Plant-pest surveys on acarid mites damaging vegetable seedlings were conducted in 63 districts of Hokkaido for 10 years from 1981 to 1990. In 39 districts, at least one of five species of the acarid mites,
Tyrophagus putrescentiae (SCHRANK),
T. similis VOLGIN,
T. perniciosus ZACHVATKIN,
Acarus immobilis GRIFFITHS and
Rhizoglyphus robini CLAPARÈDE, was collected from seedlings, soil, rice chaff and/or rice straw. Three species of the genus
Tyrophagus were damaging to vegetable seedlings:
T. putrescentiae; melon, watermelon cucumber and pumpkin,
T. similis; melon, watermelon, cucumber, pumpkin, tomato, sweet pepper and welsh onion,
T. perniciosus; melon, cucumber and pumpkin. The symptoms of damage by acarid mites are summarized as follows: (a) melon, cucumber, pumpkin; numerous small holes and yellowish spots on young leaves which later become deformed, (b) watermelon, tomato, sweet pepper; leaves become lustrous, discolored and deformed, (c) tomato, welsh onion; seedlings become dwarfed and stunted. A survey on the source of acarid mites in greenhouses showed that the mites originate from rice chaff and/or rice straw used for lagging so these mites are introduced into the greenhouse. A tarsonemid mite,
Tarsonemus bilobatus SUSUKI, which may be harmful to vegetables, was also collected from melon, watermelon, cucumber and Chinese cabbage. The symptom of damage is lustrous, discolored and deformed leaves with irregular folding of the upper surface.
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