Structure And Diversity Of Arthroped Communities In The Seed Field Of Alfalfa In Jingtai County

In this paper, there are two main research items: first, diversity and structure of the arthropod community. The species and their occurrence were found out basically through the systematic investigation on the arthropod community in seed field of alfalfa in Jingtai County, Baiyin, China. The dynamics of the arthropod community structure were also studied using community diversity analysis, path analysis, principal analysis and optimization cut-apart analysis. Second research item is temporal niche breadths and overlaps of the dominant pests and natural enemies. By the analysis of temporal niche breadths and overlaps, this paper discusses the interaction between dominant pests and natural enemies. All the researches can provide alfalfa pests control with the scientific data for Jingtai County. The results are as follows: (1)Primary investigation on arthropod community in seed field of alfalfa 108 species of arthropod were collected and identified in alfalfa seed field, belonging to 49 families, 11 orders. Of which 61 species belonging to 30 families are the pest sub-community. The dominant species of insects are Mirids, Predacious bugs, Aphids, Thrips, Seed-weevils, Ladybirds and Spiders. (2)The dynamics and structure analysis of arthropod community Relative abundance of pest sub-community and natural enemy sub-community in the seed field of alfalfa are 0.90035 and 0.09965. The relative abundance of the Thrips and Aphids are higher, the next in the order are Seed weevils and Mirids in pest sub-community; while in natural enemy sub-community Ladybirds and Spiders are higher, the next in the order are Predacious bugs. The dominant concertrations of the arthropod communities show the order: pest sub-community (0.40458)>natural enemy sub-community (0.18802), which indicate the population dynamics of Thrips and Aphids in pest sub-community are important factors that had influence on arthropod community. The diversity index such as Simpson index (J) show the order: arthropod community>pest sub-community of alfalfa> natural enemy sub-community, the other indices include Shannon-Wiener index (H'), Brillouin index (H), McIntosh index (DMc) indicate the same trend, natural enemy sub-community>arthropod community >pest sub-commnuity. (3)The seasonal dynamic of arthropod community and different sub -communities The pest sub-community and the arthropod community have the same trend in terms of their individual numbers. The individual numbers of arthropod community are affected by pest sub-community. The species numbers of arthropod community and different sub-communities have the same trend, the species numbers reached the peak in the end of June. This indicates that species numbers of arthropod community are affected by pest and natural sub-community. The analysis of the population trend in structure of arthropod show that the diversity index and evenness have the same trend, and that the pest sub-community is the dominant community in the arthropod community. (4)The diversity of arthropod community and different sub-communities, and the relation with ecological index The results of Path Analysis show that increasing the species, the evenness and decreasing the individual number can enhance the diversity of arthropod community; decreasing the dominant concentration, individual number and increasing the species are most important to enhance the diversity of pest sub-community and natural enemy sub-community. (5)The Cluster Analysis and Principal Components Analysis are also used to evaluate development of community structure and the important position of pest in the arthropod community The Cluster Analysis show the arthropod community, pest sub-community and natural enemy sub-community can be grouped into 3 clusters repectively in growing period of alfalfa. The result also reflect a dissociation trend among different classes, complex in the the community structure and a transition state sometimes in the community dynamics. The principal components analysis indicates Planar Compositor Characteristic of the arthropod community is more obvious in time series and the Shannon-Wiener (H') and Simpson (J) index are important indices in analysis of community structure. (6)Analysis of arthropod community dynamics in the seed field of alfalfa by optimization cut-apart Based on the result of optimization cut apart to time of arthropod communtiy, the period is divided into 6 stages (the end of April~the first ten days of May, the middle ten days of May~the last ten days of May, the first and the middle ten days of June, the last ten days of June, the last ten days of July, the last ten days of July~the first ten days of August) from April 22 to August 1, 2004. In the first stage, the development of the pest population should be closelymonitored and agriculture mesaure should be the main measure to control the dominant pests. The second stage is a key period of pest management, reduce the number of dominant pests by the chemical control. During the third stage, manage the pests by combining cultural, biological and chemical measures, for example, spray chemicals before bloom, while use biological agents after bloom; During the fourth stage, the natural enemy could control the pests well, so that pollinator insects and natural enemies can be protected. In the fifth stage, the seed weevils became the dominant pest and be controlled with insecticide; The sixth stage, hygiene the field after alfalfa harvest and reduce the population of insects for the next year. (7)Analysis of temporal niche breadths and overlaps of the dominant pests and the natural enemies in the seed alfalfa field The result show that the breadths index of Thrips is maximum (0.5517) and chrysomelids (Flea beetle) is minimum (0.3488) among the pests, breadths index of Spiders was maximum (0.5994) and predacious bug is minimum (0.2381) among the natural enemies. Thrips and Spiders have wide distribution resource and have powerful ability of resource-utilizing. While Flea beetle and Predacious bug have small distribution rescource. The overlaps index of Thips vs Orius minutu and Aphids vs Spiders are higher, the index is 0.7157 and 0.8227 respectively. Orius minutu and Spiders have powerful ability of preying on Thrips and Aphids respectively; they could be used to control the Thrips and Aphids in alfalfa pests management.