The silkworm, Bombyx mori L., most important source of natural silk has played an important role in the life of man ever since its discovery by the Chinese some 4000 years ago. It is also traditionally associated with the socio-economic life of many countries. Even today it enjoys the supremacy over all other fabrics and it is appropriately called “Queen of Textiles” for its unparallel lustre, delicacy, elasticity, crispness and artistic appearance.
According to Krishnaswami et al. (1973), the mulberry silkworm, Bombyx mori L., is the greatest source of natural silk. Generally we have two type of silkworm, viz. mulberry and non-mulberry silkworm. The silkworm larva passes trough five different stages called instars. The final or 5th instars spin to form cocoon within which the larva undergoes pupation. The cocoon shell is the source of the natural silk, which is the driest form of that secretion. Basically there are two proteins which go to form the silk fibre viz., “fibroin” which constitutes the core of the fibre and “sericin” a waxy substance which encases the fibroin.
The origin of sericulture, silk production and weaving is ancient and clouded with legend. They are closely interlinked with the emergence of China as one of the great civilizations. Chinese mythology ascribes the invention of this art to the mythical Empress Shih Xiling, the consort of the no less mythical “Yellow Emperor”, Shi Huang Ti, one of the fathers of Chinese civilization. Ancient Chinese annals give an exact date for Empress Xiling’s discovery of silkworm rearing, namely 2640 B.C. It is said that one day 14-year-old queen was enjoying a tea ceremony in her palace garden with her friends and maids under a mulberry tree, suddenly a golden coloured cocoon dropped in her teacup. When she tried to remove the cocoon from the teacup, an end of the filament comes out of the cocoon and it was a continuous one. Then she collected some more cocoons, carried them to her palace, preserved them till emergence and reared in the next generation and later invented the method of reeling (Sarkar, 1958).
In reality, sericulture evolved through a gradual process and by the middle of the third millennium B.C. silk was in human use. China mastered this art well before any other civilization, and for nearly 5000 years China was believed to have been the sole possessor of the techniques of silk production. For many centuries the Chinese zealously guarded the source and methods of production of silk for about 3000 years. According to legend, in about 140 B.C., sericulture as well as silk spread overland form China to India (Krishnaswami et al., 1973).
Some sources believed that the first country after China to learn the secret of silk production was Korea where Chinese immigrant started sericulture in about 1200 B.C. The industry later spread to Japan and other countries. The transfer of silkworm and silk is also flooded fables where many stories have taught from mouth to mouth. The silk industry flourished in our sub-continent in the beginning of the 17th century. At that time Bengal was known as the store house of silk (Sarkar, 1958). A large quantity of silk and silk fabrics was exported from Bengal to various parts of Mughal Empires, to the neighbouring countries and to Europe.
On the other hand, others thought that the Hindus discovered it in the sub-Himalayan regions. The history of which was older than the Chinese (Sarkar, 1958). According to them B. mori was first domesticated at the foothills of Himalayas. Latter on silkworm rearing began to spread and established on the bank to the rivers Brahmaputra and the Ganges (Krishnaswami, et al., 1973).
As the demand of silk expanded the industry set on spreading. The silk industry flourished in our territory in the beginning of the seventeenth century. At that time Bengal was known as the storehouse of silk (Sarkar, 1958). A large quantity of silk and silk fabrics was exported from Bengal to various parts of the Mughal Empire, to the neighbouring countries and to Europe.
The lending silk producing countries are China, Japan, India, South Korea, U.S.S.R and Brazil and that is to say 97% of raw silk of the world is produced in these countries. During the middle of the eighteenth century as many as six varieties of raw silk and silk clothes were exported to Europe and to the markets of the West Asia including Basra, Mecca, Jedda, Pegu and Malacca. Silkworm is reared in Bangladesh particularly in the district of Rajshahi and Nawabganj where silkworm is cultivated extensively as a major source of earning for the rural people.
Sericulture is well known as a highly employment and a low capital intensive activity ideally suited for the transfer of wealth from richer sections of the society to poorer sections since silk is consumed mostly by the affluent people and the money so spent on purchase of silk is distributed among the Seri culturists, reelers, twisters, weavers and traders. Persons from the age of 8 to 80 may engage themselves in the activity of silk production and earn their livelihood.
Mulberry silkworm is variously classified according to the difference in their physiological and ecological
characteristic. According to the voltinism they are classified into univoltine, bivoltine and multivotine or polyvotine, while on the basis of moulting, they are classified into trimoulter, teramoulter, pentamoulter and rarely bio-moulter as well as hexamoulter. Further more, they are grouped into Japanese race, Chinese race, European race and Tropical race on the basis of place of origin and geographical distribution.
