Field efficacy and transmission of fast- and slow-killing nucleopolyhedroviruses that are infectious to Adoxophyes honmai (Lepidoptera: Tortricidae)

Morphological, Biological, and Molecular Characterization of Type I Granuloviruses of Spodoptera frugiperda

Granuloviruses (GVs) Betabaculovirus associated with the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), especially those of the type I, have scarcely been studied. These GVs might be an effective alternative for the biocontrol of this insect. In this study, the native GVs SfGV-CH13 and SfGV-CH28 were isolated from FAW larvae and characterized for morphology, molecular traits, and insecticidal activity. The elapsed time between symptomatic infection of larvae and stop feeding as well as the weight of larvae before death or prior to pupation were also evaluated. Both GVs had ovoid shape and a length of 0.4 µm. They had the same DNA restriction profiles and their genome sizes were about 126 kb. The symptomatic infection with the tested GVs mainly caused flaccidity of larva body and discoloration of integument. The integument lysis was only observed in 8% of infected larvae. Infected larvae gradually stopped feeding. Overall, these symptoms are characteristic of infections caused by type I GVs, which are known as monoorganotropic or slow-killing GVs. The median lethal dose (LD50) values for SfGV-CH13 and SfGV-CH28 isolates were 5.4 × 102 and 1.1 × 103 OBs/larva, respectively. The median lethal time (LT50) ranged from 17 to 24 days. LT50 values decreased as the viral dose was increased. The elapsed time from symptomatic infection until pupation and body weight of larvae (third instar) were higher with SfGV-CH28 than SfGV-CH13. Both granulovirus isolates were able to kill the FAW larvae from the 12th day.

Identification and Field Evaluation of Sex Pheromone Components and Its Antagonist Produced by a Major Tea Pest, Archips strojny (Lepidoptera: Tortricidae)

Pesticide application is the only known control method for the tea tortrix Archips strojny (Lepidoptera: Tortricidae), which is a major pest of spring tea in China. To develop sex pheromone-based, environmentally safe control strategies, here we identified the sex pheromone components of this species. The male moths' antennae responded electrophysiologically to two compounds in female pheromone gland extracts. Gas chromatography-mass spectrometry analysis indicated that the two bioactive compounds were (Z)-11-tetradecenyl acetate (Z11-14:Ac) and (Z)-11-tetradecenyl alcohol (Z11-14:OH). Field trapping assays showed that lures baited with only the major component Z11-14:Ac were the most attractive to male moths, and the attractiveness decreased significantly when the lure was impregnated with increased relative ratios of the minor component Z11-14:OH. Our study demonstrated that Z11-14:Ac was the major attractant in the A. strojny sex pheromone, and the minor component Z11-14:OH seemed to serve as an antagonist. The results indicate that lures baited with 1 mg of Z11-14:Ac could be used as a monitoring or mass trapping tool for A. strojny management in Chinese tea plantations. Furthermore, Z11-14:Ac was identified as a common sex pheromone attractant of nine Archips species; these results lay the foundation for developing mating disruption techniques that target multiple leafroller pests.

Special Issue "Insect Viruses and Pest Management"

Most revues consider the work on Lymantria monarcha in central Europe [...].

Elucidating the genetic diversity of Phthorimaea operculella granulovirus (PhopGV)

Twelve complete genome sequences of Phthorimaea operculella granulovirus (PhopGV) isolates from four different continents (Africa, South America, Asia and Europe) were analysed after Illumina next-generation sequencing (NGS). The isolates have a circular double-stranded DNA genome that is 118 355 to 119 177 bp in length and all of them encode 130 open reading frames (ORFs). Analysis of single-nucleotide polymorphisms (SNPs) revealed a unique set of SNP positions for every tested isolate. The genome sequences of the investigated PhopGV isolates were classified into a new system of four (1-4) groups according to the presence of group-specific SNPs as well as insertions and deletions. These genome groups correlated with phylogenetic lineages inferred from minimum-evolution trees of the whole-genome consensus nucleotide sequences. All members of group 3 originated from the Mediterranean area, whereas the geographical origin and the group assignment did not correlate for isolates belonging to genome groups 1, 2 or 4. The high degree of coverage facilitated the determination of variant nucleotide frequencies. We conclude that the geographical isolates of PhopGV are genetically highly similar. On the other hand, they were rarely genetically homogenous and in most cases appeared to be mixtures of multiple genotypes.

