Population dynamics of major defoliator on soybean and their relation with abiotic factors

Introduction

Soybean (Glycine max L. Merrill) is one of the most important oilseed and leguminous crops in the world, valued for its high protein (about 40%) and oil content (around 20%). In India, soybean plays a vital role in enhancing farmers' income and improving soil fertility through biological nitrogen fixation. Despite its economic importance, soybean productivity is considerably constrained by the attack of various insect pests throughout its growth period. The soybean ecosystem harbors a wide range of insect pests, which defoliators the most destructive and occur regularly from the vegetative to reproductive stages of the crop. Defoliators such as the tobacco caterpillar Spodoptera litura (Fabricius), green semilooper Chrysodeixis acuta (Walker) and other lepidopteran larvae cause severe foliage damage, leading to reduced photosynthetic area and ultimately significant yield losses. Heavy infestation during early crop growth may even result in complete defoliation.

Material and Methods

The experiment was conducted at the Research cum Instructional Farm of Indira Gandhi Krishi Vishwavidyalaya; Raipur, Chhattisgarh (India) with the soybean variety RSC 10-46. Record of population of major defoliators of soybean crop was done in the Control plot during the Kharif of 2022-2023 and Kharif of 2023-24.

Experimental details

Crop

Results and Discussions

In the present investigation, numbers of defoliators were recorded infesting the soybean crop. Among the lepidopteran defoliators were the tobacco caterpillar (Spodoptera litura Fab.) and the green semilooper (Chrysodeixis acuta Walker). Data on the population dynamics of the major insect pests were presented in tables no 1.

Population dynamics of tobacco caterpillar, Spodoptera litura F. with abiotic factor

The population of S. litura ranged from 0.40 to 9.10 larvae per mrl, as presented in table 1. The incidence of the tobacco caterpillar was began during the third week of August (33 rd SMW) with a mean population of 0.50 larvae per mrl. It was gradually increased and reached its highest peaked density at 9.10 larvae per mrl in the second week of September (36 th SMW). During this period, the maximum temperature 33.25°C, minimum temperature 25.34°C, rainfall 9.57 mm, morning relative humidity 90.57 percent, evening relative humidity 68.42 percent, wind speed was 1.07 km/hr and sunshine hours 6.92 hrs were recorded. Later, the tobacco caterpillar population declined until the crop reached maturity recorded on the soybean crop during Kharif 2022-23. During Kharif 2023-24, depicted in table 1 revealed that the population ranged from 0.10 to 8.20 larvae per mrl. The larvae of the tobacco caterpillar were first observed during the second week of August (32 nd SMW) with mean population of 0.10 larvae per mrl. The peaked population 8.20 larvae per mrl during the third week of September (37 th SMW). The maximum temperature recorded 31.46°C, the minimum temperature 25.77°C, rainfall measured 5.63 mm, morning relative humidity 87.35 percent, evening relative humidity 70.35 percent, wind speed 5.53 km/hr and sunshine 6.79 hours were recorded during the 2023-24 season. Thereafter, the population declined until the end of the season. The pooled mean population of the tobacco caterpillar during Kharif 2022-23 and 2023-24 ranged from 0.05 to 8.65 larvae per mrl. The population peaked at 8.65 larvae per mrl during the second week of September (36 th SMW) and the seasonal mean population was 3.31 larvae per mrl depicted in table 1. During this period, the maximum temperature recorded 31.34°C, the minimum temperature 24.21°C, rainfall measured 7.41 mm, morning relative humidity reached 89.46 percent, evening relative humidity reached 65.66 percent, wind speed 2.73 km/hr and sunshine 5.52 hours were recorded. The present findings were in agreement with those of Sapekar et al. (2020), who reported that the incidence of tobacco caterpillar was recorded during the 31 st standard week (0.66 larvae/mrl), with the highest population observed at 3.66 larvae/mrl during the 36 th SMW. Munje et al. (2022) also supported these findings, stating that the presence of tobacco caterpillar was recorded at the 31 st SMW and continued up to the 34 th SMW, ranging from 0.40 larvae/mrl. The maximum population of the tobacco leafeating caterpillar was recorded between the 31 st and 34 th SMW at 0.40 larvae/mrl. Panwar et al. (2021) mentioned in their study that the peaked activity of semilooper (5 larvae/mrl) and tobacco caterpillar (1.33 larvae/mrl) was observed during the 37 th and 38 th SMW, respectively. Sunder et al. (2018) also reported the highest tobacco caterpillar infestation during the Kharif season, with a mean population of 4.30 larvae/mrl recorded in the last week of August. Subsequently, an abrupt decline in larval population was observed in the fourth week of September, with an average of 1.20 larvae/mrl.

