Key Characteristics of Late Age Silkworms

In sericulture, the larval stage of the silkworm (Bombyx mori) is divided into five distinct instars, separated by four moults. The first three instars are classified as “young age” stages, while the fourth and fifth instars are collectively referred to as “late age” stages. This late age period is the most transformative phase of the silkworm’s larval life – the worm grows explosively, consumes the bulk of its food, and builds up the silk gland in preparation for cocoon spinning. Understanding the defining characteristics of late age silkworms is essential for any sericulturist who wants to maximize cocoon yield and silk quality.

What defines the late age silkworm?
Low tolerance to heat, humidity, and poor ventilation
Shift from tender to mature mulberry leaves
Dramatic increase in leaf consumption
Explosive body growth and silk gland development
Greater resistance to disinfectants, but more vulnerable to pests
Environmental management is non-negotiable

What defines the late age silkworm?

Late age silkworms encompass the third, fourth, and fifth instar stages, though in many sericulture systems – particularly with non-mulberry silk species – the third instar is also grouped into late age rearing protocols. These late age worms require preferably lower temperature and humidity during rearing, in direct contrast to young age silkworms which need warmer, more humid conditions. Physiologically, these larvae are in a high-demand state – their organ systems, especially the silk gland, are developing at an extraordinary rate. Every management decision made during this period has a direct bearing on the final cocoon quality.

Low tolerance to heat, humidity, and poor ventilation

One of the most critical physiological traits of late age silkworms is their reduced tolerance to high temperature, high humidity, and poor air circulation. During late age rearing, the temperature should be maintained at around 26 ± 1°C and humidity at 70 ± 5% – noticeably lower than the conditions preferred during young age rearing. When temperature and humidity rise beyond these thresholds, the rearing bed becomes a breeding ground for pathogens.

If temperature and humidity are too high in the rearing tray, the accumulation of faeces, wasted leaves, and poor air circulation creates conditions suitable for bacteria to multiply in the larvae and rearing bed. This is why proper ventilation in the rearing house is non-negotiable during the late age period. Stagnant, humid air accelerates the breakdown of organic matter in the bed and exposes the larvae to bacterial and viral pathogens that can cause widespread losses.

Sericulturists are advised to provide air circulation to avoid excess humidity inside the room, particularly during moulting when the larvae are especially vulnerable. During rainy or winter seasons, lime powder application is recommended to reduce moisture in the bed and trays.

Shift from tender to mature mulberry leaves

Young age silkworms require tender, high-moisture leaves from the shoot tip. Late age silkworms have entirely different dietary needs. Leaves of medium maturity (sixth leaf onwards) are fed in the third and fourth age, while coarser leaves are fed in the fifth age. The more developed digestive system of late age larvae is capable of processing tougher leaf tissue, and the higher dry matter content in mature leaves provides the protein and carbohydrates needed for rapid growth.

Feeding overly tender or young leaves to late age silkworms is counterproductive – such leaves have excessive moisture and insufficient protein content, which can compromise cocoon quality. On the other end, over-matured and yellow leaves should be rejected, as they may induce disease outbreaks. The sweet spot is leaves from shoots that are appropriately aged – nutritionally dense, structurally firm, and free from pest or fungal damage.

Dramatic increase in leaf consumption

Perhaps the most striking characteristic of late age silkworms is the sheer volume of food they consume. During the fourth and fifth instars, silkworms consume approximately 94% of their total leaf intake, making this period the most feed-intensive phase of the entire rearing cycle. This means almost all the mulberry leaf investment a farmer makes goes toward sustaining the worm during just these final two instars.

Silkworm larvae consume 85% of their food requirement during the fifth instar alone, and 50% of the total labour input is utilized during the last seven days of rearing. This has significant implications for planning mulberry cultivation, leaf harvest schedules, and labour deployment on silk farms. Failing to supply adequate quantities of quality leaf during this window directly reduces cocoon weight and silk ratio.

