Effect of different growth regulators on biology and economic traits of silkworm (Bombyx mori L.)

Introduction

Sericulture, an ancient industry in India and Asia, is a labour-intensive and commercially viable economic activity that falls under the cottage and small-scale sector. It involves the production of silk, which serves as the final product. This industry is particularly suitable for rural populations engaged in agriculture, as well as entrepreneurs and artisans, due to its low investment requirement and potential for relatively higher returns. It plays a crucial role in generating income and employment opportunities for the rural people, especially farmers with small land holdings and vulnerable sections of the society. Maharashtra, despite being a nontraditional sericulture state, produces Mulberry and Tasar silk. Notably, the state excels in bivoltine sericulture, accounting for 98% of its production, and ranks first among nontraditional states in India, making it a promising region for silk manufacturing (Hiware 2016) [7] . Sericulture plays a significant role in both the global and rural economies. China holds the distinction of being the largest producer of silk worldwide, while India follows closely as the second highest producer of raw silk and a prominent consumer of pure silk on the global stage. The southern state of Karnataka held the leading position as the producer of raw silk during that period (Anonymous, 2022) [2] . The Bombyx mori L., commonly known as the Mulberry silkworm, is a monophagous insect that exclusively feeds on the leaves of the Mulberry tree (Morus spp.) for its nutritional needs and produces natural protein-based silk. While the rate of food consumption increases with each larval stage, the conversion of food into body weight decreases (Bongale et al., 1997) [4] . Silkworm growth regulators (SGRs) such as juvenile and moulting hormones and their analogs (juvenoids and ecdysoids) when it is used judiciously have been found to be useful in the sericulture industry. The silkworm, Bombyx mori L. is known to exhibit a stimulatory effect on the administration of exogenous JH analogs in minute quantities which leads to enhancement in commercial traits such as cocoon weight, shell weight, and silk filament length (Akai et al. 1985; Mamatha et al; 2006) [1,8] This principle has been exploited in the sericulture industry in which the silkworm rearing and the production of cocoons can be managed effectively or manipulated positively depending on the requirement by administering bioactive compounds mimicking the circulating hormone regardless of the source. JH analogs or mimics have been a celebrated option for Sericulturists as these can control silk gland function and indirectly cause an increase in silk production. Earlier it was made clear that exogenous JH delays the silkworm larva maturation and the increase in silk yield was mainly because of this phenomenon (Akai et al. 1985) [1] . The non-uniform maturation is due to the fact that the hormone does not reach the optimum level required for spinning in all the worms simultaneously (Trivedy et al., 2006) [16] .

Materials and Methods

Investigation was carried out to know the "effect of growth regulators on growth and economic traits of bivoltine silkworm Bombyx mori L." The experiment was conducted at Sericulture Research Unit, VNMKV, Parbhani during the month July-August 2022. Methods involved and materials used are described below.

Materials

Disease free layings of bivoltine mulberry silkworm (Bombyx mori L.) race FC1xFC2 were fed the leaves of mulberry variety Victory-1 in the present investigation. The growth regulators were made available from different silk producing parts of the country. Details of material used and technique adopted are described under suitable heading.

Methods of recording observation

Observations were recorded on following parameters on ten randomly selected larvae / cocoons of silkworm hybrids from each replication of each treatment.

The mean number of larvae hatched out of the total eggs laid by the female moth per hybrid was recorded and expressed in percentage.

The total larval period measured by recording the period from hatching to the onset of spinning.

The maximum larval weight was recorded by taking the weight of matured larvae just before the onset of spinning.

The cocoon was cut at one end and shell weight will recorded after removing pupae.

Weight of 10 shell Single shell weight (g) = 10

Single shell weight Shell Ratio (%) =

x 100 Single cocoon weight

The total pupal period was measured by recording period from spinning to the emergence of moth.

Treatment details Treatment 1

Application of serimore at the rate of 1.5 ml/l of water Treatment 2

Application of serimore at the rate of 2.5 ml/l of water Treatment 3

Application of serimore at the rate of 3.5 ml/l of water Treatment 4

Application of sampoorna at the rate of 2.5 ml/l of water Treatment 5

Application of sampoorna at the rate of 5 ml/l of water Treatment 6

Application of sampoorna at the rate of 7.5 ml/l of water Treatment 7

Application of folic acid at the rate of 100ppm Treatment 8

Application of folic acid at the rate of 150ppm Treatment 9

Application of folic acid at the rate of 200ppm Treatment 10 Control

Results and Discussion

The data on effect of growth regulators on fifth instar larval duration (days) of bivoltine silkworm (B. mori L.) presented in Table 1 revealed that the shortest 5 th instar larval duration (5.678 days, 6.595 days) was recorded by treatment of Sampoorna at the rate of 7.5 ml per litre at 72 and 96 hours treatment and at 48 and 120 hours treatment shortest larval duration recorded (7.113 days) by control. The longest 5 th instar larval duration (8.77,7.837, 7.96, 8.795 days) was recorded by treatment of Serimore at the rate of 3.5 ml per litre of water. However at 96 hour treatment it was at par with the treatment of Serimore at 1.5 and 2.5 ml/l of water (7.92 and 7.933 days). The litreature pertaining to the effect of Serimore on the larval duration of bivoltine silkworm was scanty. However, several scientists reported the use of growth regulators enhanced the larval period of silkworm (Trivedy et al., 1997) [15] . Chandrakal et al. (2001) [5] reported that the administration of phytoecdysteroids during the fifth instar of the silkworm, Bombyx mori, particularly at the sample spinning stage, yielded the most beneficial results. This treatment led to several advantages, including the induction of uniform spinning, reduction in larval and molting periods, and increased uniformity in the mounting process, Nair et al.

