Cocoon Yield and Quality in Silkworm Rearing: Shell Ratio, Filament Length, Denier, Yield per 100 DFLs and How to Improve Them

Cocoon quality is judged on weight, shell ratio, filament length, reelability and denier. FAO reports 19 to 25 percent shell in newly evolved hybrids and 600 to 1,500 m of filament, about 80 percent of it reelable. Yield and quality are set by clean disinfection, warm and humid chawki, cooler and drier late age, 1,200 to 1,400 kg of good leaf per 100 DFLs, moult handling, mounting at 27 to 28 degrees C, and sorting out defective cocoons before reeling. Follow your state sericulture department for exact doses.

Yield is how many kilograms of cocoons you harvest from 100 DFLs. Quality is what the reeler measures in those cocoons: weight, shell, filament and defects. Both are set by hygiene, healthy young worms, the right climate for each instar, good leaf, careful mounting, and sorting before sale.

This guide is about those levers. The basic rearing sequence, silk types, chawki method, mounting and harvest steps are already covered in our hub post on types of silkworm rearing in India, so we do not repeat them. Here every section answers one question: what moves the cocoon number, and by how much, according to Central Silk Board (CSB), CSR&TI and TNAU.

The unit throughout is the DFL, a disease-free laying. One hundred DFLs, also called one ounce, is about 40,000 larvae according to the CSR&TI Pampore farmer FAQ.

CSB gives a house of 18 ft x 12 ft x 10 ft for 100 DFLs, built on an elevated, shady spot with a 3 ft balcony, enough windows and ventilators for free air, and nylon net over every opening. The house must also be airtight enough to disinfect and ventilated enough that carbon dioxide does not build up.

The CSB package page covers the size and netting, and CSR&TI says cross ventilation is needed because a higher share of carbon dioxide in the rearing house is injurious to the larvae. TNAU adds a layout: a main rearing hall, an 8 ft x 8 ft ante room, a leaf preservation room, and a separate chawki room if you follow a two-plot system. It also lists the principles to avoid: damp conditions, stagnant air, direct strong draughts and bright sun (TNAU rearing house page).

• Net every opening. CSB says this keeps out house flies and uzi flies, which parasitise the larvae.
• Keep a wet and dry thermometer inside. Without it you cannot check the instar tables in this guide.
• Choose a washable floor. You disinfect and wash it at least twice per crop.
• Plan equipment before brushing. TNAU lists chawki and late-age trays, ant wells, a leaf chopping board, bed-cleaning nets, shoot racks, chandrikes (mountages), paraffin paper, lime powder and bed disinfectants.

Disinfect the house and appliances twice per crop: about 3 days before rearing and again after rearing ends. CSB gives 5 percent bleaching powder or 2 percent formalin. Spray with doors, windows and ventilators closed so the room is airtight, then ventilate before the worms arrive.

Quantity is 140 ml per sq ft of floor area for a 3 m (10 ft) high room, as both CSB Raigarh and CSR&TI Pampore state. Keep the room ready 3 to 4 days ahead and open doors and windows for 24 hours before brushing so no fumes remain (TNAU).

Hygiene During The Crop: Wash hands with disinfectant before you enter. Let only the people doing the rearing go inside. Sprinkle 5 percent high-grade bleaching powder around the house every 5 to 6 days. Clean the floor with 2 percent bleaching powder solution before and after bed cleaning. Burn or bury diseased worms. Do not borrow rearing appliances and do not use any appliance without disinfecting it (CSB, CSR&TI).

Eggs that are incubated at 25 to 26 degrees C and 80 percent humidity, then darkened at blue pinhead and exposed to diffused light, hatch together. Even hatching gives even first-instar worms, and uneven batches stay uneven to the end. Brush onto tender leaf chopped to 0.5 to 1 cm.

1. Incubate. Spread the eggs in one layer on paraffin paper on a bamboo tray and cover with another sheet. Except in the rainy season, keep wet foam pads around them (CSB).
2. Darken at blue pinhead. Wrap 25 to 50 DFLs in tissue paper and keep them in a black-painted box or under black cloth for 1 to 2 days (CSB).
3. Give light next day. Expose them to diffused sunlight or shade (CSB).
4. Brush onto tender leaf. Move newly hatched worms with a soft brush or feather onto paraffin-coated paper; feed the 3rd and 4th leaf from the shoot tip, chopped to 0.5 to 1 cm (CSB).
5. Dip layings for pebrine control. TNAU recommends surface disinfection of layings in 2 percent formalin for 10 minutes before incubation.