Correlation of tobacco caterpillar population with weather parameters

The data presented in Table no 2 indicated that, in Kharif 2022-23, the larval population of the tobacco caterpillar showed with maximum temperature (r = 0.599*) significant positive correlation, minimum temperature (r = 0.387), evening relative humidity (r = 0.407), morning relative humidity (r = 0.154) and wind speed (r = 0.042) nonsignificant positive correlation. However, non-significant negative correlation was observed with rainfall (r = -0.306) and sunshine hours (r = -0.342). Furthermore, the data presented in Table 2 demonstrated that, in Kharif 2023-24, the larval population of the tobacco caterpillar exhibited a non-significant positive correlation with maximum temperature (r = 0.584*), evening relative humidity (r = 0.387), minimum temperature (r = 0.381), morning relative humidity (r = 0.185) and wind speed (r = 0.272). In contrast, negative non-significant correlation were observed with sunshine hours (r = -0.243) and rainfall (r = -0.008).

Pooled mean of during Kharif 2022-23 and 2023-24, the larval population of the tobacco caterpillar showed with maximum temperature (r = 0.623*) significant positive correlation, minimum temperature (r = 0.390), evening relative humidity (r = 0.404), morning relative humidity (r = 0.164) and wind speed (r = 0.199) non-significant positive correlation. However, non-significant negative correlation was observed with rainfall (r = -0.176) and sunshine hours (r = -0.200) depicted in table 2. The present findings agree with earlier studies on S. litura population dynamics. Bramhan et al. (2018) reported a significant positive correlation with maximum temperature (r = 0.663) and a significant negative correlation with rainfall (r = -0.570). Sapekar et al. (2020) observed nonsignificant relationships between abiotic factors and S. litura, though morning (r = 0.052) and evening relative humidity (r = 0.289) showed weak positive associations. Similarly, Panwar et al. (2021) found a significant positive correlation between larval population and maximum temperature (r = 0.55). Patidar (2020) reported a strong negative correlation of rainfall (r = -0.71) with larval incidence. Other studies also showed that maximum and minimum temperatures had positive correlations (r = 0.46 and r = 0.01), while morning relative humidity showed a negative correlation (r = -0.23) with pest population.

Regression equation tobacco caterpiller, S. litura Fab. with weather parameters

During Kharif 2022-23, regression analysis between tobacco caterpillar population and minimum temperature showed the equation y = 2.0677x -50.707 (R² = 0.359), indicating that infestation increased by 2.0677 units with every unit rise in minimum temperature. Similarly, another analysis for the same season gave y = 1.9177x -57.132 (R² = 0.3412), showing an increase of 1.9177 units in infestation per unit rise in minimum temperature. When pooled data from Kharif 2022-23 and 2023-24 were analyzed, the regression between tobacco caterpillar population and maximum temperature produced the equation y = 2.0911x -62.033 (R² = 0.361). This indicated that for every unit increase in temperature, the infestation level increased by 2.091 units, confirming temperature as an important factor influencing pest buildup. (Fig. 1

Conclusion

The study showed a clear seasonal pattern of S. litura and C. acuta on soybean during Kharif 2022-23 and 2023-24, with incidence starting in August, peaking in September (36th-37th SMW), and declining at crop maturity. S. litura was positively associated with maximum temperature, while C. acuta was mainly influenced by minimum temperature and evening relative humidity. Regression analysis confirmed a positive linear effect of temperature on both pests, identifying minimum temperature as the key abiotic factor governing their seasonal buildup.

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