Effect of ecdysteroid UDP-glucosyltransferase gene deletion on efficacy of a baculovirus against Heliothis virescens and Trichoplusia ni (Lepidoptera: Noctuidae)

Baseline Susceptibility of Spodoptera frugiperda (Lepidoptera: Noctuidae) to SfMNPV and Evaluation of Cross-Resistance to Major Insecticides and Bt Proteins

The resistance evolution of Spodoptera frugiperda (J.E. Smith) to insecticides and Bt proteins along with the intensive crop production systems adopted in Brazil make it challenging to implement integrated pest management. The adoption of alternative methods to manage pests is fundamental to the implementation of favorable integrated pest management and insect resistance management. Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) is a valuable tool for S. frugiperda control. The characterization of the baseline susceptibility of S. frugiperda populations and cross-resistance involving SfMNPV and major insecticides and Bt proteins have not yet been conducted. The objective of this study was to characterize the baseline susceptibility of S. frugiperda populations from five Brazilian States to SfMNPV (Cartugen, AgBiTech, Fort Worth, TX). Possible cross-resistance to insecticides and Bt proteins among resistant S. frugiperda strains was also assessed. There were no differences in the susceptibility of the studied populations to SfMNPV. The estimated diagnostic concentration may be utilized in future monitoring studies to SfMNPV. The SfMNPV presented no cross-resistance to the chemical insecticides and to the Bt proteins tested. Our results provide evidence of the biological activity and high potential of SfMNPV as a distinct insecticidal mode of action for use in rotation with other tools. This biological insecticide is known to have a favorable toxicological and ecotoxicological profile and will be a valuable tool in insect resistance management and integrated pest management programs for control of S. frugiperda.

Insect Biotechnology

For the purpose of this work, insect biotechnology, which is also known as yellow biotechnology, is the use of insects as well as insect-derived cells or molecules in medical (red biotechnology), agricultural (green biotechnology), and industrial (white) biotechnology. It is based on the application of biotechnological techniques on insects or their cells to develop products or services for human use. Such products are then applied in agriculture, medicine, and industrial biotechnology. Insect biotechnology has proven to be a useful resource in diverse industries, especially for the production of industrial enzymes including chitinases and cellulases, pharmaceuticals, microbial insecticides, insect genes, and many other substances. Insect cells (ICs), and particularly lepidopteran cells, constitute a competitive strategy to mammalian cells for the manufacturing of biotechnology products. Among the wide range of methods and expression hosts available for the production of biotech products, ICs are ideal for the production of complex proteins requiring extensive posttranslational modification. The progress so far made in insect biotechnology essentially derives from scientific breakthroughs in molecular biology, especially with the advances in techniques that allow genetic manipulation of organisms and cells. Insect biotechnology has grown tremendously in the last 30 years.

The killing speed of egt-inactivated Bombyx mori nucleopolyhedrovirus depends on the developmental stage of B. mori larvae

Several lines of evidence have shown that the deletion of the ecdysteroid UDP-glucosyltransferase gene (egt) from the nucleopolyhedrovirus (NPV) genome increases the killing speed of host lepidopteran larvae. However, it has not been investigated in detail whether the effects of egt deletion depend on the larval stages of the host insect. In this study, we performed bioassays using 10 continuous larval stages of the 4th- or 5th-instar Bombyx mori larvae and B. mori NPV egt mutants. The fast-killing phenotype was observed in the egt mutants only when the infection process progressed through larval-larval transition. All day-2 4th-instar larvae infected with the egt mutants entered the molting stage and died much earlier than wild-type-infected larvae. Bodies of egt mutant-infected larvae were filled with excessive fluid immediately after head capsule slippage, owing presumably to the degeneration of Malpighian tubules. Fourth- or 5th-instar larvae infected with the egt mutants at early stages of each instar died similarly to those infected with the wild-type virus. Under infection in the middle stages of the 5th-instar, the survival time of egt mutant-infected larvae was significantly longer than that of the wild-type virus-infected larvae. These results clearly show that the effects of egt deletion on killing speed of NPV are largely dependent on the developmental stage of the host larvae infected by the virus.

