In sericultural farms, Bombyx mori nucleopolyhedrovirus (BmNPV), a causal agent of the inside-stained cocoon problem, would be mainly distributed in the mounting room. We revealed this fact by a classical method of the “dust feeding bioassay”, often used in the epizootiological research for the silkworm viruses. In 2003, dust samples of 13 to 16 sericultural farms located in southern area of Ibaraki, Japan, were collected from the rearing room, mulberry stock room, mounting room and entrance of the farmer’s house immediately before the spring rearing and just after the spring, summer, early-, and late-autumn cocoon harvest, and then bioassayed with artificial diets using B. mori second instar larvae to detect BmNPV. The initial survey, immediately before the spring rearing in May, revealed that BmNPV was dispersed around the farming area and mainly detected from the mounting rooms in 8 of 13 farms. The BmNPV distributional traits just after the cocoon harvest each season also suggested that the mounting room was the most contaminated with BmNPV. In September 2013, ten years later from the first survey, we reconfirmed the BmNPV accumulation in a mounting room of a sericultural farm located in Tochigi, Japan, where the cocoon yield was heavily damaged by a nucleopolyhedrosis in the early autumn season. According to a series of our surveys, the mounting room would be the most important place for the BmNPV control strategy in the sericultural farm.
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