Task B 1.1 (Egg hatching)
EGG HATCHING CONDITIONS
Hatching of silkworm eggs very much depends on climatic parameters such as
temperature, relative humidity and light intensity. These parameters are of vital
importance since they can influence the incubation period, the development of
silkworm embryo and ultimately the cocoon production. Figure 1 describes the
embryonic development of hibernating egg. The following paragraphs discuss the
effects of different factors on hatchability of silkworm egg.
Temperature is considered to be the most important factor for silkworm egg
hatching. It affects the duration of incubation, uniformity and percentage of hatching,
quality of the cocoons and the voltinism of silkworm. In case of univoltine silkworm,
the change in temperature does not affect voltinism while in the case of bivoltine
silkworms temperature variations may change the voltinism of future generations
(Aruga, 1994; Lim et al., 1990; Pang-chuan et al., 1988).
Temperature affects the speed of the different embryonic stages of
development. The optimum method is to keep eggs at 15° C until all the embryos
have reached the starting point of the longest embryo stage. This stage represents the
fourth stage (also called C2 stage) of the fifteen stages of the whole embryonic
development. After reaching this stage, the temperature must be raised up to 20 °C to
ensure a uniform development of all the eggs embryos. Therefore, it is recommended
to transport the eggs 5 days after they are taken out from the storage. It is also advised
to carry the egg in relatively low temperature (e.g. at night or in a container with
uniform low temperature) (Pang-Chuan et al., 1988). According to this author, after
the longest stage (C2) and after the embryos have reached the final stage of
development (the so-called F5 or blue eye stage), eggs can be stored at a low
temperature (2.5 °C) to shorten the time of egg hatching.
The effect of temperature conditions on egg hatching depends also on whether
the hatching is achieved naturally or artificially by acid treatment. Some changes in
incubation conditions are required for eggs hatched by acid treatment (Aruga, 1994;
Pang-Chuan et al., 1988; Lim et al., 1990)
The first three days after the beginning of the incubation, the temperature
should be maintained between 15 and 20 °C (this can be ensured by the temperature
of environment). During the remainder days, the temperature of the incubation room
should oscillate around 25 °C. In practice it has been proved that a continuous
exposure of eggs at a temperature of 25 °C shows good results. The heating of the
incubation room can be done by a heater connected to a thermostat.
Figure 1. Embryonic development of hibernating egg (Omura 1967).
While temperature has such strong effect on incubation and egg hatching,
relative humidity has also some effect on egg hatching. Physiologically, too dry or too
wet air is harmful to the eggs. For example, under dry condition, the eggs may lose
their water, and this will result in erratic hatching and increase the percentage of dead
eggs. On the other hand, high humidity (more than 90%) produces larger larvae which
are in fact very weak. The optimum humidity ranges from 70 to 75 % at earlier stages
and from 75 to 85 % at later stages of embryo development.
The humidity can be created by a humidifier with humidity sensors or by
regular watering the floor of the incubation room.
As in the case of temperature light also has some influence in the change of
voltinism. While incubation in darkness produces non-diapause eggs, incubation in
brighter light produces diapause eggs. In some cases, temperature and light have
cumulative effect on incubation and hatching of eggs. In order to ensure a uniform
hatching, eggs should be exposed to light until the appearance of the first larvae.
According to Pang-Chuan et al. (1988), keeping eggs in darkness can inhibited them
from hatching for up to only nine (9) hours. After this time, they will hatch regardless
of darkness. Note that the eggs waiting to hatch in darkness are very sensitive to light
and once in contact with light for a short time they will start to hatch at once. After
the light has been cut off, the eggs must be kept in complete darkness. Light exposure
on the day of hatching begins at 04:00-05:00 hours (San-ming et al., 1989).
The light/dark ratio should be: 18 hours light/6 hours dark. When 50% of
embryos have completed their development, continuous darkness must be maintained
until the emergence of larvae.
Eggs resistance to CO2 is higher in early stages than in the later stages (2-3
days before collection of silkworms). A content of 0.5 % of CO2 in the rearing room
will does not harm the eggs development while a higher content may kill or weaken
the eggs, and will produce weak silkworms. In practice to produce fresh air in the
rearing room, the room can be ventilated twice a day (in the morning and afternoon),
or four times in the final stages.
Incubation of eggs
After being laid, eggs are of yellow color and several hours later, they become
darker and turn gray which is their final color (Fig. 1). The unfertilized eggs remain
yellow. Overwintering eggs are kept at low temperature (2.5 °C) till the next spring
and until the beginning of the incubation process. Egg hatching very much depends on
climatic parameters such as temperature, relative humidity and light intensity. These
parameters are of vital importance since they can influence the incubation period, the
development of silkworm embryo and ultimately the cocoon production.
The incubation takes place in special places (incubating chambers) where
suitable conditions of temperature, humidity, light and air are created. If the
incubation does not take place in an incubating chamber, then it is recommended to
carry out it in a specially adapted place (e.g. small room) where the desirable
conditions must be maintained. It is recommended to begin the incubation with the
emergence of the first mulberry leaves. If the silkworms emerge earlier they will not
have food; while if they emerge later they will not develop correctly because at this
time the leaves will be very hard. The hatching of larvae should be synchronous (in 2-
3 days) at least in 95 % of cases. Only the eggshells remain after the completion of
the egg hatching (Fig. 2).
Figure 1. Silkworm eggs that just oviposited (left) and several days later(right).
Figure 2. After proper incubation of eggs we get excellent hatching.
Rather than being controlled separately, the above conditions must be
controlled all together. A balanced control of these conditions is a prerequisite for a
high and uniform hatching of egg and therefore for a production of healthy silkworms,
which in turn would produce cocoons of good quality. Incubation and egg hatching
conditions of temperature, humidity and light for bivoltine parent eggs in spring and
autumn are illustrated in tables 1 and 2.
Aruga, H. 1994. Principles of Sericulture. A.A. Balkema, Rotterdam. 376 pp
Lim, S.H., Kim, Y. T, Lee, S.P., Rhee, I.J., Lim, J.S., Lim, B.H. 1990. Sericulture
training manual. FAO Agricultural Services Bulletin, 80.
Omura, S. 1967. Introduction to Silkworm Rearing. Sericultural Experiment Station,
Ministry of Agriculture and Forestry, Japan. 129 pp.
Pang Chuan, W., Da-chuang, C., Zuo-pu, C., Ping-zhang, L., He, T. 1988. Silkworm
rearing. FAO Agricultural Services Bulletin, 73/2.
San-ming, W., Ping-yi, L., Run-shi, Bing-sen, O. 1989. Silkworm egg production.
FAO Agricultural Services Bulletin, 73/3