The effect of imidacloprid spray on the population dynamics of primary insect pests and their natural enemies in alfalfa fields was evaluated in DingXi, Gansu Province in 2002. The average number of aphids and thrips across the entire post-spray period (From late May to August 2002) was significantly higher in imidacloprid-treated fields than in untreated fields, while the average number of plant bugs and most natural enemies including coccinellids, Orius minutus, and spiders was drastically reduced in the treated fields. The average population size of parasitic wasps was not significantly different in the treated fields compared with untreated fields across the entire post-spray period. It appeared that imidacloprid spray differentially affected the population dynamics among different insect pests and natural enemies. On the seventh day post-spray in late May, the populations of aphids and thrips in the treated fields were 4.13-fold and 5.2-fold lower than in the untreated fields, respectively. However, these pest populations recovered and increased rapidly in the treated fields. The population numbers were similar by the end of June and even significantly higher than the untreated fields by mid-July. For plant bugs, the population was significantly inhibited in the treated fields in late May, but by late July the population recuperated to untreated-field level. The natural enemies investigated were significantly affected in the treated fields compared with the untreated fields during the period from the imidacloprid spray performed in late May to the end of July. Most natural enemy populations recovered and reached untreated-field levels by early August These observations suggest that imidacloprid spray inhibited insect pest populations only temporarily. Pest populations recovered rapidly resulting in even more serious pest infestation occurring in the late season because of negative impacts of pesticide spray upon the natural enemies.