Studies On Structure Of Arthropod Community And Evaluation Of Dominant Predatory Enemies In Cabbage Fields

Based on a systematic field investigation and laboratory experiments, the structures of arthropod communities and the evalution of dominant predatory enemies in cabbage fields of different seasons were studied. The main results were as follows:1 The structures and their dynamics of arthropod communities in cabbage fields1.1 Species composition and population dynamics of each species of the arthropodcommunities in the cabbage fieldsThe research results showed that the occurrence of Pieris rapae in spring cabbage fields was relatively grave. The larva population of Pieris rapae was in rising trend during the investigation period. Lipaphis erysim and Plutella xylostella was relatively lighter, Lipaphis erysimi had the peak period in the middle of May .There were totally 11 species of predatory enemies, including 3species of predatory insects and 8 species of spiders .The major dominant species of predatory enemies were Sitticus sinensis and Erigonidium graminicola and their individuals were 73.6 percent of the total individuals of the community.There were totally 25 species of arthropod in autumn cabbage fields, including 12 species of insect pests and 13 species of natural enemies. The major dominant species of insect pests were Pieris rapae, Lipaphis erysimi and Plutella xylostella. Generally, the species composition of insect pest were more complex in autumn cabbage fields than in spring cabbage fields. The occurrence of Pieris rapae was significantly lighter in autumn cabbage fields than in spring cabbage fields, but the occurrence of Lipaphis erysimi was relatively grave, which rose steadily during the investigation period from October 9 to November 5, until the cabbage was ripe. In 13 species of predatory enemies there were 3 species of predatory insects and 10 species of spiders. The major dominant species of predatory enemies were Erigonidium graminicola and Theridion octomaculatum and their individuals were 81.4 percent of the total predatory enemy individuals of the community.1.2 Spatial patterns and their dynamics of major dominant predatory enemies ofarthropod communityThe research results showed that Pieris rapae distributed mainly on heart leaves and back of inner leaves of cabbage plants, Plutella xylostella distributed mainly on the back of cabbage leaves and Lipaphis erysimi distributed widely in cabbage plants. Among the major dominant species of predatory enemies, Sitticus sinensis distributed mainly on the front of leaves of cabbage plants in spring cabbage fields, and Misumenopos tricuspidata and Propylaea japonica distributed widely in cabbage plants. Other species of predatory enemies distributed mainly on the back of outer leaves of cabbage plant.1.3 Major characteristic indices and their dynamics of arthropod communities in cabbage fieldsThe research results showed that the total individuals of arthropod communities in spring cabbage fields were relatively few and fluctuated slightly in time sequence. The species richnesses were between 12 and 19 species. The maximum species richnesses came out in May 9 and May 12. The even nesses and concentration of the communities were rather stable and the diversities of the communities were relatively high and stable.The total individuals of predatory enemy communities in autumn cabbage fields were in declining trend. The species richnesses were between 15 and 21 species and rather stable in time sequence. The evennesses and concentration of the communities were not stable. Generally, the evennesses were relatively high in middle October .After middle October, because of the rising of population, the evennesses became lower, and the concentration became higher. The community in early growth period of cabbage was higher than those in late growth period of cabbage. Before middle October the mean value of the diversities reached 1.19, but after October 15 the mean value of the diversities were only 0.22.2 Evaluation of dominant predatory enemies in cabbage fields2.1 Evaluation of dominant predatory enemies in spring cabbage fieldsBased on the determination of quantitative equivalents,predatory equivalents,synchronous equivalents and sympathy equivalents of predatory enemies of aim pest, the control equivalents of each species of predatory enemies to Pieris rapae were calculated. The results showed that Sitticus sinensis was the most important predatory enemy of three major pests in spring cabbage fields.2.2 Evaluation of dominant predatory enemies in autumn cabbage fieldsBased on the determination of quantitative equivalents,predatory equivalents,synchronous equivalents and sympathy equivalents of major predatory enemies , the control equivalents of each species of major predatory enemies to Pieris rapae ,Plutella xylostella and lipaphis were calculated . The results showed that Erigonidium graminicola and Theridium octomaculatum were the most important predatory enemies of the three major pests in autumn cabbage fields.