Effects Of Raising Crop Diversity On The Occurrence Of Plant Diseases, Insect Pests And Their Natural Enemies

Green prevention and control of pests are the most important regulation measures in agricultural ecological system. In field ecological system, raising crop genetic diversity or plant diversity has a positive role in enhancing natural enemies, controlling pests and plant diseases, keeping yield, and increasing value. After that it has a positive role in regulating the relationship between pests and their natural enemies. In order to verify its effectiveness in the practice do a deeper research, a series of tests and experiments were designed and carried out, from scope and depth levels, in our experimental sites China Agricultural University Teaching experiment farm in Zhuozhou, Baodi district in Tianjin, and Rizhao district in Shandong, during the period of 2010 to 2013. The main contents include two parts in this study, one part is culativar-mixed planting in the field, and the other is the cucumber plant (Cucumis sativus L.) associated by whitefly-repellant plants:west celery (Apium graveloens L.) or Ceylon spinach {Gynura cusimbua (D. Don) S. Moore).The first part includes four crops such as paddy rices (Oryza sativa L.), soybeans (Glycine max (L.) Merr.), wheats (Triticum aestivum L.) and peanuts (Arachis hypogaea L.). Combining the actual conditions of agricultural mechanization, one mixed cropping treatment was comprised of different cultivars with an equal weight proportion of seeds for comparing to their monocultures. We investigated the occurrence degree and tendency of plant diseases, pest insects and their natural enemies, and discussed the relationships between pests and their natural enemies. In the second part, cucumber-celery/Ceylon spinach system (row 4:1) was established in light greenhouses. The effects of associated plants on the population of two whitefly species (Trialeurodes vaporariorum Westwood and Bemisia tabaci (Gennadius)) were researched with normal and geostatistics methoads in cucumber greenhouse. We also discussed the factors which affect the spacial distribution of two whitefly populations in cucumber plants. The main results are as followings:1. Raising the genetic diversity of crops has its universality in such aspects as regulating and control many pests, benefiting for the occurrence of the natural enemis, and improving the efficiency of biological control. In the four agricultural ecosystems, the best is mixed cropping of 4 rice cultivars, which can reduce the occurrence quantity of brown planthopper (Nilaparvata lugens (Stal)), and increased that of predatory spiders in paddy rice field. The better is the mixed cropping of 5 soybean cultivars, which can reduce the occurrence quantity of small green leafhopper (Empoasca flavescens (Fabricius)), and increase the efficiency of biological control. The next is the mixed cropping of 3 peanut culativars, which can reduce significantly the occurrence quantity of grubs. The last is the mixed cropping of 4 wheat cultivars, which increased the population density of Aphidius sp., and reduced the occurrence quantity of aphids to some extent.2. Raising the diversity of crop species can reduce significantly the occurrence quantity of T. vaporariorum, Aphis gossypii Glover and Tetranychus truncates Ehara in cucumber-celery system, and improve the correlativity of population dynamic between pests and their natural enemies. Cucumber-Ceylon spinach system can reduce the population quantity of T. truncates, and increase of adults of Aphidius sp..The two systems have a significant effectivity on the spatial distribution of B. tabaci on cucumber plant, and have a positive role in regulating the relationship between pests and their natural enemies. However, all the two systems increased the cucumber down mildew (Pseudoperonospora cubensis Berk & Curt.).3. The activity rhythm of two whitefly species populations showed a bimodal pattern and tranquil at night in the lit-greenhouse in spring. The adults of B. tabaci showed a positive taxis to some odours which have attractive to the whitefly. The sticky traps with these odours can increase the quantity of whitefly trapping.4. The adults of T. vaporariorum prefer tender leaves, and B. tabaci (Q-biotype) prefer special leaf position. The spatial distribution of two whitefly species in different leaf positon of cucumber plants significantly changed in the spring and autumn. The adults of T. vaporariorum located more in upper leaf positon, B. tabaci more in lower leaf position in spring. However, the population of the two whitefly species moved to the middle leaf position in autumn.5. Geostatistics methoads are first introduced to the research of whitefly populations’ distributions in greenhouse. Geostatistics on Q biotype of B. tabaci showed its advantage over the normal statistics in the cucumber-celery system, cucumber-Ceylon spinach system and cucumber system.