Diamondback moth ecology and management: problems, progress, and prospects

Sub-lethal effects of Vip3A toxin on survival, development and fecundity of Heliothis virescens and Plutella xylostella

The assessment of sub-lethal effects is important to interpret the overall insecticide efficacy in controlling insect pest populations. In addition to the lethal effect, sub-lethal effects may also occur in exposed insects. Vegetative insecticidal proteins (Vips) have shown a broad spectrum of insecticidal activity against many insect pest species. In this study the sub-lethal effects of the Bacillus thuringiensis vegetative insecticidal toxin Vip3A on the development and reproduction of Heliothis virescens F. and Plutella xylostella L. were evaluated in the laboratory. The results indicated that the sub-lethal concentration of Vip3A increased the duration of the larval and pupal stages as compared with the control treatment for both species. The percent pupation and percent adult emergence were significantly lower for Vip3A-treated insects. The proportion of pairs that produced eggs and the longevity of adults were not significantly different between treatments. H. virescens and P. xylostella treated with Vip3A showed an 11 and 17 % decrease in their intrinsic rate of increase (rm) respectively compared with untreated insects. The results from this study will be helpful to develop the strategy to incorporate Vip 3A containing crops in an integrated pest management programme.

Stable expression and functional characterisation of the diamondback moth ryanodine receptor G4946E variant conferring resistance to diamide insecticides

Diamides, such as flubendiamide and chlorantraniliprole, belong to a new chemical class of insecticides that act as conformation-sensitive activators of insect ryanodine receptors (RyRs). Both compounds are registered for use against lepidopteran species such as the diamondback moth, Plutella xylostella, a notorious global pest of cruciferous crops. Recently acquired resistance to diamide insecticides in this species is thought to be due to a target-site mutation conferring an amino acid substitution (G4946E), located within the trans-membrane domain of the RyR, though the exact role of this mutation has not yet been fully determined. To address this we have cloned a full-length cDNA encoding the P. xylostella RyR and established clonal Sf9 cell lines stably expressing either the wildtype RyR or the G4946E variant, in order to test the sensitivity to flubendiamide and chlorantraniliprole on the recombinant receptor. We report that the efficacy of both diamides was dramatically reduced in clonal Sf9 cells stably expressing the G4946E modified RyR, providing clear functional evidence that the G4946E RyR mutation impairs diamide insecticide binding.

Genome wide discovery of long intergenic non-coding RNAs in Diamondback moth (Plutella xylostella) and their expression in insecticide resistant strains

Long non-coding RNAs (lncRNAs) play important roles in genomic imprinting, cancer, differentiation and regulation of gene expression. Here, we identified 3844 long intergenic ncRNAs (lincRNA) in Plutella xylostella, which is a notorious pest of cruciferous plants that has developed field resistance to all classes of insecticides, including Bacillus thuringiensis (Bt) endotoxins. Further, we found that some of those lincRNAs may potentially serve as precursors for the production of small ncRNAs. We found 280 and 350 lincRNAs that are differentially expressed in Chlorpyrifos and Fipronil resistant larvae. A survey on P. xylostella midgut transcriptome data from Bt-resistant populations revealed 59 altered lincRNA in two resistant strains compared with the susceptible population. We validated the transcript levels of a number of putative lincRNAs in deltamethrin-resistant larvae that were exposed to deltamethrin, which indicated that this group of lincRNAs might be involved in the response to xenobiotics in this insect. To functionally characterize DBM lincRNAs, gene ontology (GO) enrichment of their associated protein-coding genes was extracted and showed over representation of protein, DNA and RNA binding GO terms. The data presented here will facilitate future studies to unravel the function of lincRNAs in insecticide resistance or the response to xenobiotics of eukaryotic cells.

miRNAs regulated overexpression of ryanodine receptor is involved in chlorantraniliprole resistance in Plutella xylostella (L.)

The amino acid mutations in ryanodine receptor (RyR) and elevated activity of detoxification enzymes have been associated with the diamide insecticide resistance in the diamondback moth, Plutella xylostella (L.). The up-regulation of P. xylostella RyR mRNA (PxRyR) expression has also been reported in field populations of different graphical origin. However, whether the up-regulation of PxRyR is involved in diamide resistance remains unknown. In this paper, 2.28- to 4.14-fold higher expression of PxRyR was detected in five field collected resistant populations, compared to that in a susceptible population. The expression of PxRyR was up-regulated 5.0- and 7.2-fold, respectively, after P. xylostella was treated with LC50 and LC75 of chlorantraniliprole for 12 h. Suppression of PxRyR using RNA interference restored the toxicity of chlorantraniliprole against the fourth instar larvae from the resistant population. More importantly, the expression of PxRyR is regulated by two miRNAs, miR-7a and miR-8519. These findings provide an empirical evidence of the involvement of miRNAs in the regulation of insecticide resistance, and shed light on the novel targets for the sustainable management of this devastating insect pest.

The novel ABC transporter ABCH1 is a potential target for RNAi-based insect pest control and resistance management

Insect pests cause serious crop damage and develop high-level resistance to chemical insecticides and Bacillus thuringiensis (Bt) insecticidal Cry toxins. A new promising approach for controlling them and overcoming this resistance is RNA interference (RNAi). The RNAi-based insect control strategy depends on the selection of suitable target genes. In this study, we cloned and characterized a novel ABC transporter gene PxABCH1 in diamondback moth, Plutella xylostella (L.). Phylogenetic analysis showed that PxABCH1 is closely related to ABCA and ABCG subfamily members. Spatial-temporal expression detection revealed that PxABCH1 was expressed in all tissues and developmental stages, and highest expressed in head and male adult. Midgut sequence variation and expression analyses of PxABCH1 in all the susceptible and Bt-resistant P. xylostella strains and the functional analysis by sublethal RNAi demonstrated that Cry1Ac resistance was independent of this gene. Silencing of PxABCH1 by a relatively high dose of dsRNA dramatically reduced its expression and resulted in larval and pupal lethal phenotypes in both susceptible and Cry1Ac-resistant P. xylostella strains. To our knowledge, this study provides the first insight into ABCH1 in lepidopterans and reveals it as an excellent target for RNAi-based insect pest control and resistance management.

A whole genome screening and RNA interference identify a juvenile hormone esterase-like gene of the diamondback moth, Plutella xylostella

Juvenile hormone (JH) plays a crucial role in preventing precocious metamorphosis and stimulating reproduction. Thus, its hemolymph titer should be under a tight control. As a negative controller, juvenile hormone esterase (JHE) performs a rapid breakdown of residual JH in the hemolymph during last instar to induce a larval-to-pupal metamorphosis. A whole genome of the diamondback moth (DBM), Plutella xylostella, has been annotated and proposed 11 JHE candidates. Sequence analysis using conserved motifs commonly found in other JHEs proposed a putative JHE (Px004817). Px004817 (64.61 kDa, pI=5.28) exhibited a characteristic JHE expression pattern by showing high peak at the early last instar, at which JHE enzyme activity was also at a maximal level. RNA interference of Px004817 reduced JHE activity and interrupted pupal development with a significant increase of larval period. This study identifies Px004817 as a JHE-like gene of P. xylostella.

