Correcting High Temperature, Humidity and Overcrowding in a Late-Age Rearing
A rearer has 50 dfls of a bivoltine hybrid in a 6 m × 4 m mud-walled room with an asbestos roof. At the start of the fourth instar, room temperature is 31–33°C and relative humidity 85%; there is one small window and no exhaust fan. Larvae are kept in bamboo trays stacked four high, with about 300 larvae per tray, and several trays contain wet litter and leftover chopped leaves. The rearer feeds tender top leaves four times a day and has not cleaned the beds for three days. The larvae are uneven: about 15% are still in the third instar, and a few are translucent with gut juice vomited on the leaf surface. The rearer wants to increase feeding to speed up growth and to close the window to keep the room warm. Labour is limited to two family members. Advise.
1. Identify the main rearing management faults and explain how each affects larval health.
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The main faults are high temperature (31–33°C), high RH (85%), no ventilation, overcrowding at about 300 larvae per tray, wet dirty beds, and feeding tender leaves to fourth-instar larvae. Late-age silkworms do best around 24–26°C and 65–75% RH. At 31–33°C with 85% RH, litter ferments, ammonia accumulates and bacterial flacherie spreads quickly. Tender leaves are suitable for young larvae but are low in fibre for late-age larvae, causing watery frass and digestive upset. Uneven larvae reflect uneven temperature, leaf quality and spacing. The translucent larvae with vomited gut juice are a warning of flacherie, not a reason to feed more. Closing the window would make the humidity and ammonia problem worse.
2. Give immediate corrective actions and a revised feeding and bed-cleaning schedule.
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Open the window and use a fan or exhaust fan to bring the room to 24–26°C and 65–75% RH; do not close the room. Spread the larvae into more trays at lower density, because late-age larvae need roughly three to four times the area they had in early instar. Remove wet litter and leftover leaves, clean beds daily, and dust an approved bed disinfectant. Stop feeding tender leaves; give mature, dry, chopped leaves in small quantities three or four times a day and remove uneaten leaves. Isolate and destroy the translucent or vomiting larvae. Reduce feeding until temperature and litter are corrected, because more feed now would ferment and increase disease. Keep a simple daily record of temperature, RH, feeding and mortality so the effect of the changes can be judged.
3. What disease surveillance and hygiene measures should be started now?
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Start surveillance at every feed: look for sluggish larvae, vomiting, diarrhoea, shiny skin or cadavers. Remove sick larvae with forceps into a disinfectant jar and never throw them on the floor. Wash hands in disinfectant before handling each tray, and keep separate footwear and a footbath at the door. Do not share trays, leaf baskets or nets with other rearers. Collect litter in a covered bin and dispose of it away from the rearing house. Send a few fresh cadavers to the local sericulture office or laboratory for diagnosis. After the crop, disinfect the room, trays and equipment, and sun-dry the bamboo trays. Use only disease-free layings from an approved source next time.