We assessed whether three measures of visible T. urticae damage to ivy geranium plants - leaf browning, distortion, and overall plant rating – were correlated with cumulative T. urticae density and marketability of plants at maturity. We also investigated the effects of nitrogen and phosphorus on leaf distortion and overall plant rating. Four-week-old plants were infested using two levels of T. urticae inoculum: uninfested (control) and plants inoculated with 30 female T. urticae each. Fertilizer treatment combinations used consisted of 8 or 24 mM nitrogen, and 0.32, 0.64 or 1.28 mM phosphorous. Both the leaf distortion index and the overall plant rating were correlated with cumulative T. urticae density and marketability; browning was not correlated with either. We concluded that either ivy geranium leaf distortion or overall plant quality could be used to measure economic damage resulting from T. urticae. Plants treated with 24 mM nitrogen showed significantly less distortion due to T. urticae damage than those treated with 8 mM. Phosphorus concentrations of 0.64 and 1.28 mM resulted in significantly lower overall plant ratings due to T. urticae damage than those treated with 0.32 mM. This means growers might limit the impact of T. urticae by applying 0.64 to 1.28 mM phosphorus and 24 mM nitrogen. Using overall plant ratings (overall plant appearance), preliminary action thresholds for spider mites on ivy geranium were calculated and found to be 2 spider mites per leaf for the first five weeks and 7 spider mites per leaf for the last five weeks of the production period.
Species 1: Acari Tetranychidae Tetranychus urticae (Twospotted spider mite)
Keywords: Action thresholds
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