Patent Application
20190159464
This application claims priority from South African provisional patent application number 2016/05197 filed on 26 Jul. 2016, which is incorporated by reference herein.
This invention relates to a biopesticide, more particularly a nucleopolyhedrovirus (NPV), for use in controlling insect populations.
Chemical pesticides are the most commonly used agents for controlling insect pests in agriculture and forestry. Chemical insecticides may have adverse impacts not only on their target organisms but also on beneficial insects, such as bees and other pollinators. Accumulation of chemical pesticides through the food chain of an ecosystem has also been shown to have an adverse effect on birds and other insect predators. Furthermore, residues of chemical pesticides on fruit, vegetables and nuts may cause health problems among the human population.
Concerns over the use of chemical pesticides have led to an increased interest in biological pesticides. Insect baculoviruses are one group of insect biopesticides that have been promoted as an alternative control agent to chemical pesticides. These viruses, which can be isolated from a variety of insect species, exhibit relatively narrow host ranges and have almost no adverse environmental impact due to their host specificity. The absence of chemical residues means that long-term environmental hazards and health concerns are reduced with biopesticides. However, there are several disadvantages to biopesticides, including cost of production, efficacy and stability.
Nucleopolyhedrovirus (NPV) or alphabaculovirus is a genus of baculoviruses that infect insects, mostly moths and butterflies. NPVs have been known to be useful as biopesticides for insect-infested crops. NPVs have a high species specificity and while this enhances their utility as biopesticides, it may limit their effectiveness in cases of multiple pest infestation. Furthermore, there are isolated incidents of species of moth pests developing resistance to commercially-available strains of baculoviruses.
Cryptophlebia peltastica (also known as litchi moth) is a species of moth that is an important pest of litchi trees (Litchi chinensis) in Southern Africa, Mauritius and elsewhere. C. peltastica larvae are known to damage up to 20% of fruits in commercial orchards. This moth is also a pest of macadamia and a variety of other commercially significant trees and plants. There are currently no known biopesticides that specifically target C. peltastica and to date, no NPVs have been isolated from C. peltastica or from any other species within the same genus or the lepidopteran tribe Grapholitini.
There is therefore a need for a biopesticide which is active against Cryptophlebia peltastica and/or other insect pests.
In accordance with a first aspect of the invention, there is provided a nucleopolyhedrovirus (NPV) for use in controlling insect populations, the NPV having a genome sequence with at least 85% sequence identity to SEQ ID NO: 2.
The NPV may have a polyhedrin gene with more than 93% sequence identity to SEQ ID NO: 1.
The NPV may be an isolate or genotype of the species whose genome sequence is represented by SEQ ID NO: 2.
The NPV may have a virulence host range comprising the Grapholitini tribe of the lepidopteran family, Tortricidae, and the host range may include Cryptophlebia peltastica, Thaumatotibia leucotreta, Cydia pomonella, Grapholita molesta, and Thaumatotibia batracopa.
The insects may be selected from the tribe Grapholitini, particularly the larvae of Cryptophlebia peltastica, Thaumatotibia (Cryptophlebia) leucotreta and Cydia pomonella.
The NPV may be used in combination with one or more additional biopesticides, and the one or more additional biopesticide may be a Cryptophlebia leucotreta granulovirus, a Cydia pomonella granulovirus, a Cryptophlebia batracopa granulovirus or a Grapholita molesta granulovirus.
The NPV genome sequence may contain only one recognition site for the restriction enzyme Sma1.
In accordance with a second aspect of the invention, there is provided a biopesticidal composition comprising an NPV as described above and one or more agronomically acceptable adjuvants and/or diluents.
The composition may comprise an additional biopesticide, which may be a Cryptophlebia leucotreta granulovirus, a Cydia pomonella granulovirus, a Cryptophlebia batracopa granulovirus or a Grapholita molesta granulovirus. The composition may further comprise a feeding stimulant, a mineral oil and a UV protectant.
In accordance with a third aspect of the invention, there is provided a method of controlling insect populations, the method comprising applying to the insects and/or their locus an insecticidally effective amount of an NPV as described above or a biopesticidal composition as described above.
The method may comprise simultaneously or sequentially applying, in addition to the NPV or biopesticidal composition, at least one additional biopesticide to the insects and/or their locus.
In accordance with a fourth aspect of the invention, there is provided a method of treating or preventing insect infestations on plants or fruit, the method comprising applying an insecticidally effective amount of an NPV as described above or a biopesticidal composition as described above to the plants or fruit.
In accordance with a fifth aspect of the invention, there is provided an NPV isolated from Cryptophlebia peltastica, which has a genome sequence with at least 85% sequence identity to SEQ ID NO: 2.
