Cotton, Leaf roller, S. derogata, Correlation, Population dynamics.

Cotton (Gossypium hirsutum L.) belongs to the "Malvaceae" family and "Gossypium" genus (Manjunatha et al., 2009). It is also known as 'White gold' which enjoys a predominant position amongst all cash crops in India and plays a significant role in the national economy. About 65 per cent requirements of the Indian textile industry are contributed to cotton as a vital raw material. Indian textile industries with 1500 mills occupy a significant place in the country's economy (Shivagaje et al., 2004). Introduction of the Bt cotton the minor pests (sucking pests and caterpillars) have become major nowadays. According to the report of Gujar et al. (2010), no leaf roller damage was found in the Bt hybrids however, their non-Bt counterparts showed 79 to 88 per cent plant damage and 21 to 44 per cent foliage rolled for respective hybrids. Cotton leaf roller nowadays observed especially in non-Bt cotton. The pest occurs in India, Pakistan, Bangladesh, Burma, Australia, Africa, China, Japan and Sri Lanka. The pest is active from the month of September to November. The leaf roller is a polyphagous that feeds on malvaceous plants such as G. hirsutum, A. esculentus, H. rosasinensis, U. lobata, A. rosea, S. cordifolia and M. tricuspidatum. The leaves rolled in the trumpets fastened by silken threads and feed on the green tissue in early stage and eat up a large portion of the leaf as it grows. Caterpillar infestation can vary with some leaves having as few as 5 to 6, while others may have as many as 32 young caterpillars on a single damaged leaf (Mariselvi and Manimegalai 2016). In case of heavy infestation, phyllophagous could alter the photosynthetic activity of the plant and cause complete defoliation. The fifth and sixth instar larvae consume both the parenchyma and epithelium of leaves, producing holes of varying diameters. The larvae preferred soft fresh leaves for eating and did not feed on other plant parts. Leaves that have been extensively damaged turn yellow and dry. Larvae destructive feeding activity limits photosynthetic area, as well as the quantity and quality of minerals and water uptake by the plant. If the above procedure continues, the plant will dry totally (Gahramanova et al., 2020). Up to 50 per cent yield loss due to leaf roller in cotton have been reported in the northwest ecological zone of Nigeria (Yahaya, 2008).

A study of the population dynamics and impact of weather parameters on incidence of leaf roller, S. derogata in non-Bt cotton hybrid (RCH 659), field experiment was carried out during kharif, 2022-23 at Main Cotton Research Station, Navsari Agricultural University, Surat (Gujarat). The cotton was sown in a large block of 20.40 × 20.25 m (413 m2) during second fortnight of June at a spacing of 1.20 × 0.45 m. In order record the larval population of leaf roller, the entire plot divided into ten equal quadrates and five plants were selected randomly from each quadrate. For recording the observation on seasonal incidence of the leaf roller, weekly data on the number of larvae in rolled leaves per plant on fifty randomly selected plants was recorded from seven days after germination till the removal of crop. Similarly, number of rolled or damage leaves were counted on the plants at weekly interval and per cent leaves damage by S. derogata worked out. The observations were recorded at weekly intervals throughout the cropping season. Plot was kept completely free from the insecticides spray during whole season. In order to study the instantaneous effect of weather parameters on population fluctuation of leaf roller and its damage, Week-wise data on various parameters recorded by Meteorology observatory, Main Cotton Research Station, Navsari Agricultural University, Surat.

A. Incidence of leaf roller

The periodical week wise data on larval population of leaf roller per plant, number of rolled leaves per plant and per cent leaf damage per plant are summarized in Table 1. The pest observed from fifth week of July (31st SMW) and it continued till the fourth week of December (52nd SMW). Leaf roller, S. derogata larval population, rolled leaves per plant and per cent damage leaves were ranged from 0.60 to 85.82, 1.24 to 45.96 per plant and 1.69 to 44.50 per cent damage leaves in non-Bt cotton, respectively. The incidence of leaf roller was increasing slowly up to second week of September (37th SMW) with 1.64 to 21.40 larvae/plant, 1.24 to 11.36 rolled leaves/plant and 6.30 to 17.69 per cent damage leaves. The population gradually increased from third week of September (38th SMW) to fourth week of September (39th SMW), number of larvae, rolled leaves and per cent damage leaves was 33.94 to 51.04/plant, 15.48 to 28.48/plant and 18.56 to 30.63 per cent, respectively. Population increased drastically and the peak was observed in first week of October (40th SMW) with 85.82 larvae/plant which was the highest population recorded with 45.62 rolled leaves/plant and 44.50 per cent damage leaves. A little reduction in leaf roller population was recorded in second and third week of October (41st and 42nd SMW) with 83.20 larvae/plant (45.96 rolled leaves/plant and 42.34 per cent damage leaves) and 76.36 larvae/plant (40.36 rolled leaves/plant and 34.21 per cent damage leaves), respectively. Again, second peak 81.44 larvae/plant with 40.46 rolled leaves/plant and 30.49 per cent damage leaves was observed during the fourth week of October (43rd SMW). The population of leaf roller decreased 72.14 larvae/plant with 38.26 rolled leaves/plant and 26.79 per cent damage leaves from fifth week of October (44th SMW) than gradually decrease from November. The activity of leaf roller larval population ranged from 32.14 to 60.88/plant with 22.46 to 34.22 rolled leaves/plant and 15.07 to 23.23 per cent damage leaves were observed from first week of November (45th SMW) to third week of November (47th SMW). The population of leaf roller gradually decreased 18.22 larvae/plant with 17.26 rolled leaves/plant and 11.72 per cent damage leaves from fourth week of November (48th SMW) to 0.60 larva/plant with 2.22 rolled leaves/plant and 1.69 per cent damage leaves, last week of December (52nd SMW). Thus, it was clear from data that relatively higher activity of leaf roller larva with 72.14 to 85.82 larvae/plant, 38.26 to 45.96 rolled leaves/plant and 26.79 to 44.50 per cent damage leaves observed during first week of October to fifth week of October (40th to 44th SMW) in non-Bt cotton. The incidence of leaf roller during the season was 33.10 larvae/plant with 19.63 rolled leaves/plant and 18.53 per cent damage leaves recorded in non-Bt cotton hybrid.