Cry Proteins from Bacillus thuringiensis Active against Diamondback Moth and Fall Armyworm

Biopesticides based on Bacillus thuringiensis and genetically modified plants with genes from this bacterium have been used to control Plutella xylostella (L.) and Spodoptera frugiperda (J.E. Smith). However, the selection pressure imposed by these technologies may undermine the efficiency of this important alternative to synthetic insecticides. Toxins with different modes of action allow a satisfactory control of these insects. The purpose of this study was to characterize the protein and gene contents of 20 B. thuringiensis isolates from soil and insect samples collected in several areas of Northeast Brazil which are active against P. xylostella and S. frugiperda. Protein profiles were obtained by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Polymerase chain reaction assays were used to determine toxin genes present within bacterial isolates. The protein profile of the majority of the isolates produced bands of approximately 130 kDa, suggesting the presence of Cry1, Cry8 and Cry9 proteins. The gene content of the isolates of B. thuringiensis investigated showed different gene profiles. Isolates LIIT-4306 and LIIT-4311 were the most actives against both species, with LC50 of 0.03 and 0.02 × 10(8) spores mL(-1), respectively, for P. xylostella, and LC50 of 0.001 × 10(8) spores mL(-1) for S. frugiperda. These isolates carried the cry1, cry1Aa, cry1Ab, cry1Ac, cry1B, cry1C, cry1D, cry1F, cry2, cry2A, cry8, and cry9C genes. The obtained gene profiles showed great potential for the control of P. xylostella and S. frugiperda, primarily because of the presence of several cry1A genes, which are found in isolates of B. thuringiensis active against these insects.

Superfamily of genes encoding G protein-coupled receptors in the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae)

G protein-coupled receptors (GPCRs) are the largest and most versatile superfamily of cell membrane proteins, which mediate various physiological processes including reproduction, development and behaviour. The diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), is one of the most notorious insect pests, preferentially feeding on cruciferous plants. P. xylostella is not only one of the world's most widespread lepidopteran insects, but has also developed resistance to nearly all classes of insecticides. Although the mechanisms of insecticide resistance have been studied extensively in many insect species, few investigations have been carried out on GPCRs in P. xylostella. In the present study, we identified 95 putative GPCRs in the P. xylostella genome. The identified GPCRs were compared with their homologues in Bombyx mori and Drosophila melanogaster. Our results suggest that GPCRs in different insect species may have evolved by a birth-and-death process. One of the differences among compared insects is the duplication of short neuropeptide F receptor and adipokinetic hormone receptors in P. xylostella and B. mori. Another divergence is the decrease in quantity and diversity of the stress-tolerance gene, Mth, in P. xylostella. The evolution by the birth-and-death process is probably involved in adaptation to the feeding behaviour, reproduction and stress responses of P. xylostella. Some of the genes identified in the present study could be potential targets for the development of novel pesticides.

Resistance Selection and Characterization of Chlorantraniliprole Resistance in Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella (L.), is considered one of the most damaging lepidopteran pests, and it has developed resistance to all conventional insecticide classes in the field. Chlorantraniliprole is the first commercial insecticide that belongs to the new chemical class of diamide insecticides. But, P. xylostella have already shown resistance to chlorantraniliprole in China. After 52 generations of selection with chlorantraniliprole, ∼48.17-fold resistance was observed. The resistant strain showed cross-resistance to flubendiamide (7.29-fold), abamectin (6.11-fold), and cyantraniliprole (3.31-fold). Quantitative real-time polymerase chain reaction analysis showed that the expression of the ryanodine receptor gene was higher in the resistant strain than that in the susceptible strain. Enzyme assays indicated that cytochrome P450 activity in the resistant strain was 4.26 times higher compared with the susceptible strain, whereas no difference was seen for glutathione-S-transferase and esterase. Moreover, the toxicity of chlorantraniliprole in the resistant strain could be synergized by piperonyl butoxide, but not by diethyl maleate, and S,S,S-tributyl phosphorothioate. These results can serve as an important base for guiding the use of insecticide in field and delaying the development of pests that are resistant to the insecticides.

Generation-based life table analysis reveals manifold effects of inbreeding on the population fitness in Plutella xylostella

Understanding how inbreeding affects fitness is biologically important for conservation and pest management. Despite being a worldwide pest of many economically important cruciferous crops, the influence of inbreeding on diamondback moth, Plutella xylostella (L.), populations is currently unknown. Using age-stage-specific life tables, we quantified the inbreeding effects on fitness-related traits and demographic parameters of P. xylostella. Egg hatching rate, survival and fecundity of the inbred line significantly declined compared to those of the outbred line over time. The inbred P. xylostella line showed significantly lower intrinsic rate of increase (r), net reproduction rate (R₀), and finite increase rate (λ), and increasing generation time (T). Inbreeding effects vary with developmental stages and the fitness-related traits can be profoundly affected by the duration of inbreeding. Our work provides a foundation for further studies on molecular and genetic bases of the inbreeding depression for P. xylostella.

Pest control and resistance management through release of insects carrying a male-selecting transgene

Background

Development and evaluation of new insect pest management tools is critical for overcoming over-reliance upon, and growing resistance to, synthetic, biological and plant-expressed insecticides. For transgenic crops expressing insecticidal proteins from the bacterium Bacillus thuringiensis (‘Bt crops’) emergence of resistance is slowed by maintaining a proportion of the crop as non-Bt varieties, which produce pest insects unselected for resistance. While this strategy has been largely successful, multiple cases of Bt resistance have now been reported.

One new approach to pest management is the use of genetically engineered insects to suppress populations of their own species. Models suggest that released insects carrying male-selecting (MS) transgenes would be effective agents of direct, species-specific pest management by preventing survival of female progeny, and simultaneously provide an alternative insecticide resistance management strategy by introgression of susceptibility alleles into target populations. We developed a MS strain of the diamondback moth, Plutella xylostella, a serious global pest of crucifers. MS-strain larvae are reared as normal with dietary tetracycline, but, when reared without tetracycline or on host plants, only males will survive to adulthood. We used this strain in glasshouse-cages to study the effect of MS male P. xylostella releases on target pest population size and spread of Bt resistance in these populations.

Results

Introductions of MS-engineered P. xylostella males into wild-type populations led to rapid pest population decline, and then elimination. In separate experiments on broccoli plants, relatively low-level releases of MS males in combination with broccoli expressing Cry1Ac (Bt broccoli) suppressed population growth and delayed the spread of Bt resistance. Higher rates of MS male releases in the absence of Bt broccoli were also able to suppress P. xylostella populations, whereas either low-level MS male releases or Bt broccoli alone did not.

Conclusions

These results support theoretical modeling, indicating that MS-engineered insects can provide a powerful pest population suppressing effect, and could effectively augment current Bt resistance management strategies. We conclude that, subject to field confirmation, MS insects offer an effective and versatile control option against P. xylostella and potentially other pests, and may reduce reliance on and protect insecticide-based approaches, including Bt crops.

Insight into the Migration Routes of Plutella xylostella in China Using mtCOI and ISSR Markers

The larvae of the diamondback moth, Plutella xylostella, cause major economic losses to cruciferous crops, including cabbage, which is an important vegetable crop in China. In this study, we used the mitochondrial COI gene and 11 ISSR markers to characterize the genetic structure and seasonal migration routes of 23 P. xylostella populations in China. Both the mitochondrial and nuclear markers revealed high haplotype diversity and gene flow among the populations, although some degree of genetic isolation was evident between the populations of Hainan Island and other sampling sites. The dominant haplotypes, LX1 and LX2, differed significantly from all other haplotypes both in terms of the number of individuals with those haplotypes and their distributions. Haplotypes that were shared among populations revealed that P. xylostella migrates from the lower reaches of the Yangtze River to northern China and then to northeastern China. Our results also revealed another potential migration route for P. xylostella, i.e., from southwestern China to both northwestern and southern China.

