Effects Of Different Ecological Engineering Measures On Rice Pest Management

Planthoppers,leafhoppers and stem borers are three major phytophagous insects feeding on rice.The three main species of the rice planthoppers(Nilaparvata lugens,Sogatella furcifera and Laodelphax striatellus)are primary rice pests in recent decades and they threaten the rice yields by sucking phloem sap and transmitting virus diseases.Asia accounts for over 90 percent of the worlds rice production,and China,with only 13% of the world arable land need to feed 19% of the world population.Protecting adequate rice supply from pest losses has been a major task of agricultural workers.Pesticides have been the main tactic used to control pests but with the development of pesticide resistance and side effects such as threats to human and environmental health as well as inducing secondary pest development,biological control and ecological engineering that reduce insecticide use have attracted greater attention from researchers and farmers.This thesis reports three-years' field experiments conducted to study the effects of various ecological engineering measures such as withholding insecticides,conserving bund vegetation,planting flowering plants in non-rice habitats which can act as natural enemy sources and rice-duck farming on rice pest management.The following results were obtained:(1)Conserving bund vegetation increased plant biodiversity and supplementary food source for natural enemies.There was significantly higher plant diversity in Organic and Flower treatments compared with the Conventional treatment that had used herbicides.In the fields under organic management,the plant community on the bunds were dominated by beneficial plants(the flowering period of which overlapped with the rice growing season),while planthopper host plants were the major plant groups in the conventional fields.The host plant density increased rapidly in the Conventional treatment especially after 90 days of the herbicide application.The vegetation on the paddy bund could provide habitats and shelters for natural enemies,high beneficial plant proportion provided abundant nectars and pollens as supplementary food,would help to establish and develop natural enemy population.(2)Attractive effects of eco-functional(flowering and banker)plants on hymenopteran natural enemies of planthoppers.Significantly higher numbers of Mymaridae wasps were found in areas where flowering plants were planted in the Flower treatment than in the same area of the control treatment.These parasitic wasps move from the rice towards flowering plants,indicating that the flowering plants were attracting them.One of the tested banker plants: Leersia sp.also had some attractive effects on the Mymaridae wasps as there were more parasitic wasps found in the banker plant area when compared with the rice field.Saccharosydne procerus on banker plant Zizania latifolia could have provided a rich food resource for the development of planthopper parasitic wasps.Abundant numbers of Mymaridae wasps and one species of Pteromalidae wasps(Panstenon oxylus)which fed on planthopper eggs were found on the Zizania plants.The distribution and movement directions of these two wasps were different in Zizania fields and adjacent rice fields.The abundance of Pteromalidae wasps in the Zizania fields was higher at the field edges than within the fields,while the opposite result was found in the rice fields.More Mymaridae wasps were found on the edges of the Zizania fields than within the fields,while the number of Mymaridae wasps was larger within the rice fields than on the edges.The movement directions of the Mymaridae wasps were from the rice towards the Zizania while it was the reverse with the Pteromalidae wasps.Furthermore,the community similarity of herbivores was lower than natural enemies between the two crop fields,indicating that the planting of Zizania latifolia adjacent to rice fields could increase the communication of natural enemies between the two habitats,and simultaneously promote the natural control function of pests of both crops.(3)The conservation of bund vegetation and planting of flowering plants significantly increased the abundance of predators while insecticides had negative effects on natural enemies.Significant positive relationships between the predator population density and bund plant diversity and negative correlation between herbivorous insect density and plant diversity were found,indicated that non-crop plants acted as main sources to help increase natural enemy abundance and reduce pest populations.The densities of spiders and dragonflies in Flower fields where 4 species of long flowering period and high pollen sugar content plants were planted were significantly higher than in the organic and conventional fields.There was a cumulative/delayed effect on the adverse impacts of pesticide use on predators.The predator density in conventional fields and organic fields in the early season showed no difference,but in the rice heading stage,the number of predators was significantly lower in the conventional fields than that in the organic fields.The experimental treatments had little influence on within paddy parasitoid densities,but the diversity was significantly reduced by insecticide applications.The populations of planthoppers,leafhoppers and total herbivores showed no difference between the three treatments at the early stages,and the significantly larger populations in the non-insecticide fields at the heading stage did not reach economic threshold levels.The above results indicated that withholding insecticides coupled with additional flowering plants could increase natural enemy population and had good control on pests in the early stages.The treatments had also some influences on the structure of arthropod communities.The abundance of Chalcidoidae wasps(i.e.Pteromalidae and Eulophidae)was higher than other parasitoid wasps in the non-insecticide fields.In the conventional fields,the Braconidae wasps were more abundant because of larger rice leaffolder populations.(4)Rice-duck farming was found to be overrated as a pest control tactic and it had negative impacts on the predatory natural enemies,but ducks had a good control on weeds within the paddies and rice-duck farming could gain great economic benefits through sale of organic products.The lower predator populations on the rice stem and canopies in the rice-duck farming seem to have weakened the natural control of rice pests.This is especially the case after the ducks were driven out of the paddy.Natural enemy abundance in the rice fields were low,implying that an “enemy-free space” might have been created for herbivores,especially the planthoppers and leafhoppers.The population densities of Echinochloa crusgalli and the total weeds within the rice fields were well suppressed by duck activities,indicating that ducks were effective at controlling weeds.But unlike herbicides,ducks had little effects on bund vegetation which were habitats and refugia for natural enemies.The price of rice and duck meat with organic certification or brand effect were much higher than ordinary products and thus the net incomes in rice-duck farming and natural farming were 6.9~8.1 and 5.3 times that of conventional farming,respectively,compensating for any deficiencies in rice-duck farming.(5)The key external biological factors influencing the population growth of planthoppers and leafhoppers were the early-season abundance of bund vegetation and non-pest herbivores,the early-season abundance and diversity of natural enemies and the late-season diversity of bund vegetation.There were two main planthopper generations throughout the rice growing season and the cumulative abundance of planthoppers in the first generation showed no difference in the four treatments.In the second generation however,the cumulative abundance of planthoppers in non-insecticide fields were significantly higher,although the population growth rates of planthoppers in different treatments showed no significant differences.The results of leafhoppers were similar,there were no significant differences in population growth rates despite significantly larger number of leafhoppers was captured in non-insecticide fields in the late season.The principal component analysis showed that among all external biological factors influencing planthoppers and leafhoppers' population dynamics,the early-season(June and July)abundance and late-season(August and September)diversity of bund vegetation had big influence on both planthoppers and leafhoppers.The stepwise regression analysis showed that early-season diversity of natural enemies and late-season neutral insects had larger effects on planthopper population dynamics than other factors.The key factors influencing leafhopper population growth included the abundance of non-leafhopper herbivores and spiders in the early season and the diversity of bund vegetation in September.In general,the neutral insects(detritivores),non-pest herbivores and bund vegetation in the early season could help establish the initial natural enemy bank and increase their density and diversity by providing alternative food,habitats and shelters.While the late-season abundance and diversity of bund vegetation and detritivores should be manipulated to avoid increasing pest populations.In this research,multiple sampling methods(ground traps,net sweeping,and sticky trap etc.)were used to investigate the arthropods and plants in rice agroecosystem,and to study the effects of various ecological engineering measures on natural enemy population and pest control.The results showed that though different measures had different influences on the natural enemies and pests,in general,by not using insecticides,planting flowering plants and increasing bund(natural enemy source)plant diversity can be important management tactics.These practices can significantly increase the diversity of farmland landscape,abundance of predators and diversity of parasitoids in rice natural enemy sink,enhance the natural control function on planthoppers,leafhoppers,and improve the stability of rice ecosystem.