Silkworm: Species and Diseases | Zoology

In this article we will discuss about the species of silk moth used in silk production and the diseases associated with them.

Species of Silkworm:

The silk producing machine is an insect called as silk moth locally in Hindi ‘Resham-ka-kira’. Although a number of species are found producing silk but only few species are used for sericulture industry.

1. Mulberry Silkworm (Bombyxmori):

It belongs to the family Bombycidae. China is the native place of this silkworm but now it has been introduced in all the silk producing countries like, Japan, India, Rep. Korea, Italy, France and Russia. Since the natural food of this worm is mulberry leaf, it is called as mulberry silkworm. The silk produced by this moth is white in colour.

2. Tasar Silkworm (Anthereapaphia):

It belongs to the family Saturniidae and common in India, china and Sri Lanka. The caterpillar feeds on ber, oak, sal and fig plants. The cocoon produced by this moth is hard and of hen’s egg size, which produces reelable brown coloured silk. Though it had been only a wild variety of silk moth since long, now, by cross breeding, it has been possible to produce such varieties which are reared any-how and domesticated.

But the domestication of tasar caterpillars is not so easy, so the cocoons have to be collected from the forest. The moths do not easily breed in captivity. Since breeding is not well controlled, the tasar silk industry has not reached up to mark as the mulberry silkworm industry.

3. Muga Silkworm (Antheraea assama):

It also belongs to the family Saturniidae and semi-domesticated in nature. The native place of this species is Assam where, it has now become a good source of cottage industry. The caterpillars of this worm feed on Machilus plant and the silk produced by this moth is known as Muga silk.

4. Eri Silkworm (Attacus Ricinii):

It also belongs to the family Saturniidae and produce silk in East Asia. In India sericulture scientists are trying to produce silk in East- Asia. Sericulture scientists are trying to produce such cross breeds which can provide good quality of silk and can be reared easily. It feeds on castor leaf. Cocoons cannot be reeled as in mulberry cocoons; therefore, it has to be spun. Its life history resemble with that of mulberry worms. The cocoons of this work have very loose texture and the silk produced is called as ‘Arandi silk’ locally. The threads are not glossy but much durable.

5. Oak Silkworm (Antheraeapernyi):

It belongs to the family Saturniidae and is found in China and Japan. A. roylet of the Himalaya and A. yamamai of Japan have been reared for centuries. They produce good quality of silk.

6. Gaint Silkworm (Attacus alts):

It belongs to the family Saturniidae and is found in India and Malaysia. ‘It is the largest of the living insects reaching upto eleven inches in wing-span.

Out of the above species only four are of common use for sericulture viz., Bombyx mori, Attacus ricinii, Antheraea assama and Antheraea paphia.

Mulberry-Silk Moth (Bombyxmori):

Silk and silk garment have always been the first choice of the members of Royal families and other associated with them in India. It is one amongst the oldest business being conducted in our country and a large number of families are associated with the production, look after and preparation of silk in India. It is a fact that it has never been a native of India but today the Indian silk is on the top both in quality and quantity.

The scattered information’s reveal that it was originally started in China and was looked after under top security and safety, but the security could do nothing before the monks who were successful in stealing some of the eggs in their walking sticks and thus silk insects came out of the great walls of china and spreaded all over the world.

Now they are given a warm welcome all over the world and each and every country is making its effort to produce silk, improving its quality and quantity. It is associated directly with the military of the country as the parachutes used by the soldiers are always made up of the best quality of silk.

Rearing, developing and looking after of silk insects are termed as “Sericulture”. Like other farming’s, agriculture and businesses sericulture too involves skill and scientific knowledge for rearing and development. The best part of this culture is that it involves very little amount of money and can be started in a small space. Disabled and old persons as well as handicapped people to can successfully do this job and fetch a food and handsome amount of money in a short period.

This is the reason why it is at the top in India almost all the States have industries based on sericultures and are providing north as well as in the plains. A wide range of silk is available in India. The quality and the quantity of silk depend upon the quality of leaves the larvae are fed with.

The silk is produced by a large number of insects but the best quality of silk is produced by the spp. Bombyx which is popularly known as “Reshum-ka-Kida”.

Life Cycle of Mulberry-Silk Moth (Bombyxmori):

The moths Bombyxmori are pale, cream or ashy white in colour. They are about 2.5-3 cm. long, robust and covered with small hairs. The body is well divided into head, thorax and abdomen. The head being big in size with well-marked big compound eyes and a pair of plumose antennae. Thorax is strong muscular three-segmented structure provided with spiracles 2 pair of wings and three pair of legs. The wings are about 4-6 cm.

When expanded fully but the muscles associated with it are not strong. So they cannot fly for long, instead they walk. The wings are well ornamented with dark and faint brown demarkatism bands. The abdomen is covered with scales and is highly extended in females whereas narrow, slender and cylindrical in males. They mate only few hours after their birth and start laying eggs. The sexes are separate and sexual dimorphism is clean.

The males are smaller and tender whereas the females are bigger and stout. Both the sexes do not feed. The females have life span of 2-3 days whereas males have even less days to live and sometimes die during copulation. The copulation takes about 3-4 hours. Once copulated the female can lay eggs, all fertilized, till she dies, There is no need for further copulation.

