Studies On Insects Community Diversity In Longxian Tobacco Fields, Shaanxi Province, China

Tobaccos, as a kind of goods with special requirements to tobacco leaf quality. When been suffered by pests, would highly produced a negative impact on the economic benefits of tobacco. So the timely prevention and the control of pests in tobacco fields are particularly important. In order to control the pests better, reduce the loss of tobacco production, improve the quantity and quality of tobacco production, explore the impact of altitude factor on the structure and diversity of tobacco insect community, make sure conditions of pests happen and been controlled by natural enemy insects at different altitudes tobacco fields, and to provide the basis for control of pests and diseases in tobacco fields. Selected tobacco plants adopting checkerboard sampling method in different altitudes tobacco fields in Longxian, Shaanxi province, China, recorded the number of insects individuals in each leaf and processed the data by Excel and DPS software.During each tobacco growing period, through the survey aimed to the insect community in tobacco fields at different altitudes, we found 32 species of insects, belonging to 25 families, 9 orders in Longxian tobacco fields, including 21 species of pests(18 families, 7 orders) and 11 species of natural enemy insects(8 families, 5 orders). The dominant pests were Myzus persicae Sulzer and Dolycoris baccarum (Linnaeus) in tobacco fields. The natural enemy insects mainly were Aphidius gifuensis Ashmaed, Chrysopa formosa Brauer, Episyrphus balteatus De Geer and Coccinella septempunctata Linn. The species number of natural enemy insects were almost the same in different tobacco fields; while the difference of pest species number was great. The number of insect spices in tobacco growing period is far more than the number in tabacco maturity.In tobacco matural peroid, Myzus persicae has maintained a "high - low - high" trend of large quantity level. In the early of the matural period, Episyrphus balteatus De Geer and Aphidius gifuensis Ashmaed had a large number, later, Chrysopa formosa Brauer took the place. The speed of three natural enemy insects responsed to its food (Myzus persicae) existed a great difference. The control role of these natural enemy insects on aphid is not ideal.The number of Myzus persicae and Dolycoris baccarum in low altitude tobacco field is more than the other two tobacco fields. In middle altitude tobacco field, the Myzus persicae number was more than the high altitude tobacco fields, while the Dolycoris baccarum number was lesser. The changes of Myzus persicae number in middle altitude tobacco field belonged to single peak, while the low and high altitude tobacco fields were bimodal. The Myzus persicae number in tobacco fields at high altitude was the first time reach the first peak. After 20 days, low-altitude tobacco field got to the first aphid peak. Before the tobacco harvest, at August the 14th, the Myzus persicae number of 3 tobacco fields reached the second or the first peak at the same time. Entered the matural period, the number of Myzus persicae in 3 tobacco fields were stable.Dolycoris baccarum appeared after 20 days when tobacco plants been transplanted in fields, later than Myzus persicae. But its end time was earlier, and the occurrance period was discontinuous. The Dolycoris baccarum number was relatively small, the most one was less than 10 per plant.During the tobacco growing period, Myzus persicae liked to distribute in the upper leaves of tobacco plants. With the growing of tobacco leaves increaced, the distribution of Myzus persicae in the leaves were moved up by steps. While in the tobacco matural period, Myzus persicae mainly distributed in the verso of upper leaves of tobacco plant.When insect community just established in tobacco fields in 2009, its diversity was low. In July, insect community diversity in tobacco fields reached its peak. Subsequently, the index declined in the twists and turns. Before the tobacco matural period, its diversity index has reached its peak secongdly. In matural stage, insect communities were affected by continued long-term rain, their diversity index were low. While in tobacco matural period in 2008, the species number of pests and natural enemy insects community is small and stable. The pest community structure was single. Myzus persicae as the dominant species in tobacco fields in pest community was extremely easy to rampant. The natural enemies spices number was small. The quantity of all spices had little differences. Its quantity was small, the control of pests was ineffective, but it was stable.In different altitudes tobacco fields, the populations number of insect community in high-altitude tobacco field was the largest, the distribution of spices individual number was evenest. Its insect community diversity index was good than low-altitude, the community was stable, and spices were complex. The insect communities in high-altitude tobacco field was stabler than mid-altitude fields, the low-altitude one was worst.The insect community relationships between the high and medium altitude tobacco fields was closed than the low-altitude tobacco fields, it was also closed to the relationship between the low altitude and medium altitude tobacco fields. However, the similarity degrees between them were small, which means the different altitudes and phenology that result from different altitudes were important factors that affect insect community structures in tobacco field.