Assessment of Cocoon and Egg Productivity in Selected Strains of Eri Silkworm through Consumption and Utilization Indices

Silkworm nutrition deals with the substances required for growth and production of proteinaceous silk fiber. The quantum of food consumption, utilization and conversion efficiency into body mater for production of silk mainly governed by the type of host/variety on which silkworm feeds and also strain/breed of silkworm. In the current investigation, selected eri silkworm (Samia cynthia ricini Boisduval) strains (Blue-Plain, Blue-Zebra, Blue Greenish-Plain, White-Plain, White-Zebra, Yellow-Plain, Yellow-Spotted and Yellow-Zebra) were reared using Local Pink castor leaf to assess the efficiency of ingestion, digestion and conversion of food for production of unit quantity of cocoon, shell and eggs. The study revealed that, among the eri silkworm strains White-Plain larvae exhibit significantly least ingesta and digesta to produce a gram of cocoon and shell and it was more with White-Zebra both on fresh and dry weight basis, respectively. The efficiency of conversion of ingested food and digested food into cocoon and shell both on fresh and dry weight basis was significantly better with White-Plain, least values were found with Blue-Plain and Yellow-Spotted for ingested food and digested food, respectively. In respect of leaf-egg ratio, White-Plain strain stood best over other strains with least being in Yellow-Zebra strain. The study inferred that, WhitePlain strain of eri silkworm performed better by recording higher efficiency in consumption and utilization of food for eri cocoon and egg production.


I. INTRODUCTION
Insects, like all other living organisms, require nutrients to survive, grow and reproduce. The nutrients (proteins, carbohydrates, fats, minerals and vitamins) in the food are digested and absorbed at different degrees depending on the silkworm breed, host plant variety and environmental conditions [1]. Almost all insects are host specific and select their most preferred food in order to extract the maximum benefit out of it, although most of them eat a great many varieties [2]. Different food plants may influence differently on food intake, efficiency of digestion and conversion of food to body biomass and finally on growth and development of insect [3]. Nutritional indices as well as growth and development of insect varied on different hosts. Generally varieties of the some species may exert variable effect on the relative survival of herbivore insect. The nutrition that an insect derives during the course of development plays a significant role in its commercial exploitation. The process of consumption, digestion and conversion efficiency in its broader sense underlines the physiological, behavioral, ecological and evolutionary aspects of insect life [4]. The suitability of host is determined through estimation of rate of ingestion, digestibility, conversion efficiency of food and growth rate of the animal [5].
In insects, consumption and utilization of food facilitate the understanding of the adaptability of insects. Female insects have generally lower consumption and higher conversion of food into body matter. Difference in food consumption and conversion into body matter has been understood among silkworm breeds quite early. The inferior growth expressed by the tropical races compared with the temperate races may be due to low consumption rate, nutritional inadequacy and a combination of both [3]. Among many factors attributed for silk production, the major one being the nutritional efficiency, as it has direct impact on the overall performance of economic characters of silkworm such as larval and cocoon weights, amount of silk production, pupation and reproductive traits [6]. Insects feed on a remarkably diverse number of organic substances. Even so, most species show a great degree of specificity in food selection and feeding. Competition and natural selection gradually drive and bind each insect species to a specialized food supply that it can utilize more efficiently than its competitors [7].
Food intake and conversion efficiency play a pivotal role in converting leaf substances into silk. Further, many factors determine the food intake, digestion and conversion efficiency of silkworm. Ingestion of equal amounts of leaves under different feeding and nutritional conditions impacts silkworm nutrition and silk synthesis [8]. The literature pertaining to the consumption, utilization and conversion of food per unit quantity for production of silk and egg is meager. In this regard, a research work has been undertaken to understand the relationship between food intake, digestion and food conversion efficiency vis-a-vis the production of cocoon and eggs in selected strains of eri silkworm.

Pre-rearing operations
Disinfection is an integral part of silkworm rearing, as eri silkworm has been reared under indoor conditions which lead to accumulation of disease causing germs in the rearing environment. In this regard, as a precautionary measure, rearing house was disinfected with 0.05% of Asthra @ 2.0 l/m 2 . Disease free layings of eight strains of eri silkworm (Blue-Plain, Blue-Zebra, Blue Greenish-Plain, White-Plain, White-Zebra, Yellow-Plain, Yellow-Spotted and Yellow-Zebra) was procured from the Central Sericultural Germplasm Resources Centre, Hosur and incubated at a temperature of 25±1 o C and relative humidity of 75±5%.

Rearing operations
The eggs (five DFLs) were black boxed on the day of pin head stage (8/9 day post-oviposition) to ensure uniform hatching. On the day of hatching, the eggs were exposed to diffused light at 8.00 AM for two hours. Rearing was conducted in the caged condition by feeding the leaves of Local Pink (Ricinus communis L.) castor leaf from the day of brushing till cocoon spinning. One hundred larvae in three replicates for each strain were maintained throughout the rearing period by replacing the missing larvae with the fresh ones for determination of consumption of utilization indices both on fresh and dry weight basis by gravimetric method [3,9,10,4]. Left over (uneaten) leaves and excreta were collected in each instar and dried in a hot air oven daily at about 100°C until a constant weight was reached. When the larvae stopped feeding in the final (fifth) instar, they were allowed to spin cocoons at 25 ± 2°C and 65 ± 5% RH. Cocoons were assessed after harvest for cocoon weight, shell weight and shell ratio and in addition fecundity was also recorded in different strains of eri silkmoths.

Determination of conversion efficiency of food 1. Ingesta per gram of cocoon (g) (Leaf -cocoon ratio):
The total ingesta required for the production of one gram of cocoon.