The study on seasonal dynamics of leaf roller on various host plants was carried out by various researchers. Bhatnagar et al. (1993) observed the maximum activity in last week of August (35th SMW) to last week of September (39th SMW) in cotton. Ghosh et al. (1999) showed that leaf roller appearance in okra during the first week of July with higher population was recorded until August. Hiramatsu et al. (2001) observed the activity of leaf roller was started from last week of June (26th SMW) to first week of November (45th SMW) with peak incidence reported in 3rd week of August (34th SMW) and higher activity found in September and October month and remained continue up to November. Naresh et al. (2003) observed the peak incidence (2.33 larvae/plant) during the second fortnight of August. Roychoudhury et al. (2009) recorded the leaf roller infestation in the forest tree occurred from July to September. Badiyala (2011) revealed that peak activity of leaf roller was observed during first week of August (32nd SMW) and third week of August (34th SMW) in Palampur and last week of July (31st SMW); second week of August (33rd SMW) and fourth week of July (30th SMW) in Kachhari. Boopathi et al. (2011) noted that the leaf roller infestation levels reached their highest (0.40/plant) in May. Lok et al. (2011) recorded the maximum level of rolled leaves reached from 1.60 to 2.80 per plant during 39th SMW (4th week of September). Iyamu (2012) observed the leaf roller lower infestation in July (16.10%) and August (11.30%) where highest field larval population observed during November to December was 96.20 per cent. Laxman et al. (2014) noted the leaf roller infestation was observed during August to November (1.16 to 10.49 per cent) in non-Bt cotton. Ogbalu et al. (2015) noted the larvae feed on okra pods resulting in defoliation of leaves, shedding of pods (92.40%), effect on seed viability (64.50%). Pan and Xiu (2016) reported that higher activity during September on A. theophrasti, A. rosea and A. esculentus than that on G. hirsutum. Hansda et al. (2017) showed the leaf roller occurred during July to November and its peak period was from September to October (10 larvae/plant). Nair et al. (2017) reported the pest relative abundance was 8.86 and 8.46 per cent during summer and winter, respectively. Behera (2018) mentioned the incidence was started during fourth week of August (35th SMW) and higher activity noticed during third week of October (42nd SMW) with the peak population in second week of November (46th SMW). The population started to decline from the third week of November and the incidence was observed until the second week of December in non-Bt cotton. Roy et al. (2018) reported the leaf roller population was highest (5.25 ± 3.22/plant) at seven weeks after sowing (17th SMW) and lowest (1.25 ± 0.67/plant) at four WAS (14th SMW). Das et al. (2021) revealed the peak activity of leaf roller observed during third week of March (4.50 larvae/plant) and third week of August (5.00 larvae/plant) in Rabi and Kharif season, respectively. Karpun et al. (2022) found that leaf roller was observed during end of July to early August caused severe damage to Hibiscus spp. and defoliation level ranged from 50 to 100 per cent. Mailafiya et al. (2022) observed that the plant infestation by S. derogata ranged from 84 (83 larvae/field) to 88 (69 larvae/field) per cent during 2018 and 2019, respectively. In present investigation, leaf roller activity was higher during third week of September (38th SMW) to third week of November (47th SMW) with first peak during first week of October (40th standard week) and second peak during third week of October (42nd SMW) in non-Bthybrid. Thus, above reports of Hiramatsu et al. (2001); Iyamu (2012); Hansda et al. (2017); Behera (2018); Mailafiya et al. (2022) are more or less in accordance with present investigation. Conversely Bhatnagar et al. (1993), Ghosh et al. (1999); Naresh et al. (2003); Roychoudhury et al. (2009); Badiyala (2011); Boopathi et al. (2011); Lok et al. (2011); Ogbalu et al. (2015); Pan and Xiu (2016); Nair et al. (2017); Roy et al. (2018); Das et al. (2021); Karpun et al. (2022) do not tally with present observations might be due to study on different host crops, locations, sowing time and climatic condition.