The NPV may have a polyhedrin gene with more than 93% sequence identity to SEQ ID NO: 1.
The invention provides a nucleopolyhedrovirus (NPV), also known as an alphabaculovirus, a biopesticidal composition comprising the NPV, and a method of controlling insect populations and infestations.
Throughout the specification, the term ‘plant’ is used in a general sense and is understood to refer to trees, shrubs and bushes, as well as the fruit, seeds and nuts of such trees, shrubs and bushes.
Moreover, the word ‘comprise’ or variations such as ‘comprises’ or ‘comprising’ is understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
Furthermore, ‘insecticidally effective amount’ is understood to refer to those quantities of virus which will result in a significant mortality rate in a treated group of insect pests as compared to an untreated group.
The definition of a baculovirus species as herein described is understood to refer to all isolates and genotypes that have a K-2-P distance between single and/or concatenated polh, lef-8 and lef-9 nucleotide sequences which is smaller than 0.015. K-2-P is the Kimura 2-parameter distance (K), which is determined by the Kimura two parameter model according to the following formula:
K=−1/2ln((1−2p−q)√{square root over (1−2q)})
where p is the proportion of nucleotide sites that show transitional differences and q is the proportion of nucleotide sites that show transversional differences. Transitions refer to substitutions between nucleotide bases of the same type (purine for purine or pyrimidine for pyrimidine), whereas transversions refer to substitutions of a purine for a pyrimidine or a pyrimidine for a purine. Furthermore, two viruses are considered to be different virus species if the K-2-P distance between single and/or concatenated sequences is larger than 0.050. For distances between 0.015 and 0.050, complementary information is needed to determine whether two viruses are the same or different species. This definition is explained in further detail in Jehle et al Virology 346 (2006) 180-193.
The NPV was isolated from larvae of the Cryptophlebia peltastica moth in which it proliferates and to which it is pathogenic. The virus produces polyhedrin proteins that form occlusion bodies (OBs or polyhedra), which are globular structures that protect the virus particles from the outside environment until they are ingested by an insect host. The NPV is characterized by its genome sequence which has at least 85% sequence identity to SEQ ID NO: 2. The NPV has a characteristic polyhedrin gene that has a nucleotide sequence with at least 75% sequence identity to SEQ ID NO: 1, with the proviso that SEQ ID NO: 1 does not encode the Epinotia granitalis baculovirus polyhedrin gene.
In some embodiments the NPV can be characterized by its genome sequence which can have at at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with SEQ ID NO: 2.
In further embodiments, the virus can have a polyhedrin gene with a sequence that has at least 80% sequence identity to SEQ ID NO: 1, and in further embodiments the virus can have a polyhedrin gene with at least 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with SEQ ID NO: 1. In some embodiments, the virus has a polyhedrin gene with a nucleotide sequence that has more than 93% sequence identity to SEQ ID NO: 1.
The NPV can be characterised in that it is an isolate or genotype of the NPV species whose genome sequence is represented by SEQ ID NO: 2.
The NPV may have a virulence host range that comprises the Grapholitini tribe of the lepidopteran family, Tortricidae. In particular, the NPV can have a virulence host range that includes members of the Grapholitini tribe selected from Cryptophlebia peltastica, Thaumatotibia leucotreta, Cydia pomonella, Grapholita molesta (oriental fruit moth), and Thaumatotibia batracopa (macadamia nut borer).
The nucleotide sequence of the NPV genome can have a characteristic single recognition site for the restriction enzyme, Sma1. This allows the NPV to be identified from other viruses that have more than one Sma1 recognition site.
The NPV can be applied in combination with one or more additional biopesticides, simultaneously, sequentially, or in a combined composition, in order to increase the spectrum of activity and to reduce the possibility of insect pests developing resistance to the virus. In particular, the NPV can be co-administered with another NPV or a betabaculovirus, also referred to as a granulovirus (GV), such as a Cryptophlebia leucotreta granulovirus, a Cydia pomonella granulovirus, a Cryptophlebia batracopa granulovirus or a Grapholita molesta granulovirus.
The NPV can be combined with one or more agronomically acceptable components, including adjuvants, diluents, surfactants, dispersing agents, emulsifiers, spreading agents and/or wetting agents to form a biopesticidal composition. The NPV can also be formulated to improve the stability or maintain the activity of the virus.
In particular, a composition comprising the NPV may additionally contain:
In a preferred embodiment, the composition can be formulated to comprise molasses, mineral oil, and one or more of the UV protectant Break-Thru® OE446, Uvinil and clove oil. In this and other embodiments, the formulation can further comprise glycerine and distilled water.
