Seasonal occurrence of specialist and generalist insect predators of spider mites and their response to volatiles from spider-mite-infested plants in Japanese pear orchards

摘要

In two adjacent Japanese pear orchards (orchards 1 and 2), we studied the seasonal occurrence of the Kanzawa spider mite, Tetranychus kanzawai, and its predators. Also the response of these predators to the volatiles from kidney bean plants infested with T. kanzawai was investigated using trap boxes in orchard 1. The mite density in orchard 1 was unimodal, with one peak at the end of August. In this orchard, population development of the specialist insect predators, Scolothrips takahashii, Oligota kashmirica benefica and Stethorus japonicus, was almost synchronized with that of the spider mites. These predators disappeared when the density of their prey became very low in mid-September. Both S. takahashii and O. kashmirica benefica abruptly increased in number in orchard 2 when the spider mite population in orchard 1 decreased. These results suggested that some of the predators migrated from orchard 1 to orchard 2. In this period, predator-traps with T. kanzawai-infested bean plants attracted significantly more S. takahashii than traps with uninfested plants. Very few individuals of S. japonicus and O. kashimirica benefica were found in the traps, despite their abundance in orchard 1. The generalist insect predator, Orius sp., was attracted to the traps throughout the experimental period irrespective of the density of spider mites, although this predator was never observed inside the orchards.