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A new description for crop pests——Pest Attack Index (PAI) the definition and its use

为害指数(PAI)是一种反映和评价农业病虫害严重性的新方法。它将常规调查的发生密度(为害率)、发生面积简化成为单一的概念性数值形式，并据此划定发生程度等级，适用于表述县级以上地区短期病虫为害状况和趋势。PAI定义：在某一时段中，害虫(病害)的密度指数与面积指数的乘积，而密度指数为病虫单位面积的发生密度(发病率)占防治指标(或历年密度均值)的倍数，面积指数为病虫发生面积(率)占历年发生面积(率)均值的倍数。

Crop Pest Attack Index (PAI) is a new method to describe damaging risks of crop disease and insect pests which predigests both pest density and occurrence area as a single digital exponent. PAI could be calculated by the equation as follows: PAI=Id·Ia. Here, Id is the ratio of real time pest population density with the pest's economical threshold (ET) or the average of population densities of past years and Ia is the ratio of real time pest occurrence area with the average of occurrence areas of past years. PAI was also used to plot out the degree of pest attacking to crops.