The minimum effective concentration of the virus is 1 OB, which can be sufficient to kill 50% of first instar larvae in a treated sample.
The biopesticidal composition can be formulated to have a concentration of the NPV of from about 1×103 to about 1×1015 viral occlusion bodies/ml. In some embodiments, the biopesticidal composition can have a concentration of the NPV of about 1×1010 viral OBs/ml.
A solution of the NPV can be produced on a commercial scale according to the following method. Jars containing cooked larval diet are placed into a biohazard chamber and sheets containing C. peltastica eggs each containing approximately 400 eggs are sterilized with formalin before being placed onto the diet in the rearing jars. The jars are then sealed. Approximately 80% of the rearing bottles containing 4th and 5th instar larvae are removed. The remaining 20% of the rearing bottles have their lids replaced with a cotton wool plug under the biohazard chamber and are left for a further ±8 d for the larvae to move out of the diet and begin pupation in the cotton wool substrate.
The pupated larvae are then removed from the jars and placed into containers containing approximately 10 mm of larval diet. Once the larvae cover the surface of the diet they are inoculated by spraying the surface of the diet with viral inoculum. The jars are then closed with a breathable lid and monitored. The diet and infected larvae are then harvested from the container after 7-8 days. The diet and harvested larvae are then homogenized in 4 litres of water per kilogram of diet and infected larvae (at pH 5.0), until the mixture appears uniformly smooth (approximately 20 min at 2400 rpm). The resulting liquid is sieved through a 100 micron nylon mesh into a clean container and re-homogenized after dilution with fresh water. The re-homogenized mixture is re-sieved and stored in cooling vats at 4° C. Storage bottles containing glycerol and distilled water are prepared before being filled with the virus suspension. The final product is stored at 4° C.
The biopesticidal composition can include one or more additional biopesticides, such as another NPV or GV. In some embodiments, the additional biopesticide is a Cryptophlebia leucotreta granulovirus, a Cydia pomonella granulovirus, a Cryptophlebia batracopa granulovirus or a Grapholita molesta granulovirus.
The NPV or composition is suitable for use as a biopesticide for controlling insect populations, particularly populations of moth and butterfly larvae of the tribe Grapholitini. Although isolated from C. peltastica, the virus also has activity against a range of other insect pests and can be used as a biopesticide for controlling Thaumatotibia leucotreta (false codling moth) and Cydia pomonella (codling moth) species. Thaumatotibia leucotreta is sometimes referred to as Cryptophlebia leucotreta and the two names can be used interchangeably. Furthermore, the virus can have activity against at least some strains of codling moth larvae that are resistant to Cydia pomonella granulovirus.
The NPV or composition can be applied to trees or plants as a biopesticide for treating or preventing insect infestations. Suitable examples include citrus (all varieties), grape (all varieties), peaches, plums, nectarines, apples, pears, walnuts, quinces, hazelnuts, pecans, macadamia, avocado, litchis, lychees, peppers, ornamentals, cotton, karalla, olives, grape vines, pepper bushes, pomegranates and persimmons.
The NPV or composition can be applied to the insects themselves and/or to the locus of the insects. For example, the NPV can be applied to the fruit, leaves, flowers and/or bark of the trees or plants which may be occupied or anticipated to be occupied by insect pests. In some embodiments, the NPV can be applied by spraying, although any suitable application method can be used.
The insecticidal activity or effectiveness of the NPV arises when an occlusion body (OB) of the virus is ingested by an insect host during feeding. Once inside the insect gut, the polyhedrin sac of the OB is solubilised by action of the alkaline digestive juices (pH 9.5 to 11.5) and by enzymatic degradation, which releases viral particles. Primary infection occurs in cells in the gut, followed by other cell types including haemocytes, tracheal cells, and fat cells. In advanced stages of viral infection, the cells contain multiple viral particles which are present in the nucleus. Eventually the nucleus and the cell rupture, liberating a plurality of viral OBs. The external appearance of infected larvae changes during viral infection. The larvae become sluggish and their growth is stunted. As infection progresses, the integument of the larvae may change colour. Eventually the integument ruptures and a greyish-white fluid is released containing multiple viral OBs. When healthy larvae feed on plant parts that have been contaminated with this fluid, they ingest the viral OBs and become infected themselves, leading to further propagation of the virus.
The invention will now be described in more detail by way of the following non-limiting examples.
Methods and Materials
A laboratory culture of C. peltastica was established at Rhodes University, Waainek research facility, South Africa. During rearing, larvae showing symptoms of ‘wilting’ disease were collected in microtubes and placed at −25° C. for future use. These symptoms were associated with colour change: milky pink appearance (early stage) and black/brown appearance (late stage).
