Third Instar Mortality in a Bivoltine Batch
Rajeswari rears two boxes of bivoltine silkworm in Salem. On day 2 of the third instar, 15% of larvae are sluggish, some vomit gut juice, and faecal matter is soft and stained. Rearing room temperature is 30–32°C and relative humidity 55%. Leaf from a four-year-old mulberry garden was harvested at 10 a.m. and kept in wet gunny bags for six hours before feeding; it is now wilted. She disinfected the rearing house with 2% formalin before rearing but has not cleaned the beds for two days. She feeds twice a day. She asks whether the problem is disease, leaf quality or weather, and what to do immediately and for the next batch.
1. Is the problem mainly disease, leaf quality or weather? Support your judgement.
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It is likely a combination, with leaf quality and weather as the main triggers and disease as a secondary consequence. The rearing room is 30–32°C with 55% relative humidity; bivoltine silkworms do best around 24–26°C and 75–85% relative humidity, so the air is both too warm and too dry. Leaf harvested at 10 a.m., held in wet gunny bags for six hours and then fed wilted has lost moisture and may have fermented; wilted, heated leaf is poorly digested and irritates the gut. The soft, stained faeces and vomiting of gut juice suggest flacherie-type bacterial trouble rather than grasserie, which usually shows white milky skin. The two days without bed cleaning allowed faecal contamination and humidity to build up. Formalin disinfection alone cannot prevent this.
2. What immediate actions should she take in the rearing house?
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First, cool and humidify the room: increase ventilation, spread wet gunny cloths or use a humidifier to raise relative humidity toward 80%, and bring temperature down toward 26°C. Remove diseased and dead larvae, do bed cleaning now, and dust the beds lightly with lime powder. Stop feeding the wilted leaf; collect fresh leaf, shade it, and feed in small quantities more frequently. Do not heap leaf in wet bags for long. Separate affected trays from healthy ones and wash hands between trays. Disinfect the rearing room, implements and leaf chamber after cleaning. If mortality continues, consult the local sericulture extension officer before using any drug, because the cause may be viral or bacterial and treatment differs.
3. What should she change for the next rearing to prevent a repeat?
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Plan the rearing house for the season. Disinfect with 2% formalin or the recommended bleaching powder solution, then dust with lime, and allow it to dry before rearing. Maintain 24–26°C and 75–85% relative humidity throughout, using ventilation and wet gunny cloths in hot weather. Harvest mulberry in the cool morning or evening, transport it in baskets rather than tightly packed wet bags, and feed it fresh; avoid leaf that has heated or wilted. Feed four times a day in the early instars and clean beds daily. Keep appropriate spacing so larvae are not crowded. Use disease-free eggs from an approved grainage, and follow a disinfection schedule for hands, trays and the leaf room. Keep a simple record of temperature, humidity and mortality.