Insect resistance to Bt crops: lessons from the first billion acres

Host-derived artificial miRNA-mediated silencing of ecdysone receptor gene provides enhanced resistance to Helicoverpa armigera in tomato

Helicoverpa armigera causes huge crop losses due to its polyphagous nature. The present study demonstrates the use of artificial microRNA (amiRNA) mediated gene silencing approach to generate insect resistant tomato plants. Ecdysone receptor (HaEcR) gene of the target pest, H. armigera, which is involved in the regulation of all developmental stages of the insect life cycle, was silenced by sequence-specific amiRNA (amiRNA-HaEcR). Continuous feeding on detached tomato leaves expressing the amiRNA-319a-HaEcR resulted in reduced target gene transcripts and affected the overall growth and survival of H. armigera. Not only the target gene was down-regulated but, the feeding also affected the expression of down-stream genes involved in the ecdysone signaling pathway. The resistant trait was also observed in T₁ generation of tomato transgenic lines. These results further established the role of EcR as a master regulator in insect development and effectiveness of amiRNA technology for efficient control of H. armigera.

Combining refuges with transgenic insect releases for the management of an insect pest with non-recessive resistance to Bt crops in agricultural landscapes

Reinforcing the high-dose/refuge strategy with releases of transgenic insects has been suggested as a method for simultaneously managing agricultural pest populations and resistance to transgenic crops. Theoretical and empirical studies have shown that these approaches can work when deployed against closed populations and the assumptions of the HDR strategy are met. However, field-evolved resistance is often linked to non-recessive resistance or refuge non-compliance, and pest management regimes are likely to take place at the landscape-level. It is therefore important to understand how effective such strategies are when resistance is non-recessive, and how they could be employed in agricultural landscapes. We developed a spatially-explicit model to investigate the efficacy of strategies combining refuges with transgenic insect releases to manage a pest with non-recessive resistance in agricultural landscapes. We compared two release strategies, area-wide releases and localised releases targeted at population hotspots, and analysed the effects of refuge and release parameters on population and resistance dynamics. Area-wide releases reliably achieved landscape-level pest eradication. Localised releases also eradicated the pest when low release thresholds were combined with high release ratios, and maintained the pest at low densities when insufficient to achieve extinction. Reinforcing refuges with localised releases also greatly enhanced the probability of resistance extinction. However, when resistance remained in the population, localised releases prevented resistance from reaching fixation rather than greatly delaying or reversing resistance evolution. Our work indicates that combining refuges with simple release policies is effective for landscape-level pest suppression when the HDR assumptions are violated, but more nuanced release strategies may be required to enhance the benefits to resistance management.

Agronomic and compositional assessment of genetically modified DP23211 maize for corn rootworm control

DP23211 maize was genetically modified (GM) to express DvSSJ1 double-stranded RNA and the IPD072Aa protein for control of corn rootworm (Diabrotica spp.). DP23211 maize also expresses the phosphinothricin acetyltransferase (PAT) protein for tolerance to glufosinate herbicide, and the phosphomannose isomerase (PMI) protein that was used as a selectable marker. A multi-location field trial was conducted during the 2018 growing season at 12 sites selected to be representative of the major maize-growing regions of the U.S. and Canada. Standard agronomic endpoints as well as compositional analytes from grain and forage (e.g., proximates, fibers, minerals, amino acids, fatty acids, vitamins, anti-nutrients, secondary metabolites) were evaluated and compared to non-GM near-isoline control maize (control maize) and non-GM commercial maize (reference maize). A small number of agronomic endpoints were statistically significant compared to the control maize, but were not considered to be biologically relevant when adjusted using the false discovery rate method (FDR) or when compared to the range of natural variation established from in-study reference maize. A small number of composition analytes were statistically significant compared to the control maize. These analytes were not statistically significant when adjusted using FDR, and all analyte values fell within the range of natural variation established from in-study reference range, literature range or tolerance interval, indicating that the composition of DP23211 maize grain and forage is substantially equivalent to conventional maize represented by non-GM near-isoline control maize and non-GM commercial maize.

Assessment of the 2018 post-market environmental monitoring report on the cultivation of genetically modified maize MON 810 in the EU

Following a request from the European Commission, the EFSA assessed the 2018 post-market environmental monitoring (PMEM) report on the cultivation of Cry1Ab-expressing maize event MON 810. Like previous years, there was partial compliance with refuge requirements by Spanish farmers growing MON 810 varieties. European and Mediterranean corn borer populations collected from north-eastern Spain during the 2018 maize growing season and tested for Cry1Ab susceptibility show no symptoms of resistance to maize MON 810. The assessment of farmer questionnaires and relevant scientific publications does not indicate any unanticipated adverse effects on human and animal health or the environment arising from the cultivation of maize MON 810. The report does not provide information about the use of existing networks involved in environmental monitoring. Overall, EFSA concludes that the evidence reported in the 2018 PMEM report does not invalidate previous EFSA evaluations on the safety of maize MON 810. However, as in previous years, EFSA identifies shortcomings on resistance monitoring that need revision in future reports. In particular, the monitoring plan, as implemented in 2018, is not sufficiently sensitive to detect the recommended 3% resistance allele frequency. Consequently, EFSA strongly recommends the consent holder to: (1) achieve full compliance with refuge obligations in areas where adoption of maize MON 810 is high; (2) increase the sensitivity of the monitoring plan and address previously mentioned limitations for resistance monitoring; and (3) perform an F2 screen on corn borer populations from north-eastern Spain. A fit-for-purpose farmer alert system may help to detect unexpected adverse effects associated with the cultivation of MON 810 varieties and be an alternative to the current farmer survey system. Moreover, relevant stakeholders should implement a methodological framework to enable making the best use of existing networks involved in environmental monitoring for the general surveillance of genetically modified plants.

Loss of control of the culturable bacteria in the hindgut of Bombyx mori after Cry1Ab ingestion

Bt protein, produced by Bacillus thuringiensis, can bind receptors to destroy the physiological functions of the insect midgut. It is unknown whether Bt can also target the hindgut and influence its defense against fecal bacteria. Here we show that Crystal protein 1Ab (Cry1Ab), a Bt protein, was detected in the larval hindgut contents of Bombyx mori after ingestion of this toxin protein. The number of fecal bacteria that can be inhibited by the hindgut prophenoloxidase-induced melanization was significantly enhanced after oral ingestion of Cry1Ab. Although the hindgut contents became brown, the activity of hindgut phenoloxidase was decreased. LC-MS/MS analysis of the hindgut lumen contents revealed that many new proteins including several proteases were newly secreted. The enhanced secretion of proteases cleaved prophenoloxidase to decrease its activity, including the corresponding activity to inhibit the fecal bacteria. In addition, after ingestion of Cry1Ab, the pylorus (between the midgut and hindgut) could not autonomously contract due to the physical detachment of the acellular cuticle-like membrane from the epidermal cells, which prevented the movement of food from the midgut to the hindgut. Some cells in the cryptonephry of the hindgut became swollen and degraded, possibly due to the presence of Cry1Ab in the hindgut. These findings demonstrate that the inhibition of feces bacteria by the hindgut prophenoloxidase-induced melanization is out of control after Cry1Ab ingestion.

Differences in midgut transcriptomes between resistant and susceptible strains of Chilo suppressalis to Cry1C toxin

Background

Chilo suppressalis is a widespread rice pest that poses a major threat to food security in China. This pest can develop resistance to Cry toxins from Bacillus thuringiensis (Bt), threatening the sustainable use of insect-resistant transgenic Bt rice. However, the molecular basis for the resistance mechanisms of C. suppressalis to Cry1C toxin remains unknown. This study aimed to identify genes associated with the mechanism of Cry1C resistance in C. suppressalis by comparing the midgut transcriptomic responses of resistant and susceptible C. suppressalis strains to Cry1C toxin and to provide information for insect resistance management.

Results

A C. suppressalis midgut transcriptome of 139,206 unigenes was de novo assembled from 373 million Illumina HiSeq and Roche 454 clean reads. Comparative analysis identified 5328 significantly differentially expressed unigenes (DEGs) between C. suppressalis Cry1C-resistant and -susceptible strains. DEGs encoding Bt Cry toxin receptors, aminopeptidase-P like protein, the ABC subfamily and alkaline phosphatase were downregulated, suggesting an association with C. suppressalis Cry1C resistance. Additionally, Cry1C resistance in C. suppressalis may be related to changes in the transcription levels of enzymes involved in hydrolysis, digestive, catalytic and detoxification processes.

