Resistance to Bt maize in Mythimna unipuncta (Lepidoptera: Noctuidae) is mediated by alteration in Cry1Ab protein activation

MiR-2b-3p Downregulated PxTrypsin-9 Expression in the Larval Midgut to Decrease Cry1Ac Susceptibility of the Diamondback Moth, Plutella xylostella (L.)

Crystal (Cry) toxins, produced by Bacillus thuringiensis, are widely used as effective biological pesticides in agricultural production. However, insects always quickly evolve adaptations against Cry toxins within a few generations. In this study, we focused on the Cry1Ac protoxin activated by protease. Our results identified PxTrypsin-9 as a trypsin gene that plays a key role in Cry1Ac virulence in Plutella xylostella larvae. In addition, P. xylostella miR-2b-3p, a member of the micoRNA-2 (miR-2) family, was significantly upregulated by Cry1Ac protoxin and targeted to PxTrypsin-9 downregulated its expression. The mRNA level of PxTrypsin-9, regulated by miR-2b-3p, revealed an increased tolerance of P. xylostella larvae to Cry1Ac at the post-transcriptional level. Considering that miR-2b and trypsin genes are widely distributed in various pest species, our study provides the basis for further investigation of the roles of miRNAs in the regulation of the resistance to Cry1Ac and other insecticides.

A novel Cry1A resistance allele of fall armyworm in the new invaded region

The fall armyworm, Spodoptera frugiperda, is a devastating pest in its native range Western Hemisphere and has become a major invasive pest around the globe. Transgenic crops producing Bt toxins have been widely used to control S. frugiperda. However, the evolution of resistance threatens the sustainability of Bt crops. Field-evolved S. frugiperda resistance to Bt crops was observed in America, whereas, no case of field-resistance was reported in its newly invaded East Hemisphere. Here we investigated the molecular mechanism of a Cry1Ab-resistant LZ-R strain of S. frugiperda, which selected 27-generations using Cry1Ab after being collected in corn fields from China. Complementation tests between LZ-R strain and SfABCC2-KO strain, which have been knockout SfABCC2 gene and confer 174-fold resistance to Cry1Ab, showed a similar level of resistance in the F₁-progeny as their parent stains, indicating that a common locus of SfABCC2 mutation in LZ-R stain. Sequencing of the full length of SfABCC2 cDNA from LZ-R strain, we characterize a novel mutation allele of SfABCC2. Cross-resistance results showed that Cry1Ab-resistance strain also confers >260-fold resistance to Cry1F, with no cross-resistance to Vip3A. These results provided evidence of a novel SfABCC2 mutation allele in the newly invaded East Hemisphere of S. frugiperda.

Monitoring Insect Resistance to Bt Maize in the European Union: Update, Challenges, and Future Prospects

Transgenic maize producing the Cry1Ab toxin of Bacillus thuringiensis (Bt maize) was approved for cultivation in the European Union (EU) in 1998 to control the corn borers Sesamia nonagrioides (Lefèbvre) and Ostrinia nubilalis (Hübner). In the EU since then, Cry1Ab is the only Bt toxin produced by Bt maize and Spain is the only country where Bt maize has been planted every year. In 2021, about 100,000 hectares of Bt maize producing Cry1Ab were cultivated in the EU, with Spain accounting for 96% and Portugal 4% of this area. In both countries, Bt maize represented less than 25% of all maize planted in 2021, with a maximum regional adoption of 64% Bt maize in northeastern Spain. Insect resistance management based on the high-dose/refuge strategy has been implemented in the EU since 1998. This has been accompanied by monitoring to enable early detection of resistance. The monitoring data from laboratory bioassays show no decrease in susceptibility to Cry1Ab had occurred in either pest as of 2021. Also, control failures have not been reported, confirming that Bt maize producing Cry1Ab remains effective against both pests. Conditions in the EU preventing approval of new genetically modified crops, including maize producing two or more Bt toxins targeting corn borers, may limit the future effectiveness of resistance management strategies.

Up-regulated serpin gene involved in Cry1Ac resistance in Helicoverpa armigera

Insect resistance to Bacillus thuringiensis (Bt) is a critical limiting factor for applying the Bt crops. Some studies indicated that decreased protoxin activation because of lower enzymatic activities of trypsin and chymotrypsin and increased expression of serpin might involve in Bt resistance. Our previous study identified an endogenous serpin could inhibit the midgut proteases to activate Cry1Ac and reduce the insecticide activity to Helicoverpa armigera. We hypothesis that up-regulated serpin involve in resistance via inhibiting enzymatic activities of trypsin and chymotrypsin to decrease protoxin activation. Herein, we found the serpin-e gene relative expression in midgut was significantly higher in the LF30 resistant strain than that in the susceptible strain during all developmental stages. Importantly, RNAi-mediated silencing of serpin-e gene expression caused 4.46-fold mortality changes in LF30 strain, but the trypsin and chymotrypsin proteases activities were only changed 0.79-fold and 2.22-fold. In addition, although proteases activities were significantly lower in LF30 strain than that in the susceptible strain, the resistance ratios of LF30 to Cry1Ac protoxin and to activated Cry1Ac toxin were no difference. The results indicated serpins caused insect resistance to Cry1Ac protoxins partly through inhibiting the trypsin and chymotrypsin proteases activities, but it also existed other mechanisms in LF30.

