SilkWorm: Diet, Life Cycle, and Disease

By Eman Abdallah Kamel

Eman is a writer and engineer. She is interested in writing about natural fibers and their sources.

Silk is made from the threads spun by silkworms to form their cocoons. Here, you will learn about silkworms, their diet, life cycle, diseases, and care.

Source of Images: Stock.adobe.com

SilkWorm

The threads that silkworms spin to form their cocoons are used to make silk, a natural fiber. The silkworm’s life cycle consists of five stages: egg, silkworm, cocoon formation, pupa, and butterfly. It belongs to the class of insects that go through complete metamorphosis. The silkworm’s life cycle starts with the egg, which develops into a larva. The silkworm develops through five stages, or instars, and feeds on mulberry leaves. It turns into a pupa by spinning a cocoon after the last stage. At last, an adult butterfly appears, deposits its eggs, and the cycle restarts.

The most important countries producing silkworm cocoons ready for spinning are India, China, Japan, Uzbekistan, Syria, Egypt, and South Korea.

Illustration of the silkworm diet.

Diet

Silkworms of Bombyx mori primarily feed on mulberry leaves. Other silkworm species have different diets—Tasar silkworms feed on Arjun and oak leaves, while some species consume pine and castor plant leaves.

Silkworms typically feed on mulberry leaves, but artificial mulberry feed, known as “silkworm chow,” made from mulberry leaf powder, is now commercially available. This artificial feed has made year-round silkworm production possible.

Illustration of the silkworm life cycle.

LifeCycle

The silkworm undergoes complete metamorphosis and goes through five life stages during its lifespan, which are

Egg,
Larvae,
Cocoon Formation,
Pupa,
And moth.

1. Egg Stage

The life cycle begins when female moths lay eggs during summer or early autumn. These eggs are extremely small, approximately the size of an ink dot. After a period of dormancy, larvae hatch from the eggs in spring.

2. SilkWorm (Larvae) Stage

The larva stage lasts about 27 days. Newly hatched silkworms are tiny, hairy, and measure about one-eighth of an inch. During this stage, they feed continuously on mulberry leaves. They undergo five growth stages, known as instars, and molt four times. During the first molt, they shed their hair to develop softer skin. By the final stage, the silkworm stops feeding and begins searching for a suitable surface to spin its cocoon.

3. Cocoon Formation

Silkworms produce silk from two modified salivary glands. A liquid protein is secreted through the spinnerets and hardens, forming fine threads. Two main proteins are involved in this process: fibroin, which forms the skeletal core, and sericin, which acts as an adhesive. The color of the cocoon ranges from white to golden yellow, depending on the diet. Cocoon formation takes between three and six days and protects the next stage.

4. Pupa Stage

Inside the cocoon, the caterpillar transforms into a pupa. At this stage, the organism transforms into an adult butterfly within 15 to 21 days.

5. Adult Moth

The adult moth emerges by secreting an enzyme that dissolves sericin, allowing it to break through the cocoon. The moth lives for only a few days, during which time reproduction occurs.

After mating, the female lays eggs and dies, completing the life cycle. The entire cycle typically lasts between 55 and 60 days, after which a new generation begins.

Disease

Diseases that affect silkworms include the following:

1. Grasserie

The silkworm polynuclear decomposition virus causes this disease (Bombyx mori). It is present year-round, but its severity increases during the summer and rainy seasons. High temperatures, low humidity, and poor-quality mulberry leaves contribute to the spread of infection.

Sources of infection include:

Silkworms become infected when they feed on contaminated mulberry leaves.
The milky-white fluid secreted by “grasserie” larvae,
Contaminated silkworm rearing houses,
And the equipment used.

Symptoms:

The skin of infected larvae becomes shiny before molting, which then fails.
Swelling appears between the segments, and the body turns yellowish.
Infected larvae move nervously in rearing tanks/along the edges of trays.
The body of an infected larva ruptures easily, releasing a cloudy, white, bloody fluid.

2. Pebrine

Caused by various strains of microsporidia. This disease is not seasonal.

Sources of infection:

Consuming contaminated mulberry leaves
Infected silkworms,
And contaminated breeding houses and equipment.

Symptoms:

Irregular hatching of silkworm eggs.
Black, pepper-like spots appear on the bodies of infected silkworms.
Irregular body size and molting of the larva.
Infected larvae lose their appetite, become lethargic, and develop wrinkled skin.

3. Flacherie

The infectious flacherie virus of the silkworm (Bombyx mori) or several pathogenic bacteria, including Bacillus thuringiensis, Serratia marcescens, Streptococcus sp., and Staphylococcus sp., either alone or in combination, are the cause of this illness.

Poor leaf quality, high humidity, and temperature swings are predisposing factors. The disease is prevalent during the summer and rainy seasons.

Source of Infection:

Silkworms become infected by ingesting contaminated mulberry leaves.
Infected dead silkworms, their feces, intestinal juices, and body fluids are sources of bacterial contamination.
Infection can also occur through injuries or wounds.

Symptoms:

Larvae become soft and weak.
The growth of infected larvae slows; they become lethargic and regurgitate gastric juices.
Their feces become soft and moist.
The larva’s head and thorax become transparent.
Sometimes, a series of droppings and an anal protrusion are observed.

4. Muscardine

White muscardine disease is a common fungal disease caused by the fungus Beauveria bassiana.

This disease is prevalent during the rainy and winter seasons. Low temperatures and high humidity are predisposing factors.

Source of Infection:

Infection begins when fungal spores come into contact with the silkworm’s body.
Breeding houses and contaminated equipment.

Symptoms:

Larvae lose their appetite and become lethargic.
The larva vomits and becomes flabby.
Moist spots appear on their skin.
The larva gradually hardens after death, and its body turns chalky white as aerobic fungi and spores grow on it, causing mummification.

How silkworm farmers raise millions of silkworms to produce silk fibers.

SilkWorm Care

If you are raising silkworms, consider the following points:

Pay attention to the continuous cleanliness of breeding boxes and the equipment used.
Before handling silkworms or their food, wash and dry your hands thoroughly. If you don’t handle them properly, the silkworms can become susceptible to bacteria.
A plastic tub or container with a lid that has ventilation holes is ideal for raising silkworms.
Keep the humidity at about 50%. If you leave them in an open box, they will quickly dry out, and their food will spoil.
Feed the silkworms using tongs or bowls, or release them briefly to roam around their environment and feed. Leave some fresh mulberry leaves for them to eat.
When the silkworms are pupating, provide them with toilet paper tubes, plastic containers, boxes, mulberry tins, twigs, or similar materials to spin their silk. Transfer them to a separate box after pupation is complete, where they can emerge from their cocoons, reproduce, and lay eggs.

To learn more about how to care for silkworms, visit Care and Raising Tips for Silkworms.

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