Biological pest control by enhancing populations of natural enemies in organic farming systems

In nature, ecological balance is maintained by biodiversity through a stable food-web. This concept has been adopted to integrated pest inam gement in organic crop production to conserve natural enemies. However, trials are needed to find an appropriate cropping regime and the coincided field management for a specific crop and the dynamic relations of the pests and natural enemies under the conservation practices need to be clarified. Therefore, in the present study, with a relay-intercropping regime as a strategy to enrich field vegetation diversity and habitat improvement by plant residual mulching as a strategy to enrich field fauna diversity, a cabbage crop experiment was conducted to monitor the population dynamics of pests and native natural enemies and to confirm the effectiveness of relay-intercropping and plant residual mulching in natural enemies conservation and the consequent biological pest control. A summer cabbage was intercropped with tomato, and plant residues were mulched on the soil surfiic:: as the habitat for natural enemies. During the growth period of crops, the density of Lepidoptera larvae and three kind of predators including frog, spider and carabid were examined and analyzed with mathematic model as P-1= P-B1 + {P-max-P-B1-P-Y1[1-beta(1)(t-tau)(2)])EXP[-alpha(1)(t-tau)(2)] + P-Y1[1-beta(t)(t-tau)(2)](t <=tau) and P-2= P-B2 + {P-max-P-B2-P-Y2[1-beta(2)(t-tau)(2)])EXP[-alpha(2)(t-tau)(2)] + P-Y2[1-beta(2)(t-tau)(2)](t >=tau). The density of pests was lower in intercropping plots than in monocultured plots and lower in plots with habitat management than in those without. The density of natural enemies was higher in intercropping plots than in mono-cultured plots and higher in plots with habitat management than in those without. The damage index of cabbage and tomato was proportional to population density of pests. It was concluded that tomato as a shelter crop lowered the risk of pest incidence in the intercropped cabbage crop and residual mulching as habitat management in the crop field could favour natural enemies to control pest insects.