Chemicals: Formalin, Bleaching powder, Lime powder, Potassium hydroxide. Equipment and Accessories: Spray machine, BOD incubator, Microscope, Micro-slides & cover slip, Moth crushing set, Mask and Gloves, Moth cage, Foam pad, Measuring cylinder, Balance (readability- 0.1 g) Scissor, Muslin cloth, Bucket , Thread, etc.
Grainage (from French graine “seed” collectively) is the establishment of healthy silkworm eggs for production. A systematic approach not only reduces mortality and saves labour but also improves the progenies. The selection and preservation of seed cocoons, the preparation of disease-free layings and their disinfection and incubation are among the important aspects of grainage.
The wild nature of the tropical tasar silkworm is reflected in its disuniform and erratic emergence, coupling, egg laying, hatching and other habits. Nevertheless, the techniques evolved during the past decade ensure systematic grainage operations and a stable crop.
After harvesting, the healthy, well formed and tough cocoons are selected visually for seed, and the dead, deformed and otherwise unsuitable cocoons are discarded, in the absence of an organized seed sector, the selection of seed cocoons from the market should be based on sample testing. Stock showing not more than 5-10 % infection can be considered for seed.
The pupal stage of a nondiapausing crop lasts hardly a month, during which the atmospheric temperature and humidity are kept within the optimum range (25 – 30 ˚c and 70 – 80 % R.H) for their development; however, when a stock is preservrd over the winter, the pupal stage lasts 5 -7 months.
Most of the eco-races are bivoltine; only a few are univoltine and trivoltine. In univoltines the dispause extends from august to june, and in bivoltines and trivoltines from November to june and from January to june, respectively. During these periods the pupae experience extremes of temperature and relative humidity (10-46˚C and 30-100%), not only causing 25-30% mortality but also adversely affecting the vigour of the ensuing progenies. The optimum temperature and relative humidity for seed cocoon preservation are 25-26˚C and 45-50%
Simple type of seed preservation house reduces pupal mortality by as much as 5%. This well-ventilated mud house with walls 60 cm thick, rustic tile roofing, a false ceiling of split bamboo or straw mats and a corridor 2.5 m wide on all sides can maintain a difference of 5-7˚C between room and atmospheric temperature. A hut 5x11 m can easily accommodate 100000 seed cocoons arranged in hanging garlands, and the corridors provide adequate working space for the various grainage operations.
Utmost care is required in transporting seed cocoons to distant places. The cocoons should be packed loosely in split-bamboo baskets or in perforated plywood boxes. Severe jerking and direct sunlight on the box or basket should be avoided. On consignment the cocoons must be unpacked and hung in the graining room. Seed cocoons should be transported during diapauses, preferably in December and January.
The development of pupa to moth takes about 30 days, during which optimum temperature (28-30˚C) and relative humidity (75-85%) are desirable. Indoor conditions normally satisfy these requirements. In the first crop of cocoons the development continues without interruption, whereas in the over- wintering population it begins a month before actual emergence.
Emergence in the diapausing stocks generally coincides with the onset of the monsoon (the last two weeks of June), and in the second and third crops it takes place in august and September and in November and December, respectively. Usually the moths start emerging late in the afternoon, but the peak period is 19:00-21:00 hours.
A proteolytic enzyme secreted by the maxillary galeae of the fresh moths facilities emergence when applied to the peduncle end, rendering it soft and enabling the moth to force its way by making a hole.
Emergence in the natural population takes a month; but with cocoons from the first crop this period can be shortened and the emergence from those harvested on successive days can be synchronized for effective grainage by low temperature treatment (5˚C) of different duration (Table 9).
After 2-3 hours of emergence the moths start to couple, the peak period being from midnight to 2:00. Generally, a number of male and female moths are kept in large bamboo baskets for mass coupling; however, it is advisable to adjust the moth population to the size of the basket, as the coupling aptitude tends to decrease with density. For experimental purposes, a small bamboo basket as a “monia” is used for a single pair of moths.
Recent studies show the advantages of outdoor coupling for commercial grainage; it is mating aptitude between two species. The males and females of the two species are introduced into two compartments, one of which is meant for cross coupling and the other for reciprocal coupling. The males, on being excited by the sex phormone emitted by their counterparts in the other compartment, couple with the females of the other species.
Tasar silk moths generally prefer to couple in darkness at a comparatively lower temperature (24-26˚C) and high relative humidity (75-85%). A blast of cool air (20-22˚C) for 30 minutes to one hour helps raise the coupling percentage during hot, humid weather. Up to 30 minutes of premating flight exercise for male moths also induces a better coupling aptitude.
Although coupling continues for 10-12 hours, a period of 2-4 hours is adequate (3) for ensuring normal fecundity and fertility. The duration of coupling has no direct relationship to the preoviposition period.
In case of a shortage of male moths, they can be refrigerated at 10˚C for 2-3 days and then safely utilized a second time.
After the desired period of coupling, the moths are decoupled by hand. The mated females are then placed singly in small cages (monias) or together in large bamboo baskets for egg laying. Moths prefer darkness for egg laying. It has been observed that in complete darkness nearly 50% of the eggs are laid on the first day, whereas eith total illumination laying is delayed by one day
The eggs are deposited in batches of 5-10. The moths from the diapausing crop have a lower ovipositional capacity than those from the nondiapausing crop- the average being 200. The moths normally lay eggs for 6-7 days, but only the eggs deposited within the first 72 hours (85-90%) are considered for rearing.
A mild electric shock (50-100 V) administered to the mother moths showing no tendency to oviposit will induce normal egg laying within 24 – 48 hours without impairmrnt of the economic characters.
