Damage caused by Tetranychus urticae and Frankliniella occidentalis on rose plant: effect of different initial population densities

The western flower thrips, Frankliniella occidentalis (Pergand) (Thysanoptera: Thripidae), and the two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), are major economic pests of many ornamental plants including roses. We investigated the effects of the initial population densities of 10, 15, 20, 25, 30, and 35 T. urticae, F. occidentalis, and T. urticae/F. occidentalis (co-infestation) on the final pest densities on leaves and petals, the leaf chlorophyll content, and flower shelf life on Rosa hybrida, under controlled conditions. We also evaluated F. occidentalis damage to the flower petals in the presence and absence of T. urticae. The evaluations were made 5, 10, or 15 days after infestations. The final population densities of T. urticae and F. occidentalis on leaves and petals were influenced by the initial population density treatment and the duration of infestation. Tetranychus urticae primarily colonized the leaf tissue and considerably fewer mites were present on petals whereas F. occidentalis primarily colonized flower petals; these patterns remained consistent for the T. urticae/F. occidentalis co-infestations. The chlorophyll content was influenced by the initial population density and the infestation duration of T. urticae and T. urticae/F. occidentalis. However, the initial population density and the infestation duration of F. occidentalis did not significantly affect the leaf chlorophyll content. Although flower shelf life was not influenced by the T. urticae presence or density, the initial population densities of 10, 15, 20, 25, 30, and 35 F. occidentalis reduced flower shelf life by 6.4, 17.5, 23.9, 32.6, 36.7, and 43.5%, respectively, in flowers harvested 5 days after infestation. The initial density of F. occidentalis also influenced the necrotic surface area and the number of damaged petals in the absence or presence of T. urticae. The damage threshold in cut roses is considerably lower for F. occidentalis compared to T. urticae and preventive methods are recommended to minimize losses to this pest.