Effect of weather parameters on incidence of leaf roller. To study the relationship between leaf roller incidence and weather parameters, the pest population dynamics data were correlated with meteorological parameters data. The results obtained are presented in Table 2. The results on correlation between incidence of leaf roller larva and different weather parameters revealed that only maximum temperature (r = 0.439*) showed significant positive correlation whereas, minimum temperature (r = 0.033), evening relative humidity (r = 0.107) and bright sunshine hours (r = 0.314) had a non-significant positive correlation with the leaf roller population. Morning relative humidity (r = -0.166), rainfall (r = -0.215) and rainy days (r = -0.237) showed non-significant negative association with the incidence of leaf roller infesting non-Bt cotton. The results on correlation between number of rolled leaves by S. derogata and different weather parameters revealed that only maximum temperature (r = 0.469*) showed significant positive correlation association whereas morning relative humidity (r = -0.302), rainfall (r = -0.309) and rainy days (r = -0.357) showed non-significant negative association. Bright sunshine hours (r = 0.422), minimum temperature (r = 0.114) and evening relative humidity (r = 0.005) had a non-significant positive correlation (Table 2). Correlation for per cent leaves damage by S. derogata with abiotic factors revealed that maximum temperature (r = 0.234), minimum temperature (r = 0.243), evening relative humidity (r = 0.323) and bright sunshine hours (r = 0.106) had non-significant positive correlation. Rainfall (r = -0.124), morning relative humidity (r = -0.045) and rainy days (r = -0.052) showed non-significant negative association. 

The study on correlation of weather parameters with population of S. derogata was carried out by various researchers in different hosts. Bhatnagar et al. (1993) mentioned that maximum temperature (31.3°C to 36.2°C), minimum temperature (19.7°C to 25.1°C) and relative humidity (60.50 to 76.30%) with presence of intermittent light to moderate rainfall were favorable for the multiplication of S. derogata. Ghosh et al. (1999) revealed that negative and non-significant correlation with maximum temperature (r = -0.565) while positive and significant correlation with minimum temperature (r = 0.804*) and relative humidity (r = 0.588*) to leaf roller incidence. Badiyala (2011) reported the relative humidity had significant positive, bright sunshine hours exhibited a significant negative while, maximum temperature had negative whereas minimum temperature had positive correlation but non-significant against pest activity. Lok et al. (2011) observed that the larvae of cotton leaf roller exhibited non-significant positive association with relative humidity, rainfall and bright sunshine hours whereas, temperature had non-significant negative correlation. Iyamu (2012) noted that rainfall and temperature during early, mid and later season had significant negative correlation while relative humidity had significant positive correlation. Laxman et al. (2014) noted that maximum temperature (r = -0.216) had non-significant negative correlation while, rainfall (r = 0.026) had non-significant positive correlation. The minimum temperature (r = -0.424**), morning relative humidity (r = 0.516**) and evening relative humidity (r = 0.706**) had highly significant positive correlation with the leaf roller incidence and damage. Behera (2018) reported that the maximum temperature had significant negative association minimum temperature, relative morning and evening humidity and rainfall had non-significant negative correlation with leaf roller population in non-Bt cotton. Roy et al. (2018) noted that the maximum (r = 0.465**) and minimum temperature (r = 0.592**) had highly positive significant whereas maximum relative humidity (-0.128), minimum relative humidity (0.237), rainfall (0.111), sunshine hours (-0.170) and wind speed (0.025) had non-significant correlation. Das et al. (2021) observed that minimum temperature (r = 0.017) exhibited a positive correlation while maximum temperature (r = -0.350), rainfall (r = -0.229), bright sunshine hours (r = -0.530), morning relative humidity (r = -0.357) and evening relative humidity (r = -0.410) were negatively correlated with leaf roller population. In present investigation maximum temperature had significant positive correlation. Minimum temperature, evening relative humidity and bright sunshine hours had positive correlation. Morning relative humidity, rainfall and rainy days had significant negative correlation with leaf roller larval population, rolled leaves/plant and per cent leaves damage. Therefore, results of Bhatnagar et al. (1993); Lok et al. (2011) ; Das et al. (2021) are strongly accordance with the results of present investigation. Moreover, results of Ghosh et al. (1999), Badiyala (2011); Iyamu (2012); Laxman et al. (2014); Behera (2018); Roy et al. (2018) do not tally with present experiment results.

Table 1: Population dynamics of leaf roller, S. derogata in non-Bt cotton.

Table 2: Correlation between incidence of S. derogata and weather parameters in non-Bt cotton.