Fitness Costs of Two Maize Lepidopteran Pests Fed on Bacillus thuringiensis (Bt) Diets Enriched with Vitamins A and C

Simple Summary

Biotechnologists are designing new transgenic plants enriched with micronutrients and vitamins that are resistant to insects. These new plants could favor the development of some pest insects. This work aims to discover the effect of adding two vitamins, A and C, to insect diets prepared with Bt and no-Bt maize in two maize insect pests. M. unipuncta was less sensitive to the toxin, although ingestion of the Bt diet resulted in longer larval development and lower pupal weight, which were not mitigated by any of the vitamins. However, the two vitamins reduced the mortality of H. armigera larvae fed on the Bt diet. In addition, we found evidence of the antioxidant function of vitamin A. The results obtained here indicate that crops enriched with these vitamins will hardly favor the development of H. armigera and suggest that they do not affect M. unipuncta’s development at all.

Abstract

Serious malnutrition problems occur in developing countries where people’s diets are mainly based on staple crops. To alleviate this, high-production crops are being developed that are better adapted to climate change, enriched in micronutrients and vitamins, or resistant to pests. In some cases, new varieties have been developed with several of the characteristics mentioned above, such as biofortified and pest-resistant crops. The development of biofortified Bacillus thuringiensis (Bt) crops raises the question of whether vitamin enrichment of Bt crops can in any way favor those pests that are not very susceptible to the Bt toxin that feed on these crops, such as Helicoverpa armigera (Hübner) or Mythimna unipuncta (Haworth) (Lepidoptera: Noctuidae). In this study, the response to a Bt diet enriched with vitamins A (β-carotene) and C (ascorbic acid) was somewhat different between the two species. M. unipuncta was less sensitive to the toxin than H. armigera, although the ingestion of the Bt diet resulted in oxidative stress (longer larval development and lower pupal weight) which was not mitigated by the vitamins. However, the two vitamins reduced the mortality of H. armigera larvae fed on a Bt-enriched diet; in addition, ß-carotene reduced the activity of the antioxidant glutathione S-transferase (GST) of both species, suggesting it has an antioxidant role. The results obtained here indicate that biofortified Bt crops will not favor the development of H. armigera very much and will not affect M. unipuncta’s development at all, although the effect of the increase in vitamins may be very variable and should be studied for each specific phytophagous.

Baseline Susceptibility and Laboratory Selection of Resistance to Bt Cry1Ab Protein of Chinese Populations of Yellow Peach Moth, Conogethes punctiferalis (Guenée)

Yellow Peach Moth (YPM), Conogethes punctiferalis (Guenée), is one of the most destructive maize pests in the Huang-Huai-Hai summer maize region of China. Transgenic Bacillus thuringiensis (Bt) maize provides an effective means to control this insect pest in field trials. However, the establishment of Bt resistance to target pests is endangering the continued success of Bt crops. To use Bt maize against YPM, the baseline susceptibility of the local populations in the targeted areas needs to be verified. Diet-overlay bioassay results showed that all the fourteen YPM populations in China are highly susceptible to Cry1Ab. The LC₅₀ values ranged from 0.35 to 2.38 ng/cm² over the two years of the collection, and the difference between the most susceptible and most tolerant populations was sevenfold. The upper limit of the LC₉₉ estimates of six pooled populations produced >99% larval mortality for representative eight populations collected in 2020 and was designated as diagnostic concentrations for monitoring susceptibility in YPM populations in China. Hence, we evaluated the laboratory selection of resistance in YPM to Cry1Ab using the diet-overlay bioassay method. Although the resistant ratio was generally low, YPM potentially could evolve resistance to Cry1Ab. The potential developmentof resistance by target pests points out the necessity to implement resistance management strategies for delaying the establishment of pest resistance to Bt crops.

