Effects of incubation temperature on the dose-survival time relationship of Trichoplusia ni larvae infected with Autographa californica nucleopolyhedrovirus

Experimental infection trials with European North Atlantic ranavirus (Iridoviridae) isolated from lumpfish (Cyclopterus lumpus, L.)

European North Atlantic ranavirus (ENARV, Iridoviridae), is a ranavirus species recently isolated from lumpfish (Cyclopterus lumpus, L.), which are used as cleaner fish in Atlantic salmon (Salmo salar) farming in Northern Europe. This study aimed to investigate (1) the virulence of ENARV isolates from Ireland, Iceland and the Faroe Islands to lumpfish; (2) horizontal transmission between lumpfish; and (3) virulence to Atlantic salmon parr. Lumpfish were challenged in a cohabitation model using intraperitoneally (IP) injected shedders, and naïve cohabitants. IP challenge with isolates from Iceland (1.9 × 10⁷ TCID₅₀  ml^-1 ) and the Faroe Islands (5.9 × 10⁷ TCID₅₀  ml^-1 ) reduced survival in lumpfish, associated with consistent pathological changes. IP challenge with the Irish strain (8.6 × 10⁵ TCID₅₀  ml^-1 ) did not significantly reduce survival in lumpfish, but the lower challenge titre complicated interpretation. Horizontal transmission occurred in all strains tested, but no clinical impact was demonstrated in cohabitants. Salmon parr were challenged by IP injection with the Irish isolate, no virulence or virus replication were demonstrated. A ranavirus qPCR assay, previously validated for fish ranaviruses, was first used to detect ENARV in tissues of both in lumpfish and Atlantic salmon. This study provides the first data on the assessment of virulence of ENARV isolates to lumpfish and salmon, guidelines for the diagnosis of ENARV infection, and poses a basis for further investigations into virulence markers.

Optimization of In Vivo Production of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV)

Insect viruses have been used to protect crops and forests worldwide for decades. Among insect viruses, isolates of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) have proven potential for the control of Spodoptera frugiperda (J. E. Smith) (FAW) (Lepidoptera: Noctuidae), a pest of many economically essential crops across several continents. Mass production of SfMNPV depends on an in vivo system using host insect rearing. However, many factors can limit its production, including abiotic factors and host characteristics, such as the stage of development and an antagonist intraspecific interaction. Thus, to improve in vivo production, we verified the most suitable larval age to inoculate the virus and the influence of incubation temperature on viral production. Subsequently, cannibal behavior was verified in FAW larvae reared at different densities, while reproducing the conditions of the best treatments. The highest viral yield occurred when FAW larvae were inoculated at 10 and 8 days old and incubated at 22 °C and 25 °C, respectively. Nonetheless, survival (lethal period in days) and cannibal behavior were positively influenced by larval development, which potentially increases the load of contamination and requires larval individualization for these production conditions. In contrast, 4-day-old larvae, which were inoculated and incubated at 31 °C, also demonstrated high viral production, with lower rates of cannibalism and death on the same day, thereby showing potential. The information presented in this study is useful for the optimization of the in vivo production systems of SfMNPV.

Determinant Factors in the Production of a Co-Occluded Binary Mixture of Helicoverpa armigera Alphabaculovirus (HearNPV) Genotypes with Desirable Insecticidal Characteristics

A co-occluded binary mixture of Helicoverpa armigera nucleopolyhedrovirus genotypes HearSP1B and HearLB6 at a 1:1 ratio (HearSP1B+HearLB6) was selected for the development of a virus-based biological insecticide, which requires an efficient large-scale production system. In vivo production systems require optimization studies in each host-virus pathosystem. In the present study, the effects of larval instar, rearing density, timing of inoculation, inoculum concentration and temperature on the production of HearSP1B+HearLB6 in its homologous host were evaluated. The high prevalence of cannibalism in infected larvae (40-87%) indicated that insects require individual rearing to avoid major losses in OB production. The OB production of recently molted fifth instars (7.0 x 109 OBs/larva), combined with a high prevalence of mortality (85.7%), resulted in the highest overall OB yield (6.0 x 1011 OBs/100 inoculated larvae), compared to those of third or fourth instars. However, as inoculum concentration did not influence final OB yield, the lowest concentration, LC80 (5.5 x 106 OBs/ml), was selected. Incubation temperature did not significantly influence OB yield, although larvae maintained at 30°C died 13 and 34 hours earlier than those incubated at 26°C and 23°C, respectively. We conclude that the efficient production of HearSP1B+HearLB6 OBs involves inoculation of recently molted fifth instars with a LC80 concentration of OBs followed by individual rearing at 30°C.