Conclusion

Our study identified genes potentially involved in Cry1C resistance in C. suppressalis by comparative transcriptome analysis. The assembled and annotated transcriptome data provide valuable genomic resources for further study of the molecular mechanisms of C. suppressalis resistance to Cry toxins.

Transgenic insect-resistant Bt cotton expressing Cry1Ac/CpTI does not affect the mirid bug Apolygus lucorum

Common varieties of genetically modified (GM) cotton increasingly display insect-resistant properties via expression of bacterial-derived toxins from Bacillus thuringiensis (Bt). This necessitates a deeper understanding of the possible effects of these crops on non-target insects. The mirid bug Apolygus lucorum is a major pest in cotton production in China, however, the effect of GM cotton on this non-target species is currently virtually unknown. This insect is exposed to these transgenic plants by consuming genetically modified (GM) leaves. In this study, laboratory experiments were conducted to assess the toxicity of CCRI41 and CCRI45, (genetically modified cotton varieties which express the toxins Cry1Ac and CpTI (Cowpea Trypsin Inhibitor)) on nymphs and adults of A. lucorum. There was no detectable increase in mortality after A. lucorum fed on GM cotton leaves for 20 days. While we detected trace amounts of Cry1Ac proteins in both A. lucorum nymphs and adults (<10 ng/g fresh weight), the expression of genes related to detoxification did not detectably differ from those feeding on non-GM cotton. Our binding assays did not show Cry1Ac binding to receptors on the midgut brush border membrane from either A. lucorum nymphs or adults. Our findings collectively indicate that feeding on leaves of the GM cotton varieties CCRI41 and CCRI45 have few toxic effects on A. lucorum.

Augmentation and compatibility of Beauveria bassiana with pesticides against different growth stages of Bemisia tabaci (Gennadius); an in vitro and field approach

BACKGROUND

Bemisia tabaci is a notorious agricultural pest that causes serious damage to many crops via herbivory and spread of viral diseases. Effective control measures are, therefore, required. Integrating entomopathogenic fungi into the chemical control system offers promise for B. tabaci management.

RESULTS

In-vitro assays on the augmentative effect of Beauveria bassiana GHA strain with insecticides, and its compatibility with fungicides were tested. Varying egg mortality was observed for most insecticides except for milbemectin. Pyrifluquinazon was less effective against the nymphs. Flonicamid showed no insecticidal effect on either nymphs or adults. However, increased mortality in nymphs and adults was observed when flonicamid was mixed with Beauveria bassiana GHA strain. Furthermore, no significant synergistic effect of mixing pesticides with B. bassiana GHA strain was observed. Most insecticides (when not mixed with B. bassiana) showed high (>80%) mortality against B. tabaci nymphs and adults. Most fungicides tested showed no inhibitory effects on B. bassiana GHA strain against B. tabaci nymphs and adults. Fungistatic effect was observed in the mycelial and spore germination inhibition studies. Weekly rotation of some pesticides with B. bassiana GHA strain in greenhouse conditions yielded significant reduction in different growth stages of B. tabaci. However, no significant difference in viral incidence was observed.

CONCLUSION

Laboratory and field tests showed positive effects of augmenting pesticides and B. bassiana GHA strain against B. tabaci nymphs and adults. Therefore, augmentation of pesticides and B. bassiana GHA strain is one prospect towards developing an effective B. tabaci IPM system. © 2020 Society of Chemical Industry.

The Effects of Succinate Dehydrogenase Inhibitor Fungicide Dose and Mixture on Development of Resistance in Venturia inaequalis

Understanding how fungicide application practices affect selection for fungicide resistance is imperative for continued sustainable agriculture. Here, we examined the effect of field applications of the succinate dehydrogenase inhibitor (SDHI) fluxapyroxad at different doses and mixtures on the SDHI sensitivity of Venturia inaequalis, the apple scab pathogen. Fungicide applications were part of selection programs involving different doses (high or low) and mixtures (with a second single-site fungicide or a multisite fungicide). These programs were tested in two apple orchards over 4 years to determine potential cumulative selection effects on resistance. Each year after program applications, apple scab lesions were collected, and relative growth assays were conducted to understand shifts in fluxapyroxad sensitivity. After 4 years, there was a trend toward a reduction in sensitivity to fluxapyroxad for most selection programs in comparison to that in the non-selective-pressure control. In most years, the selection program plots treated with low-dose fluxapyroxad applications resulted in a larger number of isolates with reduced sensitivity, supporting the use of higher doses for disease management. Few significant differences (P < 0.05) in fungicide sensitivity were observed between isolates collected from plots where fungicide mixtures were applied compared to that in untreated plots, supporting the use of multiple modes of action in field applications. In all, appropriate doses and mixtures may contribute to increased longevity of SDHI fungicides used on perennial crops like apples.IMPORTANCE Of much debate is the effect of fungicide application dose on resistance development, as fungicide resistance is a critical barrier to effective disease management in agricultural systems. Our field study in apples investigated the effect of fungicide application dose and mixture on the selection of succinate dehydrogenase inhibitor resistance in Venturia inaequalis, a fungal pathogen that causes the economically important disease apple scab. Understanding how to best delay the development of resistance can result in increased efficacy, fewer applications, and sustainable fungicide use. Results from this study may have relevance to other perennial crops that require multiple fungicide applications and that are impacted by the development of resistance.

Genetic Differentiation of a New World Screwworm Fly Population from Uruguay Detected by SNPs, Mitochondrial DNA and Microsatellites in Two Consecutive Years

The New World screwworm (NWS) fly, Cochliomyia hominivorax (Diptera: Calliphoridae), is an economically important ectoparasite currently distributed in South America and in the Caribbean basin. The successful eradication of this species in USA, Mexico and continental Central America was achieved by a control program based on the sterile insect technique (SIT). In order to implement a genetic control strategy over the NWS fly's current area of occurrence, first, it is necessary to understand the species dynamics and population structure. In order to address this objective, the spatial genetic structure of the NWS fly was previously reported in South America based on different genetic markers; however, to date, no study has investigated temporal changes in the genetic composition of its populations. In the current study, the temporal genetic structure of a NWS fly population from Uruguay was investigated through two consecutive samplings from the same locality over an interval of approximately 18 generations. The genetic structure was accessed with neutral and under selection SNPs obtained with genotyping-by-sequencing. The results gathered with these data were compared to estimates achieved with mitochondrial DNA sequences and eight microsatellite markers. Temporal changes in the genetic composition were revealed by all three molecular markers, which may be attributed to seasonal changes in the NWS fly's southern distribution. SNPs were employed for the first time for estimating the genetic structure in a NWS fly population; these results provide new clues and perspectives on its population genetic structure. This approach could have significant implications for the planning and implementation of management programs.

Current Insights on Vegetative Insecticidal Proteins (Vip) as Next Generation Pest Killers

Bacillus thuringiensis (Bt) is a Gram negative soil bacterium. This bacterium secretes various proteins during different growth phases with an insecticidal potential against many economically important crop pests. One of the important families of Bt proteins is vegetative insecticidal proteins (Vip), which are secreted into the growth medium during vegetative growth. There are three subfamilies of Vip proteins. Vip1 and Vip2 heterodimer toxins have an insecticidal activity against many Coleopteran and Hemipteran pests. Vip3, the most extensively studied family of Vip toxins, is effective against Lepidopteron. Vip proteins do not share homology in sequence and binding sites with Cry proteins, but share similarities at some points in their mechanism of action. Vip3 proteins are expressed as pyramids alongside Cry proteins in crops like maize and cotton, so as to control resistant pests and delay the evolution of resistance. Biotechnological- and in silico-based analyses are promising for the generation of mutant Vip proteins with an enhanced insecticidal activity and broader spectrum of target insects.

Baseline Plant-to-Plant Larval Movement of Spodoptera eridania in Bt and Non-Bt Soybean and Its Possible Impacts on IRM

The widespread use of transgenic technologies has led to the emergence of insect populations resistant to Bt toxins. Some lepidopteran pest species also appear to naturally have some level of tolerance to certain proteins, such as some species of Spodoptera to Cry1Ac. One of the main strategies to manage resistance is the use of refuge areas, the success of which is in part dependent on larval movement of the target pest. Thus, in order to assess the viability of a refuge strategy addressing Spodoptera eridania Stoll (Lepidoptera: Noctuidae) in Bt soybean, it was evaluated the larval movement across plants in Bt and non-Bt soybean, as well as the larval development and mortality on Bt and non-Bt soybean cultivars. We concluded that apparent S. eridania incomplete resistance resulting from high larval mortality and low adaptability on Bt plants, high larval dispersal, nondirectional larval movement, and random larval spatial dispersion suggest that structured refuge is more suitable than mixed refuge for managing resistance in S. eridania populations.

