Host Transcriptome Analysis of Spodoptera frugiperda Larvae Parasitized by Microplitis manilae.

It has been extensively found that parasitoids manipulate host physiology to benefit the survival and development of their offspring. However, the underlying regulatory mechanisms have not received much attention. To reveal the effects of parasitization of the larval solitary endoparasitoid Microplitis manilae (Hymenoptera: Braconidae) on host Spodoptera frugiperda (Lepidoptera: Noctuidae), one of the most destructive agricultural pests in China, deep-sequencing-based transcriptome analysis was conducted to compare the host gene expression levels after 2 h, 24 h, and 48 h parasitization. A total of 1861, 962, and 108 differentially expressed genes (DEGs) were obtained from the S. frugiperda larvae at 2 h, 24 h, and 48 h post-parasitization, respectively, compared with unparasitized controls. The changes in host gene expressions were most likely caused by the injection of wasp parasitic factors, including PDVs, that were injected along with the eggs during oviposition. Based on the functional annotations in GO and KEGG databases, we revealed that most DEGs were implicated in host metabolism and immunity. Further analysis of the common DEGs in three comparisons between the unparasitized and parasitized groups identified four genes, including one unknown and three prophenoloxidase (PPO) genes. Moreover, 46 and 7 common DEGs involved in host metabolism and immunity were identified at two or three time points after parasitization, respectively. Among these, most DEGs showed increased expressions at 2 h post-wasp parasitization while exhibiting significantly decreased expression levels at 24 h post-parasitization, demonstrating the expression regulations of M. manilae parasitization on host metabolism and immune-related genes. Further qPCR verification in 20 randomly selected DEGs confirmed the accuracy and reproducibility of the gene expression profiles generated from RNA-seq. This study reveals the molecular regulatory network about how host insects respond to wasp parasitism, laying a solid foundation for revealing the physiological manipulation of wasp parasitization on host insects, which facilitates the development of biological control practices for parasitoids.

Transcriptional regulation of host insulin signaling pathway genes controlling larval development by Microplitis manilae parasitization.

Idiobiont parasitoids using other insects as hosts sabotage the host growth and development to ensure their offspring survival. Numerous studies have discovered that insect development is subtly regulated by the conserved insulin signaling pathway. However, little is known about how wasp parasitization disrupts host development controlled by the insulin signaling pathway. Here we address this study to determine the effect of wasp parasitism on host Spodoptera frugiperda development using the idiobiont parasitoid Microplitis manilae as a model. Upon M. manilae parasitization, the body weight, body length, and food consumption of host insect were dramatically reduced compared to the unparasitized S. frugiperda. We next identified the core genes involved in host insulin signaling pathway and further analyzed the domain organizations of these genes. Phylogenetic reconstruction based on the insulin receptors clustered S. frugiperda together with other noctuidae insects. In the latter study, we profiled the expression patterns of host insulin signaling pathway genes in response to M. manilae parasitization at 2, 24, and 48 h, significant decreases in mRNA levels were recorded in S. frugiperda larvae upon 24 and 48 h parasitization. These current findings substantially add to our understanding of the physiological interaction between parasitoid and host insects, thus contributing to revealing the molecular mechanism of parasitic wasps regulating host development.

Effect of Pupal Cold Storage on Reproductive Performance of Microplitis manilae (Hymenoptera: Braconidae), a Larval Parasitoid of Spodoptera frugiperda (Lepidoptera: Noctuidae).

As a major invasive pest in China, Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae) has caused great damage to crops. Hymenopteran parasitoids, especially the braconid wasps, play crucial roles in depressing pest populations. However, there was little information about the ideal storage of parasitoids to achieve their mass-rearing. Here, we identified a dominant parasitoid of S. frugiperda, Microplitis manilae (Ashmead) (Hymenoptera: Braconidae), in the Hainan province of China with a field parasitism rate of 5.66-19.10%. The investigation of biological parameters revealed that the parasitism rate of M. manilae significantly decreased with an increase in both wasp adult longevity and host age, and the wasp of 1-3 d post eclosion performed best on the first instar of host larvae, showing the highest parasitism rate. We also discovered that the decreased temperature from 30 to 20 °C greatly extended the longevity of wasp adults, and a similar result was observed after feeding on 10% sucrose water compared with sterile water. Then, the effects of different pupal cold storage temperatures (4 and 10 °C), storage period (prepupa, middle-aged pupa, late-aged pupa) and storage time (5, 10 or 20 d) on the emergence rate, parasitism rate, female proportion and longevity of M. manilae were investigated. The results demonstrated that the middle-aged wasp pupae stored at 10 °C for 5-10 d possessed a stronger parasitic ability and longer longevity. These findings may promote the flexibility and efficacy of large-scale production of M. manilae, thus contributing to its biological field control against S. frugiperda.