Laboratory and field evaluations of the efficacy of a fast-killing baculovirus isolate from Spodoptera frugiperda

Three biopesticide parameters were evaluated for a fast-killing isolate (3AP2) and a wild-type isolate (Sf3) of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV). Both isolates were evaluated for virus production using in vivo methods, for speed of kill based on bioassay of applications to glasshouse-grown and field-grown plants, and for residual insecticidal activity of unformulated virus and an encapsulating formulation to provide UV protection. Two inoculation rates comparing relative in vivo production of the isolates demonstrated 3AP2 inoculated larvae were significantly smaller than Sf3 inoculated larvae at death. At the lower inoculation rate, Sf3 inoculated larvae produced approximately twofold more occlusion bodies as the 3AP2 inoculated larvae. A model system of applications to cabbage plants and a bioassay to observe mortality of neonate S. frugiperda (J.E. Smith) after feeding on samples of treated leaves was used to evaluate speed of kill and residual insecticidal activity. The LT(50) for the 3AP2 isolate was at least 30 h less than the LT(50) for the Sf3 isolate when applied to either glasshouse-grown or field-grown plants. The spray-dried lignin encapsulating formulation provided similar benefits to both virus isolates when exposed to simulated sunlight in the laboratory and to natural sunlight in the field. For treatment applications to field grown cabbage in June, the half-life for efficacy of unformulated virus was <7.5 h compared with a half-life of >26.7 h for encapsulated virus. These results demonstrate that improved technologies can be combined to address characteristics which otherwise can limit the commercial potential of microbial-based biological insecticides.

Sequence comparison between three geographically distinct Spodoptera frugiperda multiple nucleopolyhedrovirus isolates: Detecting positively selected genes

The complete genomic sequence of a Nicaraguan plaque purified Spodoptera frugiperda nucleopolyhedrovirus (SfMNPV) genotype SfMNPV-B was determined and compared to previously sequenced isolates from United States (SfMNPV-3AP2) and Brazil (SfMNPV-19). The genome of SfMNPV-B (132,954bp) was 1623bp and 389bp larger than that of SfMNPV-3AP2 and SfMNPV-19, respectively. Genome size differences were mainly due to a deletion located in the SfMNPV-3AP2 egt region and small deletions and point mutations in SfMNPV-19. Nucleotide sequences were strongly conserved (99.35% identity) and a high degree of predicted amino acid sequence identity was observed. A total of 145 open reading frames (ORFs) were identified in SfMNPV-B, two of them (sf39a and sf110a) had not been previously identified in the SfMNPV-3AP2 and SfMNPV-19 genomes and one (sf57a) was absent in both these genomes. In addition, sf6 was not previously identified in the SfMNPV-19 genome. In contrast, SfMNPV-B and SfMNPV-19 both lacked sf129 that had been reported in SfMNPV-3AP2. In an effort to identify genes potentially involved in virulence or in determining population adaptations, selection pressure analysis was performed. Three ORFs were identified undergoing positive selection: sf49 (pif-3), sf57 (odv-e66b) and sf122 (unknown function). Strong selection for ODV envelope protein genes indicates that the initial infection process in the insect midgut is one critical point at which adaptation acts during the transmission of these viruses in geographically distant populations. The function of ORF sf122 is being examined.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Genomic sequence and biological characterization of a nucleopolyhedrovirus isolated from the summer fruit tortrix, Adoxophyes orana

Adoxophyes orana nucleopolyhedrovirus (AdorNPV) was isolated from overwintering larvae from an orchard in the UK. The nucleotide sequence of the AdorNPV DNA genome was determined and analysed. The genome contains 111724 bp and has a G+C content of 35.0 mol%. The analysis predicted 121 ORFs of 150 nt or larger. Of these putative genes, 118 were homologous to genes identified previously in the Adoxophyes honmai nucleopolyhedrovirus (AdhoNPV) genome (83.3-100 % aa identity), and three AdorNPV ORFs were unique. There were four small homologous regions that consisted of a similar core sequence and at the same relative positions in the genome as AdhoNPV, but they differed in the number of repeats and orientation. Some genes that have been reported to have major roles in baculovirus biology were either absent or truncated in the AdorNPV genome. These included chitinase, which is involved in the liquefaction of the host, and the C-terminal of the ecdysteroid UDP-glucosyltransferase (egt) protein, which was truncated by 149 aa compared with AdhoNPV, with essential amino acids absent. The AdorNPV genome encoded two inhibitor of apoptosis (iap) genes compared with three in AdhoNPV and three bro genes compared with four in AdhoNPV. The susceptibility of A. orana larvae to AdorNPV was evaluated in laboratory bioassays using inoculation by microdroplet feeding and applied dose assays. LD50 for neonates was 56 occlusion bodies rising to 2.3x10(4) for fifth instar larvae. Median survival time values using an LD80 dose were 8.8 days for neonates and 7.0 days for fifth instar larvae.

