Melanization immune response of Spodoptera frugiperda to an endoparasitoid Microplitis pallidipes

Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) is a worldwide invasive pest. Microplitis pallidipes Szépligeti (Hymenoptera: Braconidae) is an endoparasitoid of this pest. However, the melanization immune response of this pest to M. pallidipes, has not yet been explored. We found that the parasitism of S. frugiperda by M. pallidipes severely inhibited hemolymph melanization and phenoloxidase activity of larvae. Transcriptomic and proteomic analysis indicated that 1128 genes and 594 proteins were significantly altered in expression following parasitism. These included 12 of the 23 candidate genes involved in melanization in S. frugiperda (i.e., 7 recognition protein genes, 10 prophenoloxidase (PPO) cascade genes, and 6 melanin synthesis genes). Eight of these 12 melanization genes were significantly downregulated in S. frugiperda larvae 24 h or 48 h after treatment with siRNAs. Of these, RNAi knockdown of PPO1, PPO2 and TH (tyrosine hydroxylase) increased parasitism rate by 13.3 %–23.3 % and decreased hemymphatic melanization rate by 36.7–66.7 %. Our results provide insight into the melanization response of S. frugiperda and identify genes that appear to play a key role in this process. These findings shed light on the mechanisms by which insect herbivores respond to parasitism, and have applied implications for the biological control of insect pests by parasitic wasps.