In an attempt to isolate a baculovirus from the symptomatic larvae, a protocol for a granulovirus (GV) crude extract was adapted from Parnell et al. (2002). However, no virus was observed using this method and therefore an NPV protocol adapted from Grzywacz et al. (2007) was used instead. Insect cadavers were homogenised in 1 ml of 0.1% SDS in a microtube to disrupt the cuticle. The microtube was then vortexed for approximately 2 minutes. The homogenate was then centrifuged at 100×g for 10 to 20 seconds to remove insect debris. The supernatant was collected and placed into a separate microtube. The pellet was re-suspended in 1 ml 0.1% SDS and again centrifuged at 100×g for 10 to 20 seconds. The supernatant was pooled and the pellet discarded. The pooled supernatant was centrifuged at 2 500×g for 5 minutes to pellet the virus. The supernatant was discarded and the pellet was re-suspended in 1 ml ddH2O and centrifuged at 2 500×g for 5 minutes. The supernatant was again discarded and the pellet re-suspended in approximately 100 to 200 μl ddH2O.
Purification of Virus Occlusion Bodies by Glycerol Gradient Centrifugation
To distinguish the morphology of the baculovirus, occlusion bodies (OBs) were purified using a glycerol gradient. A 50 to 60% glycerol gradient was prepared in a microtube, following a method adapted from Grzywacz et al (2007), by placing 500 μl of 60% glycerol into a 2 ml tube overlaid with 500 μl of 50% glycerol. The pellet from a crude extract was re-suspended in 500 μl of 20% glycerol. 100 μl of this sample was then placed on top of the gradient and centrifuged at maximum speed for 15 minutes. Both the band and pellet were collected and re-suspended in 1 ml ddH2O and centrifuged at maximum (12 100×g) speed for 15 minutes. The supernatant was discarded and the pellet was re-suspended and centrifuged again as described above. The final pellet was re-suspended in ddH2O.
Transmission Electron Microscopy (TEM)
A drop of the prepared sample (±2 μl) was placed on a Forvar carbon coated grid for 30 seconds. Filter paper was used to drain off the excess sample. A drop of uranyl acetate was then placed on the grid and left for 20 seconds. Filter paper was used to remove the excess stain and the grid was left overnight. The grid was observed using a Zeiss Libra 120 transmission electron microscope at 80 000 kV. The images were analysed using Mega View (G2) Olympus analyses software. The size, width and diameter of the OBs was determined by measuring 100 OBs from the images taken.
CTAB DNA Extraction
A CTAB DNA extraction protocol was adapted from Aspinall et al. (2002). Using 200 μl of crude/purified occlusion bodies extracted, 90 μl of 1M sodium carbonate (Na2CO3) was added and incubated at 37° C. for 30 minutes. 120 μl Tris-HCl (1M, pH 6.8), 90 μl of 10% SDS and 50 μl proteinase K (25 mg/ml) were added and samples incubated for a further 30 minutes at 37° C. 10 μl RNAse A (10 mg/ml) was then added and incubated for another 30 minutes at 37° C. The solution was then centrifuged at 12 100×g for 3 minutes. The supernatant was transferred to a new 1.5 ml tube and the pellet was discarded. 400 μl of warm CTAB buffer was then added to the supernatant and incubated at 70° C. for 60 minutes. 400 μl of 4° C. chloroform was added and the sample centrifuged at 6 500×g for 10 minutes. The upper phase was transferred into a new 1.5 ml tube and 400 μl of −25° C. isopropanol was added. The sample was then left overnight at −25° C. The sample was then centrifuged at 12 100×g for 20 minutes and the supernatant discarded. 1 ml of cold 70% ethanol was added to the pellet and the sample centrifuged at 12 100×g for 5 minutes. The supernatant was discarded and the pellet was incubated at 50° C., until dry. The pellet was re-suspended in 20 μl 10 mM Tris-HCl (pH 8.0).
Polymerase Chain Reaction (PCR)
Degenerate oligonucleotide primers, prPH-1 and prPH-2 designed by Lange et al. (2004) were used to genetically characterise the isolated NPV (Table 1). The primers were used to amplify the polyhedrin gene using DNA extracted from the purified OBs.
PCR reactions were set up for a positive control, negative control and C. peltastica NPV. The positive control consisted of DNA extracted from CrleGV (Cryptophlebia leucotreta granulovirus) to ensure correct amplification with the degenerate primers and the negative control contained no DNA to determine the presence of contamination. The DNA concentration for C. peltastica NPV was determined using a Nanodrop spectrophotometer (Thermo Scientific®) to be approximately 84 ng/μl. This was considered when mixing the reagents.
