Winter Energy Budget of Larva of Indian Tropical Tasar Silk Insect Antheraea mylitta Drury Living in the Host Plant Shorea robusta

E-mail: amulyakdash@gmail.com Abstract: Energy budget of larva of Antheraea mylitta Drury living in Shorea robusta Gaertn host plant was prepared in winter season. During fifth instar the rate of food energy consumption, absorption, body growth, silk gland growth and respiration increased suddenly in comparison to other instars. The amount of consumption, absorption, body tissue growth and silk gland growth during fifth instar only was about 82, 83, 82 and 97% respectively of the total amount used in the entire larval period. At fifth instar the female larva showed significantly higher overall efficiencies than male larva. There was a gradual increase in amount of energy stored per mg dry body weight from first to fifth instar. Female larva showed higher value than male larva. The absorption and growth efficiency was lowest in 2nd instar. So it is the most vulnerable instar needing more care during rearing. Highest allround efficiency was observed in case of fifth instar larva. So optimum care during feeding and rearing should be given to fifth instar larva in order to maximize silk productivity.


Introduction
Antheraea mylitta is a polyphagous insect and its larva feeds on Sal host plant which is considered to be a primary food plant on the basis of cocoon crop performances (Dash et al., 1992). The lepidopteran larva act as energy transformer and shows certain level of efficiency in particular ecological conditions. So study of energetic parameters of A. mylitta is important to trace out clues for optimizing silk production efficiency of larva since it is the only food energy consuming stage of its life cycle. So a thorough knowledge of trend of energy budget of larva is essential for knowing energy allocation for body growth and silk productivity.
Some literatures are available on ecological energetics of some species of silk worm like Bombyx mori (Hiratsuka, 1920;Horie and Watanabe, 1985), Philosamia ricini (Poonia, 1978;Reddy and Alfred, 1979), Antheraea proylei Jolly (Rana et al., 1987) and A. mylitta raised on secondary host plant Ziziphus jujuba Gaertn in rainy season (Dash, 2013). Pattanayak and Dash (2000a;2000b) reported pupal energy budget on some food plants. Dash et al. (2012) reported the effect of altitude on pupa and Jena et al. (2015) the effect of altitude on growth of female larva of A. mylitta during rainy season. However no concrete literature is available on energy budget of A. mylitta feeding on S. robusta during winter season although it is crucial to know the energy budgeting trend of insect and the costing aspect of insect for different activities during the ensuing diapauses period beginning from January to June of the year. So this investigation was carried out.

Materials and Methods
The present study was conducted at State Tasar Research Farm, Durgapur in the District of Mayurbhanj of Odisha state during winter (November-December) season. For energy budget preparation, the methodology suggested by Petrusewicz and Macfadyen (1970) was followed. It is represented as C=P+R+F+U where 'C' is food consumption, 'P' is production, 'R' is energy loss as heat due to metabolism, 'F' is energy loss as faecal matter and 'U' is energy loss as nitrogenous excretory products. A. mylitta excretes mainly uric acid combined with faeces egested as solid pellets (Waldbauer, 1968). Here F+U indicate Faeces + Nitrogenous matter.
In this study healthy identical Sal plants (S. robusta) were selected at random and a huge population of freshly hatched hatchlings of same day was released in the Sal plants to be used as reserve batch for the experiment. The fresh and dry weight of consumed leaves, egested faecal pellets, gained body tissue, cast off exuviae and dissected out silk glands were recorded along with measurement of amount of oxygen consumed during each instar.
At the beginning of each instar (except the fifth), an experimental population of two hundred healthy' larvae were selected randomly from the large reserve batch and were reared on S. robusta during winter rearing season. During each instar the initial (just after hatching for the first instar and after moulting for the remainder) and the final (just before moulting when the gut was empty) fresh and dry (oven drying; Southwood, 1966) body weight of twenty larvae was measured by bomb calorimetry. For the fifth instar larvae the above method was followed separately with each sex. Another twenty larvae at hatchling stage were chosen at random from the experimental batch and were allowed to feed on twenty different branches of S. robust having sufficient leaves for the worms to spend their whole larval life. The area of all leaves on each branch was determined by tracing the margin on a graph paper and each leaf was marked serially. The worms were prevented from escaping by encircling the base of experimental branch with a plastic cone. The twenty larvae were kept under continuous observation. The fresh weight of the consumed leaf was determined by taking an identical leaf matching with traced area and calculating the amount of consumption by subtracting the weight of entire identical leaf collected from another plant of same age. The leaf consumption of fifth instar larva was measured separately for each sex since markings of sexual dimorphism appear at this instar. The leaf collected at each instar were oven dried (Southwood, 1966), powdered, mixed thoroughly and subjected to bomb calorimetry to know the amount of food energy consumed during each instar.
The faecal pellets egested by the above experimental larvae were collected every day for each instar by tying a polythene sheet below the branch. The fresh and dry weight of the pellets collected for each instar was determined. Then they were powdered, thoroughly mixed and twenty samples were subjected to bomb calorimetry to know the energy lost with faeces during each instar. The absorbed energy was calculated by subtracting 'F+U' from 'C'. For measuring the oxygen consumption, twenty healthy larvae at each instar were collected from the experimental batch every day and each larva was subjected to respirometry for half an hour during the early morning, noon, evening, night and mid-night. The daily rate of oxygen consumption per hour was calculated from the average value so obtained. Thus the total amount of oxygen consumption throughout each instar was estimated. For the fifth instar, the amount of oxygen consumption was measured separately for each sex. The oxyjoulerific conversion (19.64 J/ml; Brown, 1964) of consumed oxygen was made to know the energy lost in respiration during each instar.
The exuviae casted off after moulting by the twenty feeding larvae were collected at each instar. The fresh and dry weight of the collected exuviae was measured. Then these were powdered, thoroughly mixed and twenty samples were subjected to bomb calorimetry to know the energy lost in form of exuviae at each instar. For knowing silk budget, twenty larvae at beginning and end of each instar were collected and their silk glands were dissected out. The initial as well as final fresh and dry weight (oven drying, Southwood, 1966) of removed silk glands were recorded. The energy content of silk gland of each instar was measured by bomb calorimetry. The efficiency of absorption, body growth, silk gland growth of each instar larva was calculated as indiated in Table 3. The energy budgeting per mg dry body weight (J/mg) at each instar was made as shown in Table 2. The experiment was repeated for five years during winter season. Statistical analysis of data was made following Sokal and Rohlf (1969).

