Studies On Structure Of Predatory Enemy Community And Evaluation Of Dominant Predatory Enemies Of Pieris Rapae In Cabbage Fields

Based on a systematic field investigation and laboratory experiments, the structures of predatory enemy communities and the evalution of dominant predatory enemies of Pieris rapae in cabbage fields of different seasons were studied. The main results were as follows:1 Population dynamics of Pieris rapa and the structures and their dynamics of predatory enemy communities in cabbage fields1.1 Population dynamics of Pieris rapa and species composition and population dynamics of each species of the predatory enemy communities in the cabbage fieldsThe research results showed that the occurrence of Pieris rapae in spring cabbage fields was relatively grave. There were two peaks of egg population in the period of field investigation. The egg population at the second speak was especially high and reached the density of 28.3 individuals per cabbage plant. The larva population of Pieris rapae was in rising trend during the early fields investigation period and reached the density of 1.4 individuals per cabbage plant in the peak period (from the end of April to the beginning of may). There were totally 15 species of predatory enemies in spring cabbage fields. The dominant species were Sitticus sinensis,Theridium octomaculatum,Erigonidium graminicola,Dyschiriognatha quadrimaculata,Pirata subpiraticus,Harmonia axyridis,Misumenops tricuspidatusand and Propylea japonica and their individuals were 91.5 percent of the total individuals of the community.The occurrence of Pieris rapae was significantly lighter in autumn cabbage fields than in spring cabbage fields. The egg of Pieris rapae disappeared after October 20 and the larva population declined steadily in the period of field investigation. There were totally 10 species of predatory enemies in autumn cabbage fields and the dominant species were Theridium octomaculatum,Erigonidium graminicola,Misumenops tricuspidatus,Araneus sia and Pirata subpiraticus. The population of Theridium octomaculatum was very abundant and 66 percent of the total individuals of the community.1.2 Spatial patterns and their dynamics of Pieris rapae and major dominant species of predatory enenmiesThe research results showed that distribution proportions of the eggs of Pieris rapae in spring cabbage plants were back of outer leaves>back of inner leaves>front of inner leaves>front of outer leaves>heart leaves. The distribution proportions of the larva of Pieris rapae in different parts of spring cabbage plants were back of outer leaves> front of inner leaves> back of inner leaves> front of outer leaves>heart leaves. The distribution proportions of the eggs of Pieris rapae in autumn cabbage plants were back of outer leaves> back of inner leaves> front of outer leaves and no eggs distributed on the front of inner leaves and heart leaves during the period of field investigation. The distribution proportions of the larva of Pieris rapae in autumn cabbage plants were front of inner leaves> back of outer leaves> back of inner leaves front of outer leaves>heart leaves. The dominant species of predatory enemies were distributed mainly on the back of outer leaves in both spring and autumn cabbage plants.1.3 Major characteristic indices and their dynamics of predatory enemy communities in the cabbage fieldsThe research results showed that the total individuals of predatory enemy communities in spring cabbage fields were relatively few and fluctuated slightly in time sequence. The species richnesses were between 6 to 11 species and relatively stable. Generally, the indices of evennesses and diversities of the communities were relatively high and stable. This implied that the communities were rather stable.The total individuals of predatory enemy communities in autumn cabbage fields were in declining trend. The species richnesses were between 7 to 10 species and rather stable in time sequence. The evennesses and diversities of the communities were relatively stable but significant lower than those in spring cabbage fields.The indices of community evenness and diversity based on species were significantly related to those based on guilds in both spring and autumn cabbage fields.2 Evaluation of dominant predatory enemies of Pieris rapa in cabbage fields2.1 Evaluation of dominant predatory enemies of Pieris rapa in spring cabbage fieldsBased on the determination of quantitative equivalents,predatory equivalents,synchronous equivalents and sympathy equivalents of predatory enemies of Pieris rapae, the control equivalents of each species of predatory enemies to Pieris rapae were caculated. The results showed that Sitticus sinensis was the most important predatory enemy of both egg and larva of Pieris rapae in spring cabbage fields.2.2 Evaluation of dominant predatory enemies of Pieris rapa in autumn cabbage fieldsBased on the determination of quantitative equivalents,predatory equivalents,synchronous equivalents and sympathy equivalents of predatory enemies of Pieris rapae, the control equivalents of each species of predatory enemies to Pieris rapae were caculated.The results showed that Erigonidium graminicola and Theridium octomaculatum were the most important predatory enemies of both egg and larva of Pieris rapae in autumn cabbage fields.