Silkworm Cocoon Harvesting and Reeling Preparation — From Sericulture to Raw Silk Material

The life cycle of the domestic silkworm (Bombyx mori) is: egg → larva (silkworm) → pupa → adult (moth). The larval stage lasts about 25-30 days and is divided into five instars. At the end of the fifth instar, the silkworm's body becomes translucent and glossy (ripe silkworm), and it begins spinning silk to form a cocoon. It takes about 2-3 days from the start of spinning to the completion of the cocoon.

Ripe silkworms preparing to spin cocoons show the following signs: their bodies shorten, become translucent, the chest is slightly raised, they stop eating, and they crawl around looking for a place to spin. The silkworm's body color changes from milky white to a slightly yellowish tone. When these signs appear, the mounting frame (cocoon frame) should be prepared.

The mounting frame is the support on which silkworms spin their cocoons. Traditional methods use straw dragons woven from rice straw, while modern methods use plastic grid frames. Each cell of a grid frame is about 4×3 cm, and each cell holds one silkworm. Stand the frame upright and keep it well ventilated.

Gently place the ripe silkworms one by one onto the mounting frame, one silkworm per cell. The silkworms will automatically crawl into the cells and begin spinning silk. Be careful not to place them too densely—two silkworms in the same cell will produce a double cocoon, which is of poor quality and unsuitable for silk reeling.

During cocooning, maintain room temperature at 24-26°C and relative humidity at 60-70%. Ensure good ventilation but avoid direct drafts. Keep lighting even and somewhat dim. Excessively high temperature or humidity can lead to misshapen cocoons and poorer silk quality. The cocooning process lasts 2-3 days.

On the 6th–7th day after cocoon formation (counting from when the silkworms were placed on the mountage), gently shake the cocoon. If you can hear a "rustling" sound of the pupa rolling inside, it means the pupa has formed and the cocoon can be harvested. If no sound is heard, wait another 1–2 days. Harvesting too early will damage the pupa body that has not yet fully pupated.

Carefully remove the cocoons from the mounting frame. Strip away the loose outer silk (cocoon floss) without damaging the cocoon body. The cocoon floss can be used for other purposes (silk floss). Each silkworm cocoon weighs about 1.5-2.5 grams.

Cocoons are sorted by quality into top-grade, second-grade, and low-grade cocoons. Top-grade: well-shaped oval cocoons with thick shell layers, uniform color (white or pale yellow), and no stains. Second-grade: cocoons with slightly poorer shape or minor stains. Low-grade: double cocoons, thin-shell cocoons, and yellow-spot cocoons—these are unsuitable for reeling silk but can be used to make silk floss.

The pupa continues to develop inside the cocoon, and after about 10 days it will emerge as a moth and break out of the cocoon. The broken cocoon filaments snap, making it impossible to reel out continuous silk. Therefore, the pupae must be killed before emergence—to halt their development and keep the cocoon silk intact.

Spread the silkworm cocoons in a single layer on a bamboo tray and expose them to intense sun for 2-3 days. The temperature needs to reach above 40°C. Turn them over every 3-4 hours to ensure even drying. At night, bring them indoors to avoid moisture reabsorption. This is the most traditional and economical method for killing pupae, suitable for small-batch production.

If the weather does not permit sun-drying, cocoons can be dried in a drying room. Temperature should be applied in two stages: first bake at 80-90°C for 2 hours to kill the pupae, then lower to 50-60°C and dry for 6-8 hours.

Cut open a few cocoons with scissors to inspect them. If the pupae have been successfully killed, the pupal bodies will be shrunken, hardened, and dark brown. If the pupal bodies are still soft or contain liquid, further drying treatment is needed. Incomplete killing of the pupae will cause them to rot during storage, contaminating the silk.

Weigh the total weight of dried cocoons. Fresh cocoons lose much of their weight on drying (mainly due to the loss of water from the pupae). Record data such as variety, yield, and cocoon rate to provide reference for the next silkworm-raising batch. One batch of silkworm eggs (about 25,000 silkworms) can produce approximately 40-50 kilograms of fresh cocoons.

Place dried cocoons in breathable cloth bags or bamboo baskets. Storage environment should be dry (relative humidity below 65%), well-ventilated, and protected from light. Pest prevention: a small amount of camphor balls may be placed nearby (but must not directly contact the cocoons). Properly stored dried cocoons can be kept for 6-12 months.

Select the best cocoons to keep as seed cocoons—allow the pupae to emerge normally as moths for mating and egg-laying. Selection criteria for seed cocoons: the most well-formed cocoon shape, the thickest cocoon layer, and no defects whatsoever. Seed cocoons are not killed in the pupal stage; store them separately in a well-ventilated environment at a suitable temperature (25°C) while waiting for emergence.

After harvesting the cocoons, thoroughly clean the mounting frames and silkworm rooms. Disinfect all utensils, as well as the floors and walls of the silkworm room, with a lime water or bleaching powder solution. Pathogens of silkworm diseases (such as muscardine and pebrine) can survive on utensils for several months. Proper disinfection is the key to a successful silkworm rearing season next time.