Microscopic examination of the mother moths ensures disease- free layings. A suspension obtained by squeezing the lower abdominal portion of the moth in a little water is examined under a microspe at 10x and 40X. Special attention is given to microsporidian infection because of its transovarial mode of transmission. The mother moths exhibiting even the slightest hint of microsporidiosis noted and their layings are rejected. Infected mother moths and their layings should be burned or buried immediately. If other diseases are detected, the layings are rejected only in cases of severe infection.
Cellular examination of moths is advisable in parental stocks; for commercial grainage a rather large sample should be examined.
As tasar silkworms eat a portion of the eggshell during hatching, surface sterilization of eggs should be ensured so as to avoid infection through contamination. The selected layings are placed in clean cloth bags and dipped in water to remove the meconium. They are then rinsed in a 5-10% formalin solution for 5 minutes, followed by a wash in running water. The layings are hung in the incubation room for drying.
The disinfected eggs are kept in thin layers in egg boxes made of plastic with transparent top to admit light and perforated sides and bottom for aeration (figure 43).
The egg boxes are placed in a room for incubation. Incubation temperatures of 10-25 -30˚C, 25-30-35˚C and 30˚C give equally satisfactory results (80-90%) hatching, but for operational convenience a steady 30˚C is advised. A humidity range of 70-80% is suitable for hatching. Incubation results in uniform and more complete hatching.
The hatching commences early in the morning. The hatching of unincubated eggs begins on the tenth day and continues for six days. The hatching of unincubated eggs starts on the seventh or eight day and is completed within 2-3 days. The percentage of hatching is higher in the eggs laid on earlier days. In tasar silkworm eggs, unlike the mulberry silkworm, there is no “blue stage”, perhaps because of the thick chorion, and hence “black boxing” is not possible.
If necessary, hatching can be delayed a few days by subjecting the eggs to low temperature (8-10˚C); however, the duration of treatment should not exceed one week. Refrigeration should begin either at the early stage (within 48 hours of oviposition) or after the completion of embryonic development (120 hours after oviposition). Initiating the refrigeration treatment between these two stages is detrimental to the insect.
Photoperiodism: Among the different external factors, a long day photoperiod (18 hours) has the most decisive effect on the length of diapauses in Antheraea mylitta. Almost identical results can be obtained in less time with equally long photoperiodic treatment at 30˚C (80% RH). It has been observed that Antheraea mylitta has mainly of the sensitivities to photoperiod of its close relatives Antheraea pernyi Antheraea polyphemus.
The recent discovery of plant hormones which act like the insect hormone ecdysone has aroused interest in this field. Injections of the phytoecdysones in diapausing pupae of Samia cynthia and Antheraea pernyi were highly effective in provoking adult development. In Antheraea mylitta the most positive response of 88-96% emergence in 3-4 weeks and 100% in 8-10 weeks was observed with a 5-10 microgram dose of cyasterone. The other phytormones viz. ecdysterone, inokosterone, makisterone and ponasterone in a larger dose (30 micrograms) showed 68-91% emergence in 8-12 provoked development within 12-15 weeks; cyasterone once more gave the best results.
For seed the tribal cultivators depend largely either on wild cocoons or on local cocoon market. Of the current annual requirement of over 5.5 million disease free laying, the various government agencies are able to supply barely one million and that too unsystematically. Whereas, in Bihar seed is distributed in the form of dfls, in the other states mostly seed cocoons are supplied. In fact, the unavailability of quality seed is the major obstacle to expansion of the non- mulberry silk industry.
A seed sector soundly on scientific lines is urgently needed for the renewal of this neglected sector. Considering the abundant availability of the essential components of tasar culture, the demand for send is bound to multiply once a supply system is set up.
Early instar silkworms, rather than seed cocoons or dfls should be used for seed, in spite of problems encountered during grainage operations and rearing. Therefore, the organization of seed production and supply entails the following prerequisites for effective operation.
Each seed unit should be adequately equipped for grainage operations of about 100000 layings with a mud grainage hut, two ordinary microscopes, disinfectants and other essentials. For the early-instar rearing of 4000 dlfs, one hectare of economic plantation should be raised, and the block plantation for their advanced stage rearing should be about 50 ha.
Apart from ensuring the supply of early instar silkworms, the seed zones must be absolutely free of all infections which are ascertainable through regular microscopic examination. The rearing site should also be as far as possible from the commercial belts and have high concentration of foliage rich plants.
The objectives of seed organization cannot be attained unless unauthorized production and marketing are prohibited through legislation. If needed, the government can request the services of certain progressive rearers for the technically controlled preparation and supply of disease free layings or early-instar silkworms.
The objectives of seed organization should be comprised of a seed maintenance farm and a seed multiplication and supply farm. The former maintains the basic seed stock and the latter reproduces the stock and supplies the rearers with early instar silkworms.
Each unit should be charged with (a) the collection, evaluation and maintenance of superior races and hybrid stocks and (b) the acclimatization of commercially promising eco-races. From a crop of 200 kahans, 190 kahans should be distributed to the multiplication and supply farms (10 kahans per unit), and the remaining 10 kahans are retained for stock maintenance.
Multiplication for seed stock and for commercial supply should be carried out in two phases in the first, 10 kahans reproduce at least 200 kahans of seed stock. In the second the seed stock of 200 kahans reproduce about 64000 dfls, which should be distributed to twenty early instar rearing centres (3200 dfls each). In turn each of these centres should supply an equal number of third instar to the rearers for commercial rearing.
FAO Agricultural Service Bulletin-Manuals on Sericulture, reprinted by Central Silk Board, Bangalore