Economic injury level for Tetranychus urticae Koch on parthenocarpic cucumber under protected environment in north-western Indian Himalayas

Damage potential of two-spotted spider mite, Tetranychus urticae Koch (Tetranychidae), on parthenocarpic cucumber, Cucumis sativus Linnaeus, under naturally ventilated polyhouse was assessed by artificially releasing of different mite densities (2, 4, 8 and 16 mites/leaf) at 4, 6 and 8-weeks after transplanting (WAT) coinciding with early, mid and late crop stage. Mites were restricted to feed for 4-weeks (limited feeding duration) as well as throughout crop growth (unlimited feeding duration). Injury inflicted to leaves by mite feeding increased significantly with the advancement in duration of infestation. Tetranychus urticae infestation exhibited varying levels of stunting effects on plant growth. Crop yield under limited feeding duration increased over unlimited feeding duration from 10.7 to 22.4, 6.4 to 15.4 and 0.6 to 2.2% for the infestation initiated in early, mid and late crop stages, respectively, with the loss in marketable yield to vary from 7.4 to 53.3%. Relationship deduced between reduction in yield and cumulative mite days (CMDs) revealed one mite day feeding reduced the yield up to 0.112 g/plant. Whereas, on initial mite density basis, reduction in yield varied from 12.87 to 51.95 g/plant/mite at different plant growth stages. The economic injury level (EIL) determined was very low (0.83 mites/leaf) for the infestation initiated in early stage, whereas, for the infestation initiated in later part of plant growth (6 and 8-WAT), EIL of 1.25 and 3.33 mites/leaf were established.