Abstract: The success of sericulture industry is mainly based on leaf quality and appropriate environmental conditions for silkworm rearing. Mulberry leaves are exclusive source of nutrition for silkworm. This study was aimed to assess the impact of different mulberry varieties namely S1, S13, S146, S1635 and TR10 and seasons viz., spring, autumn, summer and monsoon on growth and cocoon characteristics of bivoltine silkworm (Bombyx mori). Among the seasons, the spring season showed better growth and cocoon characteristics followed by autumn, summer and monsoon seasons. The better traits observed during favourable seasons than unfavourable seasons might be due to the influence of climatic conditions. The mulberry varieties did not show significant impact on growth and economic traits studied, however the effect of different mulberry varieties on cocoon yield was observed in the following order: S1>TR10>S146>S1635>S13. The highest cocoon yield for 10000 larvae was observed in S1 (8.949 kg) while the minimum yield was observed in S13 (8.794 kg). The traits such as larval length, larval width, larval weight, shell ratio, filament length and filament weight showed positive correlation with yield. The results showed the suitability of S1 mulberry variety and spring season for commercial rearing and more productivity.
INTRODUCTION
Sericulture is known as agro-based cottage industry which plays an important role in improving the rural economy because it possesses high employment and income generation capability with minimum investment (Hiware, 2001). The Indian silk industry is a constant source of foreign exchange earnings and unique in producing all five types of silk varieties viz., Mulberry, Eri, Muga, tropical and temperate Tasar silks. However, the mulberry silk is highly produced and widely used in the country. The mulberry plants are cultivated both in tropical and temperate countries of the world. In India, the mulberry plants are cultivated mainly in Tamil Nadu, Andhra Pradesh, Jammu & Kashmir, Karnataka, Uttar Pradesh, West Bengal and northeastern states due to prevalence of favorable climatic conditions (Muthulakshmi et al., 2010). Mulberry leaves are the exclusive source of nutrition (e.g., protein, carbohydrates, vitamins, minerals etc.) for growth and development of silkworms (Tang et al., 2005). The quality of leaves fed to silkworm is considered to be the prime factor for good cocoon production (Ravikumar, 1988). Mulberry leaves contain a rich source of protein and amino acids which influence the shell weight of cocoon (Mechii and Katagiri, 1991). There are three important factors which determine the quality of leaves, viz. good variety of mulberry, appropriate agronomic practices and plant protection measures (Qader et al., 1992). However, the nutritional composition of mulberry leaves varies with different cultivars and influences on growth and cocoon production (Mahmood et al., 1987). The larval growth of silkworm is influenced by the nutritional level of different cultivars of mulberry which ultimately reflect the economic traits namely larval weight, cocoon and shell weight, effective rate of rearing etc., (Ghosh et al., 2000; Zannoon et al., 2012). The quality of mulberry leaves and environmental conditions influence the dietary efficiency and growth of silkworm (Rahmathulla et al., 2004). The performance of any mulberry variety with respect to leaf quality and cocoon production varies with agro-climatic conditions and cultivation practices. Physical and chemical properties of leaves directly influence the amount of leaf consumption and digestion (Das and Vijayaraghavan, 1990; Ghosh et al., 2000; Zannoon et al., 2012). Keeping in view of the above mentioned importance, the current study was conducted to investigate the effect of different mulberry varieties and rearing seasons on larval growth and economic traits of Bombyx mori.
