Iflavirus increases its infectivity and physical stability in association with baculovirus

The multispecies stinkbug iflavirus Halyomorpha halys virus detected in the multispecies stinkbug egg parasitoid microwasp, Telenomus podisi (Ashmead) (Hymenoptera: Platygastridae)

Wasps are important parasitoids of stinkbugs and frequently exposed to various types of microorganisms through environmental contact and fecal-oral transmission route. Many parasitize stinkbug eggs and are commercially used in the field to control insect population. The parasitoid T. podisi is known for its high parasitism capacity and ability to target multiple species of stinkbugs. In this study we asked whether T. podisi exposed to eggs infected by a multispecies asymptomatic stinkbug virus, the Halyomorpha halys virus (HhV) would get infected. HhV is a geographically distributed multispecies iflavirus previously found to infect four stinkbug hosts, including three Brazilian species, Chinavia ubica, Euschistus heros and Diceraeus melacanthus, and T. posidi can parasitize all of them. As results, RT-PCR screening revealed positive samples for the HhV genome in two out of four tested pools of T. podisi, whereas the antigenome, indicative of replicative activity, was not detected. The wasps were raised in E. heros eggs that presented both the genome and the antigenome forms of the HhV genome. Subsequent RNA-deep sequencing of HhV positive T. podisi RNA pools yielded a complete genome of HhV with high coverage. Phylogenetic analysis positioned the isolate HhV-Tp (isolate Telenomus podisi) alongside with the stinkbug HhV. Analysis of transcriptomes from several hymenopteran species revealed HhV-Tp reads in four species. However, the transmission mechanism and the ecological significance of HhV remain elusive, warranting further studies to illuminate both the transmission process and its capacity for environmental propagation using T. podisi as a potential vector.

Transmission mode predicts coinfection patterns of insect-specific viruses in field populations of the Queensland fruit fly

Insect-specific viruses (ISVs) can affect insect health and fitness, but can also interact with other insect-associated microorganisms. Despite this, ISVs are often studied in isolation from each other, in laboratory populations. Consequently, their diversity, prevalence and associations with other viruses in field populations are less known, yet these parameters are important to understanding virus epidemiology. To help address this knowledge gap, we assessed the diversity, prevalence and coinfections of three ISVs (horizontally transmitted cripavirus, biparentally transmitted sigmavirus and maternally transmitted iflavirus) in 29 field populations of Queensland fruit fly, Australia's most significant horticultural pest, in the context of their different transmission modes. We detected new virus variant diversity. In contrast to the very high virus prevalence in laboratory populations, 46.8% of 293 field flies carried one virus and 4.8% had two viruses. Cripavirus and sigmavirus occurred in all regions, while iflavirus was restricted to subtropical and tropical regions. Cripavirus was most prevalent (37.5%), followed by sigmavirus (13.7%) and iflavirus (4.4%). Cripavirus coinfected some flies with either one of the two vertically transmitted viruses. However, sigmavirus did not coinfect individuals with iflavirus. Three different modelling approaches detected negative association patterns between sigmavirus and iflavirus, consistent with the absence of such coinfections in laboratory populations. This may be linked with their maternal transmission and the ineffective paternal transmission of sigmavirus. Furthermore, we found that, unlike sigmavirus and iflavirus, cripavirus load was higher in laboratory than field flies. Laboratory and mass-rearing conditions may increase ISV prevalence and load due to increased transmission opportunities. We conclude that a combination of field and laboratory studies is needed to uncover ISV interactions and further our understanding of ISV epidemiology.

Lymantria Dispar Iflavirus 1 RNA Comprises a Large Proportion of RNA in Adult L. dispar Moths

The spongy moth virus Lymantria dispar iflavirus 1 (LdIV1), originally identified from a Lymantria dispar cell line, was detected in 24 RNA samples from female moths of four populations from the USA and China. Genome-length contigs were assembled for each population and compared with the reference genomes of the first reported LdIV1 genome (Ames strain) and two LdIV1 sequences available in GenBank originating from Novosibirsk, the Russian Federation. A whole-genome phylogeny was generated for these sequences, indicating that LdIV1 viruses observed in North American (flightless) and Asian (flighted) spongy moth lineages indeed partition into clades as would be expected per their host's geographic origin and biotype. A comprehensive listing of synonymous and non-synonymous mutations, as well as indels, among the polyprotein coding sequences of these seven LdIV1 variants was compiled and a codon-level phylogram was computed using polyprotein sequences of these, and 50 additional iflaviruses placed LdIV1 in a large clade consisting mostly of iflaviruses from other species of Lepidoptera. Of special note, LdIV1 RNA was present at very high levels in all samples, with LdIV1 reads accounting for a mean average of 36.41% (ranging from 1.84% to 68.75%, with a standard deviation of 20.91) of the total sequenced volume.

