The effect of uneven acaricide application on control of spider mites in chrysanthemum fields was examined by laboratory and field studies. The acaricide deposition index was measured using water-sensitive paper in chrysanthemum fields of five growers. The deposition was very high on the top face of the water-sensitive paper, but very low on the back. When two growers sprayed tebufenpyrad in their chrysanthemum fields, the density of spider mites did not decline in one field, and the result in the other field was unclear. When two growers sprayed milbemectin in their chrysanthemum fields, one grower could control the mite density, but the other could not. Resistance to acaricide was investigated using two strains of the Two-Spotted Spider Mite (green-form) collected from chrysanthemum fields examined for mite density. Both strains were resistant to tebufenpyrad, but susceptible to milbemectin. The mortality and fertility of mites were investigated by laboratory tests under different milbemectin deposition conditions. When the deposition index on the water-sensitive paper was 2 or more, the mite fertility was very low. Fertility was higher at an index of 1. We propose that both acaricide resistance and application deposition rate must be investigated for practical control of spider mites in chrysanthemum fields.
The densities of Two-Spotted Spider Mites at normal and concentrated application of acaricide were investigated on seven cultivars of chrysanthemums by 11 growers. The occurrence of mites in each grower's field was similar for the seven different cultivars. Even when growers used the same acaricide at a similar application frequency, mite density was different. Mite density is more severely affected by pesticide management than by cultivar.
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