The level of Cry1Ac endotoxin and its efficacy against H. armigera in Bt cotton at large scale in Pakistan

A biophysical survey was conducted in 15 cotton-growing districts of Pakistan. Four hundred cotton growers were approached and inquired about the production technology of Bt cotton. Further, 25 strip tests using combo strips (Cry1Ac, Cry2Ab, Vip3Aa and Cp4, EPSPS gene) were performed at each farmer’s field. Out of 10,000 total-tested samples, farmers claimed 9682 samples as Bt and 318 samples as non-Bt. After performing a strip test, 1009 and 87 samples were found false negative and false positive, respectively. Only 53 samples were found positive for Cry2Ab, 214 for EPSPS and none for Vip3Aa gene. Quantification of Cry endotoxin and bioassay studies were performed by taking leaves from upper, middle, and lower canopies, and fruiting parts at approximately 80 days after sowing from 89 varieties. Expression was highly variable among different canopies and fruiting parts. Moreover, Cry endotoxin expression and insect mortality varied significantly among varieties from 0.26 µg g⁻¹ to 3.54 µg g⁻¹ with mortality ranging from 28 to 97%, respectively. Highest Cry1Ac expression (3.54 µg g⁻¹) and insect mortality (97%) were observed for variety FH-142 from DG Khan. Cry endotoxin expression varied significantly across various plant parts, i.e., IUB-13 variety from upper canopy documented 0.34 µg g⁻¹ expression with 37% insect mortality in Layyah to 3.42 µg g⁻¹ expression and 96% insect mortality from DG Khan. Lethal dose, LD95 (2.20 µg g⁻¹) of Cry1Ac endotoxin was optimized for effective control of H. armigera. Our results provided evidence of practical resistance in H. armigera and way forward.

Comparison of Nanomaterials for Delivery of Double-Stranded RNA in Caenorhabditis elegans

RNA interference is a promising crop protection technology that has seen rapid development in the past several years. Here, we investigated polyamino acid biopolymers, inorganic nanomaterials, and hybrid organic-inorganic nanomaterials for delivery of dsRNA and efficacy of gene knockdown using the model nematode Caenorhabditis elegans. Using an oral route of delivery, we are able to approximate how nanomaterials will be delivered in the environment. Of the materials investigated, only Mg-Al layered double-hydroxide nanoparticles were effective at gene knockdown in C. elegans, reducing marker gene expression to 66.8% of that of the control at the lowest tested concentration. In addition, we identified previously unreported injuries to the mouthparts of C. elegans associated with the use of a common cell-penetrating peptide, poly-l-arginine. Our results will allow the pursuit of further research into promising materials for dsRNA delivery and also allow for the exclusion of those with little efficacy or deleterious effects.

Low Mismatch Rate between Double-Stranded RNA and Target mRNA Does Not Affect RNA Interference Efficiency in Colorado Potato Beetle

RNA interference (RNAi)-based technology has been proven as a novel approach for insect pest control. However, whether insects could evolve resistance to RNAi and the underlying mechanism is largely unknown. The target gene mutations were thought to be one of the potential ways to develop the resistance. Here we predicted the effective siRNA candidates that could be derived from dsRNA against the Colorado potato beetle (CPB) β-Actin gene (dsACT). By site-directed mutagenesis, we synthesized the dsRNAs with the defect in generation of effective siRNAs (and thus were supposed to have comparable low RNAi efficacy). We showed that, with mismatches to the target gene, all the dsRNA variants caused similar levels of silencing of target gene, mortality and larval growth retardation of CPB. Our results suggest that when the mismatch rate of dsACT and target β-Actin mRNA is less than 3%, the RNAi efficiency is not impaired in CPB, which might imply the low possibility of RNAi resistance evolving through the sequence mismatches between dsRNA and the target gene.

Analysis of Cry1Ah Toxin-Binding Reliability to Midgut Membrane Proteins of the Asian Corn Borer

Evolution of insect resistance to Bt toxins challenges the use of Cry toxins to control agricultural pests. In lepidopterans, Cry toxin affinity towards multiple midgut epithelial receptors has become a matter of dispute. Cry1Ah toxin-binding proteins were identified in the larval midgut of susceptible (ACB-BtS) and resistant (ACB-AhR) strains of the Asian corn borer (ACB). A pull-down assay was performed using biotinylated Cry1Ah toxin, and the binding proteins were identified by employing liquid chromatography–tandem mass spectrometry (LC-MS/MS). This study aimed to find the binding consistency of the midgut epithelial protein to the Cry1Ah toxin. The binding proteins from different fractions of SDS-PAGE showed a different pattern. We observed an isoform of prophenoloxidase PPO1b (UniProt Acc No. A0A1Q1MKI0), which was found only in the ACB-AhR fractions. Prophenoloxidase (proPO) is an extraordinary defense molecule activated in insect species during pathogen invasion and the wound healing process. Importantly, this prophenoloxidase might have direct/indirect interaction with the Cry1Ah toxin. Our data also suggest that factors like techniques, enrichment of binding proteins in the sample and the reversible and irreversible nature of the brush border membrane vesicles (BBMVs) to Cry toxins could cause the inconsistency in the protein–protein interactions. Moreover, inside the larva midgut, the influence of the Cry toxins under physiological conditions might be different from the laboratory procedures.

MAPK-dependent hormonal signaling plasticity contributes to overcoming Bacillus thuringiensis toxin action in an insect host

The arms race between entomopathogenic bacteria and their insect hosts is an excellent model for decoding the intricate coevolutionary processes of host-pathogen interaction. Here, we demonstrate that the MAPK signaling pathway is a general switch to trans-regulate differential expression of aminopeptidase N and other midgut genes in an insect host, diamondback moth (Plutella xylostella), thereby countering the virulence effect of Bacillus thuringiensis (Bt) toxins. Moreover, the MAPK cascade is activated and fine-tuned by the crosstalk between two major insect hormones, 20-hydroxyecdysone (20E) and juvenile hormone (JH) to elicit an important physiological response (i.e. Bt resistance) without incurring the significant fitness costs often associated with pathogen resistance. Hormones are well known to orchestrate physiological trade-offs in a wide variety of organisms, and our work decodes a hitherto undescribed function of these classic hormones and suggests that hormonal signaling plasticity is a general cross-kingdom strategy to fend off pathogens.

Bacillus thuringiensis (Bt) is an important bioinsecticide, but high-level resistance has been rapidly evolving in agricultural pests. Here, Guo et al. show that the MAPK cascade can be activated by enhanced upstream insect hormone signals to counter Bt virulence in the diamondback moth.

Txp40, a protein from Photorhabdus akhurstii, conferred potent insecticidal activity against the larvae of Helicoverpa armigera, Spodoptera litura and S. exigua

BACKGROUND

Txp40, a 37 kDa protein, previously characterized from the Gram-negative bacterium Photorhabdus akhurstii (symbiotically associates with insect-parasitic nematode, Heterorhabditis indica), conferred insecticidal activity against Galleria mellonella. Here, the biological activity of Txp40 was evaluated against economically important insects, including Helicoverpa armigera, Spodoptera litura and S. exigua.

RESULTS

When both intra-hemocoel injected and orally fed to test insects, comparatively greater oral LD₅₀ (187.7-522 ng g^-1 ) than injection LD₅₀ (32.33-150.6 ng g^-1 ) was obtained with Txp40 derived from P. akhurstii strain IARI-SGMG3. Injection of purified Txp40 caused a dose-dependent reduction in the total circulatory hemocytes and hemocyte viability of fourth-instar larvae of the test insects at 12 h post incubation; unlike healthy cells toxin-treated ones displayed aggregated distribution. Injection of Txp40 significantly elevated the phenoloxidase activity of insect hemolymph, which potentially led to unrestrained melanization reaction and ultimately larval death. Histological analyses showed the primary site of action of Txp40 in the insect midgut. Extensive damage to midgut epithelium 24 h after injection of the Txp40 explains the access of the toxin from hemocoel to midgut via leaky septate junctions. In silico analyses suggested that Txp40 can potentially interact with H. armigera midgut receptor proteins cadherin, ATP-binding cassettes, aminopeptidase N1 and alkaline phosphatase to exert toxicity.

CONCLUSION

We propose Txp40 as an attractive alternative to Cry toxins of Bacillus thuringiensis, the transgenic expression of which is reported to cause resistance development in insects. © 2019 Society of Chemical Industry.