Utilizing associational resistance for biocontrol: impacted by temperature, supported by indirect defence

Background

Associational herbivore resistance is potentiated by neighbouring heterogenic plant species that impact a focal plant’s attraction to herbivores or the damage that they cause. One mechanism to confer associational resistance is believed to be exposure to neighbour-emitted volatiles, the receivers of which range from intra- and interspecific neighbour plants to higher-trophic-level insects. In previous studies the passive adsorption of neighbour-emitted semivolatiles has been reported, but little is known regarding the mechanisms and ecological consequences on the receiver plant and its associated biota. To utilize volatile-based associational resistance for agricultural applications, it is imperative to know its effectiveness under varying diurnal temperatures and whether herbivore natural enemies, providing biological control, are impacted. Mimicking varying diurnal temperatures in a laboratory set-up, we assessed how the tritrophic model system Brassica oleracea var. italica (broccoli)–Plutella xylostella (crucifer specialist herbivore)–Cotesia vestalis (endoparasitoid of P. xylostella) is influenced by exposure to the natural semivolatile emitter plant Rhododendron tomentosum Harmaja.

Results

Rhododendron tomentosum-exposed B. oleracea was less susceptible to P. xylostella oviposition at both night-time (12°C) and day-time (22°C) temperatures and less favoured and damaged by P. xylostella larvae at 12°C. Exposure did not interfere with indirect defence, i.e. attraction of the natural enemy C. vestalis on host-damaged, R. tomentosum-exposed B. oleracea under 22°C, while there was a reduction in attraction (marginal preference towards host-damaged B. oleracea) under 12°C.

Conclusions

The ability of R. tomentosum exposure to render associational resistance against an agriculturally important Brassica herbivore P. xylostella without severely compromising the specialist parasitoid C. vestalis host location encourages further studies on the potential of using this naturally abundant plant for biocontrol. The generality of our finding on temperature as a potential regulating mechanism for the efficacy of semivolatile emitter-based associational resistance towards specialist pest larval damage should be further studied in natural and agricultural associations. Our study emphasizes the need to develop techniques to compare volatiles at the leaf versus air interface and associate their appearance and ecological role with times of activity and level of specialisation of herbivores and their natural enemies.

Chronic Sublethal Effects of Cantharidin on the Diamondback Moth Plutella xylostella (Lepidoptera: Plutellidae)

The diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae), is a major pest of cruciferous vegetables worldwide. Cantharidin, a natural toxin isolated from blister beetles, has been reported to be toxic to P. xylostella. However, little is known on the chronic sublethal effects of cantharidin on this species. In this study, we assessed the changes of susceptibility, development, reproduction and other demographic parameters in both the selected P. xylostella strain (Sub, selected by LC25 cantharidin for consecutive 12 generations) and the revertant strain (SubR, derived from the Sub strain without being exposed to cantharidin for 12 generations). Results revealed that the two strains maintained a relatively high-level susceptibility to cantharidin. Severe adverse effects on the population dynamics and fitness in Sub strain were observed. In addition, repeated exposure of P. xylostella to sublethal concentration of cantharidin resulted in negative effects on adult performance and deformities in adults. Although morphologically normal for individuals, the SubR strain exhibited a disadvantage in population growth rate. Our results showed that sublethal concentration of cantharidin exhibited severe negative effects on population growth for longtime. These findings would be useful for assessing the potential effects and risk of cantharidin on P. xylostella and for developing effective integrated pest management.

MAPK signaling pathway alters expression of midgut ALP and ABCC genes and causes resistance to Bacillus thuringiensis Cry1Ac toxin in diamondback moth

Insecticidal crystal toxins derived from the soil bacterium Bacillus thuringiensis (Bt) are widely used as biopesticide sprays or expressed in transgenic crops to control insect pests. However, large-scale use of Bt has led to field-evolved resistance in several lepidopteran pests. Resistance to Bt Cry1Ac toxin in the diamondback moth, Plutella xylostella (L.), was previously mapped to a multigenic resistance locus (BtR-1). Here, we assembled the 3.15 Mb BtR-1 locus and found high-level resistance to Cry1Ac and Bt biopesticide in four independent P. xylostella strains were all associated with differential expression of a midgut membrane-bound alkaline phosphatase (ALP) outside this locus and a suite of ATP-binding cassette transporter subfamily C (ABCC) genes inside this locus. The interplay between these resistance genes is controlled by a previously uncharacterized trans-regulatory mechanism via the mitogen-activated protein kinase (MAPK) signaling pathway. Molecular, biochemical, and functional analyses have established ALP as a functional Cry1Ac receptor. Phenotypic association experiments revealed that the recessive Cry1Ac resistance was tightly linked to down-regulation of ALP, ABCC2 and ABCC3, whereas it was not linked to up-regulation of ABCC1. Silencing of ABCC2 and ABCC3 in susceptible larvae reduced their susceptibility to Cry1Ac but did not affect the expression of ALP, whereas suppression of MAP4K4, a constitutively transcriptionally-activated MAPK upstream gene within the BtR-1 locus, led to a transient recovery of gene expression thereby restoring the susceptibility in resistant larvae. These results highlight a crucial role for ALP and ABCC genes in field-evolved resistance to Cry1Ac and reveal a novel trans-regulatory signaling mechanism responsible for modulating the expression of these pivotal genes in P. xylostella.

Biopesticide and transgenic crops based on Bacillus thuringiensis (Bt) Cry toxins are widely used worldwide, yet the development of field resistance seriously threatens their sustainability. Unraveling these resistance mechanisms are of great importance for delaying insect field resistance evolution. The diamondback moth was the first insect to evolve field resistance to Bt biopesticides and it is an excellent model for the study of Bt resistance mechanisms. In this work, we present strong empirical evidence supporting that (1) field-evolved resistance to Bt in P. xylostella is tightly associated with differential expression of a membrane-bound alkaline phosphatase (ALP) and a suite of ATP-binding cassette transporter subfamily C (ABCC) genes, and (2) a constitutively transcriptionally-activated upstream gene (MAP4K4) in the MAPK signaling pathway is responsible for this trans-regulatory signaling mechanism. These findings identify key resistance genes and provide the first comprehensive mechanistic description responsible for the field-evolved Bt resistance in P. xylostella. Given that expression alterations of multiple receptor genes result in Bt resistance in many other insects, it can now be tested to determine whether the previously unidentified trans-regulatory mechanism characterized in this study is also involved in these cases.

Influences of Cry1Ac broccoli on larval survival and oviposition of diamondback moth

Larval survival and oviposition behavior of three genotypes of diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), (homozygous Cry1Ac-susceptibile, Cry1Ac-resistant, and their F1 hybrids), on transgenic Bacillus thuringiensis (Bt) broccoli expressing different levels of Cry1Ac protein were evaluated in laboratory. These Bt broccoli lines were designated as relative low, medium, and high, respectively, according to the Cry1Ac content. Untransformed brocccoli plants were used as control. Larval survival of diamondback moth on non-Bt leaves was not significantly different among the three genotypes. The Cry1Ac-resistant larvae could survive on the low level of Bt broccoli plants, while Cry1Ac-susceptible and F1 larvae could not survive on them. The three genotypes of P. xylostella larvae could not survive on medium and high levels of Bt broccoli. In oviposition choice tests, there was no significant difference in the number of eggs laid by the three P. xylostella genotypes among different Bt broccoli plants. The development of Cry1Ac-susceptible and Cry1Ac-resistant P. xylostella on intact Bt plants was also tested in greenhouse. All susceptible P. xylostella larvae died on all Bt plants, while resistant larvae could survive on broccoli, which expresses low Cry1Ac protein under greenhouse conditions. The results of the greenhouse trials were similar to that of laboratory tests. This study indicated that high dose of Bt toxins in broccoli cultivars or germplasm lines is required for effective resistance management.