Gut-specific arylphorin mediates midgut regenerative response against Cry-induced damage in Achaea janata

Development of insect resistance to biopesticides is a current and pertinent global issue. Earlier, it was established that lepidopteran larvae can recover from Bt intoxication via a midgut regenerative response and subsequently generate resistance. Molecular aspects of restoration of the midgut integrity following toxin exposure are emerging recently. In the present study, we bring out the pivotal role of gut arylphorin in mediating the midgut regenerative response following sublethal Bt exposure in Achaea janata. Bt-induced midgut damage was characterized by microscopic analysis using differential interference contrast (DIC) and immunofluorescence (IF). Extensive disruption of brush-border membrane, associated with the underlying cytoskeletal alterations including F-actin, α-actin and β-tubulin was observed. Single-photon fluorescence microscopy combined with fluorescence lifetime imaging (FLIM) established the metabolic state associated with enhanced stem cell proliferation and migration from the basal side towards the luminal side following the damage. In-silico analysis revealed the phylogenetic relationship of gut arylphorin with closely related insect species and indicated the presence of two different subunits. Transient RNAi knockdown of the arylphorin resulted in diminished expression of mitotic Cyclin B mRNA levels. Human monoclonal Cyclin B antibody cross-reactivity with the Cyclin B located in the stem cells further validate the role of arylphorin as the mitogenic factor responsible for stem cell proliferation and epithelial regeneration. An in-depth understanding of resistance mechanisms will aid in the design of new strategies for the long-term usage and efficacy of Bt technology against pest control.

Response Mechanisms of Invertebrates to Bacillus thuringiensis and Its Pesticidal Proteins

Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.

The Common Fruit-Piercing Moth in the Pacific Region: A Survey of the Current State of a Significant Worldwide Economic Pest, Eudocima phalonia (Lepidoptera: Erebidae), with a Focus on New Caledonia

Simple Summary

Fruit-piercing moths have long been cited as important pests in tropical and subtropical countries but genus as Eudocima, has recently gained in significance, and more specifically Eudocima phalonia (Linneaus). An overview of the current pest control proposed in the literature pointed the lack of sustainable integrated pest management. A synthesis of available data opens the research per-spectives that need to be encouraged in the ecological transition of our agricultural models.

Abstract

When referring to fruit-piercing moths, the genus Eudocima, and more specifically Eudocima phalonia (Linneaus), is cited as a worldwide crop pest. Damages associated with this pest are substantial on more than 100 fruit species, wherever it is encountered. In New Caledonia, the once occasional pest has become a serious threat to the current fruit arboriculture. Particularly devastating during outbreak periods, it has become an urgent need to find a suitable solution able to support farmers in the ecological transition of our agricultural models. This review proposes a synthesis of the existing data and publications on E. phalonia, worldwide and especially in New Caledonia, with recent observations. The assessment of this knowledge and the dynamics of the species in the territory of New Caledonia provide key information for a better prospect of adapted solutions.

Mechanisms of Resistance to Insecticidal Proteins from Bacillus thuringiensis

Insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are used in sprayable formulations or produced in transgenic crops as the most successful alternatives to synthetic pesticides. The most relevant threat to sustainability of Bt insecticidal proteins (toxins) is the evolution of resistance in target pests. To date, high-level resistance to Bt sprays has been limited to one species in the field and another in commercial greenhouses. In contrast, there are currently seven lepidopteran and one coleopteran species that have evolved practical resistance to transgenic plants producing insecticidal Bt proteins. In this article, we present a review of the current knowledge on mechanisms of resistance to Bt toxins, with emphasis on key resistance genes and field-evolved resistance, to support improvement of Bt technology and its sustainability.

Performance of Sesamia nonagrioides on cultivated and wild host plants: Implications for Bt maize resistance management

BACKGROUND

Sesamia nonagrioides is an important maize pest in the Mediterranean basin that is effectively controlled by Cry1Ab-expressing maize (Bt maize). The continued cultivation of Bt maize in Spain exerts high selection pressure on the target pests, which could lead to the development of resistance. Provision of refuges of non-Bt plants is an essential component in the high-dose/refuge (HDR) strategy to delay resistance evolution. Here we analyze the suitability of cultivated (rice and sorghum) and wild (Johnsongrass, cattail, common reed and giant reed) plants, reported as hosts of S. nonagrioides, for larval development and oviposition of this pest compared to maize, and we evaluate their potential role in delaying resistance development to Bt maize.

RESULTS

Bioassays conducted with plant pieces or whole plants showed that the larval cycle could only be completed in the three cultivated plants and in Johnsongrass. Females showed a strong preference for ovipositing on maize in comparison with sorghum or rice. Although young larvae consumed more sorghum than maize in two-choice bioassays, both larvae and adults had a better performance (shorter larval period and higher pupal weight, fecundity and fertility) when larvae fed on maize throughout their larval stage than when they fed on sorghum or rice.