Ingesta per gram of shell (g) (Leaf -shell ratio):
The total ingesta required for the production of one gram of shell.

Digesta per gram of cocoon (g):
The total digesta required for the production of one gram of cocoon.

Digesta per gram of shell (g):
The total digesta required for the production of one gram of shell.

Efficiency of conversion of ingesta to cocoon (Leafcocoon conversion rate):
It was the expression of efficiency conversion of ingesta into cocoon, also referred to as the leaf-cocoon conversion rate.

Efficiency of conversion of ingesta to shell (Leaf -shell conversion rate):
This was the expression efficiency of conversion of ingesta into shell. It is also referred to as the leaf-shell conversion rate and is the ultimate index to evaluate superiority of strain for nutritional efficiency.

Efficiency of conversion of digesta to cocoon:
The expression of efficiency of conversion of digesta into cocoon.

Efficiency of conversion of digesta to shell:
The expression of efficiency of conversion of digesta into shell.

Leafegg ratio:
The expression of efficiency of conversion of food consumed to number of eggs laid by the moth.

Statistical analysis of the data
The data obtained in the current investigation was subjected to one-way ANOVA for test of significance (p ≤ 0.05 and p ≤ 0.01) [11] through SPSS statistical package (ver. 21.0).

III. RESULTS
The results pertaining to consumption, digestion and conversation efficiency of food into cocoon, shell and egg production in selected strains of eri silkworm are illustrated in the form of graphs and are explained in the following pages: Ingesta/g of cocoon Quantity of food required for the production of one gram of cocoon could vary considerably among the strains of eri silkworm with least amount of food was registered with White-Plain strain (8.353 and 4.301g) over other strains namely Blue-Zebra (8.553 and 4.546g), Yellow-Plain (8.727 and 4.699g), Yellow-Spotted (8.871 and 5.204g), Blue-Plain (9.039 and 5.443g), Yellow-Zebra (9.066 and 5.412g) and Blue Greenish-Plain (9.108 and 5.428g) both on fresh and dry weight basis. However, more amount of food was consumed by White-Zebra strain (9.291 and 5.563g) (Fig.  1).

Ingesta/g of shell
White-Plain strain of eri silkworm took significantly less quantity of food (53.32 and 24.32g) to produce one gram of shell and the strains Blue-Zebra (54.85 and 25.89g), Yellow-Plain (56.32 and 27.26g), Yellow-Spotted (60.45 and 34.72g), Yellow-Zebra (60.46 and 33.88g), Blue Greenish-Plain (60.91 and 34.26g), White-Zebra (61.36 and 33.77g) and Blue-Plain (62.13 and 37.64g) stood next in the order with latter strain took more amount of food both on fresh and dry weight basis, respectively (Fig. 2).

Leaf -egg ratio
Leaf -egg ratio too differed significantly among the eight strains of eri silkworm with higher value being in White-Plain strain (1: 11.96) followed by Blue Greenish-Plain

IV. DISCUSSION
The ingestion, digestion and conversion efficiency of food in commercial insect like silkworm is of greater significance as it indicates the quantum of food uptake and their utilization for the production of silk/eggs. Variations were evident among the strains of eri silkworm in utilization of castor leaf for the production of unit quantity of eri cocoon and eggs. Conversion efficiency of ingesta to larva, cocoon and shell were highest and the grams of ingesta necessary to produce one gram of cocoon and silk shell were the lowest, indicating that the larval enzymatic apparatus were used to convert nutrients to body matter and more specifically to silk proteins [6].
The variation in leaf-cocoon and egg ratio was evident among the castor genotypes when their leaf was offered as food [12,13]. Similarly, higher LCR on Aruna castor both for food offered and consumed, followed by SH-41 and it was lower on Local genotype [13]. The trend was similar on dry weight basis both for food offered and consumed. Further, variation in leaf -cocoon and silk conversion rate of eri silkworm were recorded when different castor genotypes were offered to eri silkworms [14].
Eri larvae fed on the leaves of DCH-519 castor hybrid recorded least ingesta and digesta to produce a gram of cocoon and shell. The efficiency of conversion of ingested and digested food into cocoon and shell on both fresh and dry weight basis was similar. With regard to leaf-laying ratio, eri moths resulting from the larvae offered with local green variety showed distinct superiority when compared with other castor hybrids / varieties. However, it was greater with DCH-519 on fresh weight and 48-1 variety on dry weight basis [17].
The current results mainly focuses on the screening of selected strains of eri silkworm for cocoon and egg production through their consumptive and conversion efficiency pattern and that of previous researchers were mainly confined to screening of castor genotypes/varieties/hybrids through conversion efficacy of food in eri silkworm. In both the types of investigation, greater amount of variation could be observed in ingestion and conversion efficiencies of food for the production of cocoon and eggs.

V. CONCLUSION AND RECOMMENDATION
The findings of the current study revealed that, White-Plain strain of eri larvae has consumed significantly less quantum of food to produce a gram of cocoon (8. 11.46) for production of one gram of cocoon and shell. Even for leaf-egg ratio, White-Plain strain showed superiority over other strains (1: 11.96 and 1: 64.42) both on fresh and dry weight basis, respectively. From the foregoing account, it is pertinent that food conversion efficiency showed considerable variation among selected strains of eri silkworms with superior performance by White-Plain for the production of cocoon, shell and eggs. Thus, White-Plain strain could be conveniently used for rearing purpose in order to harvest better cocoon crop.

VI. ACKNOWLEDGMENT
First author is thankful to the University Grants Commission, New Delhi, for financial assistance through Rajiv Gandhi National Fellowship for Ph.D. programme and both the authors are thankful to the Chairman, Department of Studies in Sericulture Science, University of Mysore, Mysuru, for providing the necessary facilities to carry out this study.