentage was calculated by using following formula: (Chatterjee & Mondal, 2014) White heads: From each plot, total number of white head and tiller with panicle were counted and white head percentage was calculated by using following formula:

Vegetative stage
Number of Tiller: From each plot, total number of tillers were counted on sample plant.Plant Height: Height of plant was taken at vegetative and reproductive stage.

Reproductive stage
A. Yield: From each plot, 1 m 2 including sample plant was harvested, threshed and its moisture content was measured.The yield of grain at standard moisture was calculated by using the following formula:

B. Test weight, filled and unfilled grain percentage
(1) A sub sample was taken, and sun dried until moisture content reached 12% and then 1000 weight grains were counted, and their weight was taken.
(2) From 1000 grain, filled and unfilled grain was counted, and filled and unfilled grain percentage were calculated by using following formula: The data collection was a bit difficult, time-consuming, and expensive, especially because of large sample size and external factor especially rainfall.

Data analysis
The data obtained from the field experiments were arranged in MS Excel (2010) and the data were analyzed by using GEN-STAT software (15 th edition).Mean dead heart and white head were statically analyzed after converting into square root transformation and percentage of filled and unfilled grains were converted into arcsine transformation as suggested by Gomez and Gomez (1984).The critical difference values were calculated at 5% probability level and the treatment mean values of the experiment were compared using Duncan's Multiple Range Test (DMRT).

Infestation of dead heart and white head before treatment
Statistically, significant variation was recorded for dead heart infestation of rice at 41 days after transplanting as the first observation.In the pre-treatment i.e., before the application of insecticides, the incidence of dead heart ranged from 5.44% to 10.19% dead heart/hill with an average of 7.40%, and white head ranged from 6.01% to 11.88% with an average of 7.63%, which directly shows that the Economic threshold level was reached.

Infestation of dead heart
The results of several treatments are shown the Tables 2 and 3 for dead heart and white head percentages of rice.The infected tiller and panicle were not removed till the last date of data collection.Overall performance of various insecticidal treatments after 1 st spray i.e., 7 th days indicated that treatment with Chlorantraniliprole 18.5 SC (0.4 ml/l) was found to be the most effective and significantly superior over all the treatments in reducing the dead heart to the minimum level.Chlorpyriphos 20 EC (2 ml/l) stood second in order of effectiveness which recorded 3.36 percent dead heart.Treatment with Emamectin benzoate 5 SG (0.25 g/l) proved next effective treatment by recording 5.06 percent dead heart this was followed by the Lambda cyhalothrin 5 EC (0.5 ml/l), Fipronil 5 SC (2 ml/l) and Cartap hydrochloride (2 g/l) with 5.7,6.39& 6.66 percent dead heart.Fipronil 5 SC (2 ml/l) and Cartap hydrochloride are par with each other.Highest percent of dead heart was recorded in untreated treatment i.e., control with 9 percent dead heart.
Overall performance of various insecticidal treatments after 1 st spray i.e., 14 th days after spray indicated that Chlorantraniliprole 18.5 SC was the most effective and significantly superior over all the treatments in reducing dead heart, followed by Chlorpyriphos 20 EC and Emamectin benzoate 5 SG with 5.2 percent and 7.1 percent, respectively, Chlorpyriphos 20 EC is par with Emamectin benzoate 5 SG.Cartap hydrochloride proved least effective in which the highest percent of dead hearts was recorded i.e., 8.43 percent whereas Fipronil is par with Cartap hydrochloride.The untreated control recorded 12.63 percent dead hearts.
The effect of various insecticides at 21 days after spraying revealed that Chlorantraniliprole 18.5 SC proved to be most effective against rice yellow stem borer recording the lowest percent of dead hearts than other treatments and which is at par with chlorpyriphos.The next effective treatment was chlorpyriphos followed by Emamectin benzoate 5 SG, Lambda cyhalothrin 5 EC, and Fipronil 5 SC (2 ml/l) with 6.16, 8.33, 9.43 percent respectively.There is a large effect of yellow stem borer on untreated control recorded at 16.36 percent dead heart.The mean percentage of dead hearts and white heads is illustrated in the figure 1.

