Cross-infectivity and activation studies with four baculoviruses

The History of Baculovirology in Africa

Baculovirology has been studied on the African continent for the development of insect virus-based biopesticides and, to a much lesser extent, vaccine production and delivery, since the 1960s. In this review, we focus only on baculoviruses as biopesticides for agricultural pests in Africa. At least 11 species of baculovirus have been discovered or studied on the African continent, some with several distinct isolates, with the objective in most cases being the development of a biopesticide. These include the nucleopolyhedroviruses of Helicoverpa armigera, Cryptophlebia peltastica, Spodoptera exempta, Spodoptera frugiperda, Spodoptera littoralis, and Maruca vitrata, as well as the granuloviruses of Cydia pomonella, Plutella xylostella, Thaumatotibia (Cryptophlebia) leucotreta, Choristoneura occidentalis, and Phthorimaea operculella. Eleven different baculovirus-based biopesticides are recorded as being registered and commercially available on the African continent. Baculoviruses are recorded to have been isolated, researched, utilised in field trials, and/or commercially deployed as biopesticides in at least 13 different African countries. Baculovirus research is ongoing in Africa, and researchers are confident that further novel species and isolates will be discovered, to the benefit of environmentally responsible agricultural pest management, not only in Africa but also elsewhere.

Genetic Variation and Biological Activity of Two Closely Related Alphabaculoviruses during Serial Passage in Permissive and Semi-Permissive Heterologous Hosts

Phylogenetic analyses suggest that Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and Helicoverpa armigera multiple nucleopolyhedrovirus (HearMNPV) may be strains of the same virus species. Most of the studies comparing their biological activities have been performed in their homologous hosts. A comparison of host range and stability in alternative hosts was performed. The host range of these viruses was compared using high concentrations of inoculum to inoculate second instars of six species of Lepidoptera. One semi-permissive host (Spodoptera littoralis) and one permissive host (S. exigua) were then selected and used to perform six serial passages involving a concentration corresponding to the ~25% lethal concentration for both viruses. Restriction endonuclease analysis showed fragment length polymorphisms in every host-virus system studied. In S. littoralis, serial passage of MbMNPV resulted in decreased pathogenicity and an increase in speed-of-kill, whereas no significant changes were detected for HearMNPV with respect to the initial inoculum. In contrast, both viruses showed a similar trend in S. exigua. These results highlight the low genetic diversity and a high phenotypic stability of HearMNPV with respect to the original inoculum after six successive passages in both insect hosts. This study concludes that host-baculovirus interactions during serial passage are complex and the process of adaptation to a novel semi-permissive host is far from predictable.

The phylogenetic relationship and cross-infection of nucleopolyhedroviruses between the invasive winter moth (Operophtera brumata) and its native congener, Bruce spanworm (O. bruceata)

Disease can affect biological invasions by acting as either a synergist or antagonist. Disease-mediated invasions have important implications for understanding the spread of invasive insects, which cost billions of dollars in damages annually. One such non-native, destructive insect is the winter moth, Operophtera brumata L. (Lepidoptera: Geometridae), which causes defoliation and mortality of deciduous trees in its introduced range. In the northeastern United States, winter moth populations overlap with a native congener, Bruce spanworm, Operophtera bruceata Hulst. Nucleopolyhedrovirus (NPV), appears to be an important natural enemy in Bruce spanworm and there is some evidence that the NPV infection found in winter moth in the northeastern U.S. may originate from Bruce spanworm. By sequencing two viral genes (the polyhedrin and p74 genes) from field-collected larvae of both species, we found that the winter moth virus (OpbuNPV) is distinct from the virus from Bruce spanworm (OpbrNPV). However, the two viruses do constitute a clade within the Alphabaculovirus Group 2 NPVs, indicating that they are more similar to each other than they are to other lepidopteran viruses, even other geometrid-derived NPVs. As far as we know, this is the first report of sequences from an NPV from Bruce spanworm. Results from cross infection trials suggest that cross infection is uncommon if it occurs at all. Our results show that these two closely related species have distinct viruses and, unlike previous suggestions, Bruce spanworm virus is not mediating the winter moth invasion.