Identification, characterization and prevalence in southern China of a new iflavirus in the leafhopper Recilia dorsalis (Hemiptera: Cicadellidae)

The leafhopper Recilia dorsalis (Hemiptera: Cicadellidae) is not only a significant pest in agriculture but also an important vector involved in transmitting numerous pathogens that are known to cause economic losses by affecting rice crops. Here, a new iflavirus was discovered in the leafhopper R. dorsalis by employing a transcriptomic approach. The complete viral genome was determined to be 10,711 nucleotides (nt) in length and contains a single open reading frame (ORF) encoding a putative polyprotein comprised of 3,161 amino acids (aa), which is flanked by 5' and 3' untranslated regions. The full viral genome nt and the deduced polyprotein aa sequence showed the highest similarity (71.6% and 77.8%, respectively) with Langfang leafhopper iflavirus. Phylogenetic analysis based on the RdRp domain indicated that the isolated virus, which we have tentatively named Recilia dorsalis iflavirus 2 (RdIV2), is clustered with the members of the family Iflaviridae. Moreover, the results of our surveys indicate that RdIV2 predominates in southwestern Guangdong and southeastern Guangxi, China, and was absent in the other three species of leafhoppers; Nephotettix cincticeps, N. virescens and N. nigropictus. Notably, R. dorsalis was found to be co-infected with RdIV2 and rice stripe mosaic virus (RSMV; a well-known rice-infecting virus vectored by R. dorsalis) in rice fields, although the co-infection rate is low.

Transcriptomics Reveal Several Novel Viruses from Canegrubs (Coleoptera: Scarabaeidae) in Central Queensland, Australia

Canegrubs (Coleoptera: Scarabaeidae) are major pests of sugarcane crops in Australia, but despite long-term and intensive research, no commercially viable biological control agents have been identified. We used the RNA-Seq approach to explore the viriomes of three different species of canegrubs from central Queensland, Australia to identify potential candidates for biological control. We identified six novel RNA viruses, characterized their genomes, and inferred their evolutionary relationships with other closely related viruses. These novel viruses showed similarity to other known members from picornaviruses, benyviruses, sobemoviruses, totiviruses, and reoviruses. The abundance of viral reads varied in these libraries; for example, Dermolepida albohirtum picorna-like virus (9696 nt) was built from 83,894 assembled reads while only 1350 reads mapped to Lepidiota negatoria beny-like virus (6371 nt). Future studies are essential to determine their natural incidence in different life stages of the host, biodiversity, geographical distributions, and potential as biological control agents for these important pests of sugarcane.

Characterization and Tissue Tropism of Newly Identified Iflavirus and Negeviruses in Glossina morsitans morsitans Tsetse Flies

Tsetse flies cause major health and economic problems as they transmit trypanosomes causing sleeping sickness in humans (Human African Trypanosomosis, HAT) and nagana in animals (African Animal Trypanosomosis, AAT). A solution to control the spread of these flies and their associated diseases is the implementation of the Sterile Insect Technique (SIT). For successful application of SIT, it is important to establish and maintain healthy insect colonies and produce flies with competitive fitness. However, mass production of tsetse is threatened by covert virus infections, such as the Glossina pallidipes salivary gland hypertrophy virus (GpSGHV). This virus infection can switch from a covert asymptomatic to an overt symptomatic state and cause the collapse of an entire fly colony. Although the effects of GpSGHV infections can be mitigated, the presence of other covert viruses threaten tsetse mass production. Here we demonstrated the presence of two single-stranded RNA viruses isolated from Glossina morsitans morsitans originating from a colony at the Seibersdorf rearing facility. The genome organization and the phylogenetic analysis based on the RNA-dependent RNA polymerase (RdRp) revealed that the two viruses belong to the genera Iflavirus and Negevirus, respectively. The names proposed for the two viruses are Glossina morsitans morsitans iflavirus (GmmIV) and Glossina morsitans morsitans negevirus (GmmNegeV). The GmmIV genome is 9685 nucleotides long with a poly(A) tail and encodes a single polyprotein processed into structural and non-structural viral proteins. The GmmNegeV genome consists of 8140 nucleotides and contains two major overlapping open reading frames (ORF1 and ORF2). ORF1 encodes the largest protein which includes a methyltransferase domain, a ribosomal RNA methyltransferase domain, a helicase domain and a RdRp domain. In this study, a selective RT-qPCR assay to detect the presence of the negative RNA strand for both GmmIV and GmmNegeV viruses proved that both viruses replicate in G. m. morsitans. We analyzed the tissue tropism of these viruses in G. m. morsitans by RNA-FISH to decipher their mode of transmission. Our results demonstrate that both viruses can be found not only in the host’s brain and fat bodies but also in their reproductive organs, and in milk and salivary glands. These findings suggest a potential horizontal viral transmission during feeding and/or a vertically viral transmission from parent to offspring. Although the impact of GmmIV and GmmNegeV in tsetse rearing facilities is still unknown, none of the currently infected tsetse species show any signs of disease from these viruses.