Baseline Susceptibility of Plutella xylostella (Lepidoptera: Plutellidae) to the Novel Insecticide Spinetoram in China

Spinetoram is a spinosyn, which is a unique class of natural insecticide. Because of its novel mode of action, spinetoram is more potent and faster acting than other insecticides, even the older spinosyn product, spinosad. On account of being efficient on insect order Lepidoptera, spinetoram provides a new alternative for control of Plutella xylostella (L.) (Lepidoptera: Plutellidae), which are resistant to other chemicals. To determine the current situation of resistance of P. xylostella to spinetoram, the susceptibility of 16 P. xylostella populations from different regions of China or different time in addition to the population from laboratory was assessed using a leaf dip bioassay. The variation in spinetoram susceptibility among the 16 field populations was narrow, with median lethal concentrations (LC50 values) ranging from 0.131 to 1.001 mg/liter. Toxicity ratios (TRs) ranged from 1.5 to 7.6 and were 5.6 and 7.6 for populations SY-2 and FX-1, respectively, indicating some low level of tolerance in these populations. A discriminating concentration (a concentration that can detect the occurrence of resistance in a population) of 10 mg/liter, which was identified based on the pooled toxicological data, caused 100% mortality in all nine tested populations. The baseline susceptibility data reflect the natural variation of the P. xylostella populations to spinetoram rather than variation caused by previous exposure.

Down-regulation of a novel ABC transporter gene (Pxwhite) is associated with Cry1Ac resistance in the diamondback moth, Plutella xylostella (L.)

Biopesticides or transgenic crops based on Cry toxins from the soil bacterium Bacillus thuringiensis (Bt) effectively control agricultural insect pests. The sustainable use of Bt biopesticides and Bt crops is threatened, however, by the development of Cry resistance in the target pests. The diamondback moth, Plutella xylostella (L.), is the first pest that developed resistance to a Bt biopesticide in the field, and a recent study has shown that the resistance of P. xylostella to Cry1Ac is caused by a mutation in an ATP-binding cassette (ABC) transporter gene (ABCC2). In this study, we report that down-regulation of a novel ABC transporter gene from ABCG subfamily (Pxwhite) is associated with Cry1Ac resistance in P. xylostella. The full-length cDNA sequence of Pxwhite was cloned and analyzed. Spatial-temporal expression detection revealed that Pxwhite was expressed in all tissues and developmental stages, and highest expressed in Malpighian tubule tissue and in egg stage. Sequence variation analysis of Pxwhite indicated the absence of constant non-synonymous mutations between susceptible and resistant strains, whereas midgut transcript analysis showed that Pxwhite was remarkably reduced in all resistant strains and further reduced when larvae of the moderately resistant SZ-R strain were subjected to selection with Cry1Ac toxin. Furthermore, RNA interference (RNAi)-mediated suppression of Pxwhite gene expression significantly reduced larval susceptibility to Cry1Ac toxin, and genetic linkage analysis confirmed that down-regulation of Pxwhite gene is tightly linked to Cry1Ac resistance in P. xylostella. To our knowledge, this is the first report indicating that Pxwhite gene is involved in Cry1Ac resistance in P. xylostella.

Identification and characterisation of multiple glutathione S-transferase genes from the diamondback moth, Plutella xylostella

BACKGROUND

The diamondback moth (DBM), Plutella xylostella, is one of the most harmful insect pests on crucifer crops worldwide. In this study, 19 cDNAs encoding glutathione S-transferases (GSTs) were identified from the genomic and transcriptomic database for DBM (KONAGAbase) and further characterized.

RESULTS

Phylogenetic analysis showed that the 19 GSTs were classified into six different cytosolic classes, including four in delta, six in epsilon, three in omega, two in sigma, one in theta and one in zeta. Two GSTs were unclassified. RT-PCR analysis revealed that most GST genes were expressed in all developmental stages, with higher expression in the larval stages. Six DBM GSTs were expressed at the highest levels in the midgut tissue. Twelve purified recombinant GSTs showed varied enzymatic properties towards 1-chloro-2,4-dinitrobenzene and glutathione, whereas rPxGSTo2, rPxGSTz1 and rPxGSTu2 had no activity. Real-time quantitative PCR revealed that expression levels of the 19 DBM GST genes were varied and changed after exposure to acephate, indoxacarb, beta-cypermethrin and spinosad. PxGSTd3 was significantly overexpressed, while PxGSTe3 and PxGSTs2 were significantly downregulated by all four insecticide exposures.

CONCLUSION

The changes in DBM GST gene expression levels exposed to different insecticides indicate that they may play individual roles in tolerance to insecticides and xenobiotics.

Characterization and expression of the cytochrome P450 gene family in diamondback moth, Plutella xylostella (L.)

Cytochrome P450 monooxygenases are present in almost all organisms and can play vital roles in hormone regulation, metabolism of xenobiotics and in biosynthesis or inactivation of endogenous compounds. In the present study, a genome-wide approach was used to identify and analyze the P450 gene family of diamondback moth, Plutella xylostella, a destructive worldwide pest of cruciferous crops. We identified 85 putative cytochrome P450 genes from the P. xylostella genome, including 84 functional genes and 1 pseudogene. These genes were classified into 26 families and 52 subfamilies. A phylogenetic tree constructed with three additional insect species shows extensive gene expansions of P. xylostella P450 genes from clans 3 and 4. Gene expression of cytochrome P450s was quantified across multiple developmental stages (egg, larva, pupa and adult) and tissues (head and midgut) using P. xylostella strains susceptible or resistant to insecticides chlorpyrifos and fiprinol. Expression of the lepidopteran specific CYP367s predominantly occurred in head tissue suggesting a role in either olfaction or detoxification. CYP340s with abundant transposable elements and relatively high expression in the midgut probably contribute to the detoxification of insecticides or plant toxins in P. xylostella. This study will facilitate future functional studies of the P. xylostella P450s in detoxification.

Characterization and expression profiling of glutathione S-transferases in the diamondback moth, Plutella xylostella (L.)

Background

Glutathione S-transferases (GSTs) are multifunctional detoxification enzymes that play important roles in insects. The completion of several insect genome projects has enabled the identification and characterization of GST genes over recent years. This study presents a genome-wide investigation of the diamondback moth (DBM), Plutella xylostella, a species in which the GSTs are of special importance because this pest is highly resistant to many insecticides.

Results

A total of 22 putative cytosolic GSTs were identified from a published P. xylostella genome and grouped into 6 subclasses (with two unclassified). Delta, Epsilon and Omega GSTs were numerically superior with 5 genes for each of the subclasses. The resulting phylogenetic tree showed that the P. xylostella GSTs were all clustered into Lepidoptera-specific branches. Intron sites and phases as well as GSH binding sites were strongly conserved within each of the subclasses in the GSTs of P. xylostella. Transcriptome-, RNA-seq- and qRT-PCR-based analyses showed that the GST genes were developmental stage- and strain-specifically expressed. Most of the highly expressed genes in insecticide resistant strains were also predominantly expressed in the Malpighian tubules, midgut or epidermis.

Conclusions

To date, this is the most comprehensive study on genome-wide identification, characterization and expression profiling of the GST family in P. xylostella. The diversified features and expression patterns of the GSTs are inferred to be associated with the capacity of this species to develop resistance to a wide range of pesticides and biological toxins. Our findings provide a base for functional research on specific GST genes, a better understanding of the evolution of insecticide resistance, and strategies for more sustainable management of the pest.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1343-5) contains supplementary material, which is available to authorized users.

