Virus entry or the primary infection cycle are not the principal determinants of host specificity of Spodoptera spp. nucleopolyhedroviruses

The distribution of covert microbial natural enemies of a globally invasive crop pest, fall armyworm, in Africa: Enemy release and spillover events

Invasive species pose a significant threat to biodiversity and agriculture world‐wide. Natural enemies play an important part in controlling pest populations, yet we understand very little about the presence and prevalence of natural enemies during the early invasion stages.

Microbial natural enemies of fall armyworm Spodoptera frugiperda are known in its native region, however, they have not yet been identified in Africa where fall armyworm has been an invasive crop pest since 2016. Larval samples were screened from Malawi, Rwanda, Kenya, Zambia, Sudan and Ghana for the presence of four different microbial natural enemies; two nucleopolyhedroviruses, Spodoptera frugiperda NPV (SfMNPV) and Spodoptera exempta NPV (SpexNPV); the fungal pathogen Metarhizium rileyi; and the bacterium Wolbachia. This study aimed to identify which microbial pathogens are present in invasive fall armyworm, and determine the geographical, meteorological and temporal variables that influence prevalence.

Within 3 years of arrival, fall armyworm was exposed to all four microbial natural enemies. SfMNPV probably arrived with fall armyworm from the Americas, but this is the first putative evidence of host spillover from Spodoptera exempta (African armyworm) to fall armyworm for the endemic pathogen SpexNPV and for Wolbachia. It is also the first confirmed incidence of M. rileyi infecting fall armyworm in Africa.

Natural enemies were localised, with variation being observed both nationally and temporally. The prevalence of SfMNPV (the most common natural enemy) was predominantly explained by variables associated with the weather; declining with increasing rainfall and increasing with temperature. However, virus prevalence also increased as the growing season progressed.

The infection of an invasive species with a natural enemy from its native range and novel pathogens specific to its new range has important consequences for understanding the population ecology of invasive species and insect–pathogen interactions. Additionally, while it is widely known that temporal and geographic factors affect insect populations, this study reveals that these are important in understanding the distribution of microbial natural enemies associated with invasive pests during the early stages of invasion, and provide baseline data for future studies.

Viruses of the Fall Armyworm Spodoptera frugiperda: A Review with Prospects for Biological Control

The fall armyworm (FAW), Spodoptera frugiperda, is a native pest species in the Western hemisphere. Since it was first reported in Africa in 2016, FAW has spread throughout the African continent and is now also present in several countries in Asia as well as Australia. The invasion of FAW in these areas has led to a high yield reduction in crops, leading to huge economic losses. FAW management options in the newly invaded areas are limited and mainly rely on the use of synthetic pesticides. Since there is a risk of resistance development against pesticides in addition to the negative environmental and human health impacts, other effective, sustainable, and cost-efficient control alternatives are desired. Insect pathogenic viruses fulfil these criteria as they are usually effective and highly host-specific with no significant harmful effect on beneficial insects and non-target organisms. In this review, we discuss all viruses known from FAW and their potential to be used for biological control. We specifically focus on baculoviruses and describe the recent advancements in the use of baculoviruses for biological control in the native geographic origin of FAW, and their potential use in the newly invaded areas. Finally, we identify current knowledge gaps and suggest new avenues for productive research on the use of viruses as a biopesticide against FAW.

Mixtures of Insect-Pathogenic Viruses in a Single Virion: towards the Development of Custom-Designed Insecticides

Alphabaculoviruses (Baculoviridae) are pathogenic DNA viruses of Lepidoptera that have applications as the basis for biological insecticides and expression vectors in biotechnological processes. These viruses have a characteristic physical structure that facilitates the transmission of groups of genomes. We demonstrate that coinfection of a susceptible insect by two different alphabaculovirus species results in the production of mixed-virus occlusion bodies containing the parental viruses. This occurred between closely related and phylogenetically more distant alphabaculoviruses. Approximately half the virions present in proteinaceous viral occlusion bodies produced following coinfection of insects with a mixture of two alphabaculoviruses contained both viruses, indicating that the viruses coinfected and replicated in a single cell and were coenveloped within the same virion. This observation was confirmed by endpoint dilution assay. Moreover, both viruses persisted in the mixed-virus population by coinfection of insects during several rounds of insect-to-insect transmission. Coinfection by viruses that differed in genome size had unexpected results on the length of viral nucleocapsids, which differed from those of both parental viruses. These results have unique implications for the development of alphabaculoviruses as biological control agents of insect pests.IMPORTANCE Alphabaculoviruses are used as biological insecticides and expression vectors in biotechnology and medical applications. We demonstrate that in caterpillars infected with particular mixtures of viruses, the genomes of different baculovirus species can be enveloped together within individual virions and occluded within proteinaceous occlusion bodies. This results in the transmission of mixed-virus populations to the caterpillar stages of moth species. Once established, mixed-virus populations persist by coinfection of insect cells during several rounds of insect-to-insect transmission. Mixed-virus production technology opens the way to the development of custom-designed insecticides for control of different combinations of caterpillar pest species.