CONCLUSION

None of the alternative hosts of S. nonagrioides tested here should be considered as natural unstructured refuges within the HDR strategy for Bt maize and this pest in Spain, as some of the necessary requirements to fulfill this strategy would not be met. © 2020 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Comprehensive analysis of Cry1Ac protoxin activation mediated by midgut proteases in susceptible and resistant Plutella xylostella (L.)

Insecticidal Cry toxins produced by Bacillus thuringiensis (Bt) have been widely used to control agricultural pests in both foliage sprays and transgenic crops. Nevertheless, rapid evolution of insect resistance to Cry toxins requires elucidation of the molecular mechanisms involved in Cry resistance. Two proposed models have been described to explain the toxicity of Cry proteins, the classic model states that Cry protoxin is activated by midgut proteases resulting in activated toxin that binds to receptors and forms a pore in the midgut cells triggering larval death, and the newly proposed dual model of the mode of action of Bt Cry toxins states that protoxin and activated toxins may have different mechanisms of action since several resistant strains to activated Cry toxins are still susceptible to the same Cry-protoxin. Protoxin activation by midgut proteases is a key step in both models. Herein, we evaluated Cry1Ac protoxin activation in a susceptible Plutella xylostella (L.) strain (DBM1Ac-S) and in the near-isogenic strain (NIL-R) with high field-evolved Cry1Ac resistance. Previous work showed that Cry1Ac resistance in NIL-R correlates with reduced binding to midgut receptors due to enhanced MAPK signaling pathway and down regulation of ABCC2 receptor. However, reduced midgut trypsin levels and altered midgut protease gene transcription were also observed in the Cry1Ac-resistant field isolated strain that is parent of the NIL-R strain. Therefore, we analyzed the midgut protease activities in both DBM1Ac-S and NIL-R strains. Detection of enzymatic activities showed that caseinolytic protease, trypsin and chymotrypsin activities were not significantly different between the susceptible and resistant strains. Furthermore, treatment with different trypsin or chymotrypsin inhibitors, such as Nα-tosyl-l-lysine chloromethyl ketone (TLCK) or Np-tosyl-L-phenylalanine chloromethyl ketone (TPCK) did not affect the susceptibility to Cry1Ac protoxin of the DBM1Ac-S and NIL-R larvae. Bioassay results indicated that the NIL-R larvae showed similar resistant levels to both Cry1Ac protoxin and trypsin-activated toxin. Taken together, our results demonstrated that high-level field-evolved Cry1Ac resistance in the NIL-R strain is independent of Cry1Ac protoxin activation and the specific protoxin mechanism of action. This discovery will strengthen our comprehensive understanding of the complex mechanistic basis of Bt resistance in different insects.

Reduced Expression of a Novel Midgut Trypsin Gene Involved in Protoxin Activation Correlates with Cry1Ac Resistance in a Laboratory-Selected Strain of Plutella xylostella (L.)

Bacillus thuringiensis (Bt) produce diverse insecticidal proteins to kill insect pests. Nevertheless, evolution of resistance to Bt toxins hampers the sustainable use of this technology. Previously, we identified down-regulation of a trypsin-like serine protease gene PxTryp_SPc1 in the midgut transcriptome and RNA-Seq data of a laboratory-selected Cry1Ac-resistant Plutella xylostella strain, SZ-R. We show here that reduced PxTryp_SPc1 expression significantly reduced caseinolytic and trypsin protease activities affecting Cry1Ac protoxin activation, thereby conferring higher resistance to Cry1Ac protoxin than activated toxin in SZ-R strain. Herein, the full-length cDNA sequence of PxTryp_SPc1 gene was cloned, and we found that it was mainly expressed in midgut tissue in all larval instars. Subsequently, we confirmed that the PxTryp_SPc1 gene was significantly decreased in SZ-R larval midgut and was further reduced when selected with high dose of Cry1Ac protoxin. Moreover, down-regulation of the PxTryp_SPc1 gene was genetically linked to resistance to Cry1Ac in the SZ-R strain. Finally, RNAi-mediated silencing of PxTryp_SPc1 gene expression decreased larval susceptibility to Cry1Ac protoxin in the susceptible DBM1Ac-S strain, supporting that low expression of PxTryp_SPc1 gene is involved in Cry1Ac resistance in P. xylostella. These findings contribute to understanding the role of midgut proteases in the mechanisms underlying insect resistance to Bt toxins.

Decreased Cry1Ac activation by midgut proteases associated with Cry1Ac resistance in Helicoverpa zea

BACKGROUND

Field-evolved resistance of Helicoverpa zea to Bacillus thuringiensis (Bt) toxin Cry1Ac was first reported more than a decade ago, yet the underlying mechanisms remain elusive. Towards understanding the mechanisms of resistance to Cry1Ac, we analyzed a susceptible (LAB-S) and two resistant (GA and GA-R) strains of H. zea. The GA strain was derived from Georgia and exposed to Bt toxins only in the field. The GA-R strain was derived from the GA strain and selected for increased resistance to Cry1Ac in the laboratory.

