经济损害水平与经济阈值.doc

经济损害水平与经济阈值THE KEYS TO DISEASE MANAGEMENTFrom Epidemiology and Plant Disease Managementby Jan C. Zadoks, Richard D. Schein. New York 1979. p350-353Let us define disease management as the total of all actions，intentional or not，that serve to regulate disease 1evels so that they remain below the economic thresho1d 1evel；the actions can be directed toward a single disease or toward all diseases threatening a crop；the actions may or may not fin to a system of supervised or even integrated contro1；disease management and pest management are parts of crop management，but they may demand a specific expertiseDevelopment of a disease management program depends on knowledge of three areas：crop economics, population dynamics, and disease control technology, With crop economics, we leave the area of natural sciences and enter an area where value judgments pertainAlthough the individuals value judgment may be highly subjective, the science of economics dealing with large groups of individuals is not soIt is the economist who studies the financial aspects of crop lossesFor detailed discussions，refer to the appropriate literature (Carlson，1971；Carlson & Main，1976；Ordish & Dufour，1969)Economic considerations are crucial in the process leading to a decision for or against action；Value judgments by authorities，consumers，merchants，and farmers play an important roleThe farmers judgment leads，implicitly or explicitly，to an idea about the economically acceptable damage1 The Economic injury level or Damage ThresholdThe strategy of disease management is to to1erate disease，but to regulate it at subeconomic levels. It follows that that the level Xt at which disease begins to adversely affect yield andor quality-the damage threshold- must be knownThe level of this threshold varies with crop, disease，and 1ocal economyOne farmer or farming region may be able to tolerate loss of attainable yield better than another and remain economically viableThe farmers judgment about the economically acceptable damage must be translated into concomitant levels of Xt. Such studies are still rare; an excellent example is given by Calpouzos et al(1976).The level of Xt at which the highest， economically acceptable damage occurs has been called the economic injury level by entomologists：it is the lowest population density that will anuse economic damage. Economic damage is the amount of injury which will justify the cost of artificial control，measures；consequently, the economic-injury 1evel may vary from area to area, season to season, or with mans changing scale of economic values (Stern et al., 1959)Here，We will call it the damage threshold (see Fig. 11.1 where it is shown as an arbitrarily placed line)Lines 1 and 2 represent disease progressing at the same rateLine 2 either originates from a lower level of Xo (a) or starts later in the，season (b)Line 1 crosses the damage threshold Xd earlier than line 2 and reaches a higher harvest level Xh of disease and，probably，damagelines 3 and 4 are for disease progressing at a much lower rate；they will not reach the damage threshold before harvest. Such a rate would result from horizontal resistance, and the use of cultivars with this quality would satisfy the requirements of disease management.True determination of the damage threshold is part of the fine tuning of a local management system and depends not only on biologic information but also on accurate knowledge of the costs of inputs to the system，the value of outputs，and the required maintenance level. The farmer can afford only that amount of disease control that will produce yield or profit exceeding the cost of the control measuresAs intensification increases，input costs generally increaseYields plotted against inputs often show an inflection in the curve after which each unit of input produces a smaller increment of yieldOne of the inputs is the cost of disease control and diminishing returns must be taken into account in judging how much control is affordable2 The Economic Threshold or Action Threshold The rate of disease development r depends on host resistance，pathogen virulence，and environmental suitabilityCultivars may have different damage thresholdsIf Xo，r, and the damage threshold are known，it can be predicted at what time