Genetic Variation and Biological Activity of Two Closely Related Alphabaculoviruses during Serial Passage in Permissive and Semi-Permissive Heterologous Hosts

Phylogenetic analyses suggest that Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and Helicoverpa armigera multiple nucleopolyhedrovirus (HearMNPV) may be strains of the same virus species. Most of the studies comparing their biological activities have been performed in their homologous hosts. A comparison of host range and stability in alternative hosts was performed. The host range of these viruses was compared using high concentrations of inoculum to inoculate second instars of six species of Lepidoptera. One semi-permissive host (Spodoptera littoralis) and one permissive host (S. exigua) were then selected and used to perform six serial passages involving a concentration corresponding to the ~25% lethal concentration for both viruses. Restriction endonuclease analysis showed fragment length polymorphisms in every host-virus system studied. In S. littoralis, serial passage of MbMNPV resulted in decreased pathogenicity and an increase in speed-of-kill, whereas no significant changes were detected for HearMNPV with respect to the initial inoculum. In contrast, both viruses showed a similar trend in S. exigua. These results highlight the low genetic diversity and a high phenotypic stability of HearMNPV with respect to the original inoculum after six successive passages in both insect hosts. This study concludes that host-baculovirus interactions during serial passage are complex and the process of adaptation to a novel semi-permissive host is far from predictable.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.

Genomic diversity in European Spodoptera exigua multiple nucleopolyhedrovirus isolates

Key virus traits such as virulence and transmission strategies rely on genetic variation that results in functional changes in the interactions between hosts and viruses. Here, comparative genomic analyses of seven isolates of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) with differing phenotypes were employed to pinpoint candidate genes that may be involved in host-virus interactions. These isolates obtained after vertical or horizontal transmission of infection in insects differed in virulence. Apart from one genome containing a piggyBac transposon, all European SeMNPV isolates had a similar genome size and content. Complete genome analyses of single nucleotide polymorphisms and insertions/deletions identified mutations in 48 ORFs that could result in functional changes. Among these, 13 ORFs could be correlated with particular phenotypic characteristics of SeMNPV isolates. Mutations were found in all gene functional classes and most of the changes we highlighted could potentially be associated with differences in transmission. The regulation of DNA replication (helicase, lef-7) and transcription (lef-9, p47) might be important for the establishment of sublethal infection prior to and following vertical transmission. Virus-host cell interactions also appear instrumental in the modulation of viral transmission as significant mutations were detected in virion proteins involved in primary (AC150) or secondary infections (ME35) and in apoptosis inhibition (IAP2, AC134). Baculovirus populations naturally harbour high genomic variation located in genes involved at different levels of the complex interactions between virus and host during the course of an infection. The comparative analyses performed here suggest that the differences in baculovirus virulence and transmission phenotypes involve multiple molecular pathways.

Baculovirus replication induces the expression of heat shock proteins in vivo and in vitro

A recent handful of studies have linked baculovirus infection with the induction of heat shock proteins, a highly conserved family of cytoprotective proteins. Here, we demonstrate baculovirus-stimulated upregulation of hsp70 transcription in the natural host, Helicoverpa zea. Larvae lethally infected with Helicoverpa zea single nucleopolyhedrovirus (HzSNPV) accumulated hsp70 transcripts throughout the 72-hour course of infection in the midgut, hemocytes, and fat body. While a maximal 17- or 15-fold induction of hsp70 was noted in the midgut and hemocytes, respectively, by 72 hours postinfection, the level of hsp70 transcription in the fat body of larvae was greater than two orders of magnitude higher than in mock-infected larvae. These results were largely mirrored in cultures of infected cells, and a potentiation effect was observed in cells that were both heat shocked and infected. In contrast, Spodoptera frugiperda multiple nucleopolyhedrovirus and ultraviolet-inactivated HzSNPV did not stimulate hsp70 transcription in these non-permissive larvae and in cell culture, respectively. Taken together, this report documents baculovirus-mediated upregulation of hsp70 in the host and demonstrates the requirement for productive infection for hsp70 induction in vitro and in vivo.