RESULTS

Resistance to MVPII, a liquid formulation containing a hybrid protoxin similar to Cry1Ac, was 110-fold for GA-R and 7.8-fold for GA relative to LAB-S. In midgut brush border membrane vesicles, activity of alkaline phosphatase and aminopeptidase N did not vary significantly among strains. The activity of total proteases, trypsin-like proteases and chymotrypsin-like proteases was significantly lower for GA-R and GA than LAB-S, but did not differ between GA-R and GA. When H. zea midgut cells were exposed to Cry1Ac protoxin that had been digested with midgut extracts, toxicity was significantly lower for extracts from GA-R and GA relative to extracts from LAB-S, but did not differ between GA-R and GA. Transcriptional analysis showed that none of the five protease genes examined was associated with the decline in Cry1Ac activation in GA-R and GA relative to LAB-S.

CONCLUSION

The results suggest that decreased Cry1Ac activation is a contributing field-selected mechanism of resistance that helps explain the reduced susceptibility of the GA-R and GA strains. Relative to the LAB-S strain, the two Cry1Ac-resistant strains had lower total protease, trypsin and chymotrypsin activities, a lower Cry1Ac activation rate, and Cry1Ac protoxin incubated with their midgut extracts was less toxic to H. zea midgut cells. © 2018 Society of Chemical Industry.

Efficacy of Chlorantraniliprole Seed Treatments Against Armyworm (Mythimna unipuncta [Lepidoptera: Noctuidae]) Larvae on Corn (Zea mays)

Mythimna unipuncta (Haworth) (Lepidoptera: Noctuidae) is an important insect pest of corn (Zea mays L.) in North America and can cause severe damage during outbreaks. Insecticides are the main control method; however, development of insecticide resistance poses management challenges and necessitates the use of novel insecticides. A synthetic insecticide, chlorantraniliprole, belonging to the anthranilic diamides, targets insect ryanodine receptors and is a potential alternative to conventional insecticides for management of M. unipuncta. We determined the efficacy of seed treatment with chlorantraniliprole alone compared with a positive control consisting of a neonicitinoid seed treatment of thiamethoxam and combinations of both compounds against M. unipuncta larvae in laboratory bioassays. Bioassays were conducted on two different growth stages of M. unipuncta larvae (instars 3 and 5) and two plant growth stages (V1 and V2 corn) in clip cages and whole plant experiments. Larval mortality, head capsule width, and feeding injury were measured. The chlorantraniliprole seed treatment alone or in combination with thimethoxam at different doses affected survival of M. unipuncta larvae. In all bioassays except one, larval mortality occurred earlier when a combination of chlorantraniliprole and thimethoxam seed treatment was used when compared with a thimethoxam alone seed treatment. Mythimna unipuncta larvae developed faster on the untreated control corn plants compared with corn with insecticide seed treatments. Foliar injury was low in both chlorantraniliprole alone and chlorantraniliprole + thimethoxam treatments compared with the control. Chlorantraniliprole thus offers potential alternative to conventionally used insecticides in the management of M. unipuncta in corn.

Insecticidal Activity and Synergistic Combinations of Ten Different Bt Toxins against Mythimna separata (Walker)

The oriental armyworm (OAW), Mythimna separata (Walker), is a destructive pest of agricultural crops in Asia and Australia. Commercialized Bt crops have performed very well against their target pests; however, very few studies have been done on the susceptibility of OAW to Bt toxins in either sprays or expressed in Bt crops. In this work, we evaluated the toxicities of Cry1Ab, Cry1Ac, Cry1Ah, Cry1Fa, Cry2Aa, Cry2Ab, Cry1Ie, Vip3Aa19, Vip3Aa16, and Vip3Ca against OAW neonate larvae, as well as the interaction between Cry and Vip toxins. The results from bioassays revealed that LC50 (lethal concentration for 50% mortality) values ranged from 1.6 to 78.6 μg/g (toxin/diet) for those toxins. Among them, Vip3 proteins, along with Cry1A proteins and Cry2Aa, were the ones with the highest potency, with LC50 values ranging from 1.6 to 7.4 μg/g. Synergism between Cry and Vip toxins was observed, being high in the combination of Vip3Aa16 with Cry1 toxins, with synergetic factors ranging from 2.2 to 9.2. The Vip3Ca toxin did not show any synergistic effect with any of the toxins tested. These results can help in designing new combinations of pyramiding genes in Bt crops, as well as in recombinant bacteria, for the control of OAW as well as for resistance management programs.