PCR parameters used for the amplification of the polyhedrin gene were adapted from Lange et al. (2004) in order to obtain a PCR product of the 507 to 510 nucleotides for analysis. The PCR amplification was performed in a thermocycler (BIO-RAD®). The PCR products were analysed by 1% AGE and band sizes estimated using GeneRuler 1 Kbp DNA Ladder (Thermo Scienticfic®) to confirm the presence and size of the virus gene.
Sequencing
The PCR products obtained were sequenced by Inqaba Biotechnology Industries (Pty) Ltd (South Africa). FinchTV® version 1.4.0 (Geospiza Inc. 2004-2006) was used to view the chromatograms in order to clean-up ambiguous nucleotides. Following the clean-up, sequences were subjected to NCBI BLAST to determine a closely matched baculovirus sequence. Sequences were analysed in MEGA® 6 (Tamura et al. 2013) to determine single nucleotide polymorphisms (SNPs). Once SNPs were determined, the sequences were translated to determine if the SNPs resulted in amino acid changes.
Results
Symptomology
Collection of symptomatic larvae from the culture began once the culture reached high densities. Symptomatic larvae were found randomly during rearing, in medium to low densities depending on how crowded the larvae were and the time the infected larvae were found. Symptomatic larvae were found on the surface of the diet, within the diet and hanging from the sides of the containers. Early symptoms of the virus were observed by a colour change in the insects, commonly from milky pink to brown-black. Once larvae had reached the brown/black stage they were prone to rupturing as the cuticle became soft and liquefied. Caution was therefore taken when these larvae were collected to avoid rupturing the cuticle.
Crude Baculovirus Extraction and Transmission Electron Microscopy
Transmission electron microscopy of a crude extract confirmed the presence of a baculovirus. The OBs were observed as amorphous black structures clumped together. The images of the OBs show the characteristic features of a NPV (
Purification of Virus Occlusion Bodies by Glycerol Gradient Centrifugation
Glycerol gradient centrifugation was used to purify OBs from the crude extract. A band forming on top of the gradient and the pellet were collected and placed in separate microtubes. TEM was then completed on both samples. Very few to no OBs were observed in the band, with numerous pure OBs found in the pelleted sample.
The pelleted sample contained the majority of the NPV OBs. Even though the sample was found in the pellet where the insect debris is usually found, the sample was pure. OBs were observed and could be morphological characterised. As shown in
DNA Extraction Using NPV Genomic DNA
Genomic DNA was successfully extracted from C. peltastica NPV crude extract of OBs. The DNA was analysed on 0.7% agarose gel electrophoresis and found to be of high molecular weight (>10 000 bp) (
PCR Amplification of the Polyhedrin Gene Using Degenerate Primers
Genomic DNA obtained was used to amplify the polyhedrin gene using degenerate primers. The amplified product was analysed on 1% agarose gel electrophoresis, illustrated in
Sequencing Analysis
Sequences obtained were subjected to a BLAST, using a blastn analysis where a closest match of 93% was observed with a partial sequence of Epinotia granitalis baculovirus polyhedrin gene region (501 nucleotides). After sequence clean-up the sequence for C. peltastica NPV polH was found to be 507 nucleotides in length (SEQ ID NO:1). Following translation and alignment between the two sequences, several SNPs were observed. Five of these SNPs resulted in amino acid changes: Isoleucine to Valine, Phenylalanine to Tyrosine, Serine to Alanine, Valine to Isoleucine and Glutamic acid to Aspartic acid. The remaining SNPs were minor and did not result in any changes.
E. granitalis, the cypress bark moth, is native to Japan and is a pest of Japanese cedar and Japanese cypress (Takatsuka 2007). There is limited research on E. granitalis baculovirus and only a partial polh/gran sequence is available for comparison. The reasons as to why the polyhedrin gene sequences of these viruses are so closely matched are unknown because they infect different insect species that are not found in the same geographic region and they do not share the same hosts.
This is the first identification of a NPV infecting C. peltastica and further characterisation through whole genome sequencing and REN analysis of genomic DNA was conducted to investigate the novel status of this virus.
Genomic Analysis of Cryptophlebia Peltastica Nucleopolyhedrovirus
Further genetic characterisation of the isolated NPV was carried out using restriction endonuclease (REN) analysis and sequencing of the full genome. REN analysis is a technique used extensively in molecular biology for genetic engineering, genome mapping and DNA sequence analysis (Roberts 1979; Bikandi et al. 2004). REN analysis is also an important technique that can be used to compare the genotypes of different virus isolates for biological control purposes.