Analagous population structures for two alphabaculoviruses highlight a functional role for deletion mutants

A natural Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) isolate from Florida shares a strikingly similar genotypic composition to that of a natural Spodoptera frugiperda MNPV (SfMNPV) isolate from Nicaragua. Both isolates comprise a high proportion of large-deletion genotypes that lack genes that are essential for viral replication or transmission. To determine the likely origins of such genotypically similar population structures, we performed genomic and functional analyses of these genotypes. The homology of nucleotides in the deleted regions was as high as 79%, similar to those of other colinear genomic regions, although some SfMNPV genes were not present in SeMNPV. In addition, no potential consensus sequences were shared between the deletion flanking sequences. These results indicate an evolutionary mechanism that independently generates and sustains deletion mutants within each virus population. Functional analyses using different proportions of complete and deletion genotypes were performed with the two viruses in mixtures of occlusion bodies (OBs) or co-occluded virions. Ratios greater than 3:1 of complete/deletion genotypes resulted in reduced pathogenicity (expressed as median lethal dose), but there were no significant changes in the speed of kill. In contrast, OB yields increased only in the 1:1 mixture. The three phenotypic traits analyzed provide a broader picture of the functional significance of the most extensively deleted SeMNPV genotype and contribute toward the elucidation of the role of such mutants in baculovirus populations.

Host-range expansion of Spodoptera exigua multiple nucleopolyhedrovirus to Agrotis segetum larvae when the midgut is bypassed

Given the high similarity in genome content and organization between Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and Agrotis segetum nucleopolyhedrovirus (AgseNPV), as well as the high percentages of similarity found between their 30 core genes, the specificity of these NPVs was analysed for the respective insect hosts, S. exigua and A. segetum. The LD(50) for AgseNPV in second-instar A. segetum larvae was 83 occlusion bodies per larva and the LT(50) was 8.1 days. AgseNPV was orally infectious for S. exigua, but the LD(50) was 10 000-fold higher than for SeMNPV. SeMNPV was not infectious for A. segetum larvae when administered orally, but an infection was established by injection into the haemocoel. Bypassing midgut entry by intrahaemocoelic inoculation suggested that the midgut is the major barrier in A. segetum larvae for infection by SeMNPV. Delayed-early genes of SeMNPV are expressed in the midgut of A. segetum larvae after oral infections, indicating that the virus is able to enter midgut epithelial cells and that it proceeds through the first phases of the infection process. The possible mechanisms of A. segetum resistance to SeMNPV in per os infections are discussed.

Bacterial, but not baculoviral infections stimulate Hemolin expression in noctuid moths

Lepidopteran larvae are regularly infected by baculoviruses during feeding on infected plants. The differences in sensitivity to these infections can be substantial, even among closely related species. For example, the noctuids Cotton bollworm (Helicoverpa zea) and Tobacco budworm (Heliothis virescens), have a 1000-fold difference in sensitivity to Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infection. Recent data were interpreted to indicate that the lepidopteran immunoglobulin protein, Hemolin, is synthesized upon viral injection and therefore to participate in anti-viral responses. To investigate whether Hemolin synthesis is affected by a natural viral infection, specific transcription in fat bodies and hemocytes of H. zea and H. virescens larvae was monitored following per os infection with the baculovirus HzSNPV (H. zea single nucleopolyhedrovirus). Both moths showed the same expression pattern as seen in uninfected animals and coincided with ecdysone responses, previously known to induce Hemolin expression. In contrast, injection of lyophilized Micrococcus luteus resulted in increased Hemolin expression supporting a role for Hemolin as an immuno-responsive protein in these species. The combined data are consistent with the suggestion that while Hemolin seems to participate in the response to virus infection in the superfamily Bombycoidea, this is not true in the Noctuoidea.