Construction and characterisation of near-isogenic Plutella xylostella (Lepidoptera: Plutellidae) strains resistant to Cry1Ac toxin

BACKGROUND

Resistance to insecticidal Bacillus thuringiensis (Bt) toxins has arisen in multiple populations of the worldwide Brassica pest Plutella xylostella (L.). To help elucidate the mechanism of resistance to Bt Cry1Ac toxin in a population from Florida, two pairs of near-isogenic lines (NILs) were developed.

RESULTS

NILs were generated using either backcross or recombinant inbred line methodologies and evaluated for near-isogenicity with inter-simple-sequence-repeat (ISSR) markers. Backcross line BC6F4 maintained a similar level of Cry1Ac resistance to parental strain DBM1Ac-R (>5000-fold) yet showed 98.24% genetic similarity to the susceptible parental strain DBM1Ac-S. Single-pair backcrosses between DBM1Ac-S and BC6F4 revealed that Cry1Ac resistance was controlled by one recessive autosomal locus. BC6F4 exhibited high levels of cross-resistance to Cry1Ab and Cry1Ah but not to Cry1Ca or Cry1Ie.

CONCLUSION

Near-isogenic strains were constructed to provide a reliable biological system to investigate the mechanism of Cry1Ac resistance in P. xylostella. These data suggest that resistance to Cry1Ac, Cry1Ab and Cry1Ah is probably caused by the alteration of a common receptor not recognised by Cry1Ca or Cry1Ie. Understanding Bt toxin cross-resistance provides valuable information to consider when developing pest control strategies to delay resistance evolution. © 2014 Society of Chemical Industry.

The midgut cadherin-like gene is not associated with resistance to Bacillus thuringiensis toxin Cry1Ac in Plutella xylostella (L.)

The Gram-positive bacterium Bacillus thuringiensis (Bt) produces Cry toxins that have been used to control important agricultural pests. Evolution of resistance in target pests threatens the effectiveness of these toxins when used either in sprayed biopesticides or in Bt transgenic crops. Although alterations of the midgut cadherin-like receptor can lead to Bt Cry toxin resistance in many insects, whether the cadherin gene is involved in Cry1Ac resistance of Plutella xylostella (L.) remains unclear. Here, we present experimental evidence that resistance to Cry1Ac or Bt var. kurstaki (Btk) in P. xylostella is not due to alterations of the cadherin gene. The bona fide P. xylostella cadherin cDNA sequence was cloned and analyzed, and comparisons of the cadherin cDNA sequence among susceptible and resistant P. xylostella strains confirmed that Cry1Ac resistance was independent of mutations in this gene. In addition, real-time quantitative PCR (qPCR) indicated that cadherin transcript levels did not significantly differ among susceptible and resistant P. xylostella strains. RNA interference (RNAi)-mediated suppression of cadherin gene expression did not affect larval susceptibility to Cry1Ac toxin. Furthermore, genetic linkage assays using four cadherin gDNA allelic biomarkers confirmed that the cadherin gene is not linked to resistance against Cry1Ac in P. xylostella. Taken together, our findings demonstrate that Cry1Ac resistance of P. xylostella is independent of the cadherin gene.

Resistance selection and molecular mechanisms of cypermethrin resistance in red hairy caterpillar (Amsacta albistriga walker)

Amsacta albistriga is one of the important pests of oilseed crops in India. This pest has developed high resistance to organophosphate (OP) insecticide in field. Therefore, cypermethrin insecticide was used as an alternative for this pest. After 20 generations of selection with cypermethrin, the LD50 value for A. albistriga was increased by 21.5-folds. The synergism ratio of piperonyl butoxide (PBO) and triphenyl phosphate (TPP) was increased by 10- and 9.6-fold in resistant strains and comparatively, 3.9 and 4.2-fold in susceptible strains. Detoxification enzyme analysis and native PAGE electrophoresis of esterase isoenzyme further revealed that esterase and mixed function oxidase may be involved in cypermethrin resistance in CypRes strain. In addition to enzyme analysis overexpression of CYP4M44, CYP9A77 and CYP6B47 (ortholog) can confer metabolic resistance in the CypRes strain. These data provide a foundation for further study of cypermethrin resistance mechanism observed in A. albistriga.

Large-scale detection and analysis of adenosine-to-inosine RNA editing during development in Plutella xylostella

Adenosine-to-inosine (A-to-I) RNA editing is site-specific modification of RNAs that increases the diversity of the transcriptome and proteome. Most insects undergo complete metamorphosis, including four life cycle stages: egg, larva, pupa and adult. Many previous studies have confirmed that RNA-editing events occur in a development-specific manner; in other words, RNA-editing levels change during metamorphosis. Here, we describe an effort to identify the developmental specificity of RNA-editing events using a large-scale computational analysis of RNA-seq data derived from four developmental stages of the diamondback moth, Plutella xylostella. One thousand one hundred and eighty-seven A-to-I RNA-editing sites were predicted to be developmental stage specific (false-discovery rate <0.01) and 1,094 of these sites were located in protein-coding regions. Editing of 152 sites resulted in an altered amino acid residue. A putative adult-specific A-to-I RNA-editing site was verified by comparing cDNA sequences with its corresponding genomic locus at different stages of the P. xylostella life cycle. Our findings will help elucidate the role of A-to-I RNA editing in the regulation of metamorphosis. Further studies detailing changes in the extent of editing are needed to establish how as yet unknown regulatory factors are involved in the editing mechanism and what biological functions' editing serves.

Identification of a novel cytochrome P450 gene, CYP321E1 from the diamondback moth, Plutella xylostella (L.) and RNA interference to evaluate its role in chlorantraniliprole resistance

Insect cytochrome P450 monooxygenases (P450s) play an important role in catalysis of many reactions leading to insecticides resistance. Our previous studies on transcriptome analysis of chlorantraniliprole-resistant development in the diamondback moth, Plutella xylostella revealed that up-regulation of cytochrome P450s are one of the main factors leading to the development of chlorantraniliprole resistance. Here, we report for the first time a novel cytochrome P450 gene CYP321E1, which belongs to the cytochrome P450 gene family CYP321. Real-time quantitative PCR (RT-qPCR) analyses indicated that CYP321E1 was expressed at all developmental stages of P. xylostella but was highest in the fourth-instar larvae; furthermore, the relatively high expression was observed in the midgut of the fourth-instar larvae, followed by fat bodies and epidermis. The expression of CYP321E1 in P. xylostella was differentially affected by three representative insecticides, including alphamethrin, abamectin and chlorantraniliprole. Among them, the exposure to chlorantraniliprole resulted in the largest transcript level of this cytochrome P450 gene. The findings suggested potential involvement of CYP321E1 in chlorantraniliprole resistance of P. xylostella. To assess the functional link of CYP321E1 to chlorantraniliprole resistance, RNA interference (RNAi)-mediated gene silencing by double stranded RNA (dsRNA) injecting was used. Results revealed that injection delivery of dsRNA can greatly reduce gene expression after 24 h. As a consequence of RNAi, a significant increment in mortality of larvae injected CYP321E1 dsRNA was observed after 24 h of exposure to chlorantraniliprole. These results strongly support our notion that this novel cytochrome P450 gene plays an important role in chlorantraniliprole detoxification in the diamondback moth and is partly responsible for its resistance.

Dominant fitness costs of abamectin resistance in Plutella xylostella

BACKGROUND

The TH-Abm strain of Plutella xylostella, exhibiting 23 670-fold resistance to abamectin, was selected from a field-evolved multiresistant population. By repeated backcrossing to a susceptible strain (Roth) and selection with abamectin, the resistance trait of TH-Abm was introgressed into Roth to generate a near-isogenic strain (Roth-Abm). Fitness costs associated with abamectin resistance were examined in Roth-Abm.