Explosive body growth and silk gland development

Late age silkworms undergo extraordinary physical transformation. From the beginning of the third instar to the end of the fifth, body volume increases by approximately 29 times. This growth is driven by continuous, intensive feeding and highly efficient food conversion. By the time the silkworm reaches its maximum weight just before spinning, it is about 10,000 times its own weight at hatching – a phenomenal growth achieved in just 20-25 days.

The silk gland, which is rudimentary in earlier instars, undergoes an even more dramatic expansion – growing to about 200 times its earlier size by the end of the fifth instar. At the fifth instar, gland hypertrophy occurs, with a significant increase in cell volume, high silk biosynthesis and secretion, and gland weight estimated to be between 20% and 40% of the total weight of the insect. This is why a mature fifth instar larva appears translucent – the enormously enlarged silk gland is visible through the body wall. In fact, the silk glands are so enlarged as to account for nearly 40% of the body itself, and this is the key indicator that the larva is ready for mounting and cocoon spinning.

Greater resistance to disinfectants, but more vulnerable to pests

Late age silkworms are more resilient to bed disinfectants and chemical treatments compared to young age worms. This is why disinfectant doses are progressively increased with each moult. Bed disinfectants are applied in increasing quantities at each moult – from 50g per 100 disease-free layings (dfls) after the first moult, up to 2,000g by the fourth day of the final instar, totalling 4,000g through the full rearing cycle. The larger, tougher body of late age worms tolerates these treatments that would be lethal to younger larvae.

However, this chemical resilience does not translate into immunity from pests. The larger body size of late age silkworms actually makes them more exposed and attractive to predators and parasites. Stress brought about by malnutrition and metabolic imbalance results in weakened larvae and increased susceptibility to bacterial infection, and the dense population of large, actively feeding worms in a rearing bed creates an environment where pest pressure can escalate rapidly. Uzi fly (Exorista sorbillans), dermestid beetles, and various bacterial pathogens are common threats during this stage. Proper bed sanitation, regular cleaning, and physical barriers against insects are essential protective measures.

Environmental management is non-negotiable

Given all the above characteristics – sensitivity to heat and humidity, high feed demand, rapid physical growth, and pest vulnerability – late age silkworm rearing demands precise environmental control. Common silkworm diseases such as Grasserie, Pebrine, Flacherie, and Muscardine are closely linked to rearing conditions, and avoiding high temperatures, low rearing temperatures, and insufficient humidity control is critical for disease prevention.

The moulting period lasts about 24 hours, during which special care is required and silkworms should not be disturbed. Lime powder is dusted in the rearing bed during this period to reduce humidity to 60-65% RH, which facilitates the moulting process. Maintaining uniform spacing in the rearing bed is equally important – overcrowded beds lead to competition for food, stress, and uneven growth that ultimately reduces the quality of the cocoon harvest.

As the fifth instar draws to a close, the ripened silkworms are identified by their characteristic movement to the corners of the rearing trays, a reduction in size by one-third, and a transparent yellow appearance. These worms must be transferred promptly to mountages for spinning – any delay wastes silk and reduces cocoon quality.

What do you think? Given that late age silkworms consume nearly 94% of their total leaf intake in just the final two instars, how should a silk farmer plan mulberry cultivation to ensure a consistent, high-quality leaf supply throughout this critical window? And with late age worms being simultaneously more resistant to disinfectants yet more vulnerable to pests, what rearing practices do you think offer the best balance between hygiene management and larval protection?

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Table of Contents

What defines the late age silkworm? 🔗

Low tolerance to heat, humidity, and poor ventilation 🔗

Shift from tender to mature mulberry leaves 🔗

Dramatic increase in leaf consumption 🔗

Explosive body growth and silk gland development 🔗

Greater resistance to disinfectants, but more vulnerable to pests 🔗

Environmental management is non-negotiable 🔗