Abstract:  【Aim】 To investigate the effects of Steinernema carpocapsae All infection on the innate immune response in larvae of the fall armyworm, Spodoptera frugiperda. 【Methods】 The hemocyte types of S. frugiperda larvae were observed and identified under an inverted microscope, and the total numbers of hemocytes in S. frugiperda larvae at different time after infection by S. carpocapsae All were counted. The encapsulation of invading S. carpocapsae All nematodes by S. frugiperda hemocytes was observed under an inverted microscope. The phagocytoic activity of fluorescent Staphylococcus aureus by hemocytes of S. frugiperda larvae was observed under an inverted fluorescence microscope. The phenoloxidase (PO) activity in the hemolymph, the relative expression levels of antibacterial peptide genes, and the antibacterial activity of plasma in S. frugiperda larvae infected with S. carpocapsae All were detected. 【Results】 Five types of hemocytes, prohemocyte, granulocyte, oenocytoide, spherulocyte and plasmatocyte, were found in S. frugiperda larvae. The total numbers of hemocytes in S. frugiperda larvae increased significantly at 9 and 12 h after injection of 1 μL S. carpocapsae All infective juveniles (IJs) at the dose of 3 IJs/μL. The hemocytes from S. frugiperda larvae failed to encapsulate the live and coldkilled S. carpocapsae All nematodes but could encapsulate heat-killed nematodes. The phagocytic activity of fluorescent S. aureus by S. frugiperda hemocytes was significantly inhibited after incubation with live S. carpocapsae All nematodes, but not with cold- and heat-killed S. carpocapsae All nematodes. The PO activity in the hemolymph of S. frugiperda larvae decreased first, then increased, and finally decreased after injection of 1 μL S. carpocapsae All at the dose of 3 IJs/μL. The relative expression levels of antimicrobial peptide genes Attacin-A2, Attacin-B1, Cecropin-B3, Cecropin-D, Gallerimycin, Gloverin-3 and Lebocin-2 in S. frugiperda larvae were significantly induced at 12 h after S. carpocapsae All infection, and then recovered to the control level or lower than the control level at 24 h after infection. The antibacterial activity of S. frugiperda plasma increased significantly at 12 h after S. carpocapsae All infection, but was not significantly different between the treatment group and the control group at 24 h after infection. 【Conclusion】 In the early stage of infection, S. carpocapsae All would inhibit the innate immune response in S. frugiperda larvae, then the immune system in S. frugiperda would be initiated for trying to defend against S. carpocapsae All, and in the late stage the immune system in S. frugiperda would be inhibited or destroyed with the successful colonization of nematodes. The results obtained in this study provide a basis for further understanding the immune mechanisms involved in the interaction between nematodes and S. frugiperda, and lay a theoretical foundation for further improving the control efficacy of entomopathogenic nematodes against S. frugiperda larvae.

Key words: Spodoptera frugiperda, entomopathogenic nematode; Steinernema carpocapsae All, innate immune, hemocyte, biological control