Responses of the cutworm Spodoptera litura (Lepidoptera: Noctuidae) to two Bt corn hybrids expressing Cry1Ab

To examine the responses of the secondary lepidopteran pest Spodoptera litura to two Bacillus thuringiensis (Bt) corn hybrids [5422Bt1 (Event Bt11), 5422CBCL (MON810)] expressing Cry1Ab, larval bioassays with Cry1Ab toxin, corn leaves or kernels and bagging on corn plants were conducted. The results showed that larvae displayed a similar performance when fed kernels, but not leaves of 5422Bt1, 5422CBCL and their near-isogenic non-Bt corn (5422). Significantly higher Cry1Ab amounts were detected in larvae fed leaves than kernels of both Bt hybrids, with different molecular weights of protein band in plants (72 and 90 kDa for 5422Bt1 and 5422CBCL, respectively), gut contents (65 kDa), feces (50 kDa), which indicated that larvae had lower ingestion, higher degradation and excretion of Cry1Ab when fed kernels not leaves of both Bt hybrids. Significantly higher levels of cadherin-like receptors and alkaline phosphatase transcripts were detected in larvae fed leaves than kernels of two Bt hybrids. Catalase, superoxide dismutase and glutathione-S-transferase activities in larvae fed 5422Bt1 leaves were significantly higher than that of 5422 treatments. Therefore, S. litura had low susceptibility to 5422Bt1 and 5422CBCL when larvae fed kernels not leaves of Bt corn. Additionally, S. litura presented a much stronger tolerance to 5422CBCL than 5422Bt1.

QTL Mapping for Yield and Resistance against Mediterranean Corn Borer in Maize

Introduction: The Mediterranean corn borer (MCB), Sesamia nonagrioides, is a major pest of maize, Zea mays, in Mediterranean countries, inflicting significant kernel yield losses. For that reason, it necessary to know the genetic mechanisms that regulate the agronomic and resistance traits. A quantitative trait loci (QTL) mapping study for yield, resistance against MCB attack, and other relevant agronomic traits was performed using a recombinant inbred line (RIL) population derived from the cross A637 × A509 that is expected to segregate for yield, and ear, and stalk resistance to MCB. 171 RILs were evaluated in 2014 and 2015 at Pontevedra, Spain, along with the two parental inbreds A637 and A509 using a 13 × 14 single lattice design with two replications. A genetic map with 285 SNP markers was used for QTL analysis. Our objectives were to detect QTL for resistance to MCB and tolerance-related agronomic traits, to gain insights on the genetic relationship between resistance to MCB attack and yield, and to establish the best way for simultaneously improving yield and resistance to MCB. Results: Twelve significant QTL were detected for agronomic and resistance traits. QTL at bins 1.10 and 5.04 were especially interesting because the same allelic variant at these QTL simultaneously improved yield and insect resistance. In contrast, in the region 8.04-8.05, QTL showed opposite effects for yield and resistance. Several QTL for indexes which combine yield and resistance traits were found especially in the region 10.02-10.03. Conclusions: Selecting genotypes with the favorable allele of QTL on chromosome 5 (bin 5.01) will decrease tunnel length without affect yield, silking and plant height and QTL on the region 5.04 could be used to improve stalk resistance and yield simultaneously. An allele of QTL on bin 9.07 will increase ear resistance to MCB attack but it could produce later varieties while favorable allele in region 1.10 could improve ear and stalk resistance and yield without secondary negative effects. The region 8.03-8.05 mainly but also the region 10.02-10.03 and 5.04 may play an important role to elucidate the association between yield, other agronomic traits and MCB resistance.

Activation of Bt Protoxin Cry1Ac in Resistant and Susceptible Cotton Bollworm

Crystalline (Cry) proteins from Bacillus thuringiensis (Bt) are used extensively for insect control in sprays and transgenic plants, but their efficacy is reduced by evolution of resistance in pests. Here we evaluated reduced activation of Cry1Ac protoxin as a potential mechanism of resistance in the invasive pest Helicoverpa armigera. Based on the concentration killing 50% of larvae (LC₅₀) for a laboratory-selected resistant strain (LF120) divided by the LC₅₀ for its susceptible parent strain (LF), the resistance ratio was 1600 for Cry1Ac protoxin and 1200 for trypsin-activated Cry1Ac toxin. The high level of resistance to activated toxin as well as to protoxin indicates reduced activation of protoxin is not a major mechanism of resistance to Cry1Ac in LF120. For both insect strains, treatment with either the trypsin inhibitor N-a-tosyl-L-lysine chloromethyl ketone (TLCK) or the chymotrypsin inhibitor N-a-tosyl-L-phenylalanine chloromethyl ketone (TPCK) did not significantly affect the LC₅₀ of Cry1Ac protoxin. Enzyme activity was higher for LF than LF120 for trypsin-like proteases, but did not differ between strains for chymotrypsin-like proteases. The results here are consistent with previous reports indicating that reduced activation of protoxin is generally not a major mechanism of resistance to Bt proteins.

