Rearing
program
In
sericulture areas which enjoy temperate or subtropical climates, silkworm
rearing is carried out twice a year in the spring and the autumn or three
times, in the spring, the summer and the autumn, coinciding with the growth and
production of mulberry leaves. In tropical areas, however, where mulberry growth
is continuous throughout the year silkworms are reared five to six times in a
year. Whether rearing is conducted frequently or only two or three times a year
it is always beneficial to have a planned program. It is advisable to rear a
greater number of lying at a time than to rear much bathos at very short
intervals. The program of silkworm rearing in a farm is determined by the
following considerations:
1. Conditions of mulberry growth and yield of
mulberry leaf.
2. Availability of labor for rearing silkworms.
3. Facilities for rearing silkworms, i.e.
suitable buildings and equipment.
Conditions
of mulberry tree growth and yield of mulberry leaf: The
time to rear silkworms is when there is a proper sprouting of mulberry leaves
and a plentiful supply of good quality leaves. The yield and the quality of the
leaf depend to a great extent on the agronomic practices followed for the
cultivation of mulberry trios namely irrigation, application of fertilizers and
proper activation.
Availability of
labor: Since sericulture is a labor‑intensive rural industry
requiring much labor the volume of rearing to be undertaken will also depend on
the adequate availability of labor at a reasonable wage. If labor costs are
high economic production costs will also increase.
Facilities
for rearing silkworms: The facilities required for
silkworm rearing are a rearing space or houses, and rearing equipment.
Rearing
space or houses: Silkworm rearing demands certain
specified environmental conditions particularly as regards temperature and
humidity. Rearing houses are planned and constructed to provide and maintain
proper environmental conditions to ensure good quality cocoons.
Equipment
for silkworm rearing: The equipment required for a model
rearing house for shelf rearing is as follows: in the model rearing house in
each of the rooms 200 ‑ 250 layings or 800‑1000 layings in the entire rearing
house can be reared under shelf rearing.
Rearing stands
Ant
wells
Rearing
trays
Rectangular
wooden trays or boxes
Paraffin
pape
Foam
rubber strips
Chop
sticks
Feathers
Chopping board
Chopping
knives
Mats
Leaf
chamber
Cleaning nets
Mountages
Hygrometers and thermometers
Feeding stands
Other
equipment: Among the other items required are feeding basins,
preferably plastic, to, hold chopped leaves, and buckets and basin stands,
etc., A sprayer is also required for disinfecting the rearing room, for
spraying water on to the leaf chambers, etc. Leaf baskets made of bamboo are
used for carrying the leaves from the garden to the rearing rooms.
Sanitation
and preparations for rearing silkworms: The silkworms must be
reared with the utmost care since they are very susceptible to disease.
Bacteria, moulds and some protozoan attack the silkworms readily and any
disease, once it breaks out, spreads quickly. To prevent disease, good
sanitation methods and hygienic rearing techniques must be meticulously
followed.
Hatching,
feeding, cleaning and spacing
Hatching:
Hatching of silkworms generally starts early in the morning. The process of
transferring the silkworms to rearing trays is called "brushing". A
suitable time for brushing is about 10.00 as most of the larvae hatch by 08.00
and develop good appetites within two hours of hatching.
Feeding
silkworms: Silkworms are fed to satisfy their
appetites and to ensure their, healthy and uniform growth. It is therefore
necessary to preserve the quality of leaf during feeding and to keep the
rearing beds clean.
Bed
cleaning: Removing the old mulberry leaves, faecal matter of
silkworms, exuviae, any dead or unhealthy silkworms, etc., from the rearing bed
is called bed cleaning. Accumulation of any such matter creates environmental
conditions detrimental to the health of silkworms.
Spacing:
Provision of adequate rearing seat space is of great
importance for the vigorous and full growth of silkworms. As the worms grow in
weight and size, the density in the rearing bed increases and conditions of
overcrowding are faced.
Overcrowding
of silkworms means insufficient space for the free movement and free feeding of
the worms and so the larvae crawl over one another.