How do toxins from Bacillus thuringiensis kill insects? An evolutionary perspective

Three-domain Cry toxins from the bacterium Bacillus thuringiensis (Bt) are increasingly used in agriculture to replace chemical insecticides in pest control. Most chemical insecticides kill pest insects swiftly, but are also toxic to beneficial insects and other species in the agroecosystem. Cry toxins enjoy the advantages of high selectivity and the possibility of the application by sprays or transgenic plants. However, these benefits are offset by the limited host range and the evolution of resistance to Bt toxins by insect pests. Understanding how Bt toxins kill insects will help to understand the nature of both problems. The recent realization that ABC transporters play a central role in the killing mechanism will play an important role in devising solutions.

Novel insecticidal chitinase from the insect pathogen Xenorhabdus nematophila

Xenorhabdus nematophila strain ATCC 19061 is an insect pathogen that produces various protein toxins which intoxicate and kill its larval host. In the present study, we have described the cloning, expression and characterization of a 76-kDa chitinase protein of X. nematophila. A 1.9 kb DNA sequence encoding the chitinase gene was PCR amplified and cloned. Further, the chitinase protein was expressed in Escherichia coli and purified by using affinity chromatography. Two highly conserved domains were identified GH18 and ChiA. The purified chitinase protein showed chitobiosidase activity, β-N-acetylglucosaminidase and endochitinase activity, when enzyme activity was measured using respective substrates. The purified chitinase protein was found to be orally toxic to the larvae of a major crop pest, Helicoverpa armigera when fed to the larvae mixed with artificial diet. It also had adverse effect on the growth and development of the surviving larvae. Surviving larvae showed 9-fold reduction in weight, as a result the transformation of larvae into pupae was adversely affected. Our results demonstrated that the chitinase protein of X. nematophila has insecticidal property and can prove to be a potent candidate for pest control in plants.

Shared and Independent Genetic Basis of Resistance to Bt Toxin Cry2Ab in Two Strains of Pink Bollworm

Evolution of pest resistance threatens the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Field populations of the pink bollworm (Pectinophora gossypiella), a global pest of cotton, have evolved practical resistance to transgenic cotton producing Bt toxin Cry2Ab in India, but not in the United States. Previous results show that recessive mutations disrupting an autosomal ATP-binding cassette gene (PgABCA2) are associated with pink bollworm resistance to Cry2Ab in field-selected populations from India and in one lab-selected strain from the United States (Bt4-R2). Here we discovered that an independently derived, lab-selected Cry2Ab-resistant pink bollworm strain from the United States (BX-R) also harbors mutations that disrupt PgABCA2. Premature stop codons introduced by mis-splicing of PgABCA2 pre-mRNA were prevalent in field-selected larvae from India and in both lab-selected strains. The most common mutation in field-selected larvae from India was also detected in both lab-selected strains. Results from interstrain crosses indicate BX-R has at least one additional mechanism of resistance to Cry2Ab that does not involve PgABCA2 and is not completely recessive or autosomal. We conclude that recessive mutations disrupting PgABCA2 are the primary, but not the only, mechanism of resistance to Cry2Ab in pink bollworm.

Identification and Functional Analysis of Two Chitin Synthase Genes in the Common Cutworm, Spodoptera litura

Chitin is one the main components of the insect cuticle, and chitin synthase (CHS) is an important enzyme required for chitin formation. CHS has been characterized in various insect species, but the structure and biochemical properties in Spodoptera litura have not been determined. In this study, we identified two CHS genes, SlCHS1 and SlCHS2, which encode proteins with 1565 and 1520 amino acid residues, respectively. Transcriptional analysis suggested that SlCHS1 has a high expression level in the integument whereas SlCHS2 showed the highest expression level in the midgut. During S. litura growth and development, SlCHS1 and SlCHS2 were both predominantly expressed in the fourth-instar larval stage. In addition, the expression of SlCHS1 and SlCHS2 could be induced by 20-hydroxyecdysone (20E). Silencing of SlCHS1 by RNA interference significantly inhibited the pupation and molting of S. litura larvae (RNAi), while knockdown of SlCHS2 had no significant effects on the S. litura phenotype. These results may provide a new molecular target for control of S. litura.

Semisynthesis and Pesticidal Activities of Derivatives of the Diterpenoid Andrographolide and Investigation on the Stress Response of Aphis citricola Van der Goot (Homoptera: Aphididae)

To discover natural-product-based pesticides, 7β-oxycarbonylandrographolide derivatives were stereoselectively constructed from a labdane diterpenoid andrographolide. Among them, 2'-(n)Pr-1',3'-dioxin-7β-oxy(m-Cl)benzoylandrographolide (IIc), 2'-(n)Pr-1',3'-dioxin-7β-oxyacetylandrographolide (IIf), 2'-(p-Me)Ph-1',3'-dioxin-7β-oxy(o-Cl)benzoylandrographolide (Vb), and 2'-(p-Me)Ph-1',3'-dioxin-7β-oxy(m-Cl)benzoylandrographolide (Vc) against Mythimna separata displayed the most promising growth inhibitory activity; 2'-(n)Pr-1',3'-dioxin-7β-oxy(o-Cl)benzoylandrographolide (IIb: LC₅₀ = 0.406 mg/mL) and IIc (LC₅₀ = 0.415 mg/mL) exhibited the most pronounced acaricidal activity (andrographolide; LC₅₀: 5.106 mg/mL) and good control effects against Tetranychus cinnabarinus; compounds Ic, IIe, and Va-c (LD₅₀ = 0.035-0.039 μg/nymph) showed potent aphicidal activity (andrographolide: LD₅₀ = 0.178 μg/nymph), and compounds IIe and Vb showed good control effects against Aphis citricola. Moreover, it was found that Hsp70 of A. citricola was an important gene involved in stress response to andrographolide and its derivatives.

Implementation of a Diagnostic-Concentration Bioassay for Detection of Susceptibility to Pyrethroids in Soybean Aphid (Hemiptera: Aphididae)

Soybean aphid, Aphis glycines Matsumura, remains the most economically damaging arthropod pest of soybean in the midwestern United States and southern Canada. Foliar applications of a limited number of insecticide modes of action have been the primary management tactic, and pyrethroid resistance was documented recently with full concentration-response leaf-dip and glass-vial bioassays. Full concentration-response bioassays can be cumbersome, and a more efficient assessment tool was needed. In this study, we implemented a diagnostic-concentration glass-vial bioassay using bifenthrin and λ-cyhalothrin. Bioassays were conducted with field-collected soybean aphid populations to assess the geographic extent and severity of resistance to pyrethroids. In 2017, 10 of 18 and 11 of 21 field populations tested with bifenthrin and λ-cyhalothrin, respectively, had mean proportion mortalities less than the susceptible laboratory population. In 2018, 17 of 23 and 13 of 23 field populations tested with bifenthrin and λ-cyhalothrin, respectively, had mean proportion mortalities less than the susceptible laboratory population. Populations collected after reported field failures of a pyrethroid insecticide generally had mean proportion mortalities less than the susceptible laboratory population. In both years, there was a strong correlation between chemistries, which suggests cross-resistance between these insecticides. The diagnostic-concentration glass-vial bioassays reported here will provide the foundation for an insecticide resistance monitoring program with the ability to determine practical levels and geographic extent of insecticide resistance.

First documentation of major Vip3Aa resistance alleles in field populations of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Texas, USA

The corn earworm, Helicoverpa zea, is a major target pest of the insecticidal Vip3Aa protein used in pyramided transgenic Bt corn and cotton with Cry1 and Cry2 proteins in the U.S. The widespread resistance to Cry1 and Cry2 proteins in H. zea will challenge the long-term efficacy of Vip3Aa technology. Determining the frequency of resistant alleles to Vip3Aa in field populations of H. zea is critically important for resistance management. Here, we provided the first F2 screen study to estimate the resistance allele frequency for Vip3Aa in H. zea populations in Texas, U.S. In 2019, 128 H. zea neonates per isofamily for a total of 114 F2 families were screened with a diagnostic concentration of 3.0 μg/cm2 of Vip3Aa39 protein in diet-overlay bioassays. The F2 screen detected two families carrying a major Vip3Aa resistance allele. The estimated frequency of major resistance alleles against Vip3Aa39 in H. zea in Texas from this study was 0.0065 with a 95% CI of 0.0014-0.0157. A Vip3Aa-resistant strain (RR) derived from the F2 screen showed a high level of resistance to Vip3Aa39 protein, with a resistance ratio of >588.0-fold relative to a susceptible population (SS) based on diet-overlay bioassays. We provide the first documentation of a major resistance allele conferring high levels of Vip3Aa resistance in a field-derived strain of H. zea in the U.S. Data generated from this study contribute to development of management strategies for the sustainable use of the Vip3Aa technology to control H. zea in the U.S.

