# Regulation of lipid metabolism in Spodoptera frugiperda by the symbiotic bracovirus of the gregarious parasitoid Cotesia ruficrus

## TL;DR

A parasitoid wasp uses a virus to increase fat levels in its insect host, helping its offspring survive.

## Contribution

Discovery of a specific bracovirus gene that manipulates lipid metabolism in a host insect for parasitoid offspring survival.

## Key findings

- PDV injection increases triglyceride concentrations and lipid droplet sizes in host larvae.
- Silencing the CrBV3–31 gene reduces lipid accumulation and lowers parasitoid pupal eclosion rates.
- The CrBV3–31 gene inhibits LSD1 expression, promoting lipid accumulation in the host.

## Abstract

Parasitoids alter host energy homeostasis to create a favorable environment for their own development. However, the mechanisms underlying this process remain largely unexplored, especially for gregarious parasitoids. Cotesia ruficrus, a gregarious endoparasitoid native to China, targets the invasive pest Spodoptera frugiperda (fall armyworm, FAW) and has been shown to effectively control FAW populations. This study investigates the role of the polydnavirus (PDV) produced by C. ruficrus in regulating lipid metabolism of FAW larvae. The results demonstrated that, following PDV injection for 5 days, both triglyceride concentrations and lipid droplet diameters in the fat bodies of FAW larvae significantly increased. RNA interference (RNAi) targeting the PDV gene CrBV3–31 led to a reduction in triglyceride concentrations and lipid droplet size, along with an upregulation of the LSD1 gene. Furthermore, silencing CrBV3–31 decreased triglyceride levels in C. ruficrus pupae and lowered its eclosion rate. These findings suggest that the PDV gene CrBV3–31 plays a crucial role in enhancing lipid accumulation in FAW larvae, thereby supporting the survival of C. ruficrus offspring. This study uncovers a novel mechanism by which gregarious endoparasitoids exploit symbiotic bracovirus genes to regulate host energy metabolism, increasing lipid levels to meet the developmental needs of their multiple offspring.

Parasitoids influence the physiological metabolism of their hosts, creating an environment conducive to their own survival. However, the mechanisms underlying this phenomenon remain largely unknown, particularly for gregarious parasitoids. In this study, we investigated the gregarious parasitoid Cotesia ruficrus, a key native natural enemy of the invasive pest Spodoptera frugiperda (fall armyworm, FAW) in China. Our results showed that C. ruficrus stimulates an increase in the lipid levels of FAW larvae, with the parasitic factor C. ruficrus bracovirus (CrBV) playing a significant role. Specifically, we identified a CrBV gene, CrBV3–31, which inhibits the expression of LSD1, leading to increased lipid accumulation in FAW larvae. Moreover, CrBV3–31 enhances the survival of C. ruficrus offspring. These findings shed light on how a symbiotic bracovirus gene manipulates host lipid metabolism in a gregarious parasitoid, offering valuable insights for the development of parasitoid conservation strategies.