Baculovirus Insecticide Production in Insect Larvae

Baculovirus-based insecticides are currently being used worldwide, and new products are in development in many countries. The most dramatic examples of successful baculovirus insecticides are found in soybean in Brazil and cotton in China. Production of baculoviruses is generally done in larvae of a convenient host species, and the level of sophistication varies tremendously between field-collection of infected insects at the one extreme and automated mass manufacturing at the other. Currently, only products with wild type baculoviruses as active ingredients are commercially available. Baculoviruses encoding insecticidal proteins are considered attractive, especially for crops with little tolerance to feeding damage, where speed-of-kill is an important characteristic. Successful field tests with such recombinant baculoviruses have been done in the past, and more tests are ongoing. However, low-cost production of recombinant baculovirus in larvae poses specific problems, due to the short survival time of the production host.In this chapter, benchtop-scale production of two typical baculoviruses is described. First, we describe the production of wild type Helicoverpa zea nucleopolyhedrovirus in bollworm (H. zea) larvae. H. zea larvae are very aggressive and need to be reared in isolation from each other. Second, we describe the production of a recombinant Autographa californica multiple nucleopolyhedrovirus in the non-cannibalistic cabbage looper, Trichoplusia ni. The recombinant baculovirus encodes the insect-specific scorpion toxin LqhIT2. The tetracycline transactivator system enables the production of wild-type quantity and quality product while toxin expression is repressed since normal toxin production would result in premature death of the production host that would limit progeny virus production.

Pathogen growth in insect hosts: inferring the importance of different mechanisms using stochastic models and response-time data

Pathogen population dynamics within individual hosts can alter disease epidemics and pathogen evolution, but our understanding of the mechanisms driving within-host dynamics is weak. Mathematical models have provided useful insights, but existing models have only rarely been subjected to rigorous tests, and their reliability is therefore open to question. Most models assume that initial pathogen population sizes are so large that stochastic effects due to small population sizes, so-called demographic stochasticity, are negligible, but whether this assumption is reasonable is unknown. Most models also assume that the dynamic effects of a host's immune system strongly affect pathogen incubation times or "response times," but whether such effects are important in real host-pathogen interactions is likewise unknown. Here we use data for a baculovirus of the gypsy moth to test models of within-host pathogen growth. By using Bayesian statistical techniques and formal model-selection procedures, we are able to show that the response time of the gypsy moth virus is strongly affected by both demographic stochasticity and a dynamic response of the host immune system. Our results imply that not all response-time variability can be explained by host and pathogen variability, and that immune system responses to infection may have important effects on population-level disease dynamics.

Effect of temperature on viral infection and its control: a mathematical approach

The interrelationship of the host-virus-environment is of critical importance in determining the amount of virus production per insect and its consequences in the overall infection process in insect community. Lower temperature inhibits the virus replication and thus infection does not persist into the system. On the other hand, virus production per insect increases with temperature increase within certain limit, and this increase of virus production brings some unforeseen consequences in the infection dynamics in pest control. We deal the problem by applying non-vulnerability concept on the mathematical model of viral infection that linked up with the temperature-viral developmental model. Using Lyapunov-like function, we determine a range of temperature and show that the system would become endemic and remain in stable situation if temperature could be maintained in this prescribed range, whereas that range depends on other parameters of the system.