Restriction endonuclease analysis is used to create a genetic ‘fingerprint’ for isolated viruses. In order to produce this ‘fingerprint’ viral DNA is digested with the use of restriction enzymes. These enzymes cut the DNA strands at certain nucleotide sequences, resulting in several pieces of DNA strands. The fragments are then analysed by agarose gel electrophoresis, which separates them according to size creating a DNA profile. It is from these ‘fingerprints’ that viral isolates can be characterised in terms of genotype.
Restriction Endonuclease Analysis
Genomic DNA was extracted and used to create REN profiles, using the following fast digest restriction enzymes BamH1, EcoR1, Hind111, Kpn1, Pst1, Sal1, Sma1, Xba1 and Xho1 (Thermo Scientific®, USA). A 30 μl reaction was set up, comprising of 200 ng/μl of template DNA, 3 μl fast digest restriction enzyme, 3 μl fast digest buffer and the remaining volume made up with ddH2O. The reaction was incubated at 37° C. for 15 minutes. REN profiles were visualised using 0.6% agarose gel electrophoresis, which were electrophoresed at 30 volts for 16 hours in 1× TAE buffer and stained with ethidium bromide. Images of the REN profiles were captured using UVIpro chemi (UVItec) UV trans-illuminator. To determine the band sizes two DNA ladders were used, a GeneRuler high range ladder (Thermo Scientific®, USA) and GeneRuler 1 Kb DNA ladder (Thermo Scientific®, USA). UviBand software (UVItec) was used to estimate the band and genome sizes.
Restriction Endonuclease Analysis Profiles
None of the REN profiles of BamH1, EcoR1, Hind111, Kpn1, Pst1, Sal1, Xba1 and Xho1 produced a characteristic profile that could be used to easily distinguish CrpeNPV from similar baculoviruses. However, the REN profile produced by Sma1 digestion produced a characteristic profile containing a single band in the agarose gel electrophoresis gel (
As no other profiles of known CrpeNPV analogues were available for comparison, REN analysis could not be used to accurately characterise CrpeNPV. Therefore the whole genome was sequenced for a more accurate method of genetic characterisation.
Genome Sequencing
The full genome (SEQ ID NO:2) of CrpeNPV, using approximately 100 ng of DNA extracted from purified OBs, was sequenced by Inqaba Biotechnical Industries (Pty) Ltd (South Africa) using a MiSeq Desktop Sequencer (Illumina). The Illumina sequencing of the full genome of CrpeNPV produced a total of 3 871 946 paired reads. Geneious (New Zealand) version R7 (Kearse et al. 2012) was used to complete a de novo assembly. In order to produce a de novo assembly, the ends of the paired reads were trimmed using the soft trimming function in Geneious. Once the paired reads were trimmed a de novo assembly was run using 20% of the data and numerous contigs were produced. The longest contig was selected and a consensus sequence was generated to create a full genome sequence for CrpeNPV.
Analysis of the CrpeNPV Genome
A total of 3 871 946 paired reads were produced from the Illumina sequencing. Of these reads 760 297 were used to complete a de novo assembly, which produced 104 532 contigs. The largest contig, contig 1, was used to create a consensus sequence of 116 646 bp. The consensus sequence had a coverage of 731.6±390.8 (Mean±StDev) and a GC content of 37.1%. The full genome of CrpeNPV (SEQ ID NO: 2) was annotated using a database of 18 NPV full genome reference sequences. A total of 105 genes with complete coding regions (stop to start condons) were identified in the genome. The 105 genes were labelled according to ORF numbers, ORF1 starting at the polyhedrin gene.
The majority of the 31 core genes identified in all baculovirus genome were recognised in the CrpeNPV genome with the exception of gp64, p6.9, odvp-6e, p95, vp80/97, gp67, p87, lef-6, ptp-2, ptp-2, arif, vef, pcna, ie-2, lef-7, pe38 and lef-3. The gp64 gene is a distinguishing feature for NPVs, as NPVs are divided into two groups, group 1 and group 2. Group 1 NPVs express the gp64 gene and fusion protein, whereas group 2 NPVs lack the gp64 gene but contain the fusion protein. Therefore the CrpeNPV is a group 2 NPV since it lacks the gp64 gene.
The results of the BLAST search revealed that there are no known polyhedrin genes having a nucleotide sequence greater than 93% sequence identity to SEQ ID NO: 1 (the CrpeNPV polyhedrin gene sequence).
Biological Activity
The bioassays were used to determine the virulence of CrpeNPV against C. peltastica, T. leucotreta and C. pomonella using a surface dose bioassay method. The first objective of the bioassay study was to determine the concentration of the viral product required to cause 50% (LC50) and 90% (LC90) mortality by using a range of doses against all three species. The second objective was to determine the speed to kill 50% (LT50) and 90% (LT90) of the population using the LC90 value obtained from the dose-response assays.
