| The pests such as diseases, insects, weeds, rats etc. are one of the primary factors which impacting on grain production security. The plant protection is the important assurance measure which carries out grain production to sustainable development and grain security. This paper systematically analyzes the plant protection impacted on grain production security in China since 1978 and its change characteristics, and searched the primary factors which influencing plant protection. It is performed to enrich the theory of economics of plant protection; and provide the theoretical references or the policy-related suggestions for the government optimizing the plant protection management of grain production, shooting for a higher security level of the grain production, helping farmers to increase their income.In this paper ,in the process of china's grain production, the total change characteristics of plant protection, the plant protection disaster reduction's characteristics of different pests, the relationship of plant protection and its related influential factors, the plant protection economic function of pesticides, the plant protection characteristics and their differences of the three kinds of staple grain crops were analyzed, and comparative studied on the situation of plant protection for grain production in 30 provinces (autonomous regions and cities directly under central government) in China. The study results are as following.First, great changes had taken place in the plant protection situation of grain production in China since 1978. Both the occurred areas of the pests since 1980 and the controlled areas since 1982 exceeding the grain-growing areas have presented a sharply increasing trend. The controlled areas increased faster than the occurred areas, the passive situation of the plant protection for grain crops has changed totally in China since 1994.The saved total grain output by plant protection always exceeded the net lost total grain output for biological disasters in 1978-2003, the annual average of output saved rate and net output lost rate were 8.57% and 2.99%.In 1996-2003, the sum of the saved total grain output by plant protection and the sum of net lost total grain output for biological disasters were 484,389,000 tons and 135,845,600 tons, equal to 100% and 22.72% to the total grain output in 2006. These facts make out that plant protection plays important roles in preventing and reducing biological disasters; promoting grain production and raising the grain security level in China. In 1978-2003, the saved total grain output by plant protection and the net lost total grain output with controlling biological disaster prevention and reduction in China were in growth tendency with time. The annual increase rate of net lost total grain output was less than the annual increase rate of saved total grain output, but more than the annual increase rate of total grain output. The fluctuation of saved total grain output went through 6 complete cycles, while the fluctuation of the net lost total grain output had 8 complete cycles; they were presented the classical type fluctuation features. But they were different from the fluctuation of total grain output. The fluctuation index average of saved total grain output was more than the fluctuation index average of total grain output, but it was less than the fluctuation index average of net lost total grain output. Therefore, the capability of biological disasters prevention and reduction on grain production by plant protection must be totally strengthened in China. However, in 1996-2003 our country had formed the relatively stable capability of plant protection to reduce disaster on grain production, its controlling the net output loss rate in 4%, and keeping the plant protection disaster reduction rate about 78%.However, the plant protection for grain production still faces the hard challenges for the net lost total grain output increasing with the saved total grain output increasing all the time. In order to realize the aim of controlling the net grain output loss rate below 3% in course of "fifteenth five-year program of agriculture development" in China, the capability of biological disasters prevention and reduction by plant protection urgently need enhance and break through.In 1983-2003, the percentage of the rural people's per capita saved grain output value and per capita share of lost grain output value to the net average plant industry income of rural people were 7.41% and 2.52%, both the rural people's per capita saved grain output value and per capita share of lost grain output value each between the net average plant industry income of rural people had the extremely remarkable relativity. The effect of plant protection for grain production plays an important role in stabling and increasing the income of rural people, and closely relates to the grain price.Second, the plant protection for grain production is gotten the influence of the multi-factors. The total grain output, the saved total grain output and the net lost total grain output in China respectively with the grain-growing area, the unit area grain output, the effective irrigated area, the chemical fertilizer usage, the pesticide usage, the agricultural membrane usage, the farm machinery gross power, the disaster area, the area of caused disaster, the occurred area of pests and the controlled area and so on 11 primary factors make up of three unattached gray relative systems. The results of the analysis of these gray relative systems are as follows:(1) The occurred area of pests and the capability of biological disasters prevention and reduction by plant protection are already leading factors of serious restricting total grain output. The occurred area of pests is the premise of the saved total grain output by plant protection, and is the initial factor to make net lost total grain output. The controlled area is the decisive factor to the saved total grain output by plant protection, so we must attach great importance to plant protection for grain production.(2) The yield characteristic of grain crop varieties, the farmland irrigation projects and the grain-growing area are the foundation to guarantee the total grain output; and the same time they are the basic conditions to decide the net lost total grain output in China .They have a certain extent influence on the saved total grain output, but they are not the determining factors on the saved total grain output by plant protection.(3) The material investment such as chemical fertilizer, pesticide and farm machinery is the important factor which impacts on the grain output, is the powerful material support and technical method to the biological disaster reduction by plant protection .In the suitable degree, the plant protection has the characteristic of everything depends on human effort. In China, the science and technology related to improving the utilization ratio of producing materials was in the stagnant state in recent years. They caused investment efficiency of producing materials obviously slowing.