Results
The amount of energy consumed, absorbed and allocated for body and silk gland growth increased gradually from first to fourth instar and rapidly during fifth instar (Table 1 and Fig. 3). Analysis of ANOVA test indicated significant difference of consumption, absorption, body growth and silk gland growth during fifth instar which was 82, 83, 82 and 97% of the total amount utilized for the entire larval period respectively. The increase was nearly five times of the recorded data for fourth instar for all above energy budget parameters except silk gland budget which was exceptionally forty times more. The t-test indicated significantly (P<0.01) higher consumption, absorption, body growth and silk gland growth in female larva than male larva. The loss of energy in faeces and respiration increased gradually from first to fourth instar and suddenly during fifth instar (Table 1 and Fig. 3). ANOVA test indicated significant difference in amount of energy loss in faeces and respiration among different instars. The t-test also showed significantly (P<0.01) higher allocation of energy for metabolism in case of female larva than the male larva. The loss of energy in egesta and metabolism during the final instar was about 82 and 83% respectively of the entire larval period. Loss of energy in feaces and respiration suddenly increased towards fifth instar and it was nearly five to six times more than fourth instar. The absorption efficiency (100 A/C), gross growth efficiency (100 P/C), net growth efficiency (100 P/A), gross silk gland growth efficiency (100 S/C) and net silk gland growth efficiency (100 S/A) were usually found lowest in second instar larva among all the instars (Table 3 and Fig. 5). The absorption and growth efficiency of body as well as silk gland increased significantly (P<0.01) from third to fourth instar. The t-test indicated significantly (P<0.01) higher all round efficiency of female than the male except silk gland gross and net growth efficiency which was higher in male than female larva. During fifth instar about 58 to 60% of the total absorbed energy was allocated for body growth and about 20 and 18% for silk gland growth by male and female larva respectively.
The mean allocation of energy per milligram dry body weight (J/mg) is given in Table 2 and Fig. 4. In general, the second instar showed highest values for all the budget parameters except the case of energy utilized for body growth being highest in fifth instar. Absorption value was highest in second instar and lowest in first instar. The 'P' value was lowest in first instar followed by second, third, fourth and fifth instar. The energy allocated for growth in fifth instar was significantly higher than other instars. The t-test indicated significantly higher energy level per mg dry body weight of the male larva than the female larva for all budget parameters except growth. The 'P' value of female was significantly higher than male. The energy allocated for growth per mg dry body tissue increased from the first to fifth instar. The energy flow budget of male and female larva is presented in Fig. 1 and 2 respectively.
The energy budget allocated for pupal life was 49.81 and 65.32 KJ in male and female larva respectively. The budgetary energy saving for pupal life was more than 65 and 71% of the total body tissue energy budget of male and female larva respectively. This is meant for budgetary allocation during diapauses life (six months from January to June) metabolism and post diapauses reproductive activities. The energy flow budget of both male and female larva is given in Fig. 1 and 2.

