Studies on Acarid Mites(Acari: Astigmata) Damaging Vegetable Plants. II. Damage to Vegetable Seedlings.

Abstract

Plant pest surveys on acarid mites damaging vegetable seedlings were conducted in 63 districts of Hokkaido for 10 years in 1981-90. In 39 districts, at least 1 of 5 species of the acarid mites, Tyrophagus putrescentiae, T. similis, T. perniciosus, Acarus immobilis and Rhizoglyphus robini, was collected from seedlings, soil, rice, chaff and/or rice straw. Three species of the genus Tyrophagus were damaging to vegetable seedlings: T. putrescentiae on melons, watermelons, cucumbers and pumpkins; T. similis on melons, watermelons, cucumbers, pumpkins, tomatoes, sweet peppers [Capsicum] and Welsh onions [Allium fistulosum]; and T. perniciosus on melons, cucumbers and pumpkins. The symptoms of damage by acarid mites are summarized as follows: on melon, cucumber and pumpkin seedlings - numerous small holes and yellowish spots on young leaves which later became deformed; on watermelon, tomato and Capsicum seedlings - leaves became lustrous, discoloured and deformed; on tomato and A. fistulosum seedlings - seedlings became dwarfed and stunted. A survey on the source of acarid mites in greenhouses showed that the mites originated from rice chaff and/or rice straw used for lagging. A tarsonemid mite, Tarsonemus bilobatus, which may be harmful to vegetables, was also collected from melon, watermelon, cucumber and Chinese cabbage seedlings. The symptoms were lustrous, discoloured and deformed leaves with irregular folding of the upper surface.