MATERIALS AND METHODS
The study was carried out in a well equipped rearing house of the Department of Applied Animal Sciences, Babasaheb Bhimrao Ambedkar University, Lucknow, Uttar Pradesh, India during spring (February-March), summer (March-April), monsoon (August-September) and Autumn (October-November) seasons of year 2010 and 2011. The growth and cocoon characteristics of silkworms fed on leaves of different mulberry varieties were studied at different seasons. Fifty Disease Free Laying (dfls) of bivoltine hybrid race (CSR2xCSR4) were procured from Silkworm Seed Production Centre (SSPC), Central Silk Board, Dehradun and Bangalore during 2010 and 2011, respectively. The newly hatched tiny larvae were brushed on rearing trays and offered chopped tender leaves to them. Chawki rearing was carried out at mass level. After 2nd instar, mulberry leaves of S1, S13, S146, S1635 and TR10 varieties were fed individually to larvae at four times in a day (6:00 a.m., 11:00 a.m., 4:00 p.m. and 10:00 p.m.). Three rearing trays consist 300 larvae were maintained for each variety of mulberry leaves. The healthy larvae in the rearing trays were enumerated and unhealthy or dead larvae removed periodically. Fully ripened worms were picked manually and mounted on bamboo spinning tray.
The effect of different mulberry varieties and seasons on growth and cocoon characteristics of B. mori was assessed based on hatching (%), larval length (mm), larval width (mm), larval weight (g), single cocoon weight (g), shell weight (g), shell ratio (%), pupal weight (g), filament length (m), filament weight (g), effective rate of rearing (%) and denier. The larval length and width were measured using digital Vernier calipers. The shell weight, pupal weight and filament weight were measured using an electronic balance (AVINET, USA). The number of fertilized eggs hatched synchronously in a brood was taken as a measure to calculate the percentage of hatching by using the following formula:
Economic traits of cocoon: The single cocoon weight (g) was recorded as an average of 10 female and 10 male cocoons at random by using the following formula:
The shell ratio (%) determines the total quantity of silk available from a single cocoon. The average ratio of 25 male and 25 female cocoon shells to the cocoon weight was used. The shell ratio (%) was calculated by using the following formula:
The effective rate of rearing (ERR%) was calculated by using the following formula:
Five cocoons randomly selected were reeled to find out the filament length of the cocoon using eprouvette and it was determined by adopting the following formula:
L = Rx1.125 |
where, L is filament length, R is No. of revolutions recorded by eprouvette, 1.125 = circumference of eprouvette reel in meters.
The Denier of silk filament was calculated by using the following formula:
Data analysis: The obtained data were pooled and analyzed with reference to different seasons and mulberry varieties. The effect of different mulberry varieties and seasons on growth and cocoon characteristics was analyzed using one way ANOVA and correlation.
RESULTS
After 48 h of incubation, the larvae of CSR2xCSR4 hatched out uniformly. The maximum hatching was observed during spring (96.3%) while the minimum hatching was observed during monsoon (91.5%) rearing seasons. The larval length of matured larvae which fed on S1, S146, TR10, S13 and S1635 mulberry varieties ranged from 62.37 mm to 63.73 during different rearing seasons. The highest mean larval length (63.73±0.08 mm) was observed during spring rearing while the least larval length (62.37±0.08 mm) was observed during monsoon rearing. The mean larval lengths of silkworms which fed on different mulberry varieties during spring, summer, autumn and monsoon seasons are given in Table 1. There was no significant difference in the larval length of larvae which fed on different mulberry varieties (F4,15 = 0.0081, p>0.05), however there was a significant difference in the larval length among different seasons (F3,16 = 688.6, p<0.05). The mean larval length showed a positive correlation with cocoon yield (r = 0.952, df = 18, p<0.05). The mean larval width of 5th instar larvae ranged from 9.13-9.35 mm.
| Table 1: | Mean Larval length of silkworms which fed on S1, S146, TR10, S13 and S1635 mulberry varieties during Spring, Summer, Monsoon and Autumn seasons |
| Table 2: | Mean Larval width of silkworms which fed on S1, S146, TR10, S13 and S1635 mulberry varieties during Spring, Summer, Autumn and Monsoon seasons |
| Table 3: | Mean single cocoon weight of larvae which fed on S1, S146, TR10, S13 and S1635 mulberry varieties during Spring, Summer, Monsoon and Autumn seasons |
The highest mean larval width (9.35±0.04 mm) was observed during spring rearing while least mean larval width (9.13±0.05 mm) was observed during monsoon rearing. The mean larval widths of larvae which fed different mulberry varieties during different seasons are given in Table 2. Though, there was a fluctuation in the larval width of larvae which fed different mulberry varieties but there was no significant difference in the larval width (F4,15 = 0.033, p>0.05). However, there was a significant difference in the larval width among different seasons (F3,16 = 344, p<0.05). The mean larval width and yield showed positive correlation (r = 0.825, df = 18, p<0.05).