Surface Dose-Response Bioassays with C. peltastica Neonate Larvae
Surface dose bioassays were conducted in glass vials. Each glass vial was filled with 2 g of false codling moth (FCM) artificial diet and 2 ml of distilled water, the diet autoclaved at 121° C. for 13 minutes. Six five-fold dilutions of purified OBs of CrpeNPV in sterilised distilled water were used for doses and sterile distilled water as a control. Forty eight larvae were treated per dose (48 glass vials per dose) and assays were replicated three times. A volume of 100 μl of each viral dilution and control was pipetted onto the centre of the diet surface. The fluid was spread evenly over the diet surface by rotating the vial at an angle. Inoculated glass vials were left for ±30 mins, until the diet had dried. One neonate larva was placed into each vial. All larvae were from the same batch of eggs, hatching at the same time. Vials were then closed with a lid and left upside down for approximately twelve hours to prevent larvae from escaping. Vials were kept in a CE room at 27° C. and a relative humidity of 60-80%. Vials were then turned upright. Approximately 24 hours later, lids were replaced with cotton wool.
After 7 days, vials were opened and diet was inspected. Larvae were recorded as dead or alive. The dose-response curve was calculated using PROBAN (Van Ark 1995) software used for probit analysis (Finney 1971). Mortality of control larvae were taken into consideration by PROBAN and corrected using Abbott's formula (Abbott 1925). From the analysis, LC50 and LC90 were calculated for each replicate. An average LC50 and LC90 were obtained from the means from the three assays.
Surface Dose-Response Bioassays with FCM Neonate Larvae
Surface dose bioassays were conducted in 24 well plates. Diet was prepared by adding 250 g of FCM artificial diet to a pyrex dish, 300 ml of dH2O was added to the diet. The diet was sterilised in an oven at 180° C. for 30 mins. Diet plugs were then cut out using a glass vial. Diet plugs were then pressed into the wells of the plate using a sterile glass rod. Six five-fold dilutions of purified OBs of CrpeNPV in sterilised distilled water were used for doses and sterile distilled water as a control. Forty eight larvae were treated per dose (two 24 well plates per dose) and assays were replicated three times. A volume of 50 μl of each viral dilution and control was pipetted onto the centre of the diet surface. The fluid was spread evenly over the diet surface by rotating the plates.
Inoculated plates were left for ±30 mins, until diet had dried. One neonate larva was placed into each well. All larvae were from the same batch of eggs, hatching at the same time. Assay plates were sealed with parafilm and kept in a CE room at 27° C. and a relative humidity of 60-80%.
After 7 days, plates were opened and diet was inspected. Larvae were recorded as dead or alive. The dose-response curve was calculated using PROBAN (Van Ark 1995) software used for probit analysis (Finney 1971). Mortality of control larvae were taken into consideration by PROBAN and corrected using Abbott's formula (Abbott 1925). From the analysis, LC50 and LC90 were calculated for each replicate. An average LC50 and LC90 were obtained from the means from the three assays.
Surface Dose-Response Bioassays with Codling Moth Neonate Larvae
Surface dose bioassays were conducted in 24 well plates. Diet was prepared by autoclaving 13 g of agar in 400 ml of distilled water. 235.27 g of pre-mixed bollworm diet was sterilised in an oven at 180° C. for 10 mins. A 1527 μl of propionic acid and 154 μl of phosphoric acid was mixed into the sterilised diet. Agar was then added to the diet, 200 to 400 ml of boiling water was added and mixed to form a uniform paste. Diet was then piped into the 24 well plates. Five five-fold dilutions of purified OBs of CrpeNPV in sterilised distilled water was used for doses and sterile distilled water as a control. Forty eight larvae were treated per dose (two 24 well plates per dose) and assays were replicated three times. A volume of 50 μl of each viral dilution and control was pipetted onto the centre of the diet surface. The fluid was spread evenly over the diet surface by rotating the plates. Inoculated plates were left for ±30 mins, until diet had dried. One neonate larva was placed into each well. All larvae were from the same batch of eggs, hatching at the same time. Wells were sealed with parafilm and kept in a CE room at 27° C. and a relative humidity of 60-80%.
After 7 days, plates were opened and diet was inspected. Larvae were recorded as dead or alive. The dose-response curve was calculated using PROBAN (Van Ark 1995) software used for probit analysis (Finney 1971). Mortality of control larvae were taken into consideration by PROBAN and corrected using Abbott's formula (Abbott 1925). From the analysis, LC50 and LC90 were calculated for each replicate. An average LC50 and LC90 were obtained from the means from the three assays.