disease will exceed the damage thresholdIf it is sufficiently before harvest，significant damage may resultFigure 11.1 illustrates thisIf the damage threshold Xd is known，and disease is present，the farmer must know when to act-the action threshold，Xa. At the right time，ta，the farmer must apply a control measure that will so reduce r that disease will not reach the damage threshold before harvestEmpirical fungicide studies show what degree of r reduction will result with different materials under different conditionsLine 3 of Figure 11.1 illustrates this idea，If disease is observed to be progressing at the rate of line 1 and if it is known that a particular treatment will reduce r the amount indicated by f. the manager or advisor can calculale the time，ta，and disease level，Xa，at which to actEpidemio1ogy provides the means to make these decisionsThe manager may also calculate，for any time in the season，the allowable r value and spray accordinglyOur term “action threshold is synonymous with the entomologists economic threshold，which is defined as the density at which control measures should be determined to prevent an increasing pest population from reaching the economic-injury levelThe economic threshold is lower than the economic injury level to permit sufficient time for the initiation of control measures and for these measures to take effect before the population reaches the economic-injury level(Stern et al. 1959)3 The Warning ThresholdMany things have to be done before a farmer can take the appropriate actionThe chemical has to be bought，the spray equipment readied，and so onSometimes a warning threshold is useful. It is the level of disease severity Xw that leads to a standby warning.The warning threshold is lower and earlier than the action threshold，and the latter is lower and earlier than the damage threshold，As the damage threshold is based on a value judgment，it is subjective，and，consequently，the other two thresholds are also subjectiveObjectivation is possible by mutual agreement，but whereas the damage threshold may vary somewhat from farm to farm，the warning thresho1d can be 1ocally orRegionally determined and watched. It is noteworthy that BLITECAST，described in Example ll.4，contains a warning threshol1d经济损害水平经济损害水平(economic injury level，简称EIL)造成经济损失的最低有害生物种群密度。 所谓经济损失是指防治费用与防治挽回损失金额的差值。针对预计流行后病情可能达到EIL的病害，如果进行防治，其收益正好等于所需防治费用。它是有害生物综合防治(IPC)或有害生物综合治理(IPM)理论的重要概念，是从经济效益出发确定的病害系统管理目标。经济损害水平最早由美国昆虫学家斯特恩(V.MStern，1959)等人提出并用于害虫综合防治。由于这个概念首次将经济学观点引入害虫防治理论，所以一经提出颇受重视。1972年黑德利(J.C.Headley)根据经济边际分析原理研究防治费用、防治收益以及防治的纯效益随着防治后害虫种群密度而变的曲线。他的结论是在不断加大防治压力降低害虫种群密度的过程中，随着种群密度不断下降，防治费用逐渐增加，越是要斩尽杀绝就越要提高防治强度和提高防治费用，而产值增长率却不断下降。在两条曲线相交之前，防治费用增长率始终低于产值增长率，增加防治压力总会提高纯效益。当防治害虫所产生的产值增殖率等于防治费用增殖率的情况下，防治的纯效益最大(见图示的Pt-n点)。这个控制后的种群密度即为经济损害水平(黑德利当时采用“经济闸值”的术语，Hall(1973)和Luckmann(1982)认为恰好是斯特恩定义的经济损害水平)。斯特恩把EIL作为权衡一场预计发生的病虫害是否值得防治的密度指标。对于最终流行程度低于这一指标的一场病害不应该进行防治，对于高于这一指标的流行进行防治则可能带来纯效益。黑德利则探求对于一场值得防治的病虫害究竟应该用多大的防治强度(防治次数、用药量)把有害生物种群控制在什么密度才能获得最好的纯效益。二者定义EIL的角度不同，推算的数值却是一致的。也可以理解为经济损害水平是一个密度指标，无论是自然发生的还是经过一定的防治以后的种群密度如果低于这一指标，都无需进行或增加防治，因为这种防治本身的费用增长量大于生产产值增长量，防治将变得不合算。 关于经济损害水平的概念还有许多不同的解释。深谷昌次(1973)认为EIL可以包含“受害水平”和“害虫密度”两方面含义。前者以农作物所表现的受害程度为单位，他提出采用“受害允许界限(水平)”(tolerable injury level)的术语；后者以害虫种群密度为单位，称“受害允许密度(tolerable pest density)。而受害允许水平又是指受害允许密度下的作物受害程度。陈杰林(1988)提出在上述二词前冠以“经济”二字，采用经济受害允许水平和经济受害允许密度与上述两个名词相对应，以明确其经济学含义。同时将受害允许密度解释为作物所能忍受的害虫密度，在这种密度下，并不引起产量损失或品质下降，它的大小完全由作物自身的耐害性及补偿能力所决定。多数学者则称这种单纯从生物学角度考虑的造成作物产量和(或)质量损失的最低有害生物种群密度为“作物损失阈值”(见损失估计)。在植物病害研究中，荷兰扎道克斯(JC. Zadoks，1979)使用“损失阈值”(damage threshold)与经济损失水平(EIL)相对应，以“病害阈值”(disease threshold)与作物损失阈值相对应。经济损害水平受生物、经济、防治效果乃至社会购物心理等因素的影响，且因作物品种、生育期而变，是动态性很强的指标。其推算过程涉及以下定量关系： 环境条件与有害生物种群动态； 种群密度与作物所受损害； 作物所受损害与作物损失； 作物损失与经济损失；防治强度与有害生物种群密度； 防治强度与防治费用等。研究的重点是有害生物种群密度与作物损失的关系。确定经济损害水平的最简单的方法是将各种因素特定化，计算固定的EIL。其计算公式为N=C(DMp)。式中，N为EIL；C为防治费用；D为单位害虫(或病害)造成的减产量；M为防治后害虫死亡率(或病害防治效果)；P为收获物单价。如果用CP计算防治费用相当的减产量，同时建立病虫害密度与作物产量的函数关系，也可以推算出EIL(郭于元，1983；陈常铭，1984)。以此类推，根据各生育期病虫情所能造成的损失情况可以制定多期固定的经济损害水平。考虑到农业生态系统内多种因素的影响，利用动态的系统模拟模型可以推算动态的和多维的EIL以及多种有害生物同时发生情况下的复合EIL。也有一种根据数年内病虫害发生概率、发生程度和损失情况经过统计制定的阈值，被称为统计的EIL(GANorton，1976)。 经济损害水平的提出和在病害防治决策中广泛采用，体现了病害防治目标由追求最高产量向争取最高经济效益的战略转移，也有利于阻止滥用农药，减少环境污染。(肖悦岩) 中国农业百科全书植物病理学卷.中国农业出版社.北京.1996.p.253-254经济阈值 经济阈值(economic threshold，简称ET)。病害合理防治时