Economic Injury Level of Two Spider Mites, Tetranychus urticae KOCH and T. kanzawai KISHIDA in Watermelon

Abstract

The Economic injury level of two species of spider mites (Tetranychus urticae KOCH and T. kanzawai KISHIDA) on watermelon were determined by releasing different number of the mite females to the fields in 1982 and 1984 in Wakayama Prefecture, the southwestern Japan. They reduced the shoot biomass, resulting in a relatively slight yield reduction by compensational responses. There were negative linear relationships between the mite density and the fruit yields and between the leaf-damage index and the fruit yields. T. kanzawai heavily damaged watermelon leaves than T. urticae. Tolerable pest density and tolerable injury level which caused 5% loss of yield at the fruit developing stage in late June were 43.6 females per leaf and 0.84 of leaf-damage index against T. urticae, 27.8 females and 1.08 against T. kanzawai.