RESULTS

Compared with Roth, Roth-Abm obtained 11 500-fold resistance to abamectin and 364 000-, 12- and 12-fold cross-resistance to emamectin benzoate, spinosad and fipronil respectively. Roth-Abm has a significantly longer pupal development time, lesser female pupal weight and lower larval survival than Roth. Female fecundity and egg viability are significantly lower in Roth-Abm than in Roth. All of the above fitness components of the F1 progeny from Roth × Roth-Abm are similar to those of Roth-Abm and are significantly lower than those of Roth. By comparing with the net replacement rate (R0 ) of Roth, the fitness of Roth-Abm, F1a (Roth male × Roth-Abm) and F1b (Roth female × Roth-Abm) are 0.50, 0.50 and 0.53 respectively.

CONCLUSION

Abamectin resistance in Roth-Abm results in significant fitness costs, and the fitness costs are autosomal and dominant. Rotation of abamectin with other insecticides without cross-resistance could be especially useful for delaying abamectin resistance in P. xylostella.

Preference of Diamondback Moth Larvae for Novel and Original Host Plant after Host Range Expansion

Utilization of a novel plant host by herbivorous insects requires coordination of numerous physiological and behavioral adaptations in both larvae and adults. The recent host range expansion of the crucifer-specialist diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Plutellidae), to the sugar pea crop in Kenya provides an opportunity to study this process in action. Previous studies have shown that larval ability to grow and complete development on sugar pea is genetically based, but that females of the pea-adapted strain do not prefer to oviposit on pea. Here we examine larval preference for the novel host plant. Larvae of the newly evolved pea-adapted host strain were offered the choice of the novel host plant sugar pea and the original host cabbage. These larvae significantly preferred pea, while in contrast, all larvae of a cabbage-adapted DBM strain preferred cabbage. However, pea-adapted larvae, which were reared on cabbage, also preferred cabbage. Thus both genetic differences and previous exposure affect larval host choice, while adult choice for the novel host has not yet evolved.

New threshold temperatures for the development of a North American diamondback moth (Lepidoptera: Plutellidae) population and its larval parasitoid, Diadegma insulare (Hymenoptera: Ichneumonidae)

The currently accepted lower threshold temperature for the development of diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae), the world's most destructive insect pest of cruciferous crops, is around 6.0°C, and there is no known upper threshold temperature. Neither are there established threshold temperatures for diamondback moth's major natural enemy, Diadegma insulare (Hymenoptera: Ichneumonidae). Laboratory studies were undertaken to determine the survival and development of a North American diamondback moth population and its parasitoid D. insulare at 20 constant temperatures ranging from 2.0 to 38.0°C. Diamondback moth completed development from second instar to adult within a temperature range of 4.0-37°C, and D. insulare completed its life cycle from egg to adult within a temperature range of 4.0-33°C. The developmental data were fitted into one linear and four nonlinear models. Using goodness-of-fit and the ability to estimate parameters of biological significance as selection criteria, the Wang model was the most acceptable among the nonlinear models to describe the relationship between temperature and development of both species. According to this model, the lower and upper threshold temperatures for diamondback moth were 2.1 and 38.0°C, respectively, and for D. insulare they were 2.1 and 34.0°C, respectively. Based on the Degree Day model, diamondback moth required 143 d above the lower threshold of 4.23°C to complete the life cycle, while D. insulare required 286 d above the lower threshold of 2.57°C. This study suggests that temperatures during the crop-growing seasons in North America are not limiting factors for development of either diamondback moth or D. insulare.

A specific glycerol kinase induces rapid cold hardening of the diamondback moth, Plutella xylostella

Insects in temperate zones survive low temperatures by migrating or tolerating the cold. The diamondback moth, Plutella xylostella, is a serious insect pest on cabbage and other cruciferous crops worldwide. We showed that P. xylostella became cold-tolerant by expressing rapid cold hardiness (RCH) in response to a brief exposure to moderately low temperature (4°C) for 7h along with glycerol accumulation in hemolymph. Glycerol played a crucial role in the cold-hardening process because exogenously supplying glycerol significantly increased the cold tolerance of P. xylostella larvae without cold acclimation. To determine the genetic factor(s) responsible for RCH and the increase of glycerol, four glycerol kinases (GKs), and glycerol-3-phosphate dehydrogenase (PxGPDH) were predicted from the whole P. xylostella genome and analyzed for their function associated with glycerol biosynthesis. All predicted genes were expressed, but differed in their expression during different developmental stages and in different tissues. Expression of the predicted genes was individually suppressed by RNA interference (RNAi) using double-stranded RNAs specific to target genes. RNAi of PxGPDH expression significantly suppressed RCH and glycerol accumulation. Only PxGK1 among the four GKs was responsible for RCH and glycerol accumulation. Furthermore, PxGK1 expression was significantly enhanced during RCH. These results indicate that a specific GK, the terminal enzyme to produce glycerol, is specifically inducible during RCH to accumulate the main cryoprotectant.

Functional analysis of a point mutation in the ryanodine receptor of Plutella xylostella (L.) associated with resistance to chlorantraniliprole

BACKGROUND

The diamondback moth, Plutella xylostella (L.) has developed extremely high resistance to chlorantraniliprole and other diamide insecticides in the field. A glycine to glutamic acid substitution (G4946E) in the P. xylostella ryanodine receptor (PxRyR) has been found in two resistant populations collected in Thailand and Philippines and was considered associated with the diamide insecticides resistance but no experimental evidence was provided. The present study aimed to clarify the function of the reported mutation in chlorantraniliprole resistance in P. xylostella.

RESULTS

We identified the same mutation (G4946E) in PxRyR from four field collected chlorantraniliprole resistant populations of Plutella xylostella in China. Most importantly, we found that the frequency of the G4946E mutation is significantly correlated to the chlorantraniliprole resistance ratios in P. xylostella (R(2)  = 0.82, P = 0.0003). Ligand binding assays showed that the binding affinities of the PxRyR to the chlorantraniliprole in three field resistant populations were 2.41-, 2.54- and 2.60-times lower than that in the susceptible one.

CONCLUSION

For the first time we experimentally proved that the G4946E mutation in PxRyR confers resistance to chlorantraniliprole in Plutella xylostella. These findings pave the way for the complete understanding of the mechanisms of diamide insecticides resistance in insects.

Parasitism performance and fitness of Cotesia vestalis (Hymenoptera: Braconidae) infected with Nosema sp. (Microsporidia: Nosematidae): implications in integrated pest management strategy

The diamondback moth (DBM) Plutella xylostella (L.) has traditionally been managed using synthetic insecticides. However, the increasing resistance of DBM to insecticides offers an impetus to practice integrated pest management (IPM) strategies by exploiting its natural enemies such as pathogens, parasitoids, and predators. Nevertheless, the interactions between pathogens and parasitoids and/or predators might affect the effectiveness of the parasitoids in regulating the host population. Thus, the parasitism rate of Nosema-infected DBM by Cotesia vestalis (Haliday) (Hym., Braconidae) can be negatively influenced by such interactions. In this study, we investigated the effects of Nosema infection in DBM on the parasitism performance of C. vestalis. The results of no-choice test showed that C. vestalis had a higher parasitism rate on non-infected host larvae than on Nosema-treated host larvae. The C. vestalis individuals that emerged from Nosema-infected DBM (F1) and their progeny (F2) had smaller pupae, a decreased rate of emergence, lowered fecundity, and a prolonged development period compared to those of the control group. DBM infection by Nosema sp. also negatively affected the morphometrics of C. vestalis. The eggs of female C. vestalis that developed in Nosema-infected DBM were larger than those of females that developed in non-infected DBM. These detrimental effects on the F1 and F2 generations of C. vestalis might severely impact the effectiveness of combining pathogens and parasitoids as parts of an IPM strategy for DBM control.