Insect viruses as control agents

Virus diseases have been reported from more than 800 species of insects and mites. Isolates of the baculovirus and cytoplasmic polyhedrosis virus groups have biological properties which should lead to their successful use as microbial control agents in integrated pest management programmes. These viruses infect the larval stages of many lepidopterous and hymenopterous pests, producing a chronic or lethal infection and the release of large quantities of relatively stable infective inclusion bodies (IBs). The IBs serve as the means by which the viruses are transmitted and persist outside the host. Younger larvae are more susceptible to infection than older stages, and this difference influences the timing of application and doses of virus needed for practical pest control. The high degree of host specificity of many isolates reduces their potential ecological hazard but also limits their use, particularly on crops where a complex of pests is established. Environmental persistence is also a limiting factor as virus is rapidly inactivated by ultra-violet light even when contained within IBs. The viruses persist for longer periods when transmitted within the host population, a feature of virus infections restricted to the insect gut.

The practical use of insect viruses in horticulture and agriculture does not utilize their full epizootic potential, but takes advantage of their high pathogenicity and specificity. The baculoviruses of codling moth, andHeliothisspp. provide satisfactory pest control, but for their most cost-effective use it is important to determine the minimum dosage rates of virus required. It is encouraging that studies of the virus control ofPierisspp. have suggested that control achieved by the insecticidal use of a virus can be closely predicted from information on dosage-mortality responses, larval feeding rates and virus persistence. The stability of forest and grassland, and their high economic thresholds makes them ideal candidates for longer-term control. Viruses of the coconut rhinoceros beetle and european spruce sawfly provide examples of classical biological control where the viruses persist for long periods, are efficiently transmitted and act as natural regulators of their hosts. Virus control of pasture, and some forest, pests may be possible by manipulating enzootic viruses without the need for direct control measures. More frequently insecticidal applications are needed, providing control of limited duration which requires periodic ‘topping-up’.

Few viruses are commercially-available; their selectivity and often small potential market, may limit industrial interest. However, improvements in virus production, formulation and a better understanding of virus epizootiology should lead to an increasing role for this group of insect pathogens in biological control.

Induction of a nonoccluded baculovirus persistently infecting Heliothis zea cells by Heliothis armigera and Trichoplusia ni nuclear polyhedrosis viruses

A nonoccluded singly enveloped baculovirus (baculovirus X) persistently infects Heliothis zea (IMC-HZ-1) cells in culture. Singly enveloped nuclear polyhedrosis viruses from H. zea and Heliothis armigera, and multiply enveloped nuclear polyhedrosis viruses from Trichoplusia ni, Spodoptera frugiperda, and Spodoptera littoralis were all found to induce baculovirus X. Experiments are reported which use metabolic inhibitors and inactivated inducing virus to show that it is probable that a structural component of the virus, most likely a protein, is responsible for inducing baculovirus X. The persistent virus is induced to replicate by uv-inactivated virus but not by heat-inactivated inducing virus. The virus is not induced to replicate by a number of metabolic inhibitors in the absence of an inducing virus. Inhibition of transcription and translation prevents the induction of the persistent virus by an inducing virus. Inhibition of DNA replication has no effect on the induction of the virus. This suggests that the persistent virus genome is present in abundance in all cells.

Comparative biological activities of a plaque-purified variant and a Turkish native isolate of SpliNPV-B against Spodoptera littoralis (Lepidoptera: Noctuidae)