Baculovirus interactions in vitro and in vivo

Baculoviruses are promising viral insecticides and are safe for the environment. Interaction of baculoviruses in vitro and in vivo is a basic molecular and ecological question that has practical applications in agriculture. Cellular secretion is also a fundamental property in cell-cell communication. Here, we review recent investigations on how baculoviruses interact with insect cells and insect hosts. We focus particularly on a new interaction mechanism in which a secretion from cells infected with one virus enhances infection by a second virus. We also discuss a hypothesis that the secreted signals may serve as ligands that bind to the receptors on the surface of the cells that harbor the suppressed genomes of Thysanoplusia orichalcea MNPV (ThorMNPV) in Sf21 and Spodoptera exigua MNPV (SeMNPV) in High 5 to initiate signal transduction leading to the activation of genome replication of ThorMNPV in Sf21 and SeMNPV in High 5. We also discuss how the enhanced replication of SeMNPV replication by Autographa californica MNPV (AcMNPV) in nonpermissive insect cells depends on the types of cells. Interaction of baculoviruses in insects focused on mutualism and antagonism, even though the mechanism is not clear on mutualism. The antagonism of a Nucleopolyhedrovirus (NPV) with a Granulovirus (GV) has been extensively studied by a metalloprotein in the capsule of GV that disrupts the peritrophic membrane, a physical barrier to NPV entry to the midgut of larvae, to facilitate NPV infection.

Highly passage of Spodoptera litura cell line causes its permissiveness to baculovirus infection

It is well known that the characteristics of cell lines possibly alter when cell lines are at high-passage number because of the environmental selection. We do not know whether non-permissive or low-permissive cell lines could become permissive or more permissive to virus infection after over-high passage. In the present studies, the alteration of the permissiveness of Spodoptera litura cell line Sl-zsu-1 to three baculovirus infection was investigated after over-high passage, and the possible mechanisms are also investigated. Vigorous apoptosis in Sl-zsu-1 cells was induced by both the recombinant Autographa californica multiple nucleopolyhedrovirus AcMNPV-GFP-actin and the celery looper Anagrapha falcifera multiple nucleopolyhedrovirus AfMNPV, suggesting the replication of the two viruses was blocked by apoptosis. However, the cells infected by S. litura multicapsid nucleopolyhedrovirus SpltMNPV did not undergo apoptosis, but the SpltMNPV titre of the supernatant was not detectable, suggesting this cell line was low-permissive for this virus infection and other factor(s) involved in blockage of the virus replication except apoptosis. However, when Sl-zsu-1 cells had been subcultured continuously for more than 4 years (high-passage cell), which was named as Sl-HP cell line afterwards, no significant apoptosis was induced by the three baculovirus in Sl-HP cells, and many replicated virions or nucleocapsids were observed in the cells. But the permissiveness of Sl-HP cells to the three viruses was very different according to the titre of viruses in the cell cultures. Interestingly, the DNA extracted from SpltMNPV could induce vigorous apoptosis of Sl-HP cells. Altogether, Sl-zsu-1 cell line becomes more permissive to baculovirus infection after over-high passage and multiple paths can block the baculovirus infectivity.

Slow cell infection, inefficient primary infection and inability to replicate in the fat body determine the host range of Thysanoplusia orichalcea nucleopolyhedrovirus

Thysanoplusia orichaceamulticapsid nucleopolyhedrovirus (ThorMNPV) carrying an enhanced green fluorescent protein (EGFP) gene expression cassette (vThGFP) was used to study host-range mechanisms. Infection kinetics showed that vThGFP replication in Sf21 cells was too slow to suppress cell growth. Wide-host-rangeAutographa californicaMNPV (AcMNPV) could speed up vThGFP infection and enhance the vThGFP infection rate in Sf21 cells. The enhancement was not due to recombination, as no recombinant virus was isolated from co-infection by plaque assay. No improvement of vThGFP infection in Sf21 was found by AcMNPV cosmid transactivation assay. However, culture medium from Sf21 cells infected with AcMNPV did enhance vThGFP replication in Sf21. Third-instar larvae ofSpodoptera frugiperda,S. exiguaandHelicoverpa zeawere not killed by feeding with vThGFP polyhedra but were killed by intrahaemocoelic injection using budded viruses (BVs). This suggested that insufficient BVs were generated during the primary infection in the midgut. vThGFP infected haemocytes, tracheae and Malpighian tubules but not fat bodies of larvae ofS. frugiperda,S. exiguaandH. zea. Third-instarS. frugiperdalarvae co-infected by injection with vThGFP and vAcDsRed2, an AcMNPV expressing a red fluorescent protein gene, showed EGFP expression in the fat body. This result suggests that vAcDsRed2 could help vThGFP to replicate in the fat body ortrans-activate EGFP expression in the fat body. All these results suggested that slow cell infection, insufficient primary infection and inability to replicate in the fat body control the host range of ThorMNPV.