Genetically modified crops: current status and future prospects

While transgenic technology has heralded a new era in crop improvement, several concerns have precluded their widespread acceptance. Alternative technologies, such as cisgenesis and genome-editing may address many of such issues and facilitate the development of genetically engineered crop varieties with multiple favourable traits. Genetic engineering and plant transformation have played a pivotal role in crop improvement via introducing beneficial foreign gene(s) or silencing the expression of endogenous gene(s) in crop plants. Genetically modified crops possess one or more useful traits, such as, herbicide tolerance, insect resistance, abiotic stress tolerance, disease resistance, and nutritional improvement. To date, nearly 525 different transgenic events in 32 crops have been approved for cultivation in different parts of the world. The adoption of transgenic technology has been shown to increase crop yields, reduce pesticide and insecticide use, reduce CO₂ emissions, and decrease the cost of crop production. However, widespread adoption of transgenic crops carrying foreign genes faces roadblocks due to concerns of potential toxicity and allergenicity to human beings, potential environmental risks, such as chances of gene flow, adverse effects on non-target organisms, evolution of resistance in weeds and insects etc. These concerns have prompted the adoption of alternative technologies like cisgenesis, intragenesis, and most recently, genome editing. Some of these alternative technologies can be utilized to develop crop plants that are free from any foreign gene hence, it is expected that such crops might achieve higher consumer acceptance as compared to the transgenic crops and would get faster regulatory approvals. In this review, we present a comprehensive update on the current status of the genetically modified (GM) crops under cultivation. We also discuss the issues affecting widespread adoption of transgenic GM crops and comment upon the recent tools and techniques developed to address some of these concerns.

Resistance to Chilo infuscatellus (Lepidoptera: Pyraloidea) in transgenic lines of sugarcane expressing Bacillus thuringiensis derived Vip3A protein

Sustainable agriculture requires management of insect pests through resistance development. The biological potential of Cry toxins and Vip protein, derived from Bacillus species, is widely recognized in this context. The identification, evaluation of new insecticidal protein genes with different mode of action and entomotoxicity against sugarcane stem borer (Chilo infuscatellus) is important to overcome evolved insect resistance. In this study, we reported the generation of transgenic sugarcane lines expressing Vip3A toxin driven by polyubiquitin promoter for resistance against sugarcane stem borer. The V₀ transgenic sugarcane plants were initially characterized by GUS histochemical staining, PCR and Southern blot assays that confirmed genetic transformation of twelve independent sugarcane lines. Variable transgene expression was found among transgenic sugarcane lines when revealed through Realtime quantitative PCR (RT-qPCR) with highest in S10 line while minimum was observed in V5 line. A similar expression pattern was observed in transgenic sugarcane lines for Vip3A protein concentration which ranged from 5.35 to 8.89 µg/mL. A direct correlation was observed between the Vip3A protein and Vip3A transgene expression in the transgenic sugarcane lines. In in-vitro insect bioassay on V₁, Vip3A transgenic sugarcane lines exhibited high resistance to C. infuscatellus with upto 100% mortality compared to the control sugarcane line. Our findings suggest that a single copy insertion of Vip3A gene in transgenic sugarcane lines render them resistant to borer and these lines can be potentially used for generation of insect resistant transgenic sugarcane and could also be employed in gene pyramiding with Bt toxin to prolong resistance.

Alteration of insulin signaling to control insect pest by using transformed bacteria expressing dsRNA

BACKGROUND

Insulin/insulin-like growth factor signaling (IIS) is known to mediate larval growth and adult reproduction in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae). Four IIS components (InR, FOXO, Akt, and TOR) play crucial roles in the IIS pathway.

RESULTS

RNA interference (RNAi) against any of these four IIS component genes was effective in suppressing each target mRNA level by either hemocoelic injection or oral administration using gene-specific double-stranded RNAs (dsRNAs). These RNAi treatments interfered with larval growth, leading to small pupae or significant larval mortality. For massive production of dsRNA, transformed bacteria expressing dsRNAs of these four IIS components were prepared with L4440 expression vector and HT115 strain of Escherichia coli. The transformed bacteria killed the larvae in a dose-dependent manner by feeding administration. An ultra-sonication pretreatment was performed to impair bacterial membrane and increase dsRNA release from the bacteria in insect intestine. This pretreatment increased the insecticidal activity of these recombinant bacteria. To further increase dsRNA toxicity, its mixture with Bacillus thuringiensis (Bt) was prepared and showed significant increase of Bt insecticidal activity in the laboratory. The bacterial mixture also showed a high control efficacy (83.3%) in an adzuki bean (Vigna angularis) field infested by M. vitrata. Furthermore, such a dsRNA effect was specific for M. vitrata, but not for non-target insects.

CONCLUSION

The bacteria expressing dsRNA specific to IIS components can be used to develop dsRNA insecticide. © 2019 Society of Chemical Industry.

Extended evaluation of Bt protein cross-pollination in seed blend plantings on survival, growth, and development of Helicoverpa zea feeding on refuge ears

BACKGROUND

A seed blend, also called refuge in a bag (RIB), is used as a strategy to provide refuge for planting pyramided Bt corn in the U.S. Corn Belt. A major concern for the RIB strategy is cross-pollination between Bt and refuge plants, which may reduce susceptible populations of ear-feeding insects like Helicoverpa zea and affect the evolution of resistance. Previous studies showed a 5:95% (Non-Bt: Bt) RIB would be unlikely to provide sufficient refuge populations of H. zea for resistance management. In this study, we extended our research and conducted multiple trials to determine if three RIB plantings with greater refuge sizes (10, 20, and 30% refuge) could provide sufficient H. zea susceptible populations to delay resistance development.

RESULTS

Experimental results showed that cross-pollination in 10:90%, 20:80%, and 30:70% RIB plantings still significantly reduced larval survival, delayed larval development and decreased the pupal size of H. zea on refuge plants. Regression analysis showed that the percent reduction of neonate-to-adult survivorship, relative to the survivorship on pure non-Bt ears, was significantly negatively correlated with the refuge percentage.

CONCLUSION

These findings suggest that, approximately a 15% non-Bt refuge plants in RIB plantings could produce a similar number of susceptible adult moths as a 5% structured refuge planting, while an approximately 30% non-Bt refuge plant in RIB plantings could provide a similar number of susceptible moths as a 20% structured refuge planting. Information generated from this study should be useful in refining resistance management strategies for Bt crop technologies. © 2019 Society of Chemical Industry.

Identification of Cry1Ah-binding proteins through pull down and gene expression analysis in Cry1Ah-resistant and susceptible strains of Ostrinia furnacalis

Bacillus thuringiensis produces insecticidal Cry toxins used in the control of multiple insect pests. Evolution of insect resistance to Bt toxins endangers the use of Cry toxins for pest control. Analysis of the Cry1Ah-binding proteins from brush border membrane vesicles (BBMV) of Ostrinia furnacalis, Asian corn borer (ACB) from the Cry1Ah-resistant (ACB-AhR) and susceptible (ACB-BtS) strains was performed by an improved pull down assay that includes coupling Cry1Ah to NHS-activated Sepharose combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Our data show that Cry1Ah bound to alkaline phosphatase (ALP), cadherin-like (CAD), actin, aminopeptidase-N (APN), prophenoloxidase (proPO), serine proteinase inhibitor (SPI), immulectin, and V-ATPase and to other proteins that were not previously characterized as Cry-binding proteins in ACB-BtS strain. Analysis of Cry1Ah-pulled down proteins of the BBMV from ACB-AhR revealed that Cry1Ah toxin did not bind to ALP in ACB-AhR strain, suggesting that this protein may correlate with the resistant phenotype of this strain. Additionally, we analyzed the expression of representative genes coding for Cry1Ah-binding proteins such as ALP, APN, CAD, proPO, SPI, and immulectin by qRT-PCR. ACB-AhR showed increased expression levels of proPO (7.5 fold), ALP (6.2 fold) and APN (1.4 fold) in comparison to ACB-BtS strain. In contrast, the cad gene showed slight decreased expression in ACB-AhR strain (0.7 fold) compared with ACB-BtS strain. Our data suggest that differences in the susceptibility to Cry1Ah toxin in the ACB-AhR strain may be associated with reduced ALP binding sites and with an increased immune response. This study also brings evidence of a possible binding interaction of Cry1Ah toxin to immune related proteins like proPO.

