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.

Sublethal infections by qPCR analysis indicated a consistently higher prevalence of sublethal infection than RT-PCR.Sublethal infection was associated with significant reductions in pupal weight, adult emergence, fecundity, and fertility (egg hatch) and significant increases in larval development time and duration of the preoviposition period. Insects taken from a persistently infected experimental population were significantly more susceptible to the OB inoculum than control insects that originated from the same virus-free colony as the persistently infected insects. We conclude that OB treatment results in rapid establishment of sublethal infections that persist between generations and which incur costs in the development and reproductive capacity of the host insect.

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Larvae of the tomato fruitworm, Helicoverpa armigera, were surveyed for nucleopolyhedrovirus (NPV) infection (referred to as HearNPV) in three different locations from the Iberian Peninsula: Olivenza and Toledo in Spain and the Oeste region in Portugal. Twenty HearNPV isolates were obtained from single field-collected larval cadavers. Restriction endonuclease (REN) profiles of the collected isolates with BglII and PstI allowed identification of six different H. armigera single-embedded NPV strains in Spain (referred to as HearSP3, HearSP4, HearSP5, HearSP6, HearSP7, and HearSP8) and two in Portugal (referred to as HearPT1 and HearPT2). No strains were shared by isolates from different geographical regions except HearSP5, which was found in isolates from Olivenza and Toledo. Cluster analysis based on the restriction fragment length polymorphisms of these strains in relation to two previously identified strains from Badajoz (HearSP1) and Cordoba (HearSP2) in Spain, showed no correlation among the strains and their geographical origin. The biological activity of HearSP2, HearSP4, HearSP7, HearSP8, HearPT1, and HearPT2 was compared in terms of pathogenicity (50% lethal concentration, LC 50) and virulence (mean time to death). HearPT2 and HearSP7 were significantly more pathogenic than HearSP2, with LC 50 values 2.8 and 2.6-fold higher than the latter, respectively, on H. armigera second instars. HearSP4 and HearPT2 killed larvae significantly faster than HearSP8, whereas HearSP2, HearSP7, and HearPT1 showed intermediate mean time to death values.

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Virus transmission and the prevalence of infection depend on multiple factors, including the interaction with other viral pathogens infecting the same host. In this study, active replication of an iflavirus, Spodoptera exigua iflavirus 1 (order Picornavirales) was observed in the offspring of insects that survived following inoculation with a pathogenic baculovirus, Spodoptera exigua multiple nucleopolyhedrovirus. Tracking the origin of the iflavirus suggested the association of this virus with the occlusion bodies of the baculovirus. Here we investigated the effect of this association on the stability and infectivity of both viruses. A reduction in baculovirus pathogenicity, without affecting its infectivity and productivity, was observed when associated with the iflavirus. In contrast, viral association increased the infectivity of the iflavirus and its resistance to ultraviolet radiation and high temperature, two of the main factors affecting virus stability in the field. In addition, electron microscopy analysis revealed the presence of particles resembling iflavirus virions inside the occlusion bodies of the baculovirus, suggesting the possible co-occlusion of both viruses. Results reported here are indicative of facultative phoresis of a virus and suggest that virus–virus interactions may be more common than currently recognized, and may be influential in the ecology of baculovirus and host populations and in consequence in the use of baculoviruses as biological insecticides.