Apoptosis is induced in the haemolymph and fat body of Spodoptera exigua larvae upon oral inoculation with Spodoptera litura nucleopolyhedrovirus

Spodoptera exigua multinucleopolyhedrovirus (SeMNPV) and Spodoptera litura nucleopolyhedrovirus (SpltNPV) are genetically similar, but the larvae of S. exigua are not susceptible to SpltNPV. The aim of this study was to identify whether any process was inhibiting SpltNPV infection at some point. S. exigua larvae infected with a high concentration of wild-type SpltNPV by oral inoculation produced a fatal infection in second- or third-instar S. exigua, but the dead larvae did not undergo liquefaction; in contrast, fourth-instar infected larvae remained healthy. RT-PCR analysis of total RNA from infected second-instar larvae targeting immediate-early (ie-0), early (dnapol), late (chit) and very late (polh) genes suggested that SpltNPV initiated infection in the non-susceptible hosts. Total DNA extracted from the haemocytes of infected larvae showed DNA ladders characteristic of apoptosis. Sections of tissue from infected third-instar larvae of S. exigua at 96 h post-inoculation, stained with haematoxylin and eosin, revealed a highly disrupted morphology in the fat body. Apoptosis in fat body tissue was detected using terminal deoxynucleotidyltransferase-mediated fluorescein–dUTP nick end labelling (TUNEL) assays. In situ hybridization revealed the presence of viral DNA within the TUNEL-positive area, indicating viral infection in this tissue. These results suggest that apoptosis limits viral propagation by reducing the number of SpltNPV-infected haemocytes and fat body cells and inhibits disseminated viral infection.

Function of Spodoptera exigua nucleopolyhedrovirus late gene expression factors in the insect cell line SF-21

We used a well established transient expression assay to test the ability of the baculovirus Spodoptera exigua M nucleopolyhedrovirus (SeMNPV) homologs of Autographa californica MNPV (AcMNPV) late expression factors (lefs) to activate a late promoter-reporter gene cassette in SF-21 cells. This insect-derived cell line is fully permissive for AcMNPV infection but not for SeMNPV. In the assay, 19 AcMNPV lefs stimulate optimal levels of late gene promoter activity. SeMNPV lef-5 successfully replaced the corresponding AcMNPV gene in the context of the remaining set of AcMNPV lefs, whereas SeMNPV dnapol and 39k exhibited partial activity. When all the SeMNPV lefs were assayed together or in the presence of four lefs encoded only in AcMNPV, it resulted in background levels of late promoter-driven reporter gene activity. However, SeMNPV genomic DNA and the four AcMNPV-specific lefs stimulated low levels of reporter gene activity. Moreover, SeMNPV IE-1, but not AcMNPV IE-1, further stimulated late gene expression in the presence of SeMNPV DNA. AcMNPV IE-1 was able to mediate early gene expression cis-linked to homologous regions (hrs) derived from AcMNPV and SeMNPV. In contrast, SeMNPV IE-1 was more specific for SeMNPV-derived hr elements.

Physical and partial genetic map of Spodoptera frugiperda nucleopolyhedrovirus (SfMNPV) genome

A Nicaraguan isolate of Spodoptera frugiperda multicapsid nucleopolyhedrovirus (SfMNPV) is undergoing field trials for control of this pest in the Americas. This isolate is composed of multiple genotypes, some of which are deletion mutants. Identification of the genetic changes in deleted genotypes cannot be accomplished without the construction of a detailed physical map. In the present study, combinations of restriction endonuclease analysis and Southern blot analysis was performed. This map was refined by sequencing the termini of cloned restriction fragments. The SfMNPV genome was estimated to be 129.3 kb, 8 kb larger than the previously characterized Sf-2 variant from the United States, due to a deletion between 14.8 and 21.0 m.u. in the physical map described in this study. A total of 27.92 kb were sequenced, which represented 21.5% of the whole genome and included 38 ORFs. Comparison with other sequenced baculoviruses revealed that SfMNPV displayed the highest sequence identity (66%) and gene arrangement (78%) with Spodoptera exigua MNPV, sharing 36 putative ORFs. In addition, the genome organization was similar to that of SeMNPV, with minor differences. Phylogenetic analysis confirmed the close relatedness between SeMNPV and SfMNPV, suggesting they evolved from a common ancestor.

Optical brighteners do not influence covert baculovirus infection of Spodoptera frugiperda

Covert infection with Spodoptera frugiperda multiple nucleopolyhedrovirus, detected by reverse transcription-PCR of virus gene transcripts (ie-0 and polh), was not significantly affected by the presence of an optical brightener (Tinopal UNPA-GX), indicating no change in virus virulence. Detection of the covert infection was dependent on insect life stage and the viral mRNA used for diagnosis.