Proteolysis activation of Cry1Ac and Cry2Ab protoxins by larval midgut juice proteases from Helicoverpa armigera

Proteolytic processing of Bacillus thuringiensis (Bt) Cry protoxins by insect midgut proteases is critical to their insecticidal activities against target insects. Although transgenic Bt cotton expressing Cry1Ac and Cry2Ab proteins have been widely used for control of the cotton bollworm (Helicoverpa armigera) in the field, the proteolytic cleavage sites in the two protoxins targeted by H. armigera midgut proteases are still not clear. In this study, the proteolysis of Cry1Ac and Cry2Ab protoxins by midgut juice prepared from midgut tissue of H. armigera larvae was investigated. Cleavage of Cry1Ac protoxin by midgut proteases formed a major protein fragment of ~65 kDa, and N-terminal sequencing revealed that cleavage occurred at Arg28 in the fore-end of helix α-1 in domain I of Cry1Ac. Cleavage of Cry2Ab protoxin by midgut juice proteases produced a major protein fragment of ~50 kDa, and the cleavage occurred at Arg139 between helices α-3 and α-4 in domain I of Cry2Ab. The amino acids Arg28 of Cry1Ac and Arg139 of Cry2Ab were predicted as putative trypsin cleavage sites. Bioassay data showed that the toxicities (LC₅₀s) of Cry1Ac and Cry2Ab protoxins were equivalent to those of their respective midgut juice-activated toxins in the susceptible SCD strain of H. armigera. Identification of the exact sites of N-terminal activation of Cry1Ac and Cry2Ab protoxins will provide a basis for a better understanding of the mode of action and resistance mechanisms based on aberrant activation of these protoxins in H. armigera.

Plant Abandonment by Busseola fusca (Lepidoptera: Noctuidae) Larvae: Do Bt Toxins Have an Effect?

Busseola fusca (Fuller; Lepidoptera: Noctuidae) is an important pest of maize in Africa and can be effectively controlled by Bt maize. However, the sustainability of this technology is threatened by resistance evolution, which necessitates the implementation of the high-dose/refuge insect resistance management (IRM) strategy. Despite the success of this IRM strategy, it is based on several assumptions about insect-hostplant interactions that are not always valid for different pest species. In this study, the plant abandonment behavior of Cry1Ab-resistant and susceptible B. fusca larvae were evaluated on a non-Bt, single toxin (Cry1Ab), and a pyramid event (Cry1.105 + Cry2Ab2) of maize over a four-day period. The aim was to determine if larvae are more likely to abandon maize plants that contain Bt-toxins than conventional non-Bt plants, and if resistance to the Cry1Ab-toxin affects this behavior. This study found that both Bt-resistant and susceptible B. fusca neonate larvae show feeding avoidance behavior and increased plant abandonment rates when exposed to Bt maize leaf tissue. The implications of these findings for the design of IRM strategies and choice of refuge structures are discussed in the context of Bt maize in Africa.

Insect-Resistant Genetically Engineered Crops in China: Development, Application, and Prospects for Use

With 20% of the world's population but just 7% of the arable land, China has invested heavily in crop biotechnology to increase agricultural productivity. We examine research on insect-resistant genetically engineered (IRGE) crops in China, including strategies to promote their sustainable use. IRGE cotton, rice, and corn lines have been developed and proven efficacious for controlling lepidopteran crop pests. Ecological impact studies have demonstrated conservation of natural enemies of crop pests and halo suppression of crop-pest populations on a local scale. Economic, social, and human health effects are largely positive and, in the case of Bt cotton, have proven sustainable over 20 years of commercial production. Wider adoption of IRGE crops in China is constrained by relatively limited innovation capacity, public misperception, and regulatory inaction, suggesting the need for further financial investment in innovation and greater scientific engagement with the public. The Chinese experience with Bt cotton might inform adoption of other Bt crops in China and other developing countries.

Resistance is not the end: lessons from pest management

The "war on cancer" began over 40 years ago with the signing of the National Cancer Act of 1971. Currently, complete eradication has proven possible in early stage premetastatic disease with increasingly successful early detection and surgery protocols; however, late stage metastatic disease remains invariably fatal. One of the main causes of treatment failure in metastatic disease is the ability of cancer cells to evolve resistance to currently available therapies. Evolution of resistance to control measures is a universal problem. While it may seem that the mechanisms of resistance employed by cancer cells are impossible to control, we show that many of the resistance mechanisms are mirrored in agricultural pests. In this way, we argue that measures developed in the agricultural industry to slow or prevent pesticide resistance could be adopted in clinical cancer biology to do the same. The agriculture industry recognized the problem of pesticide resistance and responded by developing and enforcing guidelines on resistance management and prevention. These guidelines, known as integrated pest management (IPM), do not encourage eradication of pests but instead strive to maintain pests, even with the presence of resistant strains, at a level that does not cause economic damage to the crops. Integrated pest management inspired management of metastatic cancer could result in the slowing or curtailing of widespread resistance to treatment, reducing overall drug usage, and increasing the survival and quality of life of patients with cancer. Using IPM principles as a foundation and shifting the goal of treatment of metastatic disease to long-term management will require close monitoring of evolving tumor populations, judicious application of currently available therapies, and development of new criteria of success.

Why do entomologists and plant pathologists approach trophic relationships so differently? Identifying biological distinctions to foster synthesis

Plant interactions with herbivores and pathogens are among the most widespread ecological relationships, and show many congruent properties. Despite these similarities, general models describing how plant defenses function in ecosystems, and the prioritization of responses to emerging challenges such as climate change, invasive species and habitat alteration, often differ markedly between entomologists and plant pathologists. We posit that some fundamental distinctions between how insects and pathogens interact with plants underlie these differences. We propose a conceptual framework to help incorporate these distinctions into robust models and research priorities. The most salient distinctions include features of host-searching behavior, evasion of plant defenses, plant tolerance to utilization, and sources of insect and microbial population regulation. Collectively, these features lead to relatively more diffuse and environmentally mediated plant-insect interactions, and more intimate and genetically driven plant-pathogen interactions. Specific features of insect vs pathogen life histories can also yield different patterns of spatiotemporal dynamics. These differences can become increasingly pronounced when scaling from controlled laboratory to open ecological systems. Integrating these differences alongside similarities can foster improved models and research approaches to plant defense, trophic interactions, coevolutionary dynamics, food security and resource management, and provide guidance as traditional departments increase collaborations, or merge into larger units.

Molecular characterization of Cry1F resistance in fall armyworm, Spodoptera frugiperda from Brazil

Fall armyworm, Spodoptera frugiperda (J.E. Smith) is a major lepidopteran pest of maize in Brazil and its control particularly relies on the use of genetically engineered crops expressing Bacillus thuringiensis (Bt) toxins such as Cry1F. However, control failures compromising the efficacy of this technology have been reported in many regions in Brazil, but the mechanism of Cry1F resistance in Brazilian fall armyworm populations remained elusive. Here we investigated the molecular mechanism of Cry1F resistance in two field-collected strains of S. frugiperda from Brazil exhibiting high levels of Cry1F resistance. We first rigorously evaluated several candidate reference genes for normalization of gene expression data across strains, larval instars and gut tissues, and identified ribosomal proteins L10, L17 and RPS3A to be most suitable. We then investigated the expression pattern of ten potential Bt toxin receptors/enzymes in both neonates and 2nd instar gut tissue of Cry1F resistant fall armyworm strains compared to a susceptible strain. Next we sequenced the ATP-dependent Binding Cassette subfamily C2 gene (ABCC2) and identified three mutated sites present in ABCC2 of both Cry1F resistant strains: two of them, a GY deletion (positions 788-789) and a P799 K/R amino acid substitution, located in a conserved region of ABCC2 extracellular loop 4 (EC4) and another amino acid substitution, G1088D, but in a less conserved region. We further characterized the role of the novel mutations present in EC4 by functionally expressing both wild type and mutated ABCC2 transporters in insect cell lines, and confirmed a critical role of both sites for Cry1F binding by cell viability assays. Finally, we assessed the frequency of the mutant alleles by pooled population sequencing and pyrosequencing in 40 fall armyworm populations collected from maize fields in different regions in Brazil. We found that the GY deletion being present at high frequency. However we also observed many rare alleles which disrupt residues between sites 783-799, and their diversity and abundance in field collected populations lends further support to the importance of the EC4 domain for Cry1F toxicity.