Very little attention has been given for the improvement of mulberry silkworm in Bangladesh. Silkworm, B. mori suffered very much by the environmental factors in which they are reared. Farmers in Bangladesh adopt the traditional method of silkworm rearing. Due to poor management during rearing the worms are attacked by different disease during summer and rainy rearing season the condition is more severe. The sericulture research workers have recommended of scientific technology of silkworm rearing.
Collection of varietal materials of B. mori L. and their evaluation under different environmental condition is a prerequisite for the improvement of the commercial characters. All of the Seri culturally advanced countries maintain different varieties of silkworm to serve the above objective. The different varieties of Bombyx mori L. reared by the farmers in Bangladesh are mainly indigenous in origin. Therefore, these varieties are highly resistance but shows very poor silk yielding performance with respect to both the qualitative and quantitative traits. To overcome these difficulties and to boost up silk production, the sericulture authority of Bangladesh introduced exotic and developed high yielding multivoltine varieties of silkworm for commercial rearing (Rahman et al., 1978; Islam, 1980; Salam et al., 1980; Rahman, 1983; Rahman et al., 1984).
Rearing of different varieties of silkworm is dependent on the environmental conditions like temperature, humidity, quality of mulberry leaves and population density in the rearing bed. Most of the varieties including indigenous, developed and exotic showed varied response to different environmental conditions (Sengupta, 1969; Sidhu et al., 1969; Petkov and Yolov, 1979; Biswas and Rahman, 1987; Rahman, 1995).
Correlation studies were conducted under the influence of different environment by Hayes (1922), Immer and Ausemus (1931) and Kelly et al. (1932). In breeding work, knowledge of correlation between different characters is of paramount importance. Hazel (1943) first demonstrated the estimates of genetic correlation. The genetic correlation might be due to three different causes (i) Pleiotrophy (ii) Linkage and (iii) Heterozygosity (Reeve 1953; Tallis, 1959; Mode and Robinson, 1959; Abe, 1969; Grossman 1970; Hammond and Nicholas 1972).
A series of studies on correlations between various traits were available in many insect groups. Phenotypic and genotypic correlations of egg production and longevity in wild out-bred stocks of D. melanogaster and D. simulaus had been worked out by Tantawy and EL-Helw (1966), Tantawy and Rakha (1964), Rose and Worth (1981), Giesel et al. (1982), Rose (1984), Roff and Mousseau (1987). Collins et al. (1984) worked on the correlation for several characters in the honey bee. In non-mulberry silkworms, significant positive correlation between pupal weight and fecundity were shown by Miller et al. (1982) in A. polyphemus by Singh and Prasad (1987) and in P. ricini by Kotikal et al. (1989). The correlation regression between pupal weight and fecundity in muga silkworm were studied by Barah and Sengupta (1991).
In A. mylitta, Sen et al. (1976) reported highly positive and significant correlation between cocoon weight and shell weight (r=0.86), Siddiqui et al. (1985) reported positive and significant correlation between shell weight and cocoon weight ; shell weight and larval weight ; shell weight and moth weight, both at phenotypic and genotypic level. Siddiqui et al. (1989) reported positive and significant correlation between fecundity and cocoon weight, fecundity and silk yield, cocoon weight and shell weight and cocoon weight at phenotypic level and genotypic level in a half diallel populations of A. mylitta.
References are also available on correlation studies between various quantitative characters in B. mori L. Legay (1961) reported correlation between various quantitative characters. Nataraj and Ganesh (1969) worked on the inter relationship between cocoon weight and shell weight in different breeds. Samachari and Krishnaswami (1980) studied the correlation between cocoon weight and pupal weight, shell weight and egg weight. Petkov (1981a) observed negative phenotypic correlation between cocoon weight and silk richness. Petkov (1981b) reported high and positive correlation between cocoon and shell weight (r=0.679 and 0.681). Oshiki and Sato (1986) showed the relationship between egg size and manifestation of quantitative characters, while Long and Petkov (1987) studied the correlations among different quantitative characters. On the other hand, Ozdzenska and Kremky (1987) observed positive and significant genetic correlation between
cocoon weight and silk richness. Rajanna and Sreeama (1990) studied the cocoon productivity traits such as cocoon weight, pupal weight shell weight and shell percentage on the basis of 5th instar 5th day larval weight in tropical multivoltine and bivoltine races. Jayaswal et
al. (1991) and Shaheen et al. (1992) worked on the correlation between female pupal weight and fecundity of B. mori. Giridhar et al. (1995) inferred from the study of correlation that cocoon weight, shell weight, filament length and denier could be considered as selection criteria for improvement of cocoon yield.
It is evident from the above review that there is very meagre information available on correlation among different quantitative traits in respect of silkworm, B. mori varieties of Bangladesh. Therefore, an attempt was made to study the relationship exists among different cocoon and pupal characters of four verities of B. mori L.