After brushing, cover the tray with wet foam pads and paraffin paper so the first leaf does not dry out (CSB Warangal).

Young worms need warm, humid air and older worms need cooler, drier air. CSB gives 27 to 28 degrees C and 80 to 90 percent humidity for the first two instars. CSR&TI Pampore then steps down to 26, 24 to 25 and 23 to 24 degrees C for instars 3, 4 and 5.

Avoid mounting temperatures above 28 degrees C and humidity beyond 80 percent (CSB). Swings hurt more than a small error in the average: CSB Raigarh lists temperature fluctuation, high humidity and poor leaf as the conditions that predispose silkworms to flacherie. In winter TNAU recommends charcoal stoves or heaters, and during rainy and winter seasons lime powder 60 minutes before feeding resumes to cut dampness.

The CSR&TI FAQ also explains why an instar can drag on: insufficient feeding, or wrong temperature and humidity in the bed. If a batch is slow, read the thermometer before you change anything else.

Shoot rearing, where whole branches are laid on the bed, saves up to 50 percent labour and up to 15 percent leaf according to CSR&TI Pampore. It also keeps leaf fresher, reduces handling and needs less bed area per CSR&TI. TNAU notes it needs about 30 percent more room than leaf rearing.

CSR&TI gives the bed space for shoot rearing per 100 DFLs: 45 to 90 sq ft in the third instar, 90 to 200 sq ft in the fourth and 200 to 450 sq ft in the fifth. TNAU adds practical details: harvest shoots at 1 m height, 60 to 70 days after pruning; store them upright in a dark, cool room with the cut ends in a 3 cm layer of water; lay them across the bed and alternate top and bottom ends so leaf quality mixes. Move third-instar worms on to shoots right after the moult.

CSR&TI also fixes feeding frequency for shoot rearing at two feedings every day at equal intervals in spring and autumn, adjusted to the weather. Watch the bed rather than the clock. Overcrowding or underfeeding gives small larvae (CSR&TI).

A popular hybrid needs about 1,200 to 1,400 kg of leaf per 100 DFLs (CSR&TI Pampore). The fifth instar eats about 85 percent of the total (TNAU). Young worms need tender, moist leaf; older worms need mature leaf. Leaf quality is decided in the garden before the worms are brushed.

CSB gives the early quantities: 5 kg of leaf for the first instar and 18 kg for the second. That is 23 kg out of well over a tonne, so demand is packed into the last instars. TNAU says about half the labour falls in the last seven days. Plan leaf harvest and transport for that window, and cut late-age leaf only from a plantation of 60 days or older (CSB).

• Use the right leaf by instar. Tender leaf from the 3rd and 4th position below the tip for brushing; medium-maturity leaf for instars 3 and 4; coarser leaf for instar 5 (CSB, TNAU).
• Harvest in cool hours and keep leaf moist. CSR&TI says that if shoots will travel more than 30 minutes, cover them with wet gunny or polythene.
• Store shoots upright. CSR&TI suggests bundles of 20 kg kept vertical, which loses the least moisture.
• Reject yellow, over-matured, dirty or over-stored leaf. CSB Raigarh lists these as causes of flacherie.

Thick, succulent, well-fed leaf gives better growth and cocoon weight, and thin pale leaf gives weak worms. Feed and irrigate the garden to your state sericulture department's schedule, and keep chemicals off the leaf you feed, because residue can poison the worms. This is the part of yield that is decided weeks before brushing.

The CSR&TI FAQ gives slightly acidic soil for mulberry (pH 6.2 to 6.8 in Kashmir) and says leaf quality depends on following the recommended package, integrated nutrient management and integrated pest and disease management. We could not open a mulberry fertiliser schedule from a government source, so no per-hectare dose is given here. Ask your department for the dose that suits your variety and irrigation. Our posts on reading a soil health card and soil pH and nutrient lock explain how to read the soil report, and nitrogen deficiency in plants helps tell a nutrient problem from a root problem on pale leaf.

Katyayani Organics sells general soil inputs such as organic fertiliser, manures and soil conditioners, vermicompost fertiliser, humic acid fertiliser, NPK fertiliser and bio-fertilisers. None of these pages mentions mulberry or silkworms, so read each label, test on a small plot, and ask your sericulture officer before using any input on a garden that feeds worms.