Infestation of white head
The data recorded on 7 th days after treatment Chlorantraniliprole 18.5 SC (0.4 ml/l) was found effective with 1.6 percent white head.Similarly, Chlorpyriphos 20 EC (2 ml/l) occupied the second which is at par with Emamectin benzoate 5 SG, Lambda cyhalothrin 5 EC, Fipronil 5Sc (2 ml/l), Cartap hydrochloride 50 SP (2 g/l).The highest infestations were recorded in the control plot with 9.7 % white head.
The data recorded on 14 th day after treatment, Chlorantraniliprole 18.5 SC (0.4 ml/l) was found to be most effective with 2.5 percent, white head followed by Chlorpyriphos 20 EC, Emamectin benzoate 5 SG, Lambda cyhalothrin 5 EC, Fipronil 5 SC and Cartap hydrochloride with 4. 73, 5.56, 5.6, 5.76, 6 respectively which is at par with each other.
The data recorded on the 21st day after treatment Chlorantraniliprole 18.5SC (0.4 ml/l) was found effective with 3.4 percent white head.Similarly, Chlorpyrifos 20EC occupied the second which is at par with Emamectin benzoate 5 SG, Lambda cyhalothrin 5EC, Fipronil, Cartap hydrochloride 50 SP.The highest infestations were recorded in the control plot with 11.33 % white head.
The treatment Chlorantraniliprole 18.5SC (0.4 ml/l) was highly effective at reducing yellow stem borer infestation in rice for white heads.Generally, the efficacies of the five other insecticides were in order of the effectiveness Chlorpyriphos 20 EC, Emamectin benzoate 5 SG, Lambda cyhalothrin 5 EC, Fipronil 5 SC (2 ml/l) and Cartap hydrochloride 50 SP.The insecticide Cartap hydrochloride 50 SP recorded the highest number of infestation among the chemical used so, it is less effective.

Yield (ton/ha) of rice
Statistically significant variation was recorded in terms of yield/hectare of rice due to different chemical insecticides under the present trial (Table 4).It was observed that the highest yield was recorded from Chlorantraniliprole 18.5 SC (3.313 ton/ha) which was closely followed by Chlorpyriphos 20 EC (3.243 ton/ha).Further, this was followed by Cartap hydrochloride 50 SP (2.965 ton/ha), Emamectin benzoate 5 SG (2.886 ton/ha), Lambda cyhalothrin 5 EC (2.704 ton/  ha), Fipronil 5 SC (2.440 ton/ha) whereas the lowest yield/hectare was observed from control treatment (2.357 Ton/ha).In the case of increase percent over control, the highest value was observed from Chlorantraniliprole 18.5 SC (40.56 %) and the lowest value was recorded from Fipronil 5 SC (3.52%) (Table 4).

Yield attributing characters
Test weight: Test weight of rice showed statistically significant differences due to different insecticides under the present trial (  4).
Unfilled grain percentage per panicle: Statistically, significant variation was recorded in terms of unfilled grains/panicles of rice due to different insecticides (Table 4).Data revealed that the lowest number of unfilled grains/panicles was recorded from Lambda cyhalothrin 5 EC (17.25%) which was followed by Chlorantraniliprole 18.5 SC (18.92%),Chloropyriphos (18.92%) which are at par with each other, Emamectin benzoate 5 SG (19.75%) whereas the highest unfilled grains/panicle was recorded in control (32.4%).
Plant height: All the treatments did not significantly in plant height which may be due to its genotype characters and availability of different dose of urea, DAP, MoP, Farm Yard Manure (FYM).However, Fipronil 5 SC (101.9 cm) gave an excellent performance in terms of plant height followed by Chlorpyriphos 20 Ec, Lambda cyhalothrin 5 EC with 101.6, 101.2 cm respectively.