The weight of single cocoon showed a similar trend as larval length and width. The larvae which fed S1 mulberry leaves during spring rearing produced heavier cocoons (0.895±0.057 g), while the larvae which fed S13 mulberry leaves produced smaller cocoons (0.855±0.068 g) during monsoon rearing. The mean weight of single cocoon varied among the larvae which fed different varieties of mulberry leaves but there was no significant difference (F4,15 = 0.253, p>0.05). However, there was a significant difference in the mean weight of single cocoon among seasons (F4,16 = 21.19, p<0.05). The mean weight of single cocoon at different seasons is given in (Table 3). In accordance o the single cocoon weight, the highest yield for 10000 larvae was observed in S1 (8.949 kg) compared to yield of other feeding trials viz., TR10 (8.926 kg), S146 (8.914 kg), S1635 (8.835 kg) and S13 (8.794 kg).
The average shell weight was ranged from 0.290±0.028-0.351±0.040 g during different seasons and at different mulberry varieties feeding trials (Fig. 1). The highest shell weight (0.351±0.040 g) was observed from cocoons produced by larvae fed on TR10 mulberry leaves during spring rearing. Through, there was no significant effect of different mulberry leaves on shell weight (F4,15 = 0.098, p>0.05) but there was a significant difference in the shell weight among different seasons (F3,16 = 104.86, p<0.05). The average shell ratio was ranged from 20.78-21.655% (Fig. 2). The highest shell ratio (21.655±0.518%) was observed during spring rearing. There was a positive correlation between cocoon weight and shell ratio (r = 0.654, df = 18, p<0.05).
The effective rate of rearing varied across the seasons and feeding trials varied from 74.55-80.83% (Fig. 3). The highest EER (80.83±5.64%) was observed during spring rearing. There was no significant difference in the effective rate of rearing among different mulberry varieties (F4,15 = 0.039, p>0.05), however there was an influence by different seasons on EER (F3,16 = 55.99, p<0.05).
| Fig. 1: | Mean shell weight of cocoons produced by different mulberry leaves fed larvae during Spring, Summer, Autumn and Monsoon seasons. Values are given Mean±SD |
| Fig. 2: | Mean shell ratio (%) observed from the cocoons produced by larvae fed different mulberry varieties during Spring, Summer, Autumn and Monsoon seasons. Values are given Mean±SD |
| Fig. 3: | Mean effective rate of rearing (%) observed during Spring, Summer, Autumn and Monsoon seasons, Values are given Mean±SD |
| Fig. 4: | Mean pupal weight (g) observed during Spring, Summer, Autumn and Monsoon seasons. Values are given Mean±SD |
The weight of pupae produced by different mulberry leaves fed larvae ranged from 0.252-0.330 g. The highest pupal weight (0.330±0.029 g) was observed in TR10 mulberry leaves fed larvae during spring rearing, while the least pupal weight was observed during monsoon season (Fig. 4). The pupal weight did not differ significantly among different mulberry leaves feeding trial (F4,15 = 0.194, p>0.05), however there was a significant difference in pupal weight among different seasons (F3,16 = 22.75, p<0.05).
