Distribution, sampling, and biological control of the twospotted spider mite (Tetranychus urticae Koch) on greenhouse grown roses

This study considered three aspects of the biology of the twospotted spider mite, Tetranychus urticae, as an arthropod pest of greenhouse cut roses produced using shoot-bending. First, the distribution of this mite was studied in order to develop a sampling plan as part of an integrated pest management program for cut roses. Secondly, the relationship between twospotted spider mite density and gas exchange was studied to evaluate a nominal threshold for this pest. Thirdly, the effect of interplant bridges and a mechanical dispenser on dispersal of the mite predator Phytoseiulus persimilis were studied.; To study the spatial distribution of this pest, mites were sampled weekly from at two greenhouses that used pesticides for mite control and at one greenhouse that used the specialist predator, Phytoseiulus persimilis, for mite control. Taylor's power law and Iwao's patchiness regression were used to quantify mite distribution under the two control methods. A fixed precision sampling plan based on the Taylor's power law coefficients was developed. Sixty plant samples per 929m2 were required to estimate T. urticae density at a precision of 0.25.; To evaluate an economic injury level for T. urticae, photosynthesis, stomatal conductance, and transpiration were measured in the upper canopy (flower shoots) and the lower canopy (bent shoots). There was a strong correlation between mite density and photosynthesis in the flower shoots (R2 = 0.87) and a weak correlation between these factors in the bent shoots (R2 = 0.34). Photosynthesis returned to levels comparable to untreated plants two days after mite removal from the upper shoots.; In greenhouse cut roses produced using the shoot-bending, the predatory mite Phytoseiulus persimils is widely used for control of T. urticae. Treated plants were connected with bridges made of plastic flagging tape to enhance predator dispersal, or a mechanical dispenser was used to distribute predators, while the control plants received no dispersal aid. Control of T. urticae over three weeks in the greenhouse under both dispersal methods was significantly better than the control, although there was no difference between dispersal methods.