Genomic sequence analysis of a fast-killing isolate of Spodoptera frugiperda multiple nucleopolyhedrovirus

Six clones of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV) were plaque-purified from field isolates collected in Missouri, USA. In bioassays, four of the plaque-purified isolates killed neonate S. frugiperda larvae more rapidly than the field isolates from which they were derived, with LT(50) values (mean time to kill 50 % of the test larvae) ranging from 34.4 to 49.7 h post-infection. The complete genomic sequence of one of these isolates, SfMNPV-3AP2, was determined and analysed. The SfMNPV-3AP2 genome was 131 330 bp with a G+C content of 40.2 %. A total of 144 open reading frames (ORFs) was identified and examined, including the set of 62 genes in common among lepidopteran nucleopolyhedrovirus genomes. Comparisons of ORF content, order and predicted amino acid sequences with other nucleopolyhedoviruses indicated that SfMNPV is part of a cluster of viruses within NPV group II that includes NPVs isolated from Spodoptera, Agrotis and Mamestra host species. SfMNPV-3AP2 shared a high degree of nucleotide sequence similarity with partial sequences from other SfMNPV isolates. Comparison of the SfMNPV-3AP2 genome sequence with a partial sequence from a Brazilian isolate of SfMNPV revealed that SfMNPV-3AP2 contained a deletion that removed parts of ORF sf27 and the gene encoding ecdysteroid UDP-glucosyltransferase (egt). An examination of the egt region in the other isolates revealed that the other five SfMNPV clones also contained deletions of varying length in this region. Variant genotypes with deletions extending around the egt gene have been reported previously from a Nicaraguan field isolate of SfMNPV, suggesting that the presence of such variants is a common feature of SfMNPV populations.

Structural and ultrastructural alterations of Malpighian tubules of Anticarsia gemmatalis (Hübner) (Lepidoptera: Noctuidae) larvae infected with different Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV) recombinant viruses

Malpighian tubules constitute the main excretion organ of insects. Infection by egt(-) recombinant AcMNPV baculovirus in lepidopteran larvae promotes early degeneration of these structures, which has been correlated with earlier death of the host. However, no trace of viral infection has been detected in that tissue. We constructed two AgMNPV recombinants with the egfp gene under control of the hsp70 promoter, one being egt(-), and used another two recombinants (one egt(-)) containing the lacZ gene. Morphological alterations in the tubules were analyzed by light and electron microscopies. Bioassays were conducted to compare the pathogenicity of recombinants. Results showed progressive presence of marker proteins and tissue degeneration without signals of infection in the tissue. Morphological and bioassay results showed increased pathogenicity for lacZ-containing recombinants compared to the egfp ones; as for egt(-) viruses, we noted higher intensity and earlier onset of alterations. The absence of infection led us to believe that Malpighian tubules degeneration is provoked initially by the death of tracheal cells attached to the tubules and later, by the death of Malpighian tubule cells themselves. Tubule cell death might be due to oncosis and apoptosis, which may be activated by depletion of energy reserves and by accumulation of marker proteins, respectively. Absence of the egt gene may be leading to a higher energetic expense due to molting, thus aggravating tubule cell death, resulting in faster death of host.

Genetically modified baculoviruses: a historical overview and future outlook

The concept of using genetic engineering to improve the natural insecticidal activity of baculoviruses emerged during the 1980s. Both academic and industrial laboratories have since invested a great deal of effort to generate genetically modified (GM) or recombinant baculoviruses with dramatically improved speeds of kill. Optimal production methodologies and formulations have also been developed, and the safety and ecology of the recombinant baculoviruses have been thoroughly investigated. Unfortunately, the initial excitement that was generated by these technologies was tempered when industry made a critical decision to not complete the registration process of GM baculoviruses for pest insect control. In this chapter, we summarize the developments in the field from a historical perspective and provide our opinions as to the current status and future potential of the technology. We will argue that GM baculoviruses are valuable and viable tools for pest insect control both alone and in combination with wild-type viruses. We believe that these highly effective biopesticides still have a bright future in modern agriculture as public awareness and acceptance of GM organisms, including GM baculoviruses, increases.

