Effect Of Plants Diversity On Insect Communities In Apple Orchards In The Loess Plateau

This paper deals with the insect diversity and the related plants community in apple orchards and their surroundings in Luochuan and Huangling Counties in the Loess Plateau, the well-known high quality apple-growing region of China. The insect diversities in apple orchards with different composition of plant community are investigated, and the dominant wild plant species are determined according to the species importance values (IV). The effects of wild plants and crops on the insect diversity and populations are evaluated by using biodiversity research methods; the adjusting function of wild plants to the competition relationship between dominant pests and their natural enemies are studied with the view to providing information for scientific management, biotope improvement and pest ecological control in apple orchards. The main results are presented below.1. The insect diversity is rich in the high quality apple-growing orchards in Loess Plateau. There are 219 insect species, belong to 85 families of 12 orders, were found occurring there, among which pests amount to 118 species belonging to 48 classes of seven order, natural enemy insects and arachnids amount to 72 species belonging to 27 classes of nine orders, and neutral insects amount to 29 species of 13 classes. The pests, natural enemy insects and neutral insects amount to 53.88%, 32.88% and 13.24% to the total species, respectively. Chrysopa sinica,Chrysopa formosa,Chrysopa septempunc,Harmonia axyridis,Hippodamia variegata and Coccinella sepbempunctata are the dominant species in the orchards of this region.2. The diversity of plants in apple orchards in the Loess Plateau is rich, and the dominant species in different biotopes are different. The wild plants diversity is rich in these orchards, including 84 species belonging to 69 genera of 33 families, among which 63 species belonging to 49 genera of 21 families occur under apple trees and 21 species occur in island habitats of orchards; the diversity of Asteraceae is the highest, amounting to 23.81% of the total species, and the diversities of Poaceae, Chenopodiaceace and Fabaceae are decreasing in sequence. Orchards located on slopes of valleys have the richest diversity of plants (35 species), and those in the bottom of valleys are the next (26 species). Ixeris sonchifolia, Setaria viridis (L.) , Carduus crispus L. Chenopodium glaucum, Cephalanoplos segetum and Convolvulus arvensis L. are the dominant species in the orchards, and their importance values (IV) are 0.36, 0.22, 0.22, 0.18, 0.17 and 0.14, respectively.3. Terrain and cultivating forms affect the diversity and populations of natural enemy insects in the orchards in the Loess Plateau. The insect diversity in orchards with weeds kept is richer than those with weeds cleared away, which include 58 species and the ratio of beneficial insects and pest is 1:23; while in those with weeds cleared away the ratio of beneficial insects and pest is 1:46. The biodiversity of insects of orchards located at the bottoms of valleys in the Loess Plateau is the highest, and in these orchards the diversity indices of lady bugs, lacewings, preying arachnids and preying true bugs are 1.9987, 1.9484, 2.4282 and 1.4429, respectively. In comparison, the diversity indices of the aforementioned four groups in the orchards located on the top of plateau are all among the lowest.4. Weeds affect the population and richness index of insects in orchards and some dominant species of weeds are beneficial to natural enemy insects. The species number of insects hosted in habitats with Amaranthus retroflexus is the most, including 75 species, and the next is that in habitats with Bidens pilosa. The species number of natural enemy insects hosted in habitats with Cephalanoplos segetum is the most, and attracting ability of habitats with Amaranthus spinosus,Suaeda glauca,Chenopodium glaucum is relatively low. It is beneficial to protect the diversity of natural enemy insects by keeping Cephalanoplos segetum and Convolvulus arvensis L. in orchards, and their diversity indices are 1.0735 and 0.8799, respectively. The Cs of natural enemy insects between the apple orchards and fields artificially planted with Medicago sativa L. Alfalfa and Setaria viridis (L.) together is the highest (Cs = 0.853), and then the Cs of natural enemy insects between the apple orchards and fields with wild Amaranthus retroflexus and Amaranthus spinosus L., the Cs of natural enemy insects between the apple orchards and fields with Ixeris sonchifolia is the lowest (Cs = 0.463)5. Effects of habitats with different type of surroundings on the diversity and population of insects are different. The diversity of insects in habitats with weeds around orchards is the highest, and the next is that in habitats with Cabbage; the Cs of natural enemy insects between the apple orchards and fields planted with Alfalfa is highest (Cs = 0.835), and the next is that between the apple orchards and fields with natural weeds (Cs = 0.813). Rape fields around orchards are beneficial to improving the biodiversity of natural enemy insects and also helpful to pest control in the young fruit-growing period in orchards.6. It is helpful to control pests in orchards by select and utilize dominant weed species to adjust the habitat of orchards. The diversity indices of natural enemies such as lady bugs, lace wings and preying arachnids in orchards with artificially planted weeds are all higher than that of cleared orchards, of which the diversity index of lady bugs is the highest (2.169), and the next is that of preying arachnids. The richness indices of preying natural enemies in orchards with weeds such as Cephalanoplos segetum,Convolvulus arvensis L.,Amaranthus retroflexus kept are all higher than that of cleared orchards. The Qij of Coccinella sepbempunctata toAphis citricolaand Myzus malisutus amount to 0.9365 and 0.8902, respectively; and the Qij of Chrysopa sinica to Aphis citricola and Myzus malisutus amount to 0.9142 and 0.8715, respectively. The following effects of the above two natural enemy insects are both remarkable. The Qij of lady bugs toAphis citricola is the highest in time dimension, which implies lady bugs andAphis citricolaare synchronic and lady bugs have following effect on Aphis citricola. In time and spatial dimensions, the niche breadth of Aphis citricola is the highest, and that of lady bugs is higher than that of lacewings; the overlap value between lady bugs, lacewings and Aphis citricola is the highest, which shows that the two natural enemies have evident control effect to Aphis citricola. It is concluded that keeping weeds, e.g. Cephalanoplos segetum and Convolvulus arvensis L. and Amaranthus retroflexus ect., are beneficial to improve the diversity of natural enemies insect and to control the pests in apple orchards of the Loess Plateau.