(2005) [10] investigated the varying response of the silkworm, Bombyx mori L., to Phytoecdysteroid depending on the timing of administration. The researchers found that when the larvae were treated with Phytoecdysteroid at 72 hours, there were adverse effects on the economic traits of the silkworm. However, they observed a significant reduction in larval duration as a result of the treatment. The above results are in agreement with the findings of the above workers The lowest 5 th instar larval weight (g) recorded (45.13 g, 44.47 g and 47.52 g) in Sampoorna 7.5 ml/l of water at 48h, 72 h and 96 h. However, at 48h it was at par with Sampoorna 5 ml/l of water (47.25 g) and at 96 h it was at par with all other sampoorna treatments and control (47.99 g). At 120 h lowest 5 th instar larval weight (g) recorded by control which was at par with all other treatments except Serimore at the rate 2.5 ml and 3.5 ml per litre of water (50.25 and 51.75 g). Nair et al., (2004) [11] reported that there is significant increase in larval weight of silkworm with the use of juvenoids. Plantevin et al., (1991) [12] also reported that growth regulator treated larvae were heavier than untreated silkworm larvae. Rai et al. (2002) [13] investigated the effects of folic acid administration to fifth instar silkworm larvae. They found that larval weight increased and duration decreased in all treatment groups compared to the control. The result in the present investigation are in agreement with above scientists.

The data on the effect of growth regulators on the pupal duration (days) of bivoltine silkworm (Bombyx mori L.) are presented in Table 02 revealed that shorter pupal duration (11 and10.75 days) in Sampoorna 7.5 ml/l of water at,96 h and 120 h treatment which was at par with control(11.25 days) and at 48h and 72 h shortest pupal duration (11.25 and 11.25 days) was observed in control.

The significantly longer pupal duration (15.25, 15.5, 15.75 and 15.75 days) was recorded at 48h, 72 h, 96 h and 120 h treatment timings in 5 th instar with Serimore at the rate 3.5 ml per litre water. However, at 120 h treatment in 5 th instar significantly longer pupal duration (15.75) was observed with treatment of Serimore at the rate of 1.5 ml/l of water and 25 ml/l of water. The literature on the effect of growth regulators on the pupal duration of silkworm is scanty so results could not be discussed.

The data on effect of growth regulators on the single shell weight (g) of bivoltine silkworm (B. mori L.) are presented in Table 3, The data revealed the following details. At 48 hour treatment the significantly highest Shell weight recorded (0.514 g) by the treatment of Serimore at 3.5 ml per litre and lowest shell weight recorded (0.337 g) by treatment of Sampoorna at the rate of 7.5 ml per litre. At 72 h treatment, the significantly highest shell weight was recorded (0.474 g) by treatment of Serimore at 3.5 ml per litre which was at par with treatment of Folic acid200ppm and lowest shell weight recorded (0.349 g) by treatment of Sampoorna at5 ml/l of water. At 96 h treatment, the significantly highest shell weight was recorded (0.478 g) by treatment of Serimore at 3.5 ml per litre which was at par with treatment of Sampoorna7.5 ml/l of water and lowest shell weight recorded (0.377 g) by control.

At 120 h treatment, the significantly highest shell weight was recorded (0.477 g) by treatment of Sampoorna7.5 ml/l of water which was at par with treatment of Sampoorna5 ml/l of water and lowest shell weight recorded (0.377 g) by control. Dutta et al., (2013) [6] and Nair et al. (2011) [14] reported increase in the shell weight with the application of juvenile hormones. Rai et al. (2002) [13] found the effects of folic acid administration to fifth instar silkworm larvae, it increased shell weight by 21.9%. The researchers suggested that folic acid facilitated nucleic acid synthesis in silk gland cells, increasing the silk content in the shell. Nair et al. (2005) [10] investigated the varying response of the silkworm, Bombyx mori L., to Phytoecdysteroid depending on the timing of administration. The researchers found that when the larvae were treated with Phytoecdysteroid at 72 hours, there were adverse effects on the economic traits of the silkworm. The result in the present investigation are in agreement with the research findings of the above workers.

Effect of growth regulators on the cocoon shell ratio of bivoltine silkworm (B. mori L.) (Percent).

The data on effect of growth regulators on the single shell ratio (%) of bivoltine silkworm (B. mori L.) are presented in Bharathi and Miao (1999) [3] and Sashindran et al., (2011) [14] observed that there is improvement in cocoon shell ratio with the treatment of growth regulators. Nair et al. (2005) [10] investigated the varying response of the silkworm, Bombyx mori L., to Phytoecdysteroid depending on the timing of administration. The researchers found that when the larvae were treated with Phytoecdysteroid at 72 hours, there were adverse effects on the economic traits of the silkworm. The findings of present investigation are in line with the results obtained by above worker.

Summary and Conclusion

To use Serimore at the rate 3.5 ml per liter water and Folic acid 200 ppm at the onset of fifth instar larvae, and Sampoorna at the rate of 7.5 ml/l of water before spinning stage of bivoltine silkworm in order to improve the growth and economic traits of bivoltine silkworm (Bombyx mori L.).

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