Bollworm responses to release of genetically modified Helicoverpa armigera nucleopolyhedroviruses in cotton

Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HaSNPV) has been developed as a commercial biopesticide to control the cotton bollworm, H. armigera, in China. The major limitation to a broader application of this virus has been the relative long time to incapacitate the target insect. Two HaSNPV recombinants with improved insecticidal properties were released in bollworm-infested cotton. One recombinant (HaCXW1) lacked the ecdysteroid UDP-glucosyltransferase (egt) gene and in another recombinant (HaCXW2), an insect-selective scorpion toxin (AaIT) gene replaced the egt gene. In a cotton field situation H. armigera larvae treated with either HaCXW1 or HaCXW2 were killed faster than larvae in HaSNPV-wt treated plots. Second instar H. armigera larvae, which were collected from HaCXW1 and HaCXW2 treated plots and further reared on artificial diet, showed reduced ST(50) values of 15.3 and 26.3%, respectively, as compared to larvae collected from HaSNPV-wt treated plots. The reduction in consumed leaf area of field collected larvae infected with HaCXW1 and HaCXW2 was approximated 50 and 63%, respectively, as compared to HaSNPV-wt infected larvae at 108 h after treatment. These results suggest that in a cotton field situation the recombinants will be more effective control agents of the cotton bollworm than wild-type HaSNPV.

Genetic engineering of Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus as an improved pesticide

The Helicoverpa armigera single-nucleocapsid nucleopolyhedrovirus (HearNPV) has been registered and is commercially produced in China as a biopesticide to control the bollworm in cotton. However, the virus has a relatively slow speed of action. To improve its efficacy, recombinant HearNPVs were generated by deleting the ecdysteroid UDP-glucosyltransferase (egt) gene (HaCXW1 and HaLM2) or by inserting the insect-specific toxin gene AaIT in the egt locus (HaCXW2) of HearNPV using conventional recombination strategies in insect cell culture. The various recombinants remained genetically stable when cultured in HzAM1 insect cells. Bioassay data showed a significant reduction in the time required for all HearNPV recombinants to kill second instar H. armigera larvae. The LT(50) of the egt deletion recombinants HaCXW1 and HaLM2 was about 27% faster than that of wild-type HearNPV. The largest reduction in LT(50) was achieved by inserting the gene for the insect-specific neurotoxin, AaIT, in the egt locus, giving a reduction in LT(50) of 32% compared to wild-type HearNPV. The ability to genetically improve the properties of HearNPV as a biopesticide provides a further opportunity to develop this virus into a commercially viable product to control the bollworm in China.

Mediated pathogenicity of the baculovirus AcMNPV by the venom from Euplectrus comstockii Howard (Hymenoptera: Eulophidae)

The compatibility of the venom from the parasitic species Euplectrus comstockii Howard (Hymenoptera: Eulophidae) with the pathogenicity of Autographa californica (Speyer) (Lepidoptera: Noctuidae) MNPV baculovirus (AcMNPV) was tested in third and fourth instar larvae of Trichoplusia ni (Hübner) (Lepidoptera: Noctuidae). The presence of AcMNPV did not alter the ability of the venom to arrest ecdysis in T. ni larvae. The presence of the venom delayed the rate of viruses by AcMNPV but increased the total mortality rates from days 9 to 14 in both third and fourth instar T. ni larvae. The delay in viruses was minimized by administering the virus prior to envenomation. In the presence of the venom, the final LD50 values were lower for fourth instar larvae than for third instar larvae. Surface response equations were developed to visualize the effect of the venom on the viruses caused by AcMNPV.

Assessment of the application of baculoviruses for control of Lepidoptera

Baculoviruses, among other insect viruses, are regarded as safe and selective bioinsecticides, restricted to invertebrates. They have been used worldwide against many insect pests, mainly Lepidoptera. Their application as microbial pesticides, however, has not met their potential to control pests in crops, forests, and pastures, with the exception of the nuclear polyhedrosis virus of the soybean caterpillar (Anticarsia gemmatalis), which is used on approximately 1 million ha annually in Brazil. Problems that have limited expansion of baculovirus use include narrow host range, slow killing speed, technical and economical difficulties for in vitro commercial production, timing of application based on frequent host population monitoring, variability of field efficacy due to climatic conditions, and farmers' attitudes toward pest control, which have been based on application of fast-killing chemical insecticides. Farmer education regarding use of biological insecticides and their characteristics is considered one of the major actions necessary for increased use of baculoviruses. Strategies to counteract some of the limitations of baculoviruses, especially their slow killing activity, have been investigated and are promising. These include the use of chemical or biological substances added to virus formulations and genetic engineering of the viruses themselves to express insect toxins or hormones. Such strategies can enhance viral activity and increase speed of kill as well as reduce larval feeding activity. The use of baculoviruses against Lepidoptera is reviewed, with the utilization of the nuclear polyhedrosis virus of A. gemmatalis in Brazil serving as a case-study.