Baculovirus insecticide production in insect larvae

Baculovirus-based insecticides are currently being used worldwide, and new products are in development in many countries. The most dramatic examples of successful baculovirus insecticides are found in soybean in Brazil and cotton in China. Production of baculoviruses is generally done in larvae of a convenient host species, and the level of sophistication varies tremendously between field-collection of infected insects at the one extreme and automated mass manufacturing at the other. Currently, only products with wild type baculoviruses as active ingredients are commercially available. Baculoviruses encoding insecticidal proteins are considered attractive, especially for crops with little tolerance to feeding damage, where speed-of-kill is an important characteristic. Successful field tests with such recombinant baculoviruses have been done in the past, and more tests are ongoing. However, low-cost production of recombinant baculovirus in vivo poses specific problems, because of the short survival time of the production host. In this chapter, benchtop-scale production of two typical baculoviruses is described. First, the authors describe the production of wild type Helicoverpa zea nucleopolyhedrovirus in larvae of the bollworm, H. zea. Larvae of this species are very aggressive and need to be reared in isolation from each other. The authors then describe the production of a recombinant Autographa californica multiple nucleopolyhedrovirus in the noncannibalistic cabbage looper, Trichoplusia ni. The recombinant baculovirus encodes the insect-specific scorpion toxin LqhIT2, which severely limits progeny virus production. The tetracycline transactivator system enables the production of wild-type quantity and quality product while toxin expression is repressed.

In vivo characterization of a group II nucleopolyhedrovirus isolated from Mamestra brassicae (Lepidoptera: Noctuidae) in Japan

A Japanese isolate of Mamestra brassicae nucleopolyhedrovirus (MabrNPV) was identified phylogenetically as a group II nucleopolyhedrovirus (NPV) that is related closely to other NPVs isolated from Mamestra spp. based on nucleotide sequence data of its polh, egt and lef-3 genes. The multiplication of MabrNPV in M. brassicae larvae was characterized following inoculation at various doses and in combination with the fluorescent brightener Tinopal by measuring temporal changes in the concentrations of its viral DNA using real-time quantitative PCR. The growth curves of budded-virus replication were analysed by fitting the data of viral DNA concentration in the host haemolymph to a modified Gompertz model. When fifth-instar larvae were inoculated with an LD95 equivalent dose of MabrNPV and Tinopal, the time lag between the onset of primary and secondary infection was estimated to be 25 h. Another 65 h was required to reach a plateau titre equivalent to a level of 10(9) virions ml(-1) in the haemolymph. All larvae died during the sixth instar following this inoculation regime. In contrast, following inoculation with a 1000-fold higher dose of MabrNPV and Tinopal, the time lag between the onset of primary and secondary infection was only 20 h. Subsequently, the same plateau titre was reached after a further 20 h. Following this inoculation regime, most larvae died during the fifth instar. Quantification of viral DNA by real-time quantitative PCR and application of the Gompertz model are valuable for the characterization of baculovirus replication in vivo.

Dose and host characteristics influence virulence of ranavirus infections

Parasites play a prominent role in the ecology, evolution, and more recently, conservation of many organisms. For example, emerging infectious diseases, including a group of lethal ranaviruses, are associated with the declines and extinctions of amphibians around the world. An increasingly important basic and applied question is: what controls parasite virulence? We used a dose-response experiment with three laboratory-bred clutches of tiger salamander larvae (Ambystoma tigrinum) to test how the size of inoculum and host genetic factors influence the dynamics and outcome of ranavirus infections. We found that infection rates increased with dose and were strongly affected by clutch identity and host life history stage. Case mortality increased with dose of inoculum, but was unaffected by host characteristics. Average survival time decreased with dose and differed among clutches, but this was largely due to differences in the time to onset of symptoms. Overall, our results suggest that dose of inoculum and host characteristics (life history stage and genetic background) influence the establishment and early virus replication, and therefore the virulence of ranavirus infections.

Ex vivo monitoring of protein production in baculovirus-infected Trichoplusia ni larvae with a GFP-specific optical probe

Trichoplusia ni larvae were infected with baculoviruses containing genes for the expression of ultraviolet optimized green fluorescent protein (GFPuv) and several product proteins. A GFP-specific optical probe was used to both excite the green fluorescent protein (lambda(ex) = 385 nm), and subsequently monitor fluorescence emission (lambda(em) = 514 nm) from outside the infected larvae. The probe's photodetector was connected to a voltmeter, which was used to quantify the amount of GFPuv expressed in infected larvae. Voltage readings were significantly higher for infected vs. uninfected larvae and, by Western analysis, linear with the amount of GFPuv produced. In addition, the probe sensitivity and range were sufficient to delineate infection efficiency and recombinant protein production for model proteins, chloramphenicol acetyltransferase and human interleukin-2. This work represents a critical step in developing an automated process for the production of recombinant proteins in insect larvae.