[Effects of imidacloprid on arthropod community structure and its dynamics in alfalfa field]

With species richness (S), individual number (N), Shannon's index (H') and evenness (E) as the indices of biodiversity, this paper studied the arthropod community structure and its seasonal dynamics in alfalfa field under effects of imidacloprid. The results showed that imidacloprid more affected the species number of natural enemies rather than that of insect pests, resulting in a significant increase of the dominance concentration of both total arthropod community and pest sub-community, but spraying the insecticide by the end of May had little effects on the dominance concentration of pollination bee sub-community. The numbers of dominant pests such as aphid and thrip reduced significantly at the initial stage of spraying imidacloprid, increased rapidly after 7 days, and exceeded the control after 40 days. As for the natural enemy sub-community, its species number decreased greatly and its species diversity and evenness were lower at the initial stage of spraying imidacloprid, its species number increased gradually after 7 days and approached to the level of the control thereafter, while its individual number was lower than the control all the time.