Safety: Chemicals And Silkworms: Silkworms are very sensitive to residue on leaf. TNAU warns against spraying commercial B.t. insecticides in nearby mulberry fields, because they can contribute to bacterial flacherie. B.t. is a caterpillar biopesticide (see our post on biopesticides), and the silkworm is a moth caterpillar. We found no single trustworthy waiting period between spraying and feeding, so keep sprays away from leaf you feed, avoid drift from neighbouring fields, and follow the written advice of your state sericulture department. Our post on bio versus chemical inputs explains why even bio-products must match the crop on the label.

Bed cleaning removes litter and left-over leaf so the bed stays dry. Bed disinfectant is a dust applied after each moult to cut disease. TNAU recommends about 4 kg per 100 DFLs over the crop; CSR&TI says to dust half an hour before resuming the worms after each moult and on day 4 of the fifth instar after bed cleaning.

The table is from TNAU, which names products such as Seridust, Resham Jyothi, Vijetha and Sajeevini. CSR&TI Pampore says Vijetha gave the best result among those tested. Bed cleaning in chawki is done with a cotton net of 0.5 cm mesh, and in late age you clean daily in the fourth and fifth instars and replace paraffin paper with clean newspaper (CSB). In many states the department supplies bed disinfectant to farmers free of cost (CSR&TI), so ask before buying.

Moulting is when the worm stops eating and sheds its skin. Do not disturb it. Stop feeding when 97 percent of chawki worms settle, or 90 percent of late-age worms, spread the bed to dry, dust lime if damp, and resume only when every worm is out of moult, so growth stays even.

• Stop feeding at the right moment. 97 percent settled in chawki, 90 percent in late age (CSB).
• Thin and dry the bed. Spread it for drying and aeration and dust lime powder to keep humidity low (CSB).
• Remove paraffin paper and foam pad during the chawki moult (CSB).
• Resume all at once. CSR&TI says all larvae should be resumed when all are out of moult, with an equal quantity of leaf at every part of the bed. Uneven resuming gives small, late worms.
• Avoid cold with damp. CSB Raigarh links low temperature and high humidity with muscardine; dust slaked lime during moults in the rainy season.

Uzi fly is a parasitoid that lays eggs on the larva; CSB reports 10 to 15 percent cocoon crop damage in Karnataka, Andhra Pradesh and Tamil Nadu, worst in the rainy season. Exclusion with nets and self-closing doors is the first defence, backed by traps and a biological control.

The CSB Raigarh page describes the signs as eggs or black scars on the larval body and a maggot exit hole at the cocoon tip. Control steps are: wire mesh or nylon net on windows and doors, self-closing doors with an ante room, inspecting leaf in the verandah for flies before bringing it in, uzi traps from the third instar until spinning, and release of the pupal parasitoid Nesolynx thymus on day 2 of the fifth instar at two pouches per 100 DFLs, moved to the mountages after mounting. Pack the litter in plastic bags for 15 to 20 days, or bury or burn it. Do not dump it in open pits.

Four diseases cause most crop loss: flacherie, grasserie, muscardine and pebrine. Each has a visible sign and a preventable trigger. The disease overview is in our hub post on silkworm rearing types and diseases; this table only reads the signs against yield loss.

CSR&TI notes that grasserie often shows after the fourth moult because the infection starts early. Disinfect before and after every rearing and dust bed disinfectant half an hour before feeding after each moult.

Cocoon quality is judged by weight, shell percentage, filament length, how much of it can be reeled, and the thickness and evenness of the thread. The FAO manual calls shell weight the most consequential factor because it predicts raw silk yield. Rearing and mounting conditions change every one of these.

The FAO and CSB shell figures are for different cocoon types, so read them as two reference points, not a conflict: FAO's range is for newly evolved hybrids, CSB's minimum is for multibivoltine lots.

How mounting and spinning change the cocoon

The FAO manual says low humidity during mounting makes the shell soft, high humidity makes it hard, and a hard shell lowers reelability while a soft one can multiply raw silk defects. Moderate humidity is best. Over-feeding in the fifth instar and mounting over-mature larvae lead to lousiness, hair-like projections in the silk.