Discussion
The results of our study showed that the use of insecticides significantly reduced the level of injury and yield loss caused by yellow stem borer (YSB) in rice.Among the insecticides tested, chlorantraniliprole 18.5 SC was found to be the most effective in minimizing dead heart (DH%) symptoms, reducing them from 5.93% to 4.46%, and minimizing white heads from 7.56% to 2.5% in comparison to the control treatment.Upon closer examination, the observed effectiveness of chlorantraniliprole 18.5 SC in controlling the yellow stem borer infestation can be attributed to the insecticide formulation and active ingredients.Further, the mode of action and application effectiveness chlorantraniliprole 18.5 SC may have also played a crucial role in its comparatively higher efficacy compared to other chemicals.Further research is needed to elucidate the specific factors that govern the efficacy of insecticides against the yellow stem borer.The efficacy of chlorantraniliprole 18.5 SC in controlling YSB in rice is consistent with previous studies by Rahaman and Stout (2019), Abhinandan and Gupta (2020), Sountharya and Prasad (2022), Sachan et al. (2018), and Kumbhar and Singh (2020).In particular, Sachan et al. (2018) found that chlorantraniliprole 18.5 SC (150 ml/ha) and chlorantraniliprole 0.4 GR (10 kg/ha) were the most effective treatments in their field experiments.Similarly, Suri (2011) and Sarao and Cheema (2014) also reported that Chlorantraniliprole 18.5 SC was the most effective in minimizing YSB infestation.Additionally, Fipronil 0.3% GR and Cartap hydrochloride were found to be the next most effective insecticides, consistent with the findings of Panda et al. (2004), Roshan (2006), and Singh et al. (2010).However, the variability in the effectiveness of the tested insecticides could be due to the distinct active ingredients in the treatments, as well as the ecological parameters that affect the behavior of the pest.In this study, the least amount of dead heart and white head infestation was observed in the chlorantraniliprole 18.5 SC treatment, followed by chlorpyriphos 20 EC, as shown in Table 2 & 3.These findings are consistent with the results of S and S (2008), who found that chlorpyriphos 20 EC, cartap hydrochloride 4 G, and triazophos 40 EC were significantly effective in reducing the stem borer (white ear) infestation.Further, the insecticides affected the growth and development of the rice plants in different ways, depending on their mode of action and other factors.Further research would be needed to investigate this issue.Overall, this study provides valuable insights into the effectiveness of different insecticides in controlling YSB in rice.Further research could focus on optimizing the concentration of insecticides and evaluating the long-term effects of their use on soil health and biodiversity.

Conclusion
The overall performance of all the insecticidal treatments was observed to lessen the damage caused by yellow stem borer in terms of both dead hearts (DH%) and white heads (WH%) over the untreated control.Among them, Chlorantraniliprole 18.5 SC (0.4 ml/l) and Chlorpyriphos 20 EC (2.0 ml/l) were found to be the most effective in suppressing DH% and WH%.Similarly, Emamectin Benzoate 5 SG (0.25 g/l), Lambda Cyhalothrin 5EC (0.5 ml/l), and Fipronil 5 SC (2 ml/ l) were found to be more effective against yellow stem borer (YSB) than the untreated control, but Cartap hydrochloride 50 SP (2.0 g/l) was found to be the least effective.Considering the present observations, the insecticides Chlorantraniliprole 18.5 SC (0.4 ml/l) and Chlorpyriphos 20 EC (2.0 ml/l) could be proposed to control the yellow stem borer for greater efficacy against the pest.

Table 2 . Effect of insecticides on yellow stem borer (dead hearts) after treatment on spring rice, 2022 Percent dead hearts
Note: DAS: Days after spray Values are mean of three replications at different day of observation; CV: Coefficient of variation; ***: Significant at 0.1% level of significance; *: Significant at 5% level of significance; s.e.d.: Standard error difference; LSD: Least Significant Difference; Values with the same letters in a column are not significantly different at 5% level significance by DMRT test and table indicate square root transformation values; Parenthesized values are the result of arcsine transformation.

Table 3 . Effect on insecticides on yellow stem borer (white heads) after treatment on spring rice, 2022 Percent white head
Note: DAS: Days after spray Values are the mean of three replications at different day of observation; CV: Coefficient of variation; ***: Significant at 0.1% level of significance; *: Significant at 5% level of significance; s.e.d.: Standard error difference; LSD: Least Significant Difference; Values with the same letters in a column are not significantly different at 5% level significance by DMRT test and table indicate square root transformation values; Parenthesized values are the result of arcsine transformation.

Table 4 . Effect of Different Insecticides on the plant height, filled grain, unfilled grain Yield and test Weight (1000 grains weight) of spring rice, 2022 Treatments Plant height (cm) Filled grain (%) Unfilled grain (%) Test weight (gm) Yield (t/ha) % Increase in yield
Values are mean of three replications at different day of observation; CV: Coefficient of variation; *: Significant at 5% level of significance; s.e.d.: Standard error difference; LSD: Least Significant Difference; Values with the same letters in a column are not significantly different at 5% level significance by DMRT test and table indicate square root transformation values; Parenthesized values are the result of arcsine transformation.