Effects of soil mass watering and covering on Tyrophagus similis Volgin (Acari: Acaridae) soil densities

Tyrophagus similis Volgin (Acari: Acaridae) is an important pest in the agricultural industry, causing extensive damage to new spinach leaves in greenhouses. Therefore, the development of effective control measures is necessary. The effect of mass-watered soil covered with a plastic film on T. similis densities was tested in the laboratory. Vinyl pots containing soil and released T. similis were mass watered. The soil surfaces of watered pots were closely covered with plastic film, except for the control. The densities of T. similis were significantly lower in the soil of pots that were covered for 10 or 14 d after watering than those in the control; however, no significant differences were found in the plot covered for 7 d. In 2015, the effect of mass-watered soil covered with a transparent vinyl film or a light-shading bilayer mulch film was investigated in three experiments in spinach greenhouses in Hokkaido. Although massive damage to the spinach was observed in plots covered with vinyl for 10 d, in most plots covered with vinyl and bilayer mulch for 14 d, T. similis densities were substantially lower and there was less damage than that observed in the control plots regardless of the soil temperature. However, in the plot where algae grew extensively on the soil during the covering period, T. similis densities increased rapidly and the spinach was severely damaged. Results indicated that mass watering and covering is effective to control mites; however, algal proliferation on the soil could cause mite densities to increase again, as algae is a suitable food source. T. similis density collected was determined the following morning at from 1- to 4-d intervals. The damage to spinach was observed in 25 plants in each repetition at approximately 7-d intervals using the following index: 0, no damage; 1, slightly damaged; 2, slightly deformed; 3, deformed; 4, greatly deformed, dwarfing. The extent of the damage was calculated as follows: {(the sum total of indexes/the number of observed spinach × 4) × 100}.