The
spacing to be provided for different ages of silkworms of 50 layings or a box
of 20 000 eggs are given below:
At the beginning At the end
of each age of each age
(M2) (M2)
Univoltines
and bivoltines species:
Age
lst 0.2 0.8
2nd 1.0 2.0
3rd 2.0 4.5
4th 5.0 10.0
5th 10.0 20.0
Multivoltines
and bivoltines species in tropical areas:
Age
1st 0.2 0.5
2nd 0.5 1.5
3rd 1.5 3.0
4th 3.0 9.0
5th 9.0 18.0
Care
at molting
Uniform
growth of silkworms of any batch depends on the care given to rearing. Among
the various steps involved in the rearing of silkworms the handling of
silkworms during molt is of particular importance. Silkworms under proper
rearing conditions would all settle uniformly for molt and also come out of
molt uniformly.
Environmental
conditions
Since
silkworms have been domesticated for many centuries, they are by nature quite
delicate and very sensitive to environmental conditions. In the commercial
production of cocoon crops optimum environmental requirements ensuring maximum
productivity of good quality cocoons of very high silk content are aimed at.
Temperature:
Temperature plays a vital part in the growth of silkworms. Silkworms are cold,
blooded animals and as such temperature will have a direct effect on the
various physiological activities of their systems.
Since
the temperature is in direct correlation to the growth of the silkworms’
excessive fluctuations in temperature are harmful and should be avoided. The
optimum temperatures for rearing silkworms of different instars are as follows:
In stars 0
C
lst 26
‑ 28
2nd 26
‑ 28
3rd 24
‑ 26
4th 24
‑ 25
5th 23
‑ 24
Light:
Silkworms are photosensitive and generally have a tendency to crawl towards dim
light. They do not like either strong light or complete darkness. In complete
darkness the larval period is shortened and in bright light there is a tendency
towards production of heavy cocoons.
Leaf
quality
The
mulberry leaf is the exclusive food of the silkworm Bombyx mori. It is
essential that the mulberry leaves axe not only in abundant supply but are also
of good and suitable quality. Leaf quality has much to do with the success of
silkworm rearing and the quality of the cocoons produced. Similarly in the case
of fertilizer application, higher doses of nitrogen fertilizer induce more
vigorous growth in mulberry trees and the leaves from such trees contain more
water and protein.
Harvesting
and storage of leaves: Leaves picked from mulberry
plants lose water rapidly due to withering and as a result the leaf quality is
considerably affected. Leaves that lose moisture beyond a certain level are no
longer relished by the silkworms which do not feed adequately on such leaves
with the result that they are not nourished properly.
Rearing
early-age silkworms
Different
methods of rearing the early ages are practiced. In all methods importance is
given to maintenance of leaf quality and correct humidity and temperature in
the rearing bed so that vigorous and healthy development is ensured. The two
methods used at present are: 1. paraffin‑paper rearing and 2. box rearing.
Paraffin
paper rearing: Paraffin paper is used as a bottom
layer and as a cover for the rearing beds in the usual rearing trays. The
object is to maintain the optimum humidity in the bed and to keep the leaves
fresh for a longer period by preventing moisture loss through evaporation.
Feeding is restricted to two or three times a day and so the labor required for
feeding is reduced.
Box
rearing: Boxes or trays made of wood or plastic or galvanized iron
at least 10 ‑ 15 cm deep are used for box rearing. The boxes used may be with
or without lids.
Rearing
in boxes with lids: In this case a sheet of paraffin
paper is spread over the bottom of the tray. The rearing bed is prepared over
this and covered with another sheet of paraffin paper after putting in place
the wet foam rubber pads.
Rearing
in boxes without lids: Wooden boxes of uniform size and
10 ‑ 15 cm deep are used. Each box is provided with a sheet of paraffin paper
for the base, over which the rearing bed is prepared, wet foam rubber pads axe
placed on all the four sides and the bed is covered with another sheet of
paraffin paper.
