Studies On Occurrence Dynamics And Biocontrol Techniques Of Corn Earworms In Hebei Province

In recent years,corn earworms including Ostrinia furnacalis,Helicoverpa armigera and Conogethes punctiferalis are more and mone serious.The damage rate of corn earworms in many corn plots have reached 100%,the yield of maize usually reduced by 10% to 20% percent.It is more serious that damage of corn earworms can make corn grain mildew and cause aflatoxin to exceed the standard.Not only does it seriously affect the quality of the grain,but also the strong carcinogenicity of aflatoxin also affects people health.As borer pests,corn earworms usually go into corn bracts to feed on grains or ear axes.In addition,when corn earworms damage,the corn plant is tall,and the female ear of corn is located in the middle of corn,so the general chemical control methods are difficult to work.The purpose of this experiment is to identified the best Trichogramma species and release time and evaluat field control effect of Trichogramma to corn earworms.The main results are as follows:1.Based on the systematic investigation of pests and natural enemies during the whole growth period of summer maize,the structure characteristics of insect community and the occurrence dynamics of corn earworms larvae were clarified.The survey results showed that the pests in corn field belonged to 6 orders and 11 families.The dominant population in different corn fields was different.The dominant population in Wangdu corn field was corn aphids in corn ears stage,accounting for 87.03% of the total insect population.The dominant population in the corn field of Kantai Garden in Hejian city was the corn thrips on seedling stage,accounting for 47.16% of the total insect population.The main natural enemies in corn fields had six classes,belonging to 5 orders.The spiders were the dominant population,accounting for 49.52% of the total natural enemies.The emergence time of three kinds of corn earworms larvae in summer maize field was different.H.armigera larvae continued to harm from jointing stage to corn harvesting stage,and reached the peak at corn filling stage.O.furnacalis larvae began to appear at corn heading stage and continue to harm until corn harvesting stage.C.punctiferalis larvae appeared at maize filling stage.Mixed damage of three species of corn earworms occurred after grain filling stage.2.In order to determine the release time of Trichogramma spp.for controlling three species of corn earworms,the occurrence dynamics of three kinds of corn earworms adults in different areas were investigated by using trap lamp and sex attractant respectively.The results showed that the peak of the first generation of H.armigera adult occurred in mid-late June,the second generation in late July,the third generation in late August.The overwintering generation of O.furnacalis adult in mid-late June,the first generation in mid-late July and the second generation in late August to early September.The number of overwintering generation and first generation population of peach borer was very low,so the peak period is not obvious,and it does not appear in the corn field.The number of second generation of C.punctiferalis adults increaseed,and generations overlaped seriously from late August to mid-September.Generally,the second generation of H.armigera adults and the first generation of O.furnacalis adulst overlaped in summer maize field,while the third generation of H.armigera adults,the second generation of O.furnacalis adults and C.punctiferalis adults overlaped.3.In order to evaluate the parasitic effect of T.ostriniae and T.chilonis on the eggs of three species of corn earworms,the parasitism rates of two Trichogramma to the eggs of three species of corn earworms were determined in the laboratory.The results showed that the average parasitic rates of T.chilonis to O.furnacalis,C.punctiferalis and H.armigera were 37.60%,4.63% and 51.28%,respectively.The average parasitic rates of T.ostriniae to O.furnacalis,C.punctiferalis and H.armigera were 40.4%,3.67% and 26.1%,respectively.The parasitic rate of T.chiloni to H.armigera was the highest.The parasitic rate of T.ostriniae to O.furnacalis was the highest.However,the parasitic rates of the two Trichogramma species to the eggs of C.punctiferalis all were very low.4.In order to evaluate the effect of releasing Trichogramma in the field to control corn earworms,we released mixture population of T.ostriniae and T.chilonis at different corn growth stages fields in the same time(on July 25 th and August 2nd).The pest-ear rate of early corn and late corn were 34%(control as 89.33%)and 86%(control as 80.67%)respectively.The results showed that the control effect of early corn area was better than that of late corn field.The parasitic rate of Trichogramma to O.furnacalis eggs released on July 25 and August 2,August 25 and August 31 was 92.31% and 88.24% respectively which was significantly higher than that of the control(70.79%).The parasitic rate of Trichogramma to O.furnacalis eggs released on August 13 and 17 was only 64.13% which was lower than that of the control without wasps.The pest ear rate of corn was 86%,84% and 92.67% in the three time periods respectively,while that of the control area was 80.67%.These results indicated that the release of Trichogramma wasps did not reduce the pest ear rate of corn,and most larvae on corn earworms were C.punctiferalis larvae.The field results further confirmed that T.ostriniae and T.chilonis could effectively control the population of O.furnacalis and H.armigera,but could not control the population of C.punctiferalis which resulted in a high number of C.punctiferalis larvae on the corn ears in the Trichogramma-releasing field.