Effects of different grass-growing patterns on the main pests and their natural enemies in pear orchards

Abstract:【Objective】The grass planting technology in the orchard is usually to plant perennial herbs as cover crops in the ground between the rows of fruit trees. Applying sod system can retain water, increase soil fertility, make growth environment better, and improve fruit quality. Under different ground conditions, there are various suitable types of grasses, and it is very important to choose the appropriate grass species. Vicia villosa is an excellent type of forage belonging to the legume genus Vicia, annual or over year, with strong adaptability to the environment. It is often used in agriculture to raise animals, rotate crops, and turn over as green manure. Applying sod system in apple orchards can improve soil fertility and increase the number of natural enemies. Therefore, the objective of this study is to examine the effects of different grass-growing patterns on the main pests and their natural enemies in pear orchards, and to determine whether the villous vetch is suitable for planting in pear orchards, so as to improve the ecological environment and control pests effectively.【Methods】The experimental orchards were situat- ed at Qufu city, Shandong province. The pear orchard covered 60 667 m2 and the pear trees were 5 to 9 years old. Three grass planting treatments were set up, with each of 10 to 15 667 m2. Machines were used for weeding and cutting the natural weeds on the ground between the pear tree rows. Three treat- ments were as follows: (1) Clean cultivation: there was no grass cover; (2) Natural grass cover: the grass was usually cut 2 to 3 times a year, keeping the height of the natural weeds around 20-30 cm. (3) Artificial grass cover: that was to ditch the rows of pear trees and the V. villosa was sowed, 2 kg per 666.7 m2, which subsequently covered the ground mixed with a small amount of natural weeds. Using visual, yellow board and net sweeping methods, the population dynamics of pests and natural enemies were systematically investigated in three grass- growing treatments. Statistics and calculations were made on the diversity index of natural enemies, the correlation between population dynamics of pests and natural enemies, and the temporal niche of population dynamics of pests and natural enemies.【Results】The population of aphids and Psylla chinensis in the artificial grass area was significantly con- trolled compared with the clean cultivation area. The first peak of aphids appeared on May 25, and the number of aphids in the artificial grass-growing area was the lowest (51.7±11.7 per branch), which was significantly lower than that in the clean cultivation area (116.7±20.3 per branch) and the natural grass area (86.2±10.3 per branch). In late October, the number of overwintering P. pylla populations was the highest in the year, and the number of occurrences in the artificial grass area was also the lowest (4.8± 1.2 per branch), which was significantly lower than that in the clean cultivation area (11.3±2.1 per branch). The average values of the diversity index (H) and dominance index (D) of the natural enemies in the artificial grass area and the natural grass area were 1.15, 1.00, 0.60, and 0.55, respectively, which was significantly higher than that of the clean cultivation area. The dynamic analysis of the population of natural enemies showed that the populations of Propylaea japonica, Harmonia axyridis, lacewing, syrphids and parasitic wasps on the trees in the artificial grass area increased significantly. The P. japon- ica was one of the main predatory natural enemies in the orchard. After using pesticides, the number of artificial grass areas was still 1.21 times more than the natural grass areas and 3.31 times more than the clean cultivation area, indicating that planting grass significantly reduced the adverse effects of using a certain amount of pesticides on natural enemies in the orchard. According to the population dynamics of the natural enemies in the grass, the population of the P. japonica and the lacewing in the artificial grass area increased significantly, compared with the natural grass area. And a better ecological microenviron- ment was created, including more pollen nectar and alternative insects. Planting grass in the orchard changed the correlation between the pests and natural enemies in pear orchards, but the correlation be- tween natural grass and artificial grass was not significant. Analysis showed that the orchard grass could reduce the dependence of predatory natural enemies on aphids and other short-term outbreaks of pest re- sources, while raw grass in the orchard could provide more pollen and nectar, significantly enhancing the relevance of parasitic bees on pests. The grasslands were the most relevant. Planting grass in the or- chard changed the temporal niche of the pests and natural enemies in pear orchards. The temporal niche of the insects in the artificial grass area was the lowest and the temporal niche overlap index and tempo- ral niche similarity index between pests and natural enemies in the artificial grass area also increased significantly, compared with the clean cultivation area and the natural grass area【. Conclusion】Therefore, artificial planting of V. villosa can significantly affect the occurrence dynamics and timing characteristics of natural enemies in pear orchards. Compared with the management modes of clean cultiva- tion and natural grass, the plantation of V. villosa in the pear orchard could provide a better extra habitat for natural enemies. The population of natural enemies can be more stable in the pear orchard, and the effect of controlling pests in pear orchard was more significant.