Interaction of Phthorimaea operculella granulovirus with a Nosema sp. microsporidium in larvae of Phthorimaea operculella

An antagonistic effect of a microsporidium (Nosema sp.) infection on the virulence of Phthorimaea operculella granulovirus (PhopGV) was recorded in potato tuber moth (Phthorimaea operculella) larvae with mixed infections. When the P. operculella colony was infected at a high rate (42.8-100%) with the microsporidium, it was less susceptible to the isolate PhopGV-GR1.1. A virus concentration 1.89 × 10⁵ higher was necessary to cause the same level of mortality produced in the P. operculella colony when it was uninfected or had a low level of infection with the microsporidium (0-30%). This antagonistic effect was driven by a Nosema isolate (termed Nosema sp. Phop) that was purified from microsporidian-infected P. operculella individuals. The purified microsporidium was characterised by morphological features, including size, filament coils and different developmental stages using transmission electron microscopy (TEM). On the molecular level, the partial cistron rDNA information of the small ribosomal subunit (SSU), internal transcribed spacer (ITS), and the large ribosomal subunit (LSU) were identified. Phylogenetic analyses revealed that the newly described microsporidium belongs to the "true Nosema" clade. Partial sequence information of the RNA polymerase II largest subunit (RPB1) suggested that Nosema bombycis is the closest relative (98% identity). The morphological and phylogenetic characteristics suggest that it is an isolate of N. bombycis. Interactions of microsporidia and betabaculoviruses are rarely described in the literature, although mixed infections of different pathogens seem to be rather common events, ranging from antagonistic to mutualistic interactions. The observed antagonistic relationship between the Nosema sp. and PhopGV-GR1.1 showed that pathogen interactions need to be considered when single pathogens are applied to insect populations in the context of biological control of insect pests.

Acquired resistance to a nucleopolyhedrovirus in the smaller tea tortrix Adoxophyes honmai (Lepidoptera: Tortricidae) after selection by serial viral administration

A laboratory colony of Adoxophyes honmai was selected for resistance over 156 generations by feeding neonate larvae of every generation with the LC₆₀ or LC₇₀ of its nucleopolyhedrovirus, Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV). A significant difference in LC₅₀ values between the selected (R-strain) and unselected (S1- and S2-strain) strains was first observed after three generations of selection, and the resistance level then increased continuously. The highest degree of acquired resistance, based on the ratio of the LC₅₀ values of R- and S1-strains, was more than 400,000-fold. After selection was stopped at either the 21st or the 149th generation, LC₅₀ values did not decrease significantly, suggesting that resistance of the R-strain to AdhoNPV was stable. To assess which of the two routes of baculovirus infection is affected by resistance to AdhoNPV, 5th instar larvae of the R-strain were inoculated orally and intrahemocoelically with AdhoNPV and their susceptibility was compared to that of S-strain. The ratio of the LC₂₅ values of selected and unselected strains was 91-fold when budded viruses were injected into 5th instar larvae, but was 107,000-fold after oral inoculation. These results indicate that the resistance mechanism of the R-strain of A. honmai disrupts both midgut primary infection and hemocoelic secondary infection.

Entomopoxvirus infection induces changes in both juvenile hormone and ecdysteroid levels in larval Mythimna separata

Insect viruses are among the most important pathogens of lepidopteran insects. Virus-infected larvae often show developmental defects including a prolonged larval period and a failure to pupate, but the mechanisms by which insect viruses regulate host development need further investigation. In this study, the regulation of host endocrinology by a lepidopteran entomopoxvirus (EPV), Mythimna separata EPV (MySEV), was examined. When fourth instar M. separata were inoculated with MySEV occlusion bodies, pupation was prevented and the insects died during the final (sixth) larval instar. Liquid chromatography-MS analysis revealed that juvenile hormone (JH) titres in the haemolymph of MySEV-infected sixth instars were higher than those in mock-infected larvae. JH esterase (JHE) activity was also examined by kinetic assay using a colorimetric substrate. The level of JHE activity in the haemolymph of MySEV-infected larvae was generally lower than that found in mock-infected larvae. In contrast, ecdysteroid titre in the haemolymph of final-instar MySEV-infected larvae was lower than that found in mock-infected larvae when measured by radioimmunoassay. A statistically significant difference in the release of ecdysteroids from prothoracic glands (PGs) that were dissected from MySEV- or mock-infected sixth instar Day 3 larvae was not found following prothoracicotropic hormone (PTTH) exposure. Our results indicate that the release of ecdysteroids was reduced not by infection of the PGs by MySEV, but by reduced PTTH production from the brain. Taken together our study suggests that EPVs retard host development by both reducing ecdysone titre and maintaining status quo levels of JH by preventing its metabolism.