Eggs and Oviposition of Mulberry-Silk Moth (Bombyxmori):

The female moth lays about 350-550 eggs at a time, just after fertilization. The eggs are minute, rounded and laid in clusters on the upper surface of the leaves. These eggs are well protected by means of gelatinous glue, which makes them to stick with the substratum. The newly laid eggs can be recognized by means of their pale yellow or cream coloured, unornamented structure but soon after their oviposition, as they come in contact with fresh air they become dry and hard and their colour is changed from pale yellow to dark brown or black.

This is a symptom that they are fertilized eggs. The viability of the egg depends upon the duration of copulation and number of eggs laid by the female. In the same way the number of eggs laid depends upon the health of the female moth. The adult female does not eat so the number of eggs laid depends directly on the larval health which later on develops as the female. In case the female is feeble and sick she will lay only few eggs which may have low percentages of hatchability as against the well fed and healthy female moth.

The eggs are very susceptible to temperature, light and humidity. In optimum temperature of 65°F-75°F they hatch within a period of 7-10 days provided they are well ventilated and in open shady place otherwise the period of hatching increases.

In extreme adverse atmospheric conditions they take more than 10 days and are in dormant stages. Period of incubation also depends upon the genetic condition of the female, in case the moth completes only one brood a year. The incubation period is increased but in case of B. mori the incubation days are not more.

The newly emerged larvae are minute, slender, tender, pale yellow or cream coloured with dark brown head and biting and chewing type of mouthparts. They more rapidly in search of food and are voracious feeder. They grow rapidly in size and eat continuously. A break in the food supply directly means the death of larva and loss in production of silk.

The amount of silk produced is directly proportional to the feeding of larva. Healthier the larvae maximum is the output. The newly hatched larvae are about 3 mm. in length which move very fast but the full grown larva is about 6-7 cm. dark, dull coloured and thick like thumb. The body well segmented and marked with beautiful spots like other Lepidoptera larvae it also bears pseudo legs and moves in search of fresh leaves.

The tender newly hatched larvae prefer newly formed leaves whereas the old and well-developed larvae can eat any old or new leaf. Total larval period varies according to the climatic conditions like humidity, temperature and photoperiod but in case all the above cause are favourable they take about 21-35 days for their full development and moult 4 times each after an interval of 6-7 days.

The last instar larva looks robust with a horn on the dorsal side in the posterior region which becomes more prominent when disturbed. The last instar larva possesses a pair of well-developed salivary gland on the lateral side of the body, which opens in the mouth.

The fully formed 5^th instar larva undergoes some physiological changes and becomes restless. It is done due to certain intrinsic hormonal reasons. Now the larva stops feeding and migrates on one side. It prefers dark shady places particularly the corners or the points, which are at an angle. It moves its head in whirling position. It constantly and rapidly rotates its head at its axis and in the meantime the saliva comes out through a small pore known as spinneret.

The spinneret is located on the tip of hypo-pharynx in the mouth. There is a continuous flow of saliva through this pore which when exposed to fresh air becomes dry and hard and is known as silk. This silk is wrapped round the body almost at an average speed of one round per second. This is done for a period of 2-3 days and the larva is enveloped in a case made up of thread which looks like a reel larva of the silken thread.

In these two-three days the larva remains without food and is actively engaged in spinning the silk. This reel of thread is called cocoon which provided a soft bed for the larva to pupate. The thread on the outer side of the cocoon is generally mixed and unarranged while the thread in the lower strata pure without any contamination and is arranged in an organized manner.

The total length of the thread spun by a larva varies from 1000-1500 meter which measures about 0.3-0.5 gm. The colour of cocoon depends upon the species of the moth and the leaf they are fed with. It ranges from pure white, cream to pale yellow and dirty white or dirty yellow and thickness of the thread also depends upon the species of the moth.

Thus total cocoon formation takes about 3-4 days and then the larva is changed into pupa. Pupal period ranges from 6-12 days after which the adult image comes out. It cuts open the thread and emerges out, sits on the cocoon, spreads its wings and then is ready for copulation. The adult produces certain alkaline fluid which softens the thread and the moth pushes out of cocoon.

Total life cycle is completed in about 35-50 days.

Diseases of Silkworm:

A variety of diseases are associated with the larvae of a silk moth. These enemies not only kill the larvae but spoil the forthcoming generation also.

Some of them are protozoans, viral and few are fungal:

1. Pebrene:

Is caused by protozoan and sweeps out the entire industry the larvae become pale yellow, reduce in size and finally reduces the yield.

2. Grasserie:

Makes the skin of larvae rough and blisters appear over it. The saliva becomes thick and yield is minimized.

3. Flacherie:

Caused due to indigestion so the worm become thin, inactive and regenerates occasionally.

4. Muscardine:

Mortality occurs due to heavy rain and unhygienic conditions in rainy season. The worms die soon after this disease. This is due to fungi.

Beside these the ants, crows, birds and other predators are always ready to attack the insect so the industry is always managed in such a way that these things could not reach the cage.