蚜虱净对苜蓿主要害虫及天敌种群数量的影响

在甘肃省定西市九华沟系统研究了喷施蚜虱净对苜蓿主要害虫及天敌种群数量动态的影响。结果表明，施药田蚜虫和蓟马的季节平均数量极显著高于对照田，而盲蝽和瓢虫、小花蝽、蜘蛛等大多数天敌的季节平均数量则极显著低于对照田，寄生蜂与对照田没有湿著差异。喷施蚜虱净对种群数量时序动态的影响在不同害虫和天敌种类中表现不同，5月底施药后7d对照田蚜虫和蓟马的种群数量分别为施药田的4．13倍和5．2倍，此后施药田害虫的种群数量开始迅速增加，到6月底2种害虫的种群数量均与对照田无显著差异，到7月中旬以后反而显著高于对照田。施药对盲蝽种群数量时序动态的影响与蚜虫和蓟马不同，5月底施药后盲蝽的种群数量一直显著低于对照田，直到7月下旬以后，种群数量才恢复到对照田的水平。喷施蚜虱净对天敌种群数量的时序动态有极显著的影响，大多数天敌的种群数量在5月底施药后到7月底都一直显著低于对照田，直到8月上旬才恢复到对照田的水平。施用杀虫剂在防治害虫的同时杀伤大量天敌，使害虫的种群数量迅速增加，甚至造成更严重的为害。因此在菖蓿上使用对天敌杀伤力小的杀虫剂对保持天敌的自然控制能力有重要作用。

Effects of imidacloprid spraying on the population dynamics of main insect pests and natural enemies on alfalfa

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.