• Density and climate. CSB: 40 to 45 worms per sq ft, 27 to 28 degrees C, 60 to 70 percent humidity, aeration to remove moisture, no strong light.
• Mountage type. CSB says a rotary mountage is ideal for easy self-mounting and better quality. CSR&TI warns that dried grass, straw or twigs give less cocooning, more defective cocoons and floss, and lower single cocoon and shell weight.
• Space for spinning. TNAU: the ripe worm needs space equal to the square of its body length; cramped spinning gives double cocoons.
• Stable humidity and even light. TNAU: swings in humidity thin and thicken the filament, and harsh light or dark corners crowd the worms.
• Keep mountages inclined so urine drains and does not stain cocoons (TNAU).
• Harvest on day 5 after mounting and discard flimsy, stained and irregular cocoons (CSB). Do not harvest while the pupa is amber (TNAU).

Defective cocoons are double, flimsy, melted, uzi-infected, urinated or malformed cocoons. CSB's processing guidance says to remove them on a cocoon sorting table, ideally in a dark room with fluorescent light, and not to mix lots with a wide spread in denier or reelability.

CSR&TI Pampore adds frame-printed, soiled, thin-end, deformed and thin-shelled cocoons to the list. Most are mounting faults, not rearing faults, which is why clean, shaded, inclined mountages pay back. Stained cocoons also reel poorly (FAO).

CSB also says to mix only lots with similar single-cocoon filament denier and reelability, which reelers check with a hand-operated epprouvette and balance.

Cocoons have to be dried to kill the pupa before it cuts the shell. CSB's processing page gives a stepped hot-air schedule for bivoltine cocoons from 115 down to 55 degrees C over about 5 hours, then conditioning for 7 to 10 days in a cool, dry room. Over- and under-drying both hurt reeling.

The table is from the CSB processing of mulberry cocoons page. The optimum drying degree uses your sample's shell ratio, so the shell ratio you measure for quality is the same number you need for drying. If reeling will happen within a week, the page gives a shorter partial-drying schedule (110 to 100 degrees C and 100 to 90 degrees C, 45 to 60 minutes each) and says to store partially dried cocoons in thin layers on wire-mesh racks in a ventilated room.

There is no single figure. We found no national average from CSB that we could open. The TNAU chawki-centre model assumes 65 to 70 kg of bivoltine cocoons per 100 DFLs, which is a planning figure, not a field average. Real yield changes with hybrid, season, leaf, disease and chawki quality.

The 65 to 70 kg comes from the planning table on the TNAU chawki economics page. Use it as a target to compare against, and compare also with your own department's records for the same hybrid and season. A fixed yield number from a blog or video is not a promise.

• Healthy chawki sets the ceiling. Weak first-two-instar worms cannot be fixed later.
• Leaf quantity must match the batch. A batch of 100 DFLs needs 1,200 to 1,400 kg of leaf; a shortage cuts the last instars first (CSR&TI).
• Mounting and sorting protect what you grew. Defective cocoons and stained, wet or thin-shelled lots lose value at the reeler.

Chawki rearing, meaning care of the first two instars in a dedicated centre, is meant to give farmers stronger worms. TNAU's chawki-centre model shows a thin margin per batch and a benefit-cost ratio of 1:1.1 to 1:1.24, with the main benefit going to the farmers who receive the worms. Read it as a model from a dated page.

These figures are from the TNAU chawki economics page. We left rupee amounts out because the page is dated. For what chawki rearing involves, see the hub post on silkworm rearing; here the point is only its effect on yield.

Most losses come from a short list of avoidable mistakes: poor disinfection, uneven moult handling, overcrowding, stale or over-mature leaf, temperature swings, and poor mounting. The CSR&TI Pampore FAQ gives the reason behind several of them, and each one is within the rearer's control.

Early chawki mistakes are the hardest to see and the costliest. TNAU's young-age page stresses fresh leaf kept in wet gunny cloth and correct detection of the moult for uniform growth (TNAU chawki page).

Conclusion

Cocoon yield and quality are made across the whole crop, not at the end. Disinfect twice, keep the right climate for each instar, feed 1,200 to 1,400 kg of good leaf per 100 DFLs, handle moults so growth stays even, mount only ripe worms at 27 to 28 degrees C, and sort and dry the cocoons by Central Silk Board norms. Judge your result against shell ratio, filament length and reelability, not weight alone. Keep every chemical off the leaf you feed and follow your state sericulture department for doses.