Unit 4: Pests and Diseases of Silkworms

1. Pests of Silkworm

Pests are organisms that cause damage to silkworms or their products, leading to economic losses in sericulture. They can affect silkworms at various stages of their life cycle, from larvae to pupae and cocoons.

Uzi fly (Exorista bombycis)

The Uzi fly is a highly destructive endoparasitoid of silkworms, particularly prevalent in tropical sericultural regions. It belongs to the family Tachinidae.

Definition: An Uzi fly is a parasitic fly whose larvae develop inside silkworm larvae, feeding on their internal tissues and ultimately causing the death of the host.

Identification and Life Cycle

• Adult Fly: Resembles a housefly but is larger (about 10 mm long) and darker, with prominent red eyes and black bristles on its body.
• Egg Laying: Adult female Uzi flies lay small, whitish, oval-shaped eggs (about 0.5 mm) directly on the integument (skin) of silkworm larvae, usually on the anterior segments (thoracic and first abdominal segments). A single silkworm can host multiple eggs.
• Larval Stage (Maggot): The eggs hatch in 2-3 days, and the tiny maggots bore into the silkworm's body. They feed on the internal organs and hemolymph of the silkworm, growing rapidly.
• Pupation: After 5-7 days, the fully grown maggots (about 10 mm) emerge from the silkworm's body, often killing it in the process. They drop to the ground and pupate in the soil or rearing bed.
• Emergence: Adult flies emerge from pupae in about 10-12 days, completing their life cycle.

Damage Caused

• Reduced Silk Production: Infested silkworms are weakened, consume less food, and develop poorly. This results in reduced silk gland development and the spinning of flimsy, loose, or defective cocoons.
• Larval Mortality: The emergence of maggots from the silkworm body invariably leads to the death of the host larva, causing significant direct loss.
• Qualitative Loss: Silk quality is compromised due to poor cocoon formation.

Control and Prevention

• Physical Barriers: Install fine mesh nets (Uzi nets) on doors and windows of rearing houses to prevent adult Uzi flies from entering.
• Egg Removal: Regularly inspect silkworms during the larval stages (especially from 3^rd instar onwards). Carefully remove Uzi fly eggs using a soft feather, camel hair brush, or forceps, and destroy them by burning or immersing in disinfectant.
• Traps: Use attractant traps (e.g., Uzi trap with fish extract, overripe fruits, or synthetic attractants) placed outside the rearing house to lure and capture adult flies.
• Biological Control: Release of hyperparasitoids like Nesolynx thymus or Dirhinus anthracia, which parasitize Uzi pupae, can help in population management.
• Hygiene: Maintain strict sanitation. Promptly collect and dispose of dead or pierced silkworms and cocoons to prevent the Uzi fly's life cycle from continuing.

Dermestid Beetles (e.g., Dermestes maculatus, Dermestes frischii)

Dermestid beetles are common pests of stored products and dried animal matter. In sericulture, they primarily infest stored cocoons and silkworm pupae, causing significant economic losses.

Definition: Dermestid beetles are scavenger insects whose larvae feed on dried animal matter, including silkworm pupae within cocoons, leading to damage to the cocoons and silk.

Identification and Life Cycle

• Adult Beetle: Small, oval-shaped beetles, typically dark brown or black, often with a grayish underside. They are about 5-10 mm long.
• Egg Laying: Females lay eggs on or near suitable food sources, such as stored cocoons.
• Larval Stage: The larvae are hairy, elongated, and segmented, often referred to as 'woolly bears'. They are the most damaging stage, actively feeding on dried pupae inside cocoons.
• Pupation: Larvae pupate in sheltered spots.
• Emergence: Adults emerge and continue the cycle. The entire life cycle can range from 1 to 3 months depending on conditions.

Damage Caused

• Cocoon Damage: Both larvae and adults bore holes through cocoons to feed on the pupae inside. These holes make the cocoons unsuitable for reeling, causing direct loss of silk.
• Silk Contamination: Frass (excrement) and larval skins can contaminate the silk.
• Economic Loss: Significant reduction in the quantity and quality of reelable cocoons.

Control and Prevention

• Storage Hygiene: Thoroughly clean cocoon storage rooms before storing new batches. Remove all old cocoons, pupal shells, and debris.
• Air-tight Storage: Store cocoons in airtight containers or well-sealed rooms to prevent beetle entry and infestation.
• Fumigation: Infested cocoons or storage areas can be fumigated with approved insecticides (e.g., phosphine) under strict supervision.
• Sun Drying: Exposing cocoons to direct sunlight for a few days can kill larvae and pupae within them, provided the temperatures are high enough.
• Inspection: Regular inspection of stored cocoons for signs of infestation.