Does thermal variability experienced at the egg stage influence life history traits across life cycle stages in a small invertebrate?

Although effects of thermal stability on eggs have often been considered in vertebrates, there is little data thermal stability in insect eggs even though these eggs are often exposed in nature to widely fluctuating ambient conditions. The modularity of development in invertebrates might lead to compensation across life cycle stages but this remains to be tested particularly within the context of realistic temperature fluctuations encountered in nature. We simulated natural temperate fluctuations on eggs of the worldwide cruciferous insect pest, the diamondback moth (DBM), Plutella xylostella (L.), while maintaining the same mean temperature (25°C±0°C, 25±4°C, 25±6°C, 25±8°C, 25±10°C, 25±12°C) and assessed egg development, survival and life history traits across developmental stages. Moderate fluctuations (25±4°C, 25±6°C) did not influence performance compared to the constant temperature treatment, and none of the treatments influenced egg survival. However the wide fluctuating temperatures (25±10°C, 25±12°C) slowed development time and led to an increase in pre-pupal mass, although these changes did not translate into any effects on longevity or fecundity at the adult stage. These findings indicate that environmental effects can extend across developmental stages despite the modularity of moth development but also highlight that there are few fitness consequences of the most variable thermal conditions likely to be experienced by Plutella xylostella.

Amino acid substitutions and intron polymorphism of acetylcholinesterase1 associated with mevinphos resistance in diamondback moth, Plutella xylostella (L.)

The diamondback moth, Plutella xylostella L., is the most destructive insect pest of Brassica crops in the world. It has developed resistance rapidly to almost every insecticide used for its control. Mevinphos, a fast degrading and slow resistance evocating organophosphorus insecticide, has been recommended for controlling P. xylostella in Taiwan for more than 40years. SHM strain of P. xylostella, with ca. 22-fold resistance to this chemical, has been established from a field SH strain by selecting with mevinphos since 1997. Three mutations, i.e., G892T, G971C, and T1156T/G leading to A298S, G324A, and F386F/V amino acid substitutions in acetylcholinesterase1 (AChE1), were identified in these two strains; along with three haplotype pairs and a polymorphic intron in AChE1 gene (ace1). Two genetically pure lines, i.e., an SHggt wild type with intron AS and an SHMTCN mutant carrying G892T, G971C, T1156T/G mutations and intron AR in ace1, were established by single pair mating and haplotype determination. The F1 of SHMTCN strain had 52-fold resistance to mevinphos in comparison with the F1 of SHggt strain. In addition, AChE1 of this SHMTCN population, which exhibited lower maximum velocity (Vmax) and affinity (Km), was less susceptible to the inhibition of mevinphos, with an I50 32-fold higher than that of the SHggt F1 population. These results imply that amino acid substitutions in AChE1 of SHMTCN strain are associated with mevinphos resistance in this insect pest, and this finding is important for insecticide resistance management of P. xylostella in the field.

Population-level effects of fitness costs associated with repressible female-lethal transgene insertions in two pest insects

Genetic control strategies offer great potential for the sustainable and effective control of insect pests. These strategies involve the field release of transgenic insects with the aim of introducing engineered alleles into wild populations, either permanently or transiently. Their efficacy can therefore be reduced if transgene-associated fitness costs reduce the relative performance of released insects. We describe a method of measuring the fitness costs associated with transgenes by analyzing their evolutionary trajectories when placed in competition with wild-type alleles in replicated cage populations. Using this method, we estimated lifetime fitness costs associated with two repressible female-lethal transgenes in the diamondback moth and olive fly as being acceptable for field suppression programs. Furthermore, using these estimates of genotype-level fitness costs, we were able to project longer-term evolutionary trajectories for the transgenes investigated. Results from these projections demonstrate that although transgene-associated fitness costs will ultimately cause these transgenes to become extinct, even when engineered lethality is repressed, they may persist for varying periods of time before doing so. This implies that tetracycline-mediated transgene field persistence in these strains is unlikely and suggests that realistic estimates of transgene-associated fitness costs may be useful in trialing ‘uncoupled’ gene drive system components in the field.

Insect attraction versus plant defense: young leaves high in glucosinolates stimulate oviposition by a specialist herbivore despite poor larval survival due to high saponin content

Glucosinolates are plant secondary metabolites used in plant defense. For insects specialized on Brassicaceae, such as the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), glucosinolates act as "fingerprints" that are essential in host plant recognition. Some plants in the genus Barbarea (Brassicaceae) contain, besides glucosinolates, saponins that act as feeding deterrents for P. xylostella larvae, preventing their survival on the plant. Two-choice oviposition tests were conducted to study the preference of P. xylostella among Barbarea leaves of different size within the same plant. P. xylostella laid more eggs per leaf area on younger leaves compared to older ones. Higher concentrations of glucosinolates and saponins were found in younger leaves than in older ones. In 4-week-old plants, saponins were present in true leaves, while cotyledons contained little or no saponins. When analyzing the whole foliage of the plant, the content of glucosinolates and saponins also varied significantly in comparisons among plants that were 4, 8, and 12 weeks old. In Barbarea plants and leaves of different ages, there was a positive correlation between glucosinolate and saponin levels. This research shows that, in Barbarea plants, ontogenetical changes in glucosinolate and saponin content affect both attraction and resistance to P. xylostella. Co-occurrence of a high content of glucosinolates and saponins in the Barbarea leaves that are most valuable for the plant, but are also the most attractive to P. xylostella, provides protection against this specialist herbivore, which oviposition behavior on Barbarea seems to be an evolutionary mistake.

Development and reproductive potential of diamondback moth (Lepidoptera: Plutellidae) on selected wild crucifer species

The diamondback moth, Plutella xylostella (L.), is an oligophagous insect that primarily feeds on members of the family Cruciferae. The development, survival, and reproductive potential of P. xylostella were studied on eight wild cruciferous species: Rorippa indica (L.) Hiern, Cardamine hirsuta L., Descurainia sophia (L.) Webb ex Prantl, Capsella bursa-pastoris (L.) Medic, Cardamine leucantha (Tausch) O. E. Schulz, Orychophragmus violaceus (L.) O. E. Schulz, Thlaspi arvense L., and Cardamine macrophylla Willd. Developmental durations of immatures from egg to adult emergence differed significantly among the plant species, with the longest period recorded on C. macrophylla (20.8 d) and the shortest on R. indica (15.8 d). The female pupae of P. xylostella reared on C. leucantha and T. arvense were lighter (4.2 and 4.3 mg/pupa) than those reared on other hosts (5.2-6.5 mg/pupa), and the male pupae from T. arvense were the lightest (3.1 mg/pupa) among all colonies. Survival from egg to adult emergence ranged from 95.7% on R. indica to 48.8% on T. arvense. The longevity (10.1 d) of P. xylostella female and the oviposition period (7.7 d) were the longest when larvae fed R. indica than those that fed on other wild hosts. Female adults of P. xylostella from O. violaceus, C. macrophylla, and Ca. bursa-pastoris had higher fecundity (305-351 eggs/female) than from other wild host plants, whereas that from R. indica had the lowest fecundity (134 eggs/female). C. hirsuta was the best wild host plant for P. xylostella because of the highest intrinsic rates of increase (rm = 0.2402), whereas T. arvense was the least favorable hosts with the lowest intrinsic rates of increase (rm = 0.1577). The results from this study will be useful for interpretation of the performance and population dynamics of P. xylostella on wild hosts and cultivated cruciferous vegetables.

