The seasonal fluctuation and species composition of spider mites and their predators on soybean were investigated in an agrochemical sprayed plot and an unsprayed control plot from 1999 to 2001 in Ibaraki, central Japan. Sprayed plots were treated with cypermethrin and fenitrothion (1999), acephate and imidacloprid (2000), and permethrin (2001). In both plots, spider mite populations showed three peaks in 1999 (June, August and October-November), and two in 2001 (August and September-November). In 2000, the populations peaked only once (August), because soybean flowered and fruited more than one month earlier than in other years and as a result, all leaves had dropped by late September. Of the spider mite species sampled, Tetranychus kanzawai Kishida, T. parakanzawai Ehara, T. pueraricola Ehara et Gotoh and T. urticae Koch (green form) dominated in both plots and at almost all times. Predators were phytoseiid mites, such as Gynaeseius liturivorus (Ehara), Amblyseius tsugawai Ehara, Neoseiulus californicus (McGregor), and N. womersleyi (Schicha), and insects like Scolothrips takahashii Priesner and Feltiella sp. The population density of G. liturivorus was positively related to the occurrence of phytophagous thrips until August when the thrips became scarce. Synchronized with spider mite density were phytoseiid mites other than G. liturivorus after August, and Scolothrips takahashii throughout the observation periods. Agrochemical spraying had an initial detrimental effect on phytoseiid mites and S. takahashii, but they had recovered by two-four weeks after spraying, suggesting that the effect of chemical spraying on predators was weak.

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