Antiviral Treatment Reveals a Cooperative Pathogenicity of Baculovirus and Iflavirus in Spodoptera exigua, a Lepidopteran Insect

NPVThe beet armyworm, Spodoptera exigua, is a serious insect pest infesting various vegetable crops. Two infectious insect viruses, baculovirus and iflavirus, are known to induce epizootics in S. exigua populations. Indeed, some laboratory colonies have appeared to be covertly infected by these viruses. Diagnostic PCR tests detected two different viruses: Spodoptera exigua multiple nucleopolyhedrosis virus (SeMNPV) and iflaviruses (SeIfV1 and SeIfV2). Viral extract from dead larvae of S. exigua could infect Sf9 cells and produce occlusion bodies (OBs). Feeding OBs to asymptomatic larvae of S. exigua caused significant viral disease. Interestingly, both SeIfV1 and SeIfV2 increased their titers at late larval stages. Sterilization of laid eggs with 1% sodium hypochloride significantly reduced SeMNPV titers and increased larval survival rate. Doublestranded RNA (dsRNA) specific to SeIfV1 or SeIfV2 significantly reduced viral titers and increased larval survival rate. To continuously feed dsRNA, a recombinant Escherichia coli HT115 expressing SeIfV1-dsRNA was constructed with an L4440 expression vector. Adding this recombinant E. coli to the artificial diet significantly reduced the SeIfV1 titer and increased larval survival. These results indicate that laboratory colony collapse of S. exigua is induced by multiple viral infections. In addition, either suppression of SeMNPV or SeIfV infection significantly increased larval survival, suggesting a cooperative pathogenicity between baculovirus and iflavirus against S. exigua.

Characterisation of the Viral Community Associated with the Alfalfa Weevil (Hypera postica) and Its Host Plant, Alfalfa (Medicago sativa)

Advances in viral metagenomics have paved the way of virus discovery by making the exploration of viruses in any ecosystem possible. Applied to agroecosystems, such an approach opens new possibilities to explore how viruses circulate between insects and plants, which may help to optimise their management. It could also lead to identifying novel entomopathogenic viral resources potentially suitable for biocontrol strategies. We sampled the larvae of a natural population of alfalfa weevils (Hypera postica), a major herbivorous pest feeding on legumes, and its host plant alfalfa (Medicago sativa). Insect and plant samples were collected from a crop field and an adjacent meadow. We characterised the diversity and abundance of viruses associated with weevils and alfalfa, and described nine putative new virus species, including four associated with alfalfa and five with weevils. In addition, we found that trophic accumulation may result in a higher diversity of plant viruses in phytophagous pests compared to host plants.