Time-Response Bioassays
Time response bioassays were conducted in glass vials. Fifty glass vials were inoculated with 100 μl of sterile distilled water and 50 vials were inoculated with the LC90 (calculated from the dose-response bioassays) solution of purified CrpeNPV. The diet was left for ±30 mins, until it had dried adequately. One neonate larva was placed into each vial. Vials were sealed with a lid and the same process repeated as above and kept in a CE room at 27° C. and a relative humidity of 60-80%. Bioassays were replicated three times. After 16 hours, the vials were inspected for dead larvae. Subsequently, vials were checked every 8 hours until mortality had ceased. Diet was then dissected to determine whether the remaining vials contained dead or alive larvae. Time-response was analysed using a logit regression in STATISTICA version V12 (StatSoft 2013). From the results obtained, the LT50 and LT90 were calculated.
Results of Surface Dose-Response Bioassays
The dose response curves for LC50 and LC90 were determined for each species and isolate. The results are presented in Table 2 below.
T. leucotreta and C. pomonella (South African) and C. pomonella
C. peltastica
T. leucotreta
C. pomonella
The virulence of CrpeNPV against T. leucotreta and C. pomonella was slightly more fatal than that of their homologous baculoviruses, CrleGV and CpGV-SA. The LC50 and LC90 for CrleGV against its homologous host, T. leucotreta was found to be 4.095×103 OBs/ml and 1.185×105 OBs/ml respectively (Moore et al. 2011). The LC50 and LC90 for CpGV-SA against its homologous host, C. pomonella was found to be 1.632×103 OBs/ml and 1.163×105 OBs/ml respectively (Motsoeneng 2016).
The results illustrated in Table 3 demonstrate comparable activity of CrpeNPV against CpGV-sensitive and CpGV-resistant European codling moth isolates. M is a Mexican strain of CpGV; V015 is a resistance overcoming Mexican isolate of CpGV; I07R and I12 are Iranian isolates of CpGV; S is an isolate of CpGV isolated from the commercial product, Virosoft; E2 is an English isolate of CpGV; Madex Plus is a laboratory selected isolate from CpGV-resistant codling moth.
Results of Time-Response Bioassays
Speed to kill values LT50 and LT90 were determined for each species and are illustrated in Table 4 below.
C. peltastica had an LT50 of 73.44 hours and LT90 of 89.21 hours.
T. leucotreta had a LT50 of 80.69 hours and LT90 of 96.37 hours.
C. pomonella had a LT50 of 106.09 and LT90 of 125.52 hours.
T. leucotreta and C. pomonella.
C. peltastica
T. leucotreta
C. pomonella
Speed to kill values LT50 and LT90 were also determined for two Cydia pomonella cultures and against four viral isolates, as illustrated in Table 5 below.
The speed of kill for CrpeNPV against the heterologous hosts was greater than that of their homologous granuloviruses (Table 4). As the LT50 and LT90 for CrleGV against T. leucotreta was observed to be 118 hours and 176 hours (Moore et al. 2011) and a LT50 of 135 hours for CpGV-SA against C. pomonella (Motsoeneng 2016).
From the above results it was established that CrpeNPV was more virulent against the neonate T. leucotreta and C. pomonella larvae, specifically with a significantly faster speed of kill for both species. The mean LC and LT values were slightly less than for their own homologous baculovirus, CrleGV and CpGV-SA.
Test for Infectivity
Two discriminating virus concentrations (5.8×104 and 5.8×105 OBs/ml) were used to test the infectivity of CrpeNPV against three European codling moth (C. pomonella) cultures with varying susceptibilities to Cydia pomonella granulosis virus (CpGV): CpS (susceptible), CpRR1 (resistant) and Cp5M (more resistant). CrpeNPV caused 100% mortality for all three cultures at 5.8×104 and 5.8×105 OBs/ml (see
These results indicate that CrpeNPV has activity against a wide host range including CpGV-resistant species of moth pests. This could be useful when multiple lepidopterans infest the same host because not only will the primary pest be controlled but also the secondary pests. The secondary pests are usually only observed once the primary pest has been suppressed.
The biological activity of CrpeNPV was found to be more virulent against the heterologous hosts including CpGV-resistant codling moth species with a faster speed of kill. These results indicate that CrpeNPV has the potential to be commercially developed into a biopesticide for controlling insect populations of C. peltastica, T. leucotreta and C. pomonella in trees and plants, particularly in fruit (e.g. litchi, citrus, apple and pear) and nut (e.g. macadamia and walnut) trees. Additionally, CrpeNPV could also be used to control other lepidopteran pests in the Tortricidae family.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016/05197 | Jul 2016 | ZA | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2017/054543 | 7/26/2017 | WO | 00 |
