Mixed White Muscardine and Flacherie in a Fifth-Instar Crop
In a rearing batch of 100 dfls at fifth instar, the rearer reports 18% mortality over three days. Some cadavers are hard and covered with white powdery growth; others are soft, blackish and smell putrid. The rearing room RH is 90% and temperature 25–27°C; trays are made of bamboo and were washed with water only. The previous crop had similar losses, and the same room was used without formalin or bleaching powder disinfection. Mulberry leaves come from a plot with 20% leaf spot and were fed without washing. A few larvae are sluggish with swollen abdomen and white patches on the skin. Advise.
1. Identify the likely disease or diseases, giving symptom-based reasoning and the laboratory tests that would confirm the diagnosis.
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Two syndromes are present. Hard cadavers with white powdery growth are typical of white muscardine caused by Beauveria bassiana, and the white patches on sluggish larvae support this. Soft, blackish, putrid cadavers point to bacterial flacherie, which often occurs as a secondary infection in a humid, poorly ventilated room. The 90% RH and 25–27°C favour both. Unwashed leaf-spot leaves and undisinfected bamboo trays are likely sources. Laboratory confirmation should include microscopic examination of cadavers for fungal hyphae and conidia, bacterial culture on nutrient agar, and, if viral or microsporidian disease is suspected, PCR or spore examination. Symptoms alone are not enough because mixed infections are common.
2. Outline immediate management for the current crop.
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Remove all cadavers and moribund larvae immediately with forceps into 2% formalin or bleaching powder solution; destroy them away from the rearing house. Increase ventilation and reduce RH to 70–75%, spread the larvae and remove wet litter. Stop feeding leaves from the affected plot until they are washed and dried, and use clean leaves if available. Apply an approved bed disinfectant around the larvae and on trays at the label dose. Do not handle healthy larvae after touching cadavers without disinfecting hands. Keep the crop isolated and do not share trays or cocoons. Record mortality daily. If mortality exceeds 20–25%, consult the district sericulture office before spending more on the crop.
3. Design a disinfection and prevention protocol for the next crop.
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Empty the room and clean all surfaces, cracks and corners. Wash trays, nets, mountages and utensils with detergent, then disinfect with 2% formalin or 1% bleaching powder solution, and sun-dry bamboo materials. Fumigate the closed room with formalin and potassium permanganate at the recommended dose, or use an approved fumigant, then ventilate thoroughly before rearing. Use a footbath and separate footwear. Keep the leaf-washing area away from the rearing room. Use disease-free layings, avoid overcrowding, maintain 24–26°C and 65–75% RH, and feed mature clean leaves. Destroy crop residues and litter by composting away from the rearing house. Repeat disinfection between crops, not only once a year.