In consistent to other rearing parameters, the filament length (m) also varied among different feeding trials and seasons (Fig. 5). The highest filament length (835.85±81.0 m) was observed during spring rearing while the least filament length was observed during monsoon rearing (719.30±66.50 m). There was a significant difference in the filament length observed at different rearing seasons (F3,16 = 41.02, p<0.05), however there was no significant effect due to different mulberry varieties (F4,15 = 0.111, p>0.05). The average filament weight differed across different seasons and feeding trails (Fig. 6). The filament weight showed significant difference among seasons (F3,16 = 15.95, p<0.05), however there was no significant difference among different feeding trials (F4,15 = 0.528, p>0.05).
| Fig. 5: | Mean filament length (m) observed during Spring, Summer, Autumn and Monsoon seasons. Values are given Mean±SD |
| Fig. 6: | Mean filament weight (g) observed during Spring, Summer, Autumn and Monsoon seasons. Values are given Mean±SD |
The filament length and filament weight of different seasons and feeding trails showed a positive correlation (r = 0.777, df = 18, p<0.05). The Denier varied over seasons and feeding trials from 2.628±0.090-2.836±0.077. There was a significant difference in the Denier among seasons (F3,16 = 55.0, p<0.05), however there was no significant difference across different feeding trials (F4,15 = 0.132, p>0.05).
DISCUSSION
The results of current study revealed that the cocoon yield and economic traits viz., larval length, larval width, larval weight, single cocoon weight, shell weight, shell ratio, filament length, filament weight, effective rate of rearing and denier were apparently influenced by different mulberry varieties, however, there was no significant difference on the effect of different mulberry varieties on yield and economic traits. However, the yield and economic traits were significantly influenced by various rearing seasons. All the larval characters and economic traits of cocoon positively correlated with cocoon yield. It ascertained the impact of different larval parameters on yield and economic traits. In turn the various larval parameters were greatly influenced by the nutritive contents of different mulberry varieties. The quality of leaves assumes greater importance in growth, development and health of larvae which is ultimately reflected in yield and quality of cocoon. Ray et al. (1998) opined that S1 variety was quite suitable on account of its leaf yield as well as yield and quality of cocoon. The present study confirms the suitability of S1 and TR10 for commercial rearing in Uttar Pradesh during spring as well as autumn seasons.
The results of current study revealed that spring followed by autumn rearing seasons are suitable for bivoltine silkworm rearing in Uttar Pradesh. Ghosh et al. (2000) evaluated the performance of different mulberry varieties namely S1, TR10 and Kosen and observed that Kosen showed better response regarding larval and cocoon traits than TR10 and S1 for rearing during March-April. In accordance to the current study, the traits such as larval weight, cocoon weight, shell weight, silk ratio, ERR showed positive correlation with cocoon yield (Ghosh et al., 2000). Shah et al. (2007) studied the performance of silkworm raced fed with different mulberry varieties and found the influence of nutritive value of different mulberry varieties on size of full grown larvae and shell ratio. Shah et al. (2007) found significant difference among different races of B. mori as well as among different mulberry varieties. The results of current study revealed the significant effect of different seasons on yield and various traits, however, the effect of different mulberry varieties on above traits was not significant. Qader et al. (1992) observed the effect of different mulberry leaves on larval weight, cocoon weight, shell weight, shell percentage and length of filament and deduced that the nutritive content of mulberry leaves influenced the cocoon parameters. Seidavi et al. (2005) studied the effect of leaf quality on silkworm rearing and found profound effect on growth and development of larvae and cocoon production. The current study substantiates the observations of earlier reports on the effect of different mulberry varieties and seasons on larval characters and economic traits of cocoon.
CONCLUSION
The findings of present investigation deduce the influence of different mulberry varieties and rearing seasons on growth and cocoon traits of B. mori. The leaves of S1 mulberry variety fed larvae showed optimum growth and cocoon traits than the larvae fed with the leaves of other mulberry varieties. Further, spring rearing season showed better rearing performance than other seasons.
ACKNOWLEDGMENTS
The authors are grateful to the Dr. Venkatesh Kumar, R., Department of Applied Animal Science for his cooperation during the course of research. HK and MK are recipients of UGC Rajeev Gandhi National Fellowship.