Sixteen Years of Bt Maize in the EU Hotspot: Why Has Resistance Not Evolved?

The majority of Bt maize production in the European Union (EU) is concentrated in northeast Spain, which is Europe's only hotspot where resistance might evolve, and the main target pest, Sesamia nonagrioides, has been exposed to Cry1Ab maize continuously since 1998. The cropping system in northeast Spain has some similar characteristics to those that probably led to rapid resistance failures in two other target noctuid maize pests. These include repeated cultivation of Bt maize in the same fields, low use of refuges, recurring exposure of larvae to non-high dose concentrations of Cry1Ab toxin during the first years of cultivation, low migratory potential, and production concentrated in an irrigated region with few alternative hosts. Available data reveal no evidence of resistance in S. nonagrioides after 16 years of use. We explore the possible reasons for this resistance management success using evolutionary models to consider factors expected to accelerate resistance, and those expected to delay resistance. Low initial adoption rates and the EU policy decision to replace Event 176 with MON 810 Bt maize were key to delaying resistance evolution. Model results suggest that if refuge compliance continues at the present 90%, Bt maize might be used sustainably in northeast Spain for at least 20 more years before resistance might occur. However, obtaining good estimates of the present R allele frequency and level of local assortative mating are crucial to reduce uncertainty about the future success of resistance management.

Inheritance, fitness costs, incomplete resistance and feeding preferences in a laboratory-selected MON810-resistant strain of the true armyworm Mythimna unipuncta

BACKGROUND

The low efficacy of MON810 maize against Mythimna unipuncta represents a scenario of non-compliance with the 'high-dose' strategy, raising concerns about potential resistance development and outbreaks of this secondary pest. The present study offers insight into the different components related to resistance in a laboratory-selected MON810-resistant (MR) strain of M. unipuncta.

RESULTS

The resistance in the MR strain is autosomal and inherited as a partially dominant trait. We have found a lack of fitness costs in this strain for essential life history traits, reproductive potential and most of the population growth parameters analysed, the only exception being an increment in the mean generation time. Larvae of the MR strain reared on Bacillus thuringiensis (Bt) maize took longer to develop, presented a high adult cumulative emergence time and had lower growth rate than those reared on non-Bt maize, suggesting the existence of incomplete resistance. Feeding preference assays reveal a low discrimination between Bt and conventional maize.

CONCLUSION

Both resistant and heterozygous larvae of M. unipuncta survive the Cry1Ab toxin expressed on Bt maize, with a weak fitness cost for the homozygous larvae, indicating the potential risk of field-evolved resistance and its relevance to resistance monitoring. © 2015 Society of Chemical Industry.

Dual mode of action of Bt proteins: protoxin efficacy against resistant insects

Transgenic crops that produce Bacillus thuringiensis (Bt) proteins for pest control are grown extensively, but insect adaptation can reduce their effectiveness. Established mode of action models assert that Bt proteins Cry1Ab and Cry1Ac are produced as inactive protoxins that require conversion to a smaller activated form to exert toxicity. However, contrary to this widely accepted paradigm, we report evidence from seven resistant strains of three major crop pests showing that Cry1Ab and Cry1Ac protoxins were generally more potent than the corresponding activated toxins. Moreover, resistance was higher to activated toxins than protoxins in eight of nine cases evaluated in this study. These data and previously reported results support a new model in which protoxins and activated toxins kill insects via different pathways. Recognizing that protoxins can be more potent than activated toxins against resistant insects may help to enhance and sustain the efficacy of transgenic Bt crops.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

The impact of secondary pests on Bacillus thuringiensis (Bt) crops

The intensification of agriculture and the development of synthetic insecticides enabled worldwide grain production to more than double in the last third of the 20th century. However, the heavy dependence and, in some cases, overuse of insecticides has been responsible for negative environmental and ecological impacts across the globe, such as a reduction in biodiversity, insect resistance to insecticides, negative effects on nontarget species (e.g. natural enemies) and the development of secondary pests. The use of recombinant DNA technology to develop genetically engineered insect-resistant crops could mitigate many of the negative side effects of insecticides. One such genetic alteration enables crops to express toxic crystalline (Cry) proteins from the soil bacteria Bacillus thuringiensis (Bt). Despite the widespread adoption of Bt crops, there are still a range of unanswered questions concerning longer term agro-ecosystem interactions. For instance, insect species that are not susceptible to the expressed toxin can develop into secondary pests and cause significant damage to the crop. Here, we review the main causes surrounding secondary pest dynamics in Bt crops and the impact of such outbreaks. Regardless of the causes, if nonsusceptible secondary pest populations exceed economic thresholds, insecticide spraying could become the immediate solution at farmers' disposal, and the sustainable use of this genetic modification technology may be in jeopardy. Based on the literature, recommendations for future research are outlined that will help to improve the knowledge of the possible long-term ecological trophic interactions of employing this technology.