Larval Migration Behaviour of Busseola fusca (Lepidoptera: Noctuidae) on Bt and Non-Bt Maize under Semi-Field and Field Conditions

Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is a destructive pest of maize throughout the African continent. Bt maize is an effective control measure for this pest, however, selection pressure for resistance evolution is high. This necessitates the implementation of insect resistance management (IRM) strategies such as the high-dose/refuge strategy. This IRM strategy relies on the validity of several assumptions about the behaviour of pests during insect-hostplant interactions. In this study, the migration behaviour of B. fusca larvae was evaluated in a semi-field (greenhouse) and field setting. The effect of factors such as different Cry proteins, plant growth stage at infestation, and plant density on the rate and distance of larval migration were investigated over four and five week periods. Migration of the larvae were recorded by using both a leaf feeding damage rating scale and destructive sampling at the end of the trials. Results indicated that B. fusca larval migration success was significantly affected by plant growth stage and plant density-while limited larval migration was recorded in plots inoculated with larvae at a late growth stage (V10), higher plant density facilitated increased interplant migration. The results also suggest that B. fusca larvae do not migrate extensively (rarely further than two plants from the natal plant) and that larval mortality is high. Implications for IRM strategies are discussed.

Introgression of a cry1Ab transgene into open pollinated maize and its effect on Cry protein concentration and target pest survival

In Africa, the target pests of genetically modified Bt maize are lepidopteran stem borers, notably Busseola fusca (Lepidoptera: Noctuidae). Gene flow between Bt maize hybrids and open pollinated varieties (OPVs) that do not contain the Bt trait is highly likely in areas where both types of maize are cultivated. Consequently, introgression of the cry1Ab transgene into local OPVs will result in unknown patterns of Cry1Ab protein expression in plants during follow-up seasons when recycled seed of OPVs is planted. Too low concentrations of Cry protein in such plants may result in selection for resistant alleles and accelerate resistance evolution. The aim of this study was to determine the effects of introgression of the cry1Ab transgene into an OPV, on Cry protein concentration levels and pest survival. Bt transgene introgression was done by crossing a transgenic donor hybrid containing the cry1Ab gene with a non-Bt OPV as well as with a non-Bt near-isogenic hybrid. F1 and F2 crosses as well as back crosses were done yielding 11 genotypes (treatments). Cry1Ab protein concentrations in leaf tissue of these crosses were determined by means of ELISAs. All crosses that contained the transgene had similar or higher Cry1Ab concentrations when compared to the Bt parental hybrid, except for the Bt x OPV F1-cross that had a significantly lower Cry1Ab concentration. Survival B. fusca larvae were evaluated in assays in which larvae were reared for 14 days on whorl leaf tissue of the different treatments. Larval survival did not differ between any of the maize plant treatments which contained the Bt gene. Results suggest that Bt transgene introgression into OPVs may produce plant progenies that express Cry1Ab protein at sufficient concentrations, at last up to the F2 seed, to control B. fusca larvae. Resistance evolution is however not only influenced by the frequency of pest individuals that survive exposure to the Cry proteins but also by factors such as genetics of the pest and recipient OPV, pest biology and migration behaviour.

Bifenthrin Baseline Susceptibility and Evaluation of Simulated Aerial Applications in Striacosta albicosta (Lepidoptera: Noctuidae)

Striacosta albicosta (Smith) is a maize pest that has recently expanded its geographical range into the eastern United States and southeastern Canada. Aerial application of pyrethroids, such as bifenthrin, has been a major practice adopted to manage this pest. Reports of field failure of pyrethroids have increased since 2013. Striacosta albicosta populations were collected in 2016 and 2017 from maize fields in Nebraska, Kansas, and Canada and screened with bifenthrin active ingredient in larval contact dose-response bioassays. Resistance ratios estimated were generally low in 2016 (1.04- to 1.32-fold) with the highest LC50 in North Platte, NE (66.10 ng/cm2) and lowest in Scottsbluff, NE (50.10 ng/cm2). In 2017, O'Neill, NE showed the highest LC50 (100.66 ng/cm2) and Delhi, Canada exhibited the lowest (6.33 ng/cm2), resulting in a resistance ratio variation of 6.02- to 15.90-fold. Implications of bifenthrin resistance levels were further investigated by aerial application simulations. Experiments were conducted with a spray chamber where representative S. albicosta populations were exposed to labeled rates of a commercial bifenthrin formulation. Experiments resulted in 100% mortality for all populations, instars, insecticide rates, and carrier volumes, suggesting that levels of resistance estimated for bifenthrin active ingredient did not seem to impact the efficacy of the correspondent commercial product under controlled conditions. Results obtained from this research indicate that control failures reported in Nebraska could be associated with factors other than insecticide resistance, such as issues with the application technique, environmental conditions during and/or after application, or the insect's natural behavior. Data generated will assist future S. albicosta resistance management programs.

Global Patterns of Resistance to Bt Crops Highlighting Pink Bollworm in the United States, China, and India

Crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) have advanced pest control, but their benefits have been reduced by evolution of resistance in pests. The global monitoring data reviewed here reveal 19 cases of practical resistance to Bt crops, which is field-evolved resistance that reduces Bt crop efficacy and has practical consequences for pest control. Each case represents the responses of one pest species in one country to one Bt toxin. The results with pink bollworm (Pectinophora gossypiella) and Bt cotton differ strikingly among the world's three leading cotton-producing nations. In the southwestern United States, farmers delayed resistance by planting non-Bt cotton refuges from 1996 to 2005, then cooperated in a program that used Bt cotton, mass releases of sterile moths, and other tactics to eradicate this pest from the region. In China, farmers reversed low levels of pink bollworm resistance to Bt cotton by planting second-generation hybrid seeds from crosses between Bt and non-Bt cotton. This approach yields a refuge of 25% non-Bt cotton plants randomly interspersed within fields of Bt cotton. Farmers adopted this tactic voluntarily and unknowingly, not to manage resistance, but apparently because of its perceived short-term agronomic and economic benefits. In India, where non-Bt cotton refuges have been scarce and pink bollworm resistance to pyramided Bt cotton producing Cry1Ac and Cry2Ab toxins is widespread, integrated pest management emphasizing shortening of the cotton season, destruction of crop residues, and other tactics is now essential.

The Corn-Cotton Agroecosystem in the Mid-Southern United States: What Insecticidal Event Pyramids Should be Used in Each Crop to Extend Vip3A Durability

Recent studies suggest that resistance in Helicoverpa zea (Boddie) (Lepidoptera, Noctuidae) to Cry1A(b/c) and Cry2Ab2 toxins from the bacterium Bacillus thuringiensis Berliner (Bacillales: Bacillaceae) has increased and field efficacy is impacted in transgenic corn and cotton expressing these toxins. A third toxin, Vip3A, is available in pyramids expressing two or more Bt toxins in corn hybrids and cotton varieties, but uncertainty exists regarding deployment strategies. During a growing season, H. zea infests corn and cotton, and debate arises over use of Vip3A toxin in corn where H. zea is not an economic pest. We used a three-locus, spatially explicit simulation model to evaluate when using Vip3A in corn might hasten evolution of resistance to Vip3A, with implications in cotton where H. zea is a key pest. When using a conventional refuge in corn and initial resistance allele frequencies of Cry1A and Cry2A were 10%, transforming corn with Vip3A slowed resistance to these toxins and delayed resistance evolution to the three-toxin pyramid as a whole. When Cry resistance allele frequencies exceeded 30%, transforming corn with Vip3A hastened the evolution of resistance to the three-toxin pyramid in cotton. When using a seed blend refuge strategy, resistance was delayed longest when Vip3A was not incorporated into corn and used only in cotton. Simulations of conventional refuges were generally more durable than seed blends, even when 75% of the required refuge was not planted. Extended durability of conventional refuges compared to other models of resistance evolution are discussed as well as causes for unusual survivorship in seed blends.

Seasonal Declines in Cry1Ac and Cry2Ab Concentration in Maturing Cotton Favor Faster Evolution of Resistance to Pyramided Bt Cotton in Helicoverpa zea (Lepidoptera: Noctuidae)

Under ideal conditions, widely adopted transgenic crop pyramids producing two or more distinct insecticidal proteins from Bacillus thuringiensis (Bt) that kill the same pest can substantially delay evolution of resistance by pests. However, deviations from ideal conditions diminish the advantages of such pyramids. Here, we tested the hypothesis that changes in maturing cotton producing Cry1Ac and Cry2Ab affect evolution of resistance in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), a pest with low inherent susceptibility to both toxins. In terminal leaves of field-grown Bt cotton, the concentration of both toxins was significantly higher for young, squaring plants than for old, fruiting plants. We used laboratory bioassays with plant material from field-grown cotton to test H. zea larvae from a strain selected for resistance to Cry1Ac in the laboratory, its more susceptible parent strain, and their F1 progeny. On young Bt cotton, no individuals survived to pupation. On old Bt cotton, survival to pupation was significantly higher for the lab-selected strain and the F1 progeny relative to the unselected parent strain, indicating dominant inheritance of resistance. Redundant killing, the extent to which insects resistant to one toxin are killed by another toxin in a pyramid, was complete on young Bt cotton, but not on old Bt cotton. No significant fitness costs associated with resistance were detected on young or old non-Bt cotton. Incorporation of empirical data into simulations indicates the observed increased selection for resistance on old Bt cotton could accelerate evolution of resistance to cotton producing Cry1Ac and Cry2Ab in H. zea.