Co‑operative
rearing of young silkworms: Silkworms not reared properly in
their young ages are prone to disease at later stages, and crops may even fail.
Maintenance of optimum temperature and humidity, supply of suitable leaves, and
handling of the young‑age rearing with utmost care, etc., demand highly
technical skills, which are not often available to the ordinary silkworm
rearer.
Rearing
late‑age silkworms
Late‑age
worms in the fourth and fifth instars need comparatively less humid rearing
conditions and also preferably a lower temperature. These are real feeding
stages when the worms consume about 90 ‑ 95 percent of the total feed, and
therefore, adequate spacing and an adequate amount of feed should be given to
these two ages.
Methods
of rearing late‑age worms: Three methods of rearing are
commonly known: shelf rearing, floor rearing and shoot rearing.
Shelf
rearing : Rearing of silkworms in rearing trays arranged one over the
other in tiers on rearing stands is called shelf rearing.
Floor
rearing: This is another method of rearing silkworms on fixed
rearing seats. The rearing seats are arranged in two to three tiers so as to
accommodate as many silkworms as possible.
Shoot
rearing: Shoot rearing is very similar to floor rearing. Silkworms
are reared on big branches in one or two tiers. The big shoots harvested from
the fields are fed straight away to the worms.
Mounting
and harvesting
Toward
the close of the final larval instars the silkworm stops feeding and gets ready
to build the cocoon prior to transforming itself into the pupa inside the
cocoon. The feeding during the final instars may last from five to seven days
in the case of multivoltines and bivoltines races in the tropical areas, and
seven to nine days in the case of bivoltines and univoltines races in sub‑tropical
areas. With the cessation of feeding and as the stomach contents empty, the
fully mature larva becomes translucent and yellowish in appearance which is a
clear indication that the worm is fully ripe and is ready to be mounted on to
montages or cocooning frames. Such ripening larvae normally crawl towards the
periphery of the rearing trays in search of suitable supports for building
their cocoons. Thus the process involved in picking the ripe worms and putting
them on to the suitable montages for the spinning of cocoons is called mounting
of worms.
Collecting
mature worms for mounting: Collecting mature silkworms and
mounting them on the montages is a laborious job. Usually the mature larvae are
picked by hand by skilled labor capable of identifying the mature worms, which
are collected in trays and later put on the montages.
Method
of free mounting: With this method instead of collecting
the mature worms from the beds, the montages are placed in the beds themselves
and the worms crawl up onto them.
Density
of mounting: In the case of revolving montages
density is automatically adjusted since only the number required to fill the
empty blocks in the cocooning frames are mounted. Ordinarily a single frame
contains about 156 blocks and 10 such frames are put together to form a unit of
turning cocooning frames which can mount 1560 worms.
Environmental
conditions required for mounting: Maintenance of the
required environmental conditions during mounting is important as it determines
the quality of the cocoons spun. Silkworms under mounting normally require a
comparatively drier 0 atmosphere. Ideal conditions for good spinning are
temperatures not exceeding 26 0 C and relative humidity between 60 and 70
percent. If temperatures rise above 26 C the quality of the cocoon is affected.
It is particularly important that during the first 50 hours after mounting
ideal conditions are maintained for cocoon spinning.
Harvesting
of cocoons: After spinning the cocoon the larva
undergoes metamorphosis inside the cocoon and becomes the pupa. Generally
pupation takes place on the fourth or fifth day of spinning. The pupa when
formed has a thin cuticular skin which is soft to the touch and ruptures easily
if disturbed; therefore early harvesting of cocoons should be strictly avoided.
In course of time the pupal skin hardens and turns dark brown and it is at this
stage that cocoon can be harvested. The proper time for harvesting cocoons is
on the seven or eighth day of spinning in the case of bivoltines and
univoltines races, and on the fifth day in the case of multivoltines races.
Harvesting must not be unduly delayed beyond the period mentioned above lest
any parasitic insects infecting the silkworm larvae emerge from the pupa and
damage the cocoons.
