Pathogenicity study of a strain of Cordyceps fumosorosea to Spodoptera frugiperda(Lepidoptera:Noctuidae)

The fall armyworm,Spodoptera frugiperda(J.E Smith)(Lepidoptera:Noctuidae),is a new invasive pest to China.To understand the pathogenic effect of the entomopathogenic fungus Cordyceps fumosorosea(renamed from Isaria fumosorosea)to S.frugiperda,the pathogenicity of C.fumosorosea strain SCAU-IFCF01 to S.frugiperda larvae was studied in laboratory by immersing larvae of different instars into serial concentrations of conidial suspension.The external symptoms of infected larvae were observed under light microscope as well.The results showed that larvae from 1 st to 6 th instar were infected by C.fumosorosea strain IFCF01.The whole body surface of 4 th instar larvae thickened and reduced in size after 48 h post inoculation.Hyphae colonized and emerged around the cadavers after 72 h post inoculation.Whole insect body covered with aerial mycelium and conidia after 120 h post inoculation.The mortality rates of the S.frugiperda caused by infection of C.fumosorosea increased with the concentration of conidial suspension and the time after treatment.The highest concentration(1×109 conidia/mL)treatment caused 100%,100%,100%,98.9%,50%,and 21.1%mortality for 1 st to 6 th instar larvae on the 5 th day,respectively.The estimated LC 50 values of 1 st to 5 th instar larvae on day 5 after treatment were 3.74×104、1.17×105、1.86×105、8.09×105 and 3.03×108 conidia/mL,respectively.Values of LT 50 of all tested instars were shortened with concentration.The LT 50 values of 1 st and 2 nd instar were 3.62~1.37 d and 4.51~1.65 d,respectively,in the range of 1×105~1×109 conidia/mL.The LT 50 values of 3 rd instar were 4.03~1.82 d in the range of 1×106~1×109 conidia/mL.The LT 50 values of 4 th instar were 3.47~2.52 d in the range of 1×107~1×109 conidia/mL.The LT 50 value of 5 th instar was 4.74 d in 1×109 conidia/mL.The results suggested the potential of the C.fumosorosea strain IFCF01 for use in microbial control of S.frugiperda.