Vertebrates

Various vertebrate animals can act as pests in sericulture, causing damage by directly preying on silkworms or consuming mulberry leaves, leading to economic losses.

Definition: Vertebrate pests in sericulture are animals with backbones (e.g., rodents, birds, lizards) that directly harm silkworms, consume their food, or damage rearing infrastructure.

Common Vertebrate Pests and Damage

• Rats and Mice: These rodents are highly destructive. They feed on silkworm larvae and pupae, particularly during the later instars and cocooning stages. They can also gnaw through rearing equipment and structures.
• Birds (e.g., Sparrows, Crows): Birds can enter rearing houses and feed on silkworm larvae, especially during active feeding periods.
• Lizards and Geckos: These reptiles prey on silkworms, particularly smaller larvae, and can be found hiding in crevices within rearing houses.
• Ants: While invertebrates, ants (especially red ants) can cause significant damage by attacking and carrying away young silkworms. They can also damage mulberry leaves.

Control and Prevention

• Rodent Proofing: Construct rearing houses with rodent-proof materials. Seal all cracks, crevices, and openings. Use metal guards on doors. Employ traps (snap traps, live traps) and baits (used cautiously to avoid secondary poisoning and contamination) to manage rodent populations.
• Bird Netting: Install nets on windows and doors to prevent birds from entering the rearing house.
• Lizard Control: Seal cracks and holes in walls where lizards might hide. Regular cleaning and removal of clutter can discourage them.
• Ant Barriers: Apply ant repellents (e.g., chalk lines, turmeric powder, or ant baits designed for indoor use) around rearing stands or sprinkle bleaching powder. Ensure the rearing stand legs are placed in water troughs to create a physical barrier.
• General Sanitation: Keep the rearing area clean and free from food scraps or debris that might attract pests.
• Protective Fencing: For outdoor mulberry gardens, fencing can deter larger herbivores from consuming leaves.

2. Pathogenesis of Silkworm Diseases

Silkworms are susceptible to a range of diseases caused by various pathogens, including protozoa, viruses, fungi, and bacteria. The pathogenesis refers to the mechanism by which these pathogens cause disease and the subsequent development of symptoms.

Protozoan Diseases (Pebrine)

Pebrine is the most serious and devastating disease of silkworms, capable of causing complete crop failure. It is caused by a microsporidian protozoan.

Pathogen: Nosema bombycis (a microsporidian protozoan).

Symptoms

• Early Stages: Often asymptomatic in young larvae.
• Later Stages: Infected larvae show irregular growth, poor appetite, black spots or dots (peppering) on the body integument, sluggishness, and ultimately death.
• Pupae: Infected pupae often fail to emerge or produce deformed moths.
• Moths: Infected moths show black spots on the abdomen and wings, are weak, and lay fewer eggs.

Transmission and Pathogenesis

• Primary Contamination (Transovarial): The most dangerous mode. If the mother moth is infected, the pathogen (spores) is transmitted directly to the eggs. These eggs hatch into infected larvae, ensuring the disease perpetuates across generations.
• Secondary Contamination (Oral/Environmental): Healthy larvae can become infected by consuming mulberry leaves contaminated with spores from diseased larvae, their excreta, or contaminated rearing equipment.
• Pathogenesis: Once ingested, spores germinate in the silkworm gut, releasing sporoplasms that invade gut epithelial cells. They multiply rapidly and then spread to various tissues and organs, including silk glands, fat body, muscles, and reproductive organs. The extensive multiplication of the parasite damages these tissues, impairing physiological functions and leading to the observed symptoms and eventual death.

Control and Prevention

• Mother Moth Examination (MME): This is the most critical control measure. Examine individual mother moths microscopically after egg laying to ensure they are free from Nosema bombycis spores. Only eggs from disease-free moths should be used for rearing.
• Disinfection: Thorough disinfection of rearing houses, equipment, and appliances with 2% formalin solution before and after each crop.
• Hygiene: Maintain strict hygiene throughout the rearing period. Avoid overcrowding and remove diseased larvae promptly.
• Separate Rearing: Rearing of young silkworms (chawki rearing) should be done in separate, highly hygienic facilities.