Appearances are deceptive: Three RNA viruses co-infected with the nucleopolyhedrovirus in host Lymantria dispar

The virus infection, which visually looks like typical monoinfection, in fact may hide a great complex of different species. Without detailed analysis, we may miss the important interaction between pathogens, including new species. In the current study, we found the new species inside the mix of cubic and polyhedral occlusion bodies (OBs) isolated from the gypsy moth, Lymantria dispar L. (Ld). Transmission electron microscopy (TEM) revealed that into the one cadaver were OBs which belonged to baculovirus and cypoviruses. The baculovirus produced polyhedral OBs, while cypoviruses produced polyhedral and cubic OBs. Genomic analysis detected the multiple Ld nucleopolyhedroviruses, and cypoviruses were Hubei lepidoptera virus 3 and Dendrolimus punctatus cypovirus 1. This represents the first isolation of the Hubei lepidoptera virus 3 from the gypsy moth, proposed as "Lymantria dispar cypovirus 3". The RNAseq analysis also revealed the presence of Lymantria dispar iflavirus 1. The insecticidal activity of the mixed infection was comparable to that of typical baculovirus monoinfection. Thus, we demonstrate that i) the shape of OBs identified by light microscopy cannot be a robust indicator of viral species infecting the host; ii) only specific analysis may reveal the true composition of viral infection.

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.

A Novel Iflavirus Was Discovered in Green Rice Leafhopper Nephotettix cincticeps and Its Proliferation Was Inhibited by Infection of Rice Dwarf Virus

The green rice leafhopper, Nephotettix cincticeps (Hemiptera: Cicadellidae), is a key insect vector transmitting rice dwarf virus (RDV) that causes rice dwarf disease. We discovered a novel iflavirus from the transcriptomes of N. cincticeps and named it as Nephotettix cincticeps positive-stranded RNA virus-1 (NcPSRV-1). The viral genome consists of 10,524 nucleotides excluding the poly(A) tail and contains one predicted open reading frame encoding a polyprotein of 3,192 amino acids, flanked by 5' and 3' untranslated regions. NcPSRV-1 has a typical iflavirus genome arrangement and is clustered with the members of the family Iflaviridae in the phylogenetic analysis. NcPSRV-1 was detected in all tested tissues and life stages of N. cincticeps and could be transmitted horizontally and vertically. Moreover, NcPSRV-1 had high prevalence in the laboratory populations and was widely spread in field populations of N. cincticeps. NcPSRV-1 could also infect the two-striped leafhopper, Nephotettix apicalis, at a 3.33% infection rate, but was absent in the zigzag leafhopper, Recilia dorsalis, and rice Oryza sativa variety TN1. The infection of RDV altered the viral load and infection rate of NcPSRV-1 in N. cincticeps, for which it seems that RDV has an antagonistic effect on NcPSRV-1 infection in the host.

Iflavirus Covert Infection Increases Susceptibility to Nucleopolyhedrovirus Disease in Spodoptera exigua

Naturally occurring covert infections in lepidopteran populations can involve multiple viruses with potentially different transmission strategies. In this study, we characterized covert infection by two RNA viruses, Spodoptera exigua iflavirus 1 (SeIV-1) and Spodoptera exigua iflavirus 2 (SeIV-2) (family Iflaviridae) that naturally infect populations of Spodoptera exigua, and examined their influence on susceptibility to patent disease by the nucleopolyhedrovirus Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) (family Baculoviridae). The abundance of SeIV-1 genomes increased up to ten-thousand-fold across insect developmental stages after surface contamination of host eggs with a mixture of SeIV-1 and SeIV-2 particles, whereas the abundance of SeIV-2 remained constant across all developmental stages. Low levels of SeIV-2 infection were detected in all groups of insects, including those that hatched from surface-decontaminated egg masses. SeIV-1 infection resulted in reduced larval weight gain, and an unbalanced sex ratio, whereas larval developmental time, pupal weight, and adult emergence and fecundity were not significantly affected in infected adults. The inoculation of S. exigua egg masses with iflavirus, followed by a subsequent infection with SeMNPV, resulted in an additive effect on larval mortality. The 50% lethal concentration (LC50) of SeMNPV was reduced nearly 4-fold and the mean time to death was faster by 12 h in iflavirus-treated insects. These results suggest that inapparent iflavirus infections may be able to modulate the host response to a new pathogen, a finding that has particular relevance to the use of SeMNPV as the basis for biological pest control products.