Specific binding of Bacillus thuringiensis Cry1Ea toxin, and Cry1Ac and Cry1Fa competition analyses in Anticarsia gemmatalis and Chrysodeixis includens

Anticarsia gemmatalis (velvetbean caterpillar) and Chrysodeixis includens (soybean looper) are two important defoliation pests of soybeans. In the present study, we have investigated the susceptibility and brush border membrane-binding properties of both species to Bacillus thuringiensis Cry1Ea toxin. Bioassays performed in first-instar larvae demonstrated potent activity against both soybean pests in terms of mortality or practical mortality. Competition-binding studies carried out with 125Iodine-labelled Cry1Ea, demonstrated the presence of specific binding sites on the midgut brush border membrane vesicles (BBMV) of both insect species. Heterologous competition-binding experiments indicated that Cry1Ea does not share binding sites with Cry1Ac or Cry1Fa in either soybean pest. This study contributes to the knowledge of Cry1Ea toxicity and midgut binding sites in A. gemmatalis and C. includens and sheds light on the cross-resistance potential of Cry1Ea with other Bt proteins aimed at controlling lepidopteran pests in soybeans.

Chromosomal deletions mediated by CRISPR/Cas9 in Helicoverpa armigera

Helicoverpa armigera, cotton bollworm, is one of the most disastrous pests worldwide, threatening various food and economic crops. Functional genomic tools may provide efficient approaches for its management. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system, dependent on a single guide RNA (sgRNA), has been used to induce indels for targeted mutagenesis in cotton bollworm. However, genomic deletions may be more desirable to disrupt the function of noncoding genes or regulatory sequences. By injecting two sgRNAs with Cas9 protein targeting different exons, we obtained predictable genomic deletions of several hundred bases. We achieved this type of modification with different combinations of sgRNA pairs, including HaCad and HaABCC2. Our finding indicated that CRISPR/Cas9 can be used as an efficient tool to engineer genomes with chromosomal deletion in H. armigera.

Insect Hsp90 Chaperone Assists Bacillus thuringiensis Cry Toxicity by Enhancing Protoxin Binding to the Receptor and by Protecting Protoxin from Gut Protease Degradation

Bacillus thuringiensis took advantage of important insect cellular proteins, such as chaperones, involved in maintaining protein homeostasis, to enhance its insecticidal activity. This constitutes a positive loop where the concentrations of Hsp90 and Hsp70 in the gut lumen are likely to increase as midgut cells burst due to Cry1A pore formation action. Hsp90 protects Cry1A protoxin from degradation and enhances receptor binding, resulting in increased toxicity. The effect of insect chaperones on Cry toxicity could have important biotechnological applications to enhance the toxicity of Cry proteins to insect pests, especially those that show low susceptibility to these toxins.

Bacillus thuringiensis Cry proteins are pore-forming insecticidal toxins with specificity against different crop pests and insect vectors of human diseases. Previous work suggested that the insect host Hsp90 chaperone could be involved in Cry toxin action. Here, we show that the interaction of Cry toxins with insect Hsp90 constitutes a positive loop to enhance the performance of these toxins. Plutella xylostella Hsp90 (PxHsp90) greatly enhanced Cry1Ab or Cry1Ac toxicity when fed together to P. xylostella larvae and also in the less susceptible Spodoptera frugiperda larvae. PxHsp90 bound Cry1Ab and Cry1Ac protoxins in an ATP- and chaperone activity-dependent interaction. The chaperone Hsp90 participates in the correct folding of proteins and may suppress mutations of some client proteins, and we show here that PxHsp90 recovered the toxicity of the Cry1AbG439D protoxin affected in receptor binding, in contrast to the Cry1AbR99E or Cry1AbE129K mutant, affected in oligomerization or membrane insertion, respectively, which showed a slight toxicity improvement. Specifically, PxHsp90 enhanced the binding of Cry1AbG439D protoxin to the cadherin receptor. Furthermore, PxHsp90 protected Cry1A protoxins from degradation by insect midgut proteases. Our data show that PxHsp90 assists Cry1A proteins by enhancing their binding to the receptor and by protecting Cry protoxin from gut protease degradation. Finally, we show that the insect cochaperone protein PxHsp70 also increases the toxicity of Cry1Ac in P. xylostella larvae, in contrast to a bacterial GroEL chaperone, which had a marginal effect, indicating that the use of insect chaperones along with Cry toxins could have important biotechnological applications for the improvement of Cry insecticidal activity, resulting in effective control of insect pests.

Bacteria-Mediated RNA Interference for Management of Plagiodera versicolora (Coleoptera: Chrysomelidae)

RNA interference (RNAi) has emerged as a novel and feasible strategy for pest management. Methods for cost-effective production and stable delivery of double-stranded RNA (dsRNA) to the target insects are crucial for the wide application of RNAi for pest control. In this study, we tested the expression of dsRNA in RNaseIII-deficient Escherichia coli HT115 which was then fed to Plagiodera versicolora larvae, an insect pest of Salicaceae plants worldwide. By targeting six potential genes, including actin (ACT), signal recognition particle protein 54k (SRP54), heat shock protein 70 (HSC70), shibire (SHI), cactus (CACT), and soluble N-ethylmaleimide-sensitive fusion attachment proteins (SNAP), we found that feeding bacteria-expressed dsRNA successfully triggered the silencing of the five target genes tested and the suppression of ACT and SRP54 genes caused significant mortality. Our results suggest that the oral delivery of bacteria-expressed dsRNA is a potential alternative for the control of P. versicolora, and that ACT and SRP54 genes are the potent targets.

Bt-induced hormesis in Bt-resistant insects: Theoretical possibility or factual concern?

The biphasic dose-response of a stressor where low amounts of a toxicant may stimulate some biological processes is a recent focus of attention in insecticide ecotoxicology. Nonetheless, the importance and management consequences of this phenomenon of pesticide-induced hormesis remain largely unrecognized. Curiously, the potential induction of hormesis by insecticidal proteins such as Bacillus thuringiensis toxins (i.e., Bt toxins), a major agriculture pest management tool of widespread use, has been wholly neglected. Thus, we aimed to circumvent this shortcoming while assessing the potential occurrence of hormesis induced by the Bt toxin Cry1Fa in its main target pest species - the fall armyworm Spodoptera frugiperda. Concentration-response bioassays were carried out in a Bt-susceptible and a Bt-resistant population providing the purified Cry1Fa toxin in artificial diet and recording the insect demographic parameters. As significant hormetic effect was detected in both populations with a significant increase in the net reproductive rate and the intrinsic rate of population growth, the potential occurrence of Bt-induced hormesis was subsequently tested providing the insects with leaves from transgenic Bt maize expressing the toxic protein. The performance of the Bt-resistant insects was not different in both maize genotypes, indicating that the leaf expression of the Bt protein did not promote hormesis in the resistant insects. Thus, despite the Bt-induced hormesis detected in the purified protein bioassays, the phenomenon was not detected with current levels of Bt expression in maize minimizing the risk of this additional efficacy constraint besides that of field occurrence of Bt resistance.

The many dimensions of phytochemical diversity: linking theory to practice

Research on the ecological and evolutionary roles of phytochemicals has recently progressed from studying single compounds to examining chemical diversity itself. A key conceptual advance enabling this progression is the use of species diversity metrics for quantifying phytochemical diversity. In this perspective, we extend the theory developed for species diversity to further our understanding of what exactly phytochemical diversity is and how its many dimensions impact ecological and evolutionary processes. First, we discuss the major dimensions of phytochemical diversity - richness, evenness, functional diversity, and alpha, gamma and beta diversity. We describe their potential independent roles in biotic interactions and the practical challenges associated with their analysis. Second, we re-analyse the published and unpublished datasets to reveal that the phytochemical diversity experienced by an organism (or observed by a researcher) depends strongly on the scale of the interaction and the total amount of phytochemicals involved. We argue that we must account for these frames of reference to meaningfully understand diversity. Moving from a general notion of phytochemical diversity as a single measure to a precise definition of its multidimensional and multiscale nature yields overlooked testable predictions that will facilitate novel insights about the evolutionary ecology of plant biotic interactions.