Viral Diseases

Viral diseases are common and can cause significant losses in silkworm rearing. The most prominent viral diseases are Grasserie and Flacherie.

Definition: Viral diseases in silkworms are caused by pathogenic viruses that replicate within the host cells, leading to characteristic symptoms and often death.

Grasserie (Nuclear Polyhedrosis Virus - NPV)

Pathogen: Bombyx mori Nucleopolyhedrovirus (BmNPV).

• Symptoms: Infected larvae become sluggish, lose appetite, and their integument becomes swollen and shiny. The body becomes fragile and ruptures easily, releasing a turbid, milky white fluid (hemolymph) containing numerous polyhedral inclusion bodies (PIBs). The larva hangs upside down from its prolegs.
• Transmission and Pathogenesis: Spores are orally ingested from contaminated leaves. The virus enters the gut epithelial cells, replicates in the nucleus, and forms polyhedra. These polyhedra are released and infect other cells and tissues, eventually leading to the lysis (rupture) of host cells and tissues, causing the milky fluid appearance.
• Control and Prevention: Strict hygiene, disinfection with bleaching powder (0.5%) or formalin (2%), feeding quality mulberry leaves, maintaining optimal temperature and humidity, and avoiding stress factors like overcrowding or rough handling.

Flacherie (Bacterial and Viral Flacherie)

Flacherie is a general term for diseases characterized by softening and rotting of the silkworm body. It can be caused by viruses or bacteria, or a combination of both.

Viral Flacherie Pathogen: Bombyx mori Infectious Flacherie Virus (BmIFV), or Densonucleosis Virus (BmDNV).

• Symptoms: Larvae become sluggish, lose appetite, and vomit gut fluid. The body becomes soft, flaccid, and appears dark. When ruptured, the gut contents are turbid and putrid.
• Transmission and Pathogenesis: Viruses are orally ingested. They replicate in the gut epithelial cells, causing digestive dysfunction. This leads to accumulation of undigested food and toxins, eventually causing paralysis and death. Often, secondary bacterial infections complicate viral flacherie.
• Control and Prevention: Similar to Grasserie – maintain strict hygiene, disinfect rearing rooms and equipment, provide good quality mulberry leaves, and manage environmental conditions.

Fungal Diseases (Muscardine)

Muscardine diseases are caused by various fungi, which infest the silkworm externally and internally, leading to mummification of the larva.

Pathogen: The most common is Beauveria bassiana (causing White Muscardine). Other species cause green, yellow, or black muscardine.

Symptoms

• Early Stages: Larvae become sluggish, lose appetite, and develop dark spots on the body.
• Later Stages: The silkworm body hardens and becomes chalky white (in white muscardine) due to the fungal mycelial growth. The larva becomes rigid, resembling a mummy. This mummified cadaver is covered with powdery fungal spores.

Transmission and Pathogenesis

• Primary Contamination: Fungal spores, present in the environment (soil, dust, infected leaves), adhere to the silkworm's cuticle.
• Infection: Under high humidity, the spores germinate, penetrate the cuticle, and multiply within the hemocoel (body cavity).
• Mummification: The fungus consumes the internal tissues, producing toxins and causing the larva to die. After death, the fungus grows externally, covering the entire body with a powdery layer of spores.
• Predisposing Factors: High humidity, low temperature, and poor ventilation favor fungal growth and infection.

Control and Prevention

• Hygiene and Disinfection: Thorough disinfection of rearing houses and equipment using 2% formalin solution or bleaching powder.
• Environmental Control: Maintain optimal temperature and humidity in the rearing room. Ensure proper ventilation to reduce humidity.
• Dusting: Apply antifungal dusting powders (e.g., lime powder, Dithane M-45, or Resham Keet Oushadh) over silkworms and rearing beds, especially during periods of high humidity.
• Isolation: Isolate and destroy diseased larvae immediately to prevent the spread of spores.
• Cocoon Disinfection: Disinfect newly harvested cocoons to kill any dormant spores.

Bacterial Diseases (Bacterial Flacherie, Septicemia)

Bacterial diseases are often associated with poor hygiene, contaminated food, and stressful rearing conditions. They can cause rapid mortality.

Pathogen: Various bacteria, including species of Streptococcus, Staphylococcus, Bacillus, and others.

Symptoms

• Bacterial Flacherie: Similar to viral flacherie, larvae become sluggish, lose appetite, and may vomit. The body softens and rots, often emitting a foul odor. The gut may swell and appear dark.
• Septicemia: Bacteria enter the hemocoel and multiply rapidly, leading to blood poisoning. Larvae become dull, lose appetite, and may show small black spots. Death occurs rapidly, and the body turns black after death.

