菜青虫是对蔬菜危害较重的害虫，它取食植株叶片，严重者能把叶片食光，仅剩下叶脉或叶柄。此外，菜青虫造成的伤口，易招致软腐病菌的侵入而引起病害流行。菜青虫１年有两个危害高峰，春季随气温升高，虫口迅速上升，在春夏之间达到高峰，盛夏或雨季虫口密度迅速下降，到秋季? Pieris rapae is a pest that harms vegetables. It infects the leaves of plants. In severe cases, the leaves can be eaten, leaving only veins or petioles. In addition, cabbage caterpillars caused by the wound, easily lead to the invasion of soft rot bacteria caused by epidemics. Pieris rapae has two hazardous peaks in 1 year. With the temperature rising in spring, the insect population rapidly increases and reaches the peak between spring and summer. The density of insect population in summer or rainy season drops rapidly to fall.