Discussion
In the present study it was observed that only fifth instar larva consumed more than 82% of the total food energy consumption throughout the entire larval life of A. mylitta. Waldbauer (1968) reported that lepidopterans consume more than 70% of total larval consumption during last instar only.
In A. mylitta an increasing trend of ingestion, absorption, body tissue production and food oxidation through respiration was found with advance of instars. Similar trends were recorded in P. ricini (Reddy and Alfred, 1979) and A. mylitta reared on Z. jujuba secondary food plant during rainy season (Dash, 2013). Both sexes of fifth instar larva of A. mylitta allocated more than 54% of total absorbed energy for body tissue. But 40.28% of total absorbed energy was allocated for body tissue by A. proylei (Rana et al., 1987). Higher metabolic rate was noticed in larva of A. mylitta in later stage. Earlier it was recorded in A. proylei (Rana et al., 1987).
The consumption of food energy during fourth instar was about 12% of the total food energy consumed throughout its larval life, but in B. mori it was approximately 9% (Horie and Watanabe, 1985). The efficiency of utilization of energy for the body growth during fifth instar of male and female A. mylitta larvae was 58 and 60% respectively. But in B. mori the efficiency of utilization of energy to whole body by the fifth instar male and female larvae ranged between 46.4-65.5 and 51.7-61.8% respectively (Horie and Watanabe, 1985). The net growth efficiency (100 P/A) of A. mylitta larva ranged between 51-60%. In Hyalophora cecropia larva it was found to be 53.1% (Schroeder, 1971). The gross growth efficiency (100P/C) of A. mylitta ranged between 5-9% during different instars whereas in H. cecropia it was 19.4% (Schroeder, 1971).
At fifth instar stage the male and female larva of A mylitta on average, consumed 276.58 and 237.97 joules of food energy per mg dry body weight respectively. Similar studies on male and female larva of B. mori showed consumption of 26.00 and 25.54 joules of food energy per mg dry body weight respectively (Horie and Watanabe, 1985). The male A. mylitta larva was found to show higher consumption of energy per mg dry body weight than female larva. Horie and Watanabe (1985) also found this trend in larva of B. mori. During fifth instar of A. mylitta the average absorption and metabolic loss of energy per mg dry body weight of male and female larvae was 38.52 and 37.77 and 15.90 and 14.73 J/mg respectively. It was observed that the average absorption and metabolic consumption per mg dry body weight of male and female larva of B.mori in fifth instar were 12.54 J/mg and 4.37 J/mg respectively (Horie and Watanabe, 1985). So from above data it appears that the amount of absorption and metabolic cost per mg dry body weight of A. mylitta larva is much higher than B. mori larva.
An increasing trend of stored energy per mg dry body weight was observed in successive instars of A. mylitta larva. Hiratsuka (1920) reported a similar trend in B. mori. He stated that it might be due to an increase in relative amount of fat deposition in successive instars. It was observed that the fifth instar of A. mylitta larva is very important of all instars and especially for silk production. Because the energy utilized for silk production during fifth instar was about 97% of the total amount gathered over the entire larval period. Similar trend was observed earlier in case of A.mylitta feeding on Z. jujuba (Dash, 2013) and also in B. mori (Horie and Watanabe, 1985) and P. ricini (Joshi and Mishra, 1979). The female larva of A. mylitta utilized more energy for total silk synthesis than male larva. The gross and net silk gland growth efficiencies of A. mylitta larva at fifth instar ranged between 3-4 and 20-24% respectively and it was also higher in male than female larva. Horie and Watanabe (1985) reported higher net and gross silk gland growth efficiency in male larva of B. mori than female larva and its range was within 23-27%. The male and female A. mylitta larva allocated about 34 and 29% of the accumulated body energy for silk gland respectively. The allocation of absorbed energy for silk preparation was 20 and 17% by male and female larvae respectively. Yokoyama (1962) reported that about 25% of absorbed energy of B. mori larva is contributed for silk production. Horie and Watanabe (1985) recorded that 34% of total amount of body energy is diverted for silk matter.
The amount of energy utilized for silk production gradually increased along with increase in silk gland growth efficiency in successive instars. Suddenly the value became highest in fifth instar. Similar trends were also observed in B. mori (Horie and Watanabe, 1985) and in case of A. mylitta raised on secondary host plant Z. jujuba during rainy season (Dash, 2013).

Conclusion
It was observed that the fifth instar of A. mylitta larva is the crucial stage of all instars for optimum silk production, since the energy utilized for production of silk during fifth instar was about 97% of the total amount utilized over the entire larval period. The commercial aim of the tasar crop is to yield possibly highest amount of raw silk for the economic benefit of the farmers. So, maximum care is suggested for the fifth instar period of larval life.