Identification and RNAi Profile of a Novel Iflavirus Infecting Senegalese Aedes vexans arabiensis Mosquitoes

The inland floodwater mosquito Aedes vexans (Meigen, 1830) is a competent vector of numerous arthropod-borne viruses such as Rift Valley fever virus (Phenuiviridae) and Zika virus (Flaviviridae). Aedes vexans spp. have widespread Afrotropical distribution and are common European cosmopolitan mosquitoes. We examined the virome of Ae. vexans arabiensis samples from Barkédji village, Senegal, with small RNA sequencing, bioinformatic analysis, and RT-PCR screening. We identified a novel 9494 nt iflavirus (Picornaviridae) designated here as Aedes vexans iflavirus (AvIFV). Annotation of the AvIFV genome reveals a 2782 amino acid polyprotein with iflavirus protein domain architecture and typical iflavirus 5' internal ribosomal entry site and 3' poly-A tail. Aedes vexans iflavirus is most closely related to a partial virus sequence from Venturia canescens (a parasitoid wasp) with 56.77% pairwise amino acid identity. Analysis of AvIFV-derived small RNAs suggests that AvIFV is targeted by the exogenous RNA interference pathway but not the PIWI-interacting RNA response, as ~60% of AvIFV reads corresponded to 21 nt Dicer-2 virus-derived small RNAs and the 24-29 nt AvIFV read population did not exhibit a "ping-pong" signature. The RT-PCR screens of archival and current (circa 2011-2020) Ae. vexans arabiensis laboratory samples and wild-caught mosquitoes from Barkédji suggest that AvIFV is ubiquitous in these mosquitoes. Further, we screened wild-caught European Ae. vexans samples from Germany, the United Kingdom, Italy, and Sweden, all of which tested negative for AvIFV RNA. This report provides insight into the diversity of commensal Aedes viruses and the host RNAi response towards iflaviruses.

Generation of Virus- and dsRNA-Derived siRNAs with Species-Dependent Length in Insects

Double-stranded RNA (dsRNA) molecules of viral origin trigger a post-transcriptional gene-silencing mechanism called RNA interference (RNAi). Specifically, virally derived dsRNA is recognized and cleaved by the enzyme Dicer2 into short interfering RNAs (siRNAs), which further direct sequence-specific RNA silencing, ultimately silencing replication of the virus. Notably, RNAi can also be artificially triggered by the delivery of gene-specific dsRNA, thereby leading to endogenous gene silencing. This is a widely used technology that holds great potential to contribute to novel pest control strategies. In this regard, research efforts have been set to find methods to efficiently trigger RNAi in the field. In this article, we demonstrate the generation of dsRNA- and/or virus-derived siRNAs—the main RNAi effectors—in six insect species belonging to five economically important orders (Lepidoptera, Orthoptera, Hymenoptera, Coleoptera, and Diptera). In addition, we describe that the siRNA length distribution is species-dependent. Taken together, our results reveal interspecies variability in the (antiviral) RNAi mechanism in insects and show promise to contribute to future research on (viral-based) RNAi-triggering mechanisms in this class of animals.

Phylogenetic and recombination analyses of two deformed wing virus strains from different honeybee species in China

Background

Deformed wing virus (DWV) is one of many viruses that infect honeybees and has been extensively studied because of its close association with honeybee colony collapse that is induced by Varroa destructor. However, virus genotypes, sequence characteristics, and genetic variations of DWV remain unknown in China.

Methods

Two DWV strains were isolated from Jinzhou and Qinhuangdao cities in China, and were named China1-2017 (accession number: MF770715) and China2-2018 (accession number: MH165180), respectively, and their complete genome sequences were analyzed. To investigate the phylogenetic relationships of the DWV isolates, a phylogenetic tree of the complete open reading frame (ORF), structural protein VP1, and non-structural protein 3C+RdRp of the DWV sequences was constructed using the MEGA 5.0 software program. Then, the similarity and recombinant events of the DWV isolated strains were analyzed using recombination detection program (RDP4) software and genetic algorithm for recombination detection (GARD).

Results

The complete genomic analysis showed that the genomes of the China1-2017 and China2-2018 DWV strains consisted of 10,141 base pairs (bp) and 10,105 bp, respectively, and contained a single, large ORF (China1-2017: 1,146–9,827 bp; China2-2018: 1,351–9,816 bp) that encoded 2,894 amino acids. The sequences were compared with 20 previously reported DWV sequences from different countries and with sequences of two closely related viruses, Kakugo virus (KV) and V. destructor virus-1. Multiple sequence comparisons revealed a nucleotide identity of 84.3–96.7%, and identity of 94.7–98.6% in amino acids between the two isolate strains and 20 reference strains. The two novel isolates showed 96.7% nucleotide identity and 98.1% amino acid identity. The phylogenetic analyses showed that the two isolates belonged to DWV Type A and were closely related to the KV-2001 strain from Japan. Based on the RDP4 and GARD analyses, the recombination of the China2-2018 strain was located at the 4,266–7,507 nt region, with Korea I-2012 as an infer unknown parent and China-2017 as a minor parent, which spanned the entire helicase ORF. To the best of our knowledge, this is the first study to the complete sequence of DWV isolated from Apis cerana and the possible DWV recombination events in China. Our findings are important for further research of the phylogenetic relationship of DWVs in China with DWV strains from other countries and also contribute to the understanding of virological properties of these complex DWV recombinants.