Using plant chemistry and insect preference to study the potential of Barbarea (Brassicaceae) as a dead-end trap crop for diamondback moth (Lepidoptera: Plutellidae)

Barbarea vulgaris R. Br. has been proposed as a dead-end trap crop for diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), because its larvae do not survive on this plant species despite being highly preferred for oviposition. We compared plants of several species, varieties, and types in the genus Barbarea (Brassicaceae) to study their potential as trap crops for P. xylostella. In terms of insect behavior, Barbarea plants were assessed based on the criteria of high oviposition preference by P. xylostella moths (compared to other Barbarea plants and to three Brassica oleracea L. crop varieties) and low survival of P. xylostella larvae. Barbarea plants were also assessed based on the criteria of high content of glucosinolates, which stimulate adult oviposition and larval feeding in P. xylostella, and high content of saponins, which are detrimental to survival of P. xylostella larvae. All Barbarea plants tested were preferred over cabbage by ovipositing P. xylostella. Among Barbarea plants, few significant differences in oviposition preference by P. xylostella were found. Ovipositing P. xylostella preferred B. vulgaris plants containing mainly 2-phenylethylglucosinolate over B. vulgaris plants containing mainly (S)-2-hydroxy-2-phenylethylglucosinolate, and P-type B. vulgaris var. arcuata plants over Barbarea rupicola and B. vulgaris var. variegata plants. Despite containing a lower content of saponins than other Barbarea plants tested, Barbarea verna did not allow survival of P. xylostella larvae. Our studies show that, except for B. rupicola and P-type B. vulgaris var. arcuata, which allowed survival of P. xylostella larvae, all Barbarea plants tested have potential as dead-end trap crops for P. xylostella.

Thermal tolerance limits of diamondback moth in ramping and plunging assays

Thermal sensitivity is a crucial determinant of insect abundance and distribution. The way it is measured can have a critical influence on the conclusions made. Diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an important insect pest of cruciferous crops around the world and the thermal responses of polyphagous species are critical to understand the influences of a rapidly changing climate on their distribution and abundance. Experiments were carried out to the lethal temperature limits (ULT₀ and LLT₀: temperatures where there is no survival) as well as Upper and Lower Lethal Temperature (ULT₂₅ and LLT₂₅) (temperature where 25% DBM survived) of lab-reared adult DBM population to extreme temperatures attained by either two-way ramping (ramping temperatures from baseline to LT₂₅ and ramping back again) or sudden plunging method. In this study the ULT₀ for DBM was recorded as 42.6°C and LLT₀ was recorded as −16.5°C. DBM had an ULT₂₅ of 41.8°C and LLT₂₅ of −15.2°C. The duration of exposure to extreme temperatures had significant impacts on survival of DBM, with extreme temperatures and/or longer durations contributing to higher lethality. Comparing the two-way ramping temperature treatment to that of direct plunging temperature treatment, our study clearly demonstrated that DBM was more tolerant to temperature in the two-way ramping assay than that of the plunging assay for cold temperatures, but at warmer temperatures survival exhibited no differences between ramping and plunging. These results suggest that DBM will not be put under physiological stress from a rapidly changing climate, rather access to host plants in marginal habitats has enabled them to expand their distribution. Two-way temperature ramping enhances survival of DBM at cold temperatures, and this needs to be examined across a range of taxa and life stages to determine if enhanced survival is widespread incorporating a ramping recovery method.

DBM-DB: the diamondback moth genome database

The diamondback moth Genome Database (DBM-DB) is a central online repository for storing and integrating genomic data of diamondback moth (DBM), Plutella xylostella (L.). It provides comprehensive search tools and downloadable datasets for scientists to study comparative genomics, biological interpretation and gene annotation of this insect pest. DBM-DB contains assembled transcriptome datasets from multiple DBM strains and developmental stages, and the annotated genome of P. xylostella (version 2). We have also integrated publically available ESTs from NCBI and a putative gene set from a second DBM genome (KONAGbase) to enable users to compare different gene models. DBM-DB was developed with the capacity to incorporate future data resources, and will serve as a long-term and open-access database that can be conveniently used for research on the biology, distribution and evolution of DBM. This resource aims to help reduce the impact DBM has on agriculture using genomic and molecular tools.

Database URL:http://iae.fafu.edu.cn/DBM/

Metabolic differentiation of diamondback moth ( Plutella xylostella (L.)) resistance in cabbage ( Brassica oleracea L. ssp. capitata)

The diamondback moth, Plutella xylostella (L.), is a major pest responsible for destroying cabbage and other Brassica vegetable crops. A diamondback moth-resistant cabbage line was studied by comparing its metabolite profiles with those of a susceptible cabbage. Fourier transform infrared spectroscopy analysis revealed that carbohydrates, aromatic compounds, and amides were the major factors that distinguished the resistant and susceptible genotypes. Gas chromatography-time-of-flight mass spectrometry profiled 46 metabolites, including 19 amino acids, 15 organic acids, 8 sugars, 3 sugar alcohols, and 1 amine in two genotypes and F1 hybrid cabbages. The levels of glycolic acid, quinic acid, inositol, fumaric acid, glyceric acid, trehalose, shikimic acid, and aspartic acid were found to be very significantly different between the resistant and susceptible genotypes with a P value of <0.0001. These results will provide a foundation for further studies on diamondback moth resistance in cabbage breeding and for the development of other herbivore-resistant crops.

Molecular cloning and characterization of a P-glycoprotein from the diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

Macrocyclic lactones such as abamectin and ivermectin constitute an important class of broad-spectrum insecticides. Widespread resistance to synthetic insecticides, including abamectin and ivermectin, poses a serious threat to the management of diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), a major pest of cruciferous plants worldwide. P-glycoprotein (Pgp), a member of the ABC transporter superfamily, plays a crucial role in the removal of amphiphilic xenobiotics, suggesting a mechanism for drug resistance in target organisms. In this study, PxPgp1, a putative Pgp gene from P. xylostella, was cloned and characterized. The open reading frame (ORF) of PxPgp1 consists of 3774 nucleotides, which encodes a 1257-amino acid peptide. The deduced PxPgp1 protein possesses structural characteristics of a typical Pgp, and clusters within the insect ABCB1. PxPgp1 was expressed throughout all developmental stages, and showed the highest expression level in adult males. PxPgp1 was highly expressed in midgut, malpighian tubules and testes. Elevated expression of PxPgp1 was observed in P. xylostella strains after they were exposed to the abamectin treatment. In addition, the constitutive expressions of PxPgp1 were significantly higher in laboratory-selected and field-collected resistant strains in comparison to their susceptible counterpart.

Identification and developmental profiling of microRNAs in diamondback moth, Plutellaxylostella (L.)

MicroRNAs (miRNAs) are a group of small RNAs involved in various biological processes through negative regulation of mRNAs at the post-transcriptional level. Although miRNA profiles have been documented in over two dozen insect species, few are agricultural pests. In this study, both conserved and novel miRNAs in the diamondback moth, Plutella xylostella L., a devastating insect pest of cruciferous crops worldwide, were documented. High-throughput sequencing of a small RNA library constructed from a mixed life stages of P. xylostella, including eggs, 1st to 4th (last) instar larvae, pupae and adults, identified 384 miRNAs, of which 174 were P. xylostella specific. In addition, temporal expressions of 234 miRNAs at various developmental stages were investigated using a customized microarray analysis. Among the 91 differentially expressed miRNAs, qRT-PCR analysis was used to validate highly expressed miRNAs at each stage. The combined results not only systematically document miRNA profiles in an agriculturally important insect pest, but also provide molecular targets for future functional analysis and, ultimately, genetic-based pest control practice.