Transmission and Pathogenesis

• Oral Contamination: Most bacterial infections occur through the ingestion of contaminated mulberry leaves (e.g., leaves contaminated with bacterial spores from excreta or soil).
• Wound Infection: Some bacteria can enter through wounds in the cuticle.
• Pathogenesis: Bacteria multiply in the gut or hemocoel, producing toxins that poison the silkworm and break down tissues, leading to symptoms like digestive problems, liquefaction of tissues, and ultimately death.
• Predisposing Factors: Unhygienic conditions, feeding poor quality or stale leaves, overcrowding, high humidity, and temperature fluctuations.

Control and Prevention

• Hygiene and Sanitation: Paramount for bacterial disease control. Thorough cleaning and disinfection of rearing rooms and equipment with bleaching powder or formalin.
• Feed Quality: Feed only fresh, clean, disease-free mulberry leaves. Avoid feeding wet, spoiled, or contaminated leaves.
• Environmental Management: Maintain optimal temperature and humidity. Ensure good ventilation.
• Early Detection: Promptly remove and dispose of diseased larvae to prevent further contamination.
• Antibiotics: In some cases, specific antibiotics might be used as a preventive measure, but this is less common in general practice and requires careful consideration.

Control and Prevention of Pests and Diseases

Effective management of pests and diseases is fundamental to successful sericulture. A holistic approach combining various strategies is essential to minimize losses and ensure healthy silkworm crops.

General Control and Prevention Measures

1. Hygiene and Sanitation (Aseptic Rearing)

• Disinfection: Thoroughly disinfect rearing houses, appliances, and trays before and after each crop using recommended disinfectants like 2% formalin solution or 0.5% bleaching powder. Ensure all surfaces are treated.
• Cleaning: Regular cleaning of rearing beds to remove silkworm excreta (faeces) and leftover leaves, especially at regular intervals and before each feeding.
• Waste Disposal: Prompt and proper disposal of diseased larvae, dead silkworms, and infected cocoons by burning or burying to prevent pathogen build-up and spread.

2. Environmental Management

• Temperature and Humidity Control: Maintain optimal temperature (24-28 °C) and humidity (70-85%) as specific conditions can stress silkworms and favor pathogen growth.
• Ventilation: Ensure adequate ventilation in the rearing room to provide fresh air and prevent accumulation of harmful gases and excessive humidity, which can promote fungal and bacterial growth.
• Light Management: Control light entry, as pests like Uzi fly are attracted to light.

3. Quality of Silkworm Eggs and Food

• Disease-Free Eggs: Always use silkworm eggs (layings) certified to be disease-free, particularly through rigorous Mother Moth Examination for Pebrine.
• Fresh, Clean Mulberry Leaves: Feed only healthy, fresh, and clean mulberry leaves. Avoid feeding wet, dusty, diseased, or contaminated leaves. Ensure leaves are free from pesticide residues.

4. Physical Barriers and Exclusion

• Nets and Screens: Install fine mesh nets on windows and doors of rearing houses to prevent the entry of flying pests like Uzi flies and birds.
• Rodent Proofing: Construct and maintain rearing houses to be rodent-proof, sealing all entry points.
• Ant Barriers: Use ant wells (placing rearing stand legs in water troughs) or ant repellents to protect silkworms from ant attacks.

5. Cultural Practices

• Avoid Overcrowding: Provide sufficient space for silkworms to prevent stress, competition for food, and rapid spread of disease.
• Optimal Feeding: Follow recommended feeding schedules and quantities to ensure adequate nutrition and prevent accumulation of leftover leaves.
• Careful Handling: Handle silkworms gently to avoid injuries that can serve as entry points for pathogens.

6. Early Detection and Segregation

• Regular Inspection: Regularly inspect silkworms for any signs of disease or pest infestation.
• Isolation: Immediately isolate and remove any diseased or infested silkworms from the healthy batch to prevent further spread.

7. Biological and Chemical Control (Used Cautiously)

• Biological Control: In some cases, natural enemies or parasitoids can be used to control pests (e.g., hyperparasitoids for Uzi fly).
• Chemical Control: Use of disinfectants is critical. Insecticides for pests should be used with extreme caution and only as a last resort, as silkworms are highly sensitive to most chemicals. Antifungal dusting powders can be used for muscardine.