Genome-wide association study reveals a set of genes associated with resistance to the Mediterranean corn borer (Sesamia nonagrioides L.) in a maize diversity panel

BACKGROUND

Corn borers are the primary maize pest; their feeding on the pith results in stem damage and yield losses. In this study, we performed a genome-wide association study (GWAS) to identify SNPs associated with resistance to Mediterranean corn borer in a maize diversity panel using a set of more than 240,000 SNPs.

RESULTS

Twenty five SNPs were significantly associated with three resistance traits: 10 were significantly associated with tunnel length, 4 with stem damage, and 11 with kernel resistance. Allelic variation at each significant SNP was associated with from 6 to 9% of the phenotypic variance. A set of genes containing or physically close to these SNPs are proposed as candidate genes for borer resistance, supported by their involvement in plant defense-related mechanisms in previously published evidence. The linkage disequilibrium decayed (r(2) < 0.10) rapidly within short distance, suggesting high resolution of GWAS associations.

CONCLUSIONS

Most of the candidate genes found in this study are part of signaling pathways, others act as regulator of expression under biotic stress condition, and a few genes are encoding enzymes with antibiotic effect against insects such as the cystatin1 gene and the defensin proteins. These findings contribute to the understanding the complex relationship between plant-insect interactions.

Proteolytic processing of Bacillus thuringiensis Vip3A proteins by two Spodoptera species

Vip3 proteins have been described to be secreted by Bacillus thuringiensis during the vegetative growth phase and to display a broad insecticidal spectrum against lepidopteran larvae. Vip3Aa protoxin has been reported to be significantly more toxic to Spodoptera frugiperda than to Spodoptera exigua and differences in the midgut processing have been proposed to be responsible. In contrast, we have found that Vip3Ae is essentially equally toxic against these two species. Proteolysis experiments were performed to study the stability of Vip3A proteins to peptidase digestion and to see whether the differences found could explain differences in toxicity against these two Spodoptera species. It was found that activation of the protoxin form and degradation of the 62kDa band took place at lower concentrations of trypsin when using Vip3Aa than when using Vip3Ae. The opposite effect was observed for chymotrypsin. Vip3Aa and Vip3Ae protoxins were effectively processed by midgut content extracts from the two Spodoptera species and the proteolytic activation did not produce a peptidase resistant core under these in vitro conditions. Digestion experiments performed with S. frugiperda chromatography-purified digestive serine peptidases showed that the degradation of the Vip3A toxins active core is mainly due to the action of cationic chymotrypsin-like peptidase. Although the digestion patterns of Vip3A proteins do not always correlate with toxicity, the peptidase stability of the 62kDa core is in agreement with intraspecific differences of toxicity of the Vip3Aa protein.

Response of last instar Helicoverpa armígera larvae to Bt toxin ingestion: changes in the development and in the CYP6AE14, CYP6B2 and CYP9A12 gene expression

Bt crops are able to produce Cry proteins, which were originally present in Bacillus thuringiensis bacteria. Although Bt maize is very efficient against corn borers, Spanish crops are also attacked by the earworm H. armigera, which is less susceptible to Bt maize. Many mechanisms could be involved in this low susceptibility to the toxin, including the insect's metabolic resistance to toxins due to cytochrome P450 monooxygenases. This paper examines the response of last instar H. armigera larvae to feeding on a diet with Bt and non-Bt maize leaves in larval development and in the gene expression of three P450 cytochromes: CYP6AE14, CYP6B2 and CYP9A12. Larvae fed on sublethal amounts of the Bt toxin showed reduced food ingestion and reduced growth and weight, preventing most of them from achieving the critical weight and pupating; additionally, after feeding for one day on the Bt diet the larvae showed a slight increase in juvenile hormone II in the hemolymp. Larvae fed on the non-Bt diet showed the highest CYP6AE14, CYP6B2 and CYP9A12 expression one day after feeding on the non-Bt diet, and just two days later the expression decreased abruptly, a finding probably related to the developmental programme of the last instar. Moreover, although the response of P450 genes to plant allelochemicals and xenobiotics has been related in general to overexpression in the resistant insect, or induction of the genes when feeding takes place, the expression of the three genes studied was suppressed in the larvae feeding on the Bt toxin. The unexpected inhibitory effect of the Cry1Ab toxin in the P450 genes of H. armigera larvae should be thoroughly studied to determine whether this response is somehow related to the low susceptibility of the species to the Bt toxin.