Estimating control threshold of two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae) on chrysanthemum using presence-absence sampling method

To develop an easier control decision-making system, the control threshold of Tetranychus urticae on chrysanthemum was estimated in terms of proportion of leaves with adult females. The relationship between the proportion of leaves with mites and the mean number of mites per leaf fitted the Kono-Sugino model. The relationship between the injury level of leaves at harvest (30 July) and the mean number of mites per leaf at various census dates was described by the same model. The control threshold in terms of proportion of leaves with mites at various census dates could be estimated by combining the two relationships. For example, the control threshold when assuming the economic injury level to be 50% of the leaf undersurface to be spotted was 0.1%, 15.6%, and 81.8% for 5, 16, and 26 July, respectively. However, the control threshold at more than 1 month before harvest was difficult to establish because of the high marketable quality of chrysanthemum. Wald's sequential likelihood-ratio test seemed useful in control decision-making.