Infectious Virions of Bombyx Mori Latent Virus Are Incorporated into Bombyx Mori Nucleopolyhedrovirus Occlusion Bodies

The Bombyx mori latent virus (BmLV) belongs to the unassigned plant virus family Tymoviridae and contains a positive-sense, single-stranded RNA genome. BmLV has infected almost all B. mori-derived cultured cell lines through unknown routes. The source of BmLV infection and the BmLV life cycle are still unknown. Here, we examined the interaction between BmLV and the insect DNA virus Bombyx mori nucleopolyhedrovirus (BmNPV). Persistent infection with BmLV caused a slight delay in BmNPV propagation, and BmLV propagation was enhanced in B. mori larvae via co-infection with BmNPV. We also showed that BmLV infectious virions were co-occluded with BmNPV virions into BmNPV occlusion bodies. We propose a new relationship between BmLV and BmNPV.

Cannibalism as a Possible Entry Route for Opportunistic Pathogenic Bacteria to Insect Hosts, Exemplified by Pseudomonas aeruginosa, a Pathogen of the Giant Mealworm Zophobas morio

Opportunistic bacteria are often ubiquitous and do not trigger disease in insects unless the conditions are specifically favorable for bacterial development in a suitable host. In this paper, we isolated and identified a bacterium, Pseudomonas aeruginosa, from the larvae of the giant mealworm Zophobas morio and we studied the possible entry routes by challenging larvae with per os injection and subdermal injection. We also evaluated the effect of exposing groups of larvae to P. aeruginosa inoculated in their feed and the effect of exposing wounded larvae to P. aeruginosa. We concluded that the mortality rate of Z. morio larvae is higher when P. aeruginosa gets in direct contact with the hemolymph via intracoelomic injection compared to a situation where the bacterium is force-fed. Larvae with an open wound exposed to P. aeruginosa presented higher mortality rate compared to larvae with a wound that was not exposed to the bacterium. We documented too, that cannibalism and scavenging were more prevalent among larvae in a group, when P. aeruginosa is present compared to when it is absent. We discuss hereby different aspects related with the pathogen’s entry routes to insects the complexity of pathogen´s transmission in high population densities and different ways to prevent and/or control P. aeruginosa in mass rearing systems.

Genetic and Biological Characterization of Four Nucleopolyhedrovirus Isolates Collected in Mexico for the Control of Spodoptera exigua (Lepidoptera: Noctuidae)

This study describes four multiple nucleocapsid nucleopolyhedrovirus isolates recovered from infected larvae of beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae), on crops in two different geographical regions of Mexico. Molecular and biological characterization was compared with characterized S. exigua multiple nucleopolyhedrovirus (SeMNPV) isolates from the United States (SeUS1 and SeUS2) and Spain (SeSP2). Restriction endonuclease analysis of viral DNA confirmed that all Mexican isolates were SeMNPV isolates, but molecular differences between the Mexican and the reference isolates were detected using PCR combined with restriction fragment length polymorphism (RFLP). Amplification of the variable region V01 combined with RFLP distinguished the two Mexican isolates, SeSLP6 and SeSIN6. BglII digestions showed that the majority of the isolates contained submolar bands, indicating the presence of genetic heterogeneity. Amplification of the variable regions V04 and V05 distinguished between American and the Spanish isolates. Biological characterization was performed against two laboratory colonies of S. exigua, one from Mexico, and another from Switzerland. Insects from the Mexican colony were less susceptible to infection than insects from Se-Swiss colony. In the Se-Mex colony, SeSP2 was the most pathogenic isolate followed by SeSIN6, although their virulence was similar to most of the isolates tested. In Se-Swiss colony, similar LD50 values were observed for the five isolates, although the virulence was higher for the SeSLP6 isolate, which also had the highest OB (occlusion body) yield. We conclude that the Mexican isolates SeSIN6 and SeSLP6 possess insecticidal traits of value for the development of biopesticides for the control of populations of S. exigua.

