Effects of the parasitoid Microplitis pallidipes on the menanization and phenoloxidase activity of the hemolymph of Spodoptera frugiperda larvae.
Objectives: To investigate the effects of Microplitis pallidipes on the menanization, prophenoloxidase (PPO) gene expression and phenoloxidase (PO) activity, of the hemolymph of Spodoptera frugiperda larvae, and thereby provide a foundation for the more effective use of this endoparasitoid to control this pest. Methods: The S. frugiperda larval instar most susceptible to parasitism by M. pallidipes was determined experimentally and RT-qPCR was then used to analyze the temporal and spatial expression profiles of two S. frugiperdaPPO genes. The menanization rate of larvae was counted using the film method, and the PO activity of larvae was measured using the absorbance method. The expression of SfPPO1 and SfPPO2 in the hemolymph of S. frugiperda was determined with RT-qPCR. Results: 3rd instar S. frugiperda larvae were found to be the most susceptible to being parasitized by M. pallidipes. The relative expression of the two SfPPO genes in eggs and 4th instar larvae was relatively high, and both genes were mainly expressed in the hemolymph. Hemolymph menanization rates of parasitized larvae were significantly lower than those of the non-parasitized larvae after 12, 24, 48 and 72 hours (P < 0.05). PPO expression and PO activity of parasitized larvae were also lower than those of non-parasitized larvae. There was a strong, positive linear correlation between the expression of PPO1 and PPO2 (R2=0.912 4), and there were also positive correlations among PO activity, PPO expression and the menanization rate. Conclusion: Parasitism of M. pallidipes inhibited the menanization of S. frugiperda larvae and down-regulated the expression of two PPO genes and PO activity. PO activity, PPO gene expression and menanization rate were all positively correlated.