Voltinism and Biology of Silkworm

Voltinism refers to the number of generations a silkworm breed undergoes in a year. Silkworms are categorized into two main types based on voltinism: biovoltines and multivoltines. These types are distinguished by their life cycle patterns, particularly the number of generations they complete annually.

1. Biovoltines

• Lifecycle: Biovoltine silkworms undergo only two life cycles per year. Their eggs alternate between hibernating and non-hibernating states, making them well-suited for temperate climates.
• Silk Quality: The silk produced by biovoltine silkworms is of superior quality, characterized by:
  • Higher silk content
  • Longer filament length
  • Better neatness and cleanliness
  • Lower size deviation and boil-off ratio
  • Higher tensile strength
  • Less variation in evenness of the filament
• Economic Significance: Because of the higher quality silk, biovoltines are more profitable to rear. They are generally preferred by sericulturists (silk farmers).
• Popular Breeds in India: Kalimpong, KA, NB7, NB4D2, NB18,

2. Multivoltines

• Lifecycle: Multivoltine silkworms can complete 5-6 generations in one year. They do not have a hibernation period and their eggs hatch year-round, making them suitable for tropical climates.
• Silk Quality: The silk produced by multivoltine silkworms is of lower quality compared to biovoltines: The cocoon is harder and more brittle. The silk yield is generally lower, and the quality of the filament is not as fine.
• Economic Significance: While the yield is higher in terms of the number of generations, the silk quality is inferior to biovoltine silkworms. This means that while multivoltines are less expensive to produce, they do not offer the same premium value in the market.
• Popular Breeds in India:
  • Mysore multivoltine (found in Karnataka, Tamil Nadu, Andhra Pradesh)
  • Nistari multivoltine (found in West Bengal)
• Hybridization: In some regions, hybrids of multivoltines and biovoltines are bred to combine the higher yield of multivoltines with the superior silk quality of biovoltines. For example, Mysore females are often crossed with biovoltine males like KA or NB4D2 to create hybrids.

Biology of Silkworm

The biological life cycle of the silkworm involves three main stages: the egg, larva (caterpillar), and pupa (cocoon).

1. Egg Stage

• Egg Laying: Female silkworms lay eggs, typically in clusters on the undersides of mulberry leaves. Egg Characteristics: Small, pale white, and seed-like in appearance. The number of eggs laid by a female ranges from 300-400.
• Incubation Period: In summer, the eggs hatch in 10-12 days. In winter, they can take up to 30 days to hatch due to lower temperatures.
• Hibernation: In univoltine races (mainly found in temperate regions), the eggs do not hatch during winter and remain dormant, meaning only one generation occurs each year. Multivoltine races, on the other hand, hatch throughout the year, completing 2-7 generations annually.

2. Larva (Caterpillar) Stage

• Appearance: After hatching, the caterpillar is initially white to dark in color and about 3 mm in length.
• Development: The caterpillar has three pairs of thoracic legs and five pairs of abdominal legs located on the 3rd, 4th, 5th, 6th, and 10th segments. The larvae are reared on mulberry leaves, and they grow by continuously eating the leaves. Initially, small quantities of leaves are provided several times a day, but as the larvae grow larger, a mesh net is placed over them, and the leaves are put on top of the mesh. The larvae crawl through the mesh to access the leaves.
• Molting: The larvae molt 4-5 times during their development, typically every 6-7 days.
• Mature Caterpillar: After about 30-35 days, the larvae are fully grown, and they reach a length of 75 mm. They are creamy white in color at this stage.

3. Pupa (Cocoon) Stage

• Cocoon Formation: The caterpillar spins a cocoon around itself, which is made of a single silk thread. The cocoon is typically 38 mm in length and 19 mm in width, with an oval shape. It can be white or yellow.
• Pupal Characteristics: The pupa inside the cocoon is reddish-brown in color and measures about 25 mm x 7 mm. The pupal stage lasts for about 10-15 days.
• Emergence: When the adult moth emerges, it secretes an alkaline fluid that softens the cocoon and allows the moth to break free.

Silk Yield and Feeding Requirements

• A typical cocoon weighs about 1.8-2 grams, and the shell alone weighs around 0.45 grams. To produce 1 lb (0.45 kg) of silk, approximately 2,500 cocoons are required.
• During their development, about 40,000-50,000 caterpillars can hatch from 28 grams of silk seed, requiring around 337-406 kg of mulberry leaves.