Natural Protection and Augment of Trichogramma Population for Controlling Plutella xylostella

Egg parasitism rate of diamondback moth(DBM),Plutella xylostella(L.),by Trichogramma ostriniae Pang et Chen was greatly enhanced when cruciferous vegetables intercropped with sweet corn,Zea mays(L.).During April to June,2001,egg parasitoid collected from crucifers-corn field was predominately T.ostriniae,and its parasitism varied from 4.62% to 11.47%.While parasitoid species in crucifer fields was T.confusum Viggiani,the parasitism rate of which was no more than 4%.Trap cage of black light for naturally rearing wasp also played an important role in population augment of the Trichogrammatoidea bactrae.About 100 000 Trichogramma wasps had been reared from 127 300 eggs laid by trapped DBM during April to June,2001.In the first month after the cage was set up,the parasitism rate within the cage was 87.34%.In the field the parasitism rate decreased with the increase of distance from the cage,the effective dispersion distance was no more than 20 m.However,the dispersion distance of wasps increased with the time.By the end of May 2001,two months after the trap cage was set up,Trichogramma wasps could arrive at the 30 m from the cage and the parasitism rate kept above 50% within 25 m around the cage.