The noctuid moth Spodoptera littoralis (Boisduval) is an important pest of many cultivated plants worldwide and five different geographical Nucleopolyhedrovirus (NPV) strains of this pest have been isolated to date. Two of these, a plaque-purified variant of the S. littoralis NPV from Morocco (SpliNPV-M2) and a SpliNPV isolated from field-infected S. littoralis larvae found in Turkey (SpliNPV-TR1), were compared biologically in terms of infectiveness (median lethal dose, LD50) for third instars and in terms of virulence (median lethal time, LT50) for neonates and third-instar S. littoralis larvae. The LD50 values of SpliNPV-TR1 and SpliNPV-M2 were 20.73 and 185.21 occlusion bodies (OBs)/larva, respectively, with non-overlapping confidence limits indicating they were significantly different. Thus, SpliNPV-M2 was found to be significantly less infective (about nine times higher LD50) than SpliNPV-TR1. The LT50 values of neonates for SpliNPV-M2 and SpliNPV-TR1 were 37 and 43.9 h at a concentration of 10(6) OBs ml(-1), respectively. For these same isolates, the LT50 values at a concentration of 3 x 10(6) OBs ml(-1) were calculated as 35.6 and 41.7 h, respectively. The LT(50) values of third instars for SpliNPV-M2 and SpliNPV-TR1 were 147.4 and 160.5 h, respectively, at a dose of 3000 OBs/larva and 145.4 and 152.4 h, respectively, for the same isolates at a dose of 20,000 OBs/larva. On the other hand, Helicoverpa armigera (Hübner) and Agrotis ipsilon (Hufnagel) revealed a lack of lethality of the SpliNPV-TR1 isolate.

Comparative study on the susceptibility of cutworms (Lepidoptera: Noctuidae) to Agrotis segetum nucleopolyhedrovirus and Agrotis ipsilon nucleopolyhedrovirus

The common cutworm (Agrotis segetum) and the black cutworm (Agrotis ipsilon) are serious soil pests of many vegetable and field crops all over the world. We have demonstrated the cross-infectivity of two baculoviruses, A. segetum nucleopolyhedrovirus (AgseNPV) and A. ipsilon nucleopolyhedrovirus (AgipNPV) for these two insect pests. The susceptibility of A. segetum to AgipNPV was confirmed by DNA restriction endonuclease analyses of DNA isolated from virus harvested from infected A. segetum larvae. For an initial comparison of both viruses, partial polyhedrin sequences were amplified by PCR, cloned, and sequenced. Both viruses shared a very similar polyhedrin gene sequence resulting in only three amino acid substitutions. Phylogenetic analyses clearly demonstrated that both viruses belong to NPV group II and are most closely related to a clade consisting of Spodoptera exigua NPV, Spodoptera frugiperda NPV, and Spodoptera littoralis NPV. Since AgipNPV shows high virulence for both cutworm species, it appears to be a suitable candidate as a single biological control agent of A. segetum and A. ipsilon.

Description and analysis of two internet-based databases of insect pathogens: EDWIP and VIDIL

In 1996, two searchable databases covering insect pathogens were posted on the World Wide Web: the Ecological Database of the World's Insect Pathogens (EDWIP) and the Viral Diseases of Insects in the Literature database (VIDIL). In this paper, we describe the format and contents of EDWIP and VIDIL on the World Wide Web. EDWIP contains over 9,400 pathogen-host association records, 677 negative test result or "no association" records, 4,454 host species, 2,285 pathogen species records, and 2,057 bibliographical references. Species of Coleoptera and Lepidoptera are the best represented groups in EDWIP. Lepidopteran species account for the most associations of any host order in EDWIP, over 2,500, or 27%. Of the pathogen groups, Protozoa (including microsporidia) accounted for nearly 66% of the pathogen species records and over 40% of the association records in EDWIP. Fungi account for only 18% of the pathogen species, but nearly 33% of the association records. Habitats dominated by human activities (e.g., crop, stored product, and human dwelling) account for most of the host habitats recorded in EDWIP. The United States and Japan are the most common locations and the Nearctic and Palearctic are the most common biogeographic regions reported in EDWIP. There are 4,801 annotated bibliographic records in VIDIL.