(4) The disaster area and the area of caused disaster are the primary factors which made net lost total grain output increasing, but the influence of them on the saved total grain output by plant protection are in a tendency of declining with time. Preventing and controlling the pest occurrence and damage should be in conjunction with taking measures against natural disasters.Third, there are many pests damaging grain production in China .The harm of the different pest is different, and its effect of biological disasters prevention and reduction by plant protection also is different. During 1990-2003, in China's grain production process, the occurred areas and the controlled areas of plant diseases, insects, weeds and rats all had the increasing tendency along with the time. All the occurred areas of the pests, the occurred areas of plant diseases and insects were in the majority, but the occurred areas of weeds and rats had presented the acceleration tendency. The plant diseases, insects, weeds and rats, often have a reciprocal impact, progress in one sector promoting progress in the other. The control of plant diseases and insects was with the heaviest workload and investment, and still being strengthened; the control of weeds was energetically strengthened; on the contrary, the control of rats was gradually neglected and weakened. The effects of controlling weeds and rats by plant protection were raised quickly, but the effects of controlling plant diseases and insects were enhanced steadily. The effects of controlling plant diseases and insects were better than the effects of controlling rats, but worse than the effects of controlling weeds. In China, in the cultivated process of paddy rice, wheat and maize, the occurred areas and the controlled areas of key plant diseases, key insects and weeds size order all were: key insects, key plant diseases and weeds; their average of plant protection disaster reduction rate were in turn: key insects, weeds and key plant diseases; but their average of coefficient of variation of plant protection disaster reduction rate size order were: key plant diseases, weeds and key insects. The difficulty of the key plant diseases disaster reduction by plant protection was the biggest, and its effect was most indefinite.Fourth, the pesticide plays an important role in plant protection for grain production in China. In 1978-2003, the percentage of the pesticide cost to total grain production costs was lower, but the ascending velocity of it is the quickest than other grain production cost. The pesticide cost increased with the pest occurrence rate and the control rate, also related to the excessive pesticide price and the drug efficacy of pesticide gradually declined. The pesticide cost risen not getting influence by the grain price fluctuation, while presented the bigger growth rigidity. The unit area pesticide cost of three grain crops height order were the paddy rice, the wheat, the maize; but the increasing rate of them size order actually were the maize, the wheat, the paddy rice. At present, in course of grain production, the mean value of proportion of input to output of pesticides in China is about 1:4.5; it nears to the mean value of U.S.(1: 4), but less than the mean value of other developed countries(1:6-8).The mean value of proportion of input to output of pesticides of the paddy rice, of the wheat, of the maize partly are 1:4, 1:5.5, 1:4.5.The pesticides input had made stronger influence on production output value of paddy rice and wheat, but weaker on the maize production. The pesticides input was the most stable influence on the wheat production output value, but slowly on the paddy rice, the maize production output value. The pesticides cost of grain production was closely the net lost grain output value. Therefore, it is very necessary for grain production to reduce the pesticides usage, and control the pesticides cost of grain production. Fifth, the paddy rice, the wheat, the maize are the main objects of plant protection for grain production in China. In 1987- 2003 the occurred area of plant diseases and insects of paddy rice and the maize were in increasing tendency along with the time, its of the wheat increased with fluctuation. the pests occurrence rate and the control rate size order were the paddy rice, the wheat and the maize, but the increase ratio of pests occurrence rate size order were the wheat, the maize and the paddy rice; the coefficient of variation of pests occurrence rate and control rate of wheat was more than of paddy rice, but less than of maize. The increasing rate of controlled area of plant diseases and insects was over the increasing rate of occurred area in the plant process of paddy rice and wheat; But the increasing rate of controlled area of plant diseases and insects was less than the increasing rate of occurred area in the plant process of maize, and the foundation of controlling for plant diseases and insects was the weakest than of rice and wheat. Both the net lost total grain output and the unit area net lost grain output for plant diseases and insects size order were the paddy rice, the wheat and the maize; but the net grain output loss rate and its coefficient of variation size order all were the wheat, the maize and the paddy rice. The net lost grain output value and the saved grain output value by plant protection for the paddy rice, the wheat and the maize all made severely impacted on their net production profit. The grain output saved rate and the plant protection disaster reduction rate of paddy rice both were the most than of wheat and of maize; but their coefficient of variation both were the least than of maize and of wheat. The capability of biological disasters prevention and reduction by plant protection for maize and wheat were weaker markedly than for paddy rice. From now on, in China's grain production process, will have to increase investment in the plant protection for wheat, to reduce the fluctuation of wheat grain output because of plant diseases and insects damage. The plant protection for maize should be comprehensively strengthened to promote the capability of plant diseases and insects damage prevention and reduction. The plant protection for paddy rice should shoot at the technological breakthrough.Sixth, every region has different conditions of economy, society and nature from other region in China. The situation of plant protection for grain production in 30 provinces (autonomous regions and cities directly under central government) in China has respective characteristic. The occurrence rate, the control rate and the plant protection disaster reduction rate of plant diseases and insects were taken as the representative indexes, to carry on cluster analysis for the situation of plant protection for grain production during 2002-2003 in 30 provinces (autonomous regions and cities directly under central government) in China. Its result is that the situation of plant protection for grain production in 30 provinces (autonomous regions and cities directly under central government) can be divided into 7 types. The main reasons of difference from the situation of plant protection for gram production in different region are because of the difference from level of local grain production, the level of pesticide investment in different regions, as well as regional natural condition. But the unit area pesticide usage of grain production contributed to the unit area grain output has presented an obvious tendency of diminishing returns. Therefore, integrated management should be strengthen and adopted to prevent and reduce the damage of pests, and reduce the pesticides usage, resolve negative functions with the pesticides excess usage for the grain production.Seventh, there are some plant protection strategies and policy-related suggestions for promoting grain production security as follows:Comprehensively understand the meaning of our country's grain security in the future, change the target of plant protection disaster reduction for grain production; carry out the public plant protection, to solving the principal contradictions from plant protection system of grain production; increase investment in optimizing grain producing condition ,advancing agricultural high-tech, to enhance the capability of biological disasters prevention and reduction by plant protection; perform different plant protection strategies, to improve the level of plant protection for grain production; depend on the support of science and technology, management and policy, to reduce the pesticide cost of grain production; develop plant protection ecological culture ,to promote grain security sustainable development.
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