Mortality of cutworm larvae is not enhanced by Agrotis segetum granulovirus and Agrotis segetum nucleopolyhedrovirus B coinfection relative to single infection by either virus

Effects of Mixed Baculovirus Infections in Biological Control: A Comprehensive Historical and Technical Analysis

Baculoviruses are insect-specific DNA viruses that have been exploited as bioinsecticides for the control of agricultural and forest pests around the world. Mixed infections with two different baculoviruses have been found in nature, infecting the same host. They have been studied to understand the biology of virus interactions, their effects on susceptible insects, and their insecticidal implications. In this work, we summarize and analyze the in vivo baculovirus co-infections reported in the literature, mainly focusing on pest biocontrol applications. We discuss the most common terms used to describe the effects of mixed infections, such as synergism, neutralism, and antagonism, and how to determine them based on host mortality. Frequently, baculovirus co-infections found in nature are caused by a combination of a nucleopolyhedrovirus and a granulovirus. Studies performed with mixed infections indicated that viral dose, larval stage, or the presence of synergistic factors in baculovirus occlusion bodies are important for the type of virus interaction. We also enumerate and discuss technical aspects to take into account in studies on mixed infections, such as statistical procedures, quantification of viral inocula, the selection of instars, and molecular methodologies for an appropriate analysis of baculovirus interaction. Several experimental infections using two different baculoviruses demonstrated increased viral mortality or a synergistic effect on the target larvae compared to single infections. This can be exploited to improve the baculovirus-killing properties of commercial formulations. In this work, we offer a current overview of baculovirus interactions in vivo and discuss their potential applications in pest control strategies.

Natural Coinfection between Novel Species of Baculoviruses in Spodoptera ornithogalli Larvae

Spodoptera ornithogalli (Guenée) (Lepidoptera: Noctuidae) is an important pest in different crops of economic relevance in America. For its control, strategies that include chemicals are usually used; so, the description of entomopathogens would be very useful for the formulation of biopesticides. In this regard, two different baculoviruses affecting S. ornithogalli were isolated in Colombia, with one of them being an NPV and the other a GV. Ultrastructural, molecular, and biological characterization showed that both isolates possess the 38 core genes and are novel species in Baculoviridae, named as Spodoptera ornithogalli nucleopolyhedrovirus (SporNPV) and Spodoptera ornithogalli granulovirus (SporGV). The bioassays carried out in larvae of S. ornithogalli and S. frugiperda showed infectivity in both hosts but being higher in the first. In addition, it was observed that SporGV potentiates the insecticidal action of SporNPV (maximum value in ratio 2.5:97.5). Both viruses are individually infective but coexist in nature, producing mixed infections with a synergistic effect that improves the performance of the NPV and enables the transmission of the GV, which presents a slowly killing phenotype.

Cross-Resistance of the Codling Moth against Different Isolates of Cydia pomonella Granulovirus Is Caused by Two Different but Genetically Linked Resistance Mechanisms

Cydia pomonella granulovirus (CpGV) is a widely used biological control agent of the codling moth. Recently, however, the codling moth has developed different types of field resistance against CpGV isolates. Whereas type I resistance is Z chromosomal inherited and targeted at the viral gene pe38 of isolate CpGV-M, type II resistance is autosomal inherited and targeted against isolates CpGV-M and CpGV-S. Here, we report that mixtures of CpGV-M and CpGV-S fail to break type II resistance and is expressed at all larval stages. Budded virus (BV) injection experiments circumventing initial midgut infection provided evidence that resistance against CpGV-S is midgut-related, though fluorescence dequenching assay using rhodamine-18 labeled occlusion derived viruses (ODV) could not fully elucidate whether the receptor binding or an intracellular midgut factor is involved. From our peroral and intra-hemocoel infection experiments, we conclude that two different (but genetically linked) resistance mechanisms are responsible for type II resistance in the codling moth: resistance against CpGV-M is systemic whereas a second and/or additional resistance mechanism against CpGV-S is located in the midgut of CpR5M larvae.

Comparative Pathogenesis of Generalist AcMNPV and Specific RanuNPV in Larvae of Rachiplusia nu (Lepidoptera: Noctuidae) Following Single and Mixed Inoculations

The South American soybean pest, Rachiplusia nu (Guenée), is naturally infected by Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and Rachiplusia nu nucleopolyhedrovirus (RanuNPV). We compared their pathogenicity to fourth-instar R. nu larvae, by evaluating time to death and virus spread throughout the tissues in single and mixed infections. Bioassays showed that generalist AcMNPV had a faster speed of kill than specific RanuNPV, while the mixed-virus treatment did not statistically differ from AcMNPV alone. Histopathology evidenced similar tissue tropism for both viruses, but co-inoculation resulted in mostly AcMNPV-infected cells. In sequential inoculations, however, the first virus administered predominated over the second one. Implications on baculovirus interactions and biocontrol potential are discussed.

Cryptophlebia peltastica Nucleopolyhedrovirus Is Highly Infectious to Codling Moth Larvae and Cells

Cydia pomonella granulovirus (CpGV) is a cornerstone of codling moth (Cydia pomonella) control in integrated and organic pome fruit production, though different types of resistance to CpGV products have been recorded in codling moth field populations in Europe for several years. Recently, a novel baculovirus named Cryptophlebia peltastica nucleopolyhedrovirus (CrpeNPV) was isolated from a laboratory culture of the litchi moth, Cryptophlebia peltastica, in South Africa. Along with CpGV, it is the third known baculovirus that is infectious to codling moth. In the present study, parameters of infectiveness of CrpeNPV, such as the median lethal concentration and median survival time, were determined for codling moth larvae susceptible or resistant to CpGV. In addition, the permissiveness of a codling moth cell line with respect to infection by CrpeNPV budded virus was demonstrated by infection and gene expression studies designed to investigate the complete replication cycle. Investigations of the high degree of virulence of CrpeNPV for codling moth larvae and cells are of high significant scientific and economic value and may offer new strategies for the biological control of susceptible and resistant populations of codling moth.IMPORTANCE The emergence of codling moth populations resistant to commercially applied isolates of CpGV is posing an imminent threat to organic pome fruit production. Very few CpGV isolates are left that are able to overcome the reported types of resistance, emphasizing the demand for new and highly virulent baculoviruses. Here we report the recently discovered CrpeNPV as highly infectious to all types of resistant codling moth populations with a high speed of killing, making it a promising candidate baculovirus in fighting the spread of resistant codling moth populations.