Comparative pathogenesis of the Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus in noctuid hosts of different susceptibility

Neonate larvae of the noctuid moth Spodoptera exigua were susceptible to an infection by Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HaSNPV). Biological activity (LD(50),ST(50)) of the virus was considerably reduced as compared to its activity in the homologous host, H. armigera. Pathogenesis was studied using a recombinant HaSNPV carrying a green fluorescent protein gene, which induces fluorescence in infected cells to mark infection. In larvae of H. armigera, fluorescence was pronounced in the fat body after 2.9 days post infection and could also be detected in several other tissues. In contrast, fluorescence was not observed in tissues of S. exigua until 9 days post infection and was restricted almost exclusively to cells of the ganglia. Examination of serial sections of wildtype HaSNPV-infected S. exigua-larvae revealed a similar pattern of tissue tropism. Apparently, HaSNPV does not undergo the usual steps in host invasion and infection in this insect species, but targets specifically to nervous tissue.

Persistent baculovirus infection results from deletion of the apoptotic suppressor gene p35

Infection with the wild-type baculovirus Autographa californica multiple nuclear polyhedrosis virus (AcMNPV) results in complete death of Spodoptera frugiperda (Sf) cells. However, infection of Sf cells with AcMNPV carrying a mutation or deletion of the apoptotic suppressor gene p35 allowed the cloning of surviving Sf cells that harbored persistent viral genomes. Persistent infection established with the virus with p35 mutated or deleted was blocked by stable transfection of p35 in the host genome or by insertion of the inhibitor of apoptosis (iap) gene into the viral genome. These artificially established persistently virus-infected cells became resistant to subsequent viral challenge, and some of the cell lines carried large quantities of viral DNA capable of early gene expression. Continuous release of viral progenies was evident in some of the persistently virus-infected cells, and transfection of p35 further stimulated viral activation of the persistent cells, including the reactivation of viruses in those cell lines without original continuous virus release. These results have demonstrated the successful establishment of persistent baculovirus infections under laboratory conditions and that their establishment may provide a novel continuous, nonlytic baculovirus expression system in the future.

Quantification of latent Mamestra brassicae nuclear polyhedrosis virus in M. brassicae insects using a PCR-scintillation proximity assay

A laboratory culture of Mamestra brassicae insects (MbLC) was found to harbour a latent baculovirus infection. The copy number of the occult MbNPV genome in both the MbLC larvae, and in a cell line derived from the fat body of MbLC was determined by the use of a rapid and convenient PCR-scintillation proximity assay (SPA). The SPA system relies on the use of fluomicrospheres (SPA beads) coated with acceptor molecules which are capable of binding radiolabelled ligands in solution. In the assay described, a biotinylated PCR primer is used and [3H]dNTPs are incorporated into the amplified DNA. The SPA beads are coated with streptavidin, and after binding the biotinylated primer, any amplified, radiolabelled DNA will activate the fluor. The amount of amplified DNA from the target sequence can then be directly quantified using a scintillation counter. The number of MbNPV genomes present in a persistently infected M. brassicae cell, as proposed by SPA, suggest between 13 and 20 copies of the viral genome may be present in individual fat body cells.

DNA restriction polymorphism in wild isolates of Spodoptera frugiperda nuclear polyhedrosis virus

Restriction endonuclease analysis was used to examine variation in DNA of 22 wild isolates of Spodoptera frugiperda nuclear polyhedrosis virus (SfNPV). Eleven of the 15 isolated from Louisiana were distinguishable based on restriction fragment profiles from the enzymes BamHI, HindIII, and EcoRI. There was significant genetic variation in SfNPV isolates within single agricultural fields. Nucleotide sequence divergence values, based on restriction fragment profiles, indicated that genetic variation among isolates foreign to Louisiana (Ohio, Ecuador, Mexico, Georgia, Colombia, and Venezuela) was greater than that among the Louisiana isolates. However, certain foreign isolates were similar to or identical with Louisiana isolates. Genetic variation of the viral DNA was not influenced by the insect's host plan species.

Induction of Spodoptera littoralis nuclear polyhedrosis virus in cell cultures

A nuclear polyhedrosis virus (NPV) infection was initiated in a Spodoptera littoralis cell line when haemocytes from granulosis virus (GV)-infected S. littoralis larvae and ultra-violet-inactivated Sendai virus were added.