The silkworm, Bombyx mori, is a fully domesticated insect raised for silk production, a practice known as sericulture. Sericulture has been a human activity for thousands of years, primarily originating in China. Unlike its wild relatives, the domestic silkworm is completely reliant on human care and a highly specialized diet. The quality and abundance of the food provided directly determines the success of the harvest.
The Natural and Essential Food Source
The natural diet of the Bombyx mori silkworm is almost exclusively the leaves of the mulberry tree (Morus species). Because of this singular focus, the insect is classified as monophagous. This diet is driven by specific chemical cues in the leaf that act as powerful feeding stimulants for the larva. Silkworms particularly prefer the white mulberry, Morus alba, which is cultivated globally to support sericulture.
The leaves of Morus alba contain a precise balance of compounds that trigger the silkworm’s feeding response, including specific volatile chemicals. For instance, a jasmine-scented compound called cis-jasmone, present in the leaves, activates the silkworm’s antennae. Other chemo-factors, such as the flavonoid morin and the sterol \(\beta\)-sitosterol, also act as stimulants, ensuring the silkworm consumes the plant material. This chemical dependency explains why the silkworm refuses to eat most other plants.
Nutritional Requirements and Growth Stages
Silkworm larvae require a diet rich in protein, moisture, and specific micronutrients to support their rapid growth through five larval stages. The protein content in mulberry leaves, typically 18 to 26% of the dry weight, is fundamental. Amino acids derived from these proteins are the direct precursors for the silk protein fibroin. Essential amino acids like lysine, leucine, and arginine must be obtained directly from the diet because the silkworm cannot synthesize them internally.
Moisture content, typically ranging between 70 and 77% in fresh leaves, is important, facilitating both the palatability and the efficient digestion of the ingested food. Silkworms, like all insects, cannot produce sterols on their own, making the phytosterols in the leaves necessary for their survival and development. The feeding demands of the silkworm increase dramatically throughout its life. The final two stages, the fourth and fifth instars, account for approximately 80% to 94% of the larva’s total lifetime food intake.
Artificial Diets and Substitutes
To address seasonal leaf availability and labor-intensive harvesting, researchers developed artificial diets to replace or supplement fresh mulberry leaves. These synthetic diets, often formulated as moist pellets or powders, mimic the nutritional profile of the natural food source. They are typically composed of dried mulberry leaf powder, a protein source like soybean meal, carbohydrates such as starch, and a blend of vitamins and minerals.
A major advantage of artificial diets is the ability to provide a consistent nutritional mixture throughout the year. This consistency allows for better standardization of silkworm rearing, especially in controlled laboratory settings or industrial-scale production. However, silkworms sometimes show reduced acceptance of these diets, which may result in slightly longer developmental periods or reduced silk performance compared to those fed fresh leaves.
How Diet Influences Silk Production
The quality and composition of the silkworm’s diet impact the characteristics of the spun silk fiber and cocoon. Protein deficiency in the leaves, for instance, can result in lower cocoon weight and a reduced cocoon shell percentage. The proper balance of amino acids is influential; supplementing the diet with specific amino acids, such as threonine, can increase the tensile strength and toughness of the resulting silk thread.
The diet can also determine the color of the silk, as the silkworm incorporates certain pigments from its food directly into the cocoon. Wild silkworms that consume leaves containing high levels of pigments will spin naturally gold or yellow cocoons. By introducing specific dyes or other compounds into the larval diet, such as cellulose nanofibers, it is possible to create naturally colored or enhanced silk fibers with improved mechanical properties. However, incorporating dyes can sometimes result in a slight reduction in cocoon and shell weight.