Covert Infection of Insects by Baculoviruses

Baculoviruses (Baculoviridae) are occluded DNA viruses that are lethal pathogens of the larval stages of some lepidopterans, mosquitoes, and sawflies (phytophagous Hymenoptera). These viruses have been developed as biological insecticides for control of insect pests and as expression vectors in biotechnological applications. Natural and laboratory populations frequently harbor covert infections by baculoviruses, often at a prevalence exceeding 50%. Covert infection can comprise either non-productive latency or sublethal infection involving low level production of virus progeny. Latency in cell culture systems involves the expression of a small subset of viral genes. In contrast, covert infection in lepidopterans is associated with differential infection of cell types, modulation of virus gene expression and avoidance of immune system clearance. The molecular basis for covert infection may reside in the regulation of host-virus interactions through the action of microRNAs (miRNA). Initial findings suggest that insect nudiviruses and vertebrate herpesviruses may provide useful analogous models for exploring the mechanisms of covert infection by baculoviruses. These pathogens adopt mixed-mode transmission strategies that depend on the relative fitness gains that accrue through vertical and horizontal transmission. This facilitates virus persistence when opportunities for horizontal transmission are limited and ensures virus dispersal in migratory host species. However, when host survival is threatened by environmental or physiological stressors, latent or persistent infections can be activated to produce lethal disease, followed by horizontal transmission. Covert infection has also been implicated in population level effects on host-pathogen dynamics due to the reduced reproductive capacity of infected females. We conclude that covert infections provide many opportunities to examine the complexity of insect-virus pathosystems at the organismal level and to explore the evolutionary and ecological relationships of these pathogens with major crop and forest pests.

Co-infection with iflaviruses influences the insecticidal properties of Spodoptera exigua multiple nucleopolyhedrovirus occlusion bodies: Implications for the production and biosecurity of baculovirus insecticides

Biological insecticides based on Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) can efficiently control S. exigua larvae on field and greenhouse crops in many parts of the world. Spanish wild populations and laboratory colonies of S. exigua are infected by two iflaviruses (SeIV-1 and SeIV-2). Here we evaluated the effect of iflavirus co-infection on the insecticidal characteristics of SeMNPV occlusion bodies (OBs). Overall, iflavirus co-inoculation consistently reduced median lethal concentrations (LC50) for SeMNPV OBs compared to larvae infected with SeMNPV alone. However, the speed of kill of SeMNPV was similar in the presence or absence of the iflaviruses. A reduction of the weight gain (27%) associated with iflavirus infection resulted in a 30% reduction in total OB production per larva. Adult survivors of SeMNPV OB inoculation were examined for covert infection. SeMNPV DNA was found to be present at a high prevalence in all SeIV-1 and SeIV-2 co-infection treatments. Interestingly, co-inoculation of SeMNPV with SeIV-2 alone or in mixtures with SeIV-1 resulted in a significant increase in the SeMNPV load of sublethally infected adults, suggesting a role for SeIV-2 in vertical transmission or reactivation of sublethal SeMNPV infections. In conclusion, iflaviruses are not desirable in insect colonies used for large scale baculovirus production, as they may result in diminished larval growth, reduced OB production and, depending on their host-range, potential risks to non-target Lepidoptera.

Novel RNA viruses producing simultaneous covert infections in Ceratitis capitata. Correlations between viral titers and host fitness, and implications for SIT programs

The Mediterranean fruit fly (medfly), Ceratitis capitata is a highly polyphagous pest, which infests multiple species of fruits and vegetables worldwide. In addition to the traditional control with chemical insecticides, sterile insect technique (SIT) has been implemented in integrated programs worldwide, and has become an essential measure for the control of this pest. A key issue for SIT is to release sterile males that are sufficiently competitive with males from the wild population. Using sequence information available in public databases, three novel picornaviruses infecting medflies were discovered and named as C. capitata iflavirus 1 and 2 (CcaIV1 and CcaIV2), and C. capitata noravirus (CcaNV). Additional analyses have revealed the presence of CcaIV2 and CcaNV covertly infecting most of the medfly strains used in the different SIT programs around the world, as well as in field captures in the east of Spain. High viral titers of CcaNV were associated with a reduction in the lifespan of males released to the field for the control of this pest, suggesting the possibility that CcaNV may impair the fitness of sterile flies produced by SIT programs.