A Comparison of the Vertical Transmission of High- and Low-Virulence Nucleopolyhedrovirus Strains in Lymantria Dispar L

Differential pathogenicity of heterologous nucleopolyhedroviruses on Spodoptera frugiperda corresponds to modulated expression of immune-response-related genes

The fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is a notorious invasive pest wreaking havoc on various crops globally. Nucleopolyhedroviruses (NPVs) are viral pathogens that specially target lepidopteran pests. However, the homologous virus, Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV), has not been commercialized in China. Therefore, understanding the molecular mechanisms underlying heterologous virus-host interactions can inform the design of virus-based insecticides for controlling S. frugiperda. The pathogenicity of the four heterologous NPVs on S. frugiperda varied greatly. Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) exhibited the most potent virulence on larvae and induced the most robust sublethal effects on adults. Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) infection was characterized by more moderate pathogenicity, and larvae were relatively resistant to Helicoverpa armigera single nucleopolyhedrovirus (HaSNPV) and Spodoptera litura multiple nucleopolyhedrovirus (SlMNPV). Larval mortality was virus-concentration and larval stage dependent. Specifically, the corrected mortality rate of third instar larvae after treatment with 1 × 106, 1 × 107, and 1 × 108 OBs/mL MbMNPV was 88.9 %, 100.0 %, and 100.0 %, respectively. All four NPVs negatively affected the longevity and fecundity of S. frugiperda adults. Female adults surviving treatment with MbMNPV and SeMNPV were unable to lay eggs. Transcriptomic analysis revealed that MbMNPV infection might suppress the antiviral immune response, and dysregulate biological pathways of S. frugiperda larvae to facilitate systemic infection. However, the overall transcript profiles remain unchanged after SlMNPV infection. The results reinforce the potential of NPVs, specifically MbMNPV, as potent biocontrol agents for S. frugiperda. These findings yield valuable insights into the complex arms race between S. frugiperda and NPVs that may advance the development of virus-based strategies to mitigate the destructive impact of this pest.

The Impact of a Cypovirus on Parental and Filial Generations of Lymantria dispar L

Recently, we found that the spongy moth Lymantria dispar L. is susceptible to infection by a Dendrolimus sibiricus cytoplasmic polyhedrosis virus (DsCPV-1). In the present study, we evaluated the pathogenicity of DsCPV-1 against L. dispar larvae and its impact on surviving insects after the infection. Offspring of virally challenged insects were tested for susceptibility to a stress factor (starvation). In addition, we used light microscopy and quantitative polymerase chain reaction (qPCR) to test the ability of DsCPV-1 to be transmitted vertically. We found insect mortality of the L. dispar parents following the infection was positively associated with DsCPV-1 dose. DsCPV-1 was lethal to second-instar L. dispar larvae with a 50% lethal dose (LD50) of 1687 occlusion bodies per larva. No vertical transmission of DsCPV-1 to offspring larvae was detected, while the majority of insect deaths among offspring larvae were caused by microsporidia (Vairimorpha lymantriae), which was harbored by the parents. The offspring of virally challenged parents exhibited a higher number of detected microsporidia compared to the control. Our findings suggest that the application of DsCPV-1 is effective in controlling pests in terms of transgenerational impact following virus exposure.

Susceptibility of the Gypsy Moth Lymantria dispar (Lepidoptera: Erebidae) to Nosema pyrausta (Microsporidia: Nosematidae)

Simple Summary

Microsporidia are widespread insect pathogens and a single species may infect different hosts. Nosema pyrausta from the corn borer was tested against the gypsy moth. Thirty days after larvae were fed with spores, infection was established in the salivary glands and fat body of pupae and prepupae. Up to 10% of insects became infected. The gypsy moth can be referred to as a resistant host of N. pyrausta.

Abstract

The gypsy moth, Lymantria dispar, is a notorious forest defoliator, and various pathogens are known to act as natural regulators of its population density. As a widespread herbivore with a broad range of inhabited areas and host plants, it is potentially exposed to parasitic microorganisms from other insect hosts. In the present paper, we determined the susceptibility of gypsy moth larvae to the microsporidium Nosema pyrausta from the European corn borer, Ostrinia nubilalis. Gypsy moth samples from two localities of Western Siberia were used. N. pyrausta developed infections in the salivary gland and adipose tissue of gypsy moth prepupae and pupae, forming spore masses after 30 days of alimentary exposure to the second instar larvae. Among the experimental groups, the infection levels ranged from 0 to 9.5%. Effects of a covert baculovirus infection, phenylthiourea pretreatment and feeding insects on an artificial diet versus natural foliage were not significant in terms of microsporidia prevalence levels. Thus, L. dispar showed a low level of susceptibility to a non-specific microsporidium. It can be referred to as a resistant model host and not an appropriate substitute host for laboratory propagation of the microsporidium.

Gypsy Moth Management with LdMNPV Baculovirus in Cork Oak Forest

(1) Research Highlights: Applications of a species-specific baculovirus is a promising method to control the gypsy moth and regulate its population dynamics in forest ecosystems. (2) Background and Objectives: Cork oak protection against the Lepidopteran defoliator Lymantria dispar requires an appropriate forest ecosystem management program, involving the application of eco-sustainable microbial products during population outbreaks. The species-specific multicapsid nucleopolyhedrovirus (LdMNPV), agent of natural epizootics in gypsy moth populations, represents an option that was investigated in a multi-year field study, involving viral applications either from the ground or by aerial treatment. (3) Materials and Methods: Efficacy trials against L. dispar populations were conducted in 2018 and 2019 in Sardinia, according to a randomized block design. Each year, two trials were conducted, applying a baculovirus commercial formulation with an atomizer from the ground and assessing the effects of different doses and application timing, respectively. An aerial application trial distributing LdMNPV at ultra-low volumes (2 L/ha) was also conducted in 2019 to assess the virus efficacy at a larger field scale. (4) Results: In both years, a significant increase in larval mortality was detected in plots treated with higher viral occlusion body (OB) doses and with an earlier application targeting younger larvae, in comparison with untreated controls. Due to an observed retrogradation phase of the target pest in 2019, no significant differences in larval density between areas treated from a helicopter and control were detected, but in the few weeks following application, a meaningful vitality decrease in larval samples from treated plots was observed. (5) Conclusions: Based on the results of this study, the use of LdMNPV in forest protection programs against gypsy moth can be worth consideration in multi-year integrated program strategies to modulate population dynamics.