Cry1Ab Toxin Research Articles

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.

Daphnia magna negatively affected by chronic exposure to purified Cry-toxins

Cry-toxin genes originating from Bacillus thuringiensis are inserted into genetically modified (GM) plants, often called Bt-plants, to provide insect resistance to pests. Significant amounts of Bt-plant residues, and thus Cry-toxins, will be shed to soil and aquatic environments.We exposed Daphnia magna to purified Cry1Ab and Cry2Aa toxins for the full life-span of the animals. We used single toxins in different doses and combinations of toxins and Roundup®, another potential stressor on the rise in agricultural ecosystems.Animals exposed to 4.5 mg/L (ppm) of Cry1Ab, Cry2Aa and the combination of both showed markedly higher mortality, smaller body size and very low juvenile production compared to controls. Animals exposed to 0.75 mg/L also showed a tendency towards increased mortality but with increased early fecundity compared to the controls. Roundup® stimulated animals to strong early reproductive output at the cost of later rapid mortality.We conclude that i) purified Cry-toxins in high concentrations are toxic to D. magna, indicating alternative modes-of-action for these Cry-toxins; ii) Cry-toxins act in combination, indicating that ‘stacked events’ may have stronger effects on non-target organisms; iii) further studies need to be done on combinatorial effects of multiple Cry-toxins and herbicides that co-occur in the environment.

Effects of three-year cultivation of Cry1Ab-expressing Bt maize on soil microarthropod communities

The impact of Cry1Ab-expressing Bt maize (event MON810) on non-target fauna has been a major concern since its deployment in Europe. In this paper, we have assessed the levels of Cry1Ab in rhizosphere soil samples from a Bt maize crop and evaluated the potential effects of Bt maize on soil microarthropods by a three-year trial in an experimental farm-scale field in Central Spain. The Cry1Ab toxin was detected in decaying soil organic matter (OM) from Bt maize plots up to three months after harvest, with values ranging between 0.10 and 0.18ng Cry1Ab/mg OM, but it showed low insecticidal activity. The study focused on Acari and Collembola, the two major components of the soil microarthropod community. They accounted for 88% of the total specimens collected, and they were identified at the suborder and species level, respectively. Interestingly, Cry1Ab was detected for the first time in field collected collembolans, Entomobrya spp., demonstrating their exposure to the toxin. The abundance of mites and collembolans and the frequency of occurrence of the main collembolan species did not rely on the type of maize except for Parisotoma notabilis, more abundant and frequent in Bt maize plots. However, significant differences among years were common in both groups. Noticeably, we found higher values of species richness and diversity of collembolans in Cry1Ab-expressing Bt maize than in non-Bt plots, which could be explained under different scenarios. Our results suggest that continuous cultivation of Bt maize does not negatively affect soil microarthropods, indicating that Bt maize could be compatible with this community.

Different Effects of Bacillus thuringiensis Toxin Cry1Ab on Midgut Cell Transmembrane Potential of Mythimna separata and Agrotis ipsilon Larvae.

Bacillus thuringiensis (Bt) Cry toxins from the Cry1A family demonstrate significantly different toxicities against members of the family Noctuidae for unknown reasons. In this study, membrane potential was measured and analyzed in freshly isolated midgut samples from Mythimna separata and Agrotis ipsilon larvae under oral administration and in vitro incubation with Bt toxin Cry1Ab to elucidate the mechanism of action for further control of these pests. Bioassay results showed that the larvae of M. separata achieved a LD50 of 258.84 ng/larva at 24 h after ingestion; M. separata larvae were at least eightfold more sensitive than A. ipsilon larvae to Cry1Ab. Force-feeding showed that the observed midgut apical-membrane potential (Vam) of M. separata larvae was significantly depolarized from −82.9 ± 6.6 mV to −19.9 ± 7.2 mV at 8 h after ingestion of 1 μg activated Cry1Ab, whereas no obvious changes were detected in A. ipsilon larvae with dosage of 5 μg Cry1Ab. The activated Cry1Ab caused a distinct concentration-dependent depolarization of the apical membrane; Vam was reduced by 50% after 14.7 ± 0.2, 9.8 ± 0.4, and 7.6 ± 0.6 min of treatment with 1, 5, and 10 μg/mL Cry1Ab, respectively. Cry1Ab showed a minimal effect on A. ipsilon larvae even at 20 μg/mL, and Vam decreased by 26.3% ± 2.3% after 15 min. The concentrations of Cry1Ab displayed no significant effect on the basolateral side of the epithelium. The Vam of A. ipsilon (−33.19 ± 6.29 mV, n = 51) was only half that of M. separata (−80.94 ± 6.95 mV, n = 75). The different degrees of sensitivity to Cry1Ab were speculatively associated with various habits, as well as the diverse physiological or biochemical characteristics of the midgut cell membranes.

Internal quality control of an enzyme-linked immunoassay for Cry1Ab toxin detection applied in animal tissues

Reliable determination of microbial or transgenic Cry toxins is an essential issue in food and feed analyses, and enzyme-linked immunosorbent assays (ELISAs) are the method of choice for quantifying these toxins currently in food and environmental analysis. Internal Quality Control (IQC) is an indispensable method to assess accuracy, precision, and reproducibility of analytical measurements. To assess the utility of the ELISA method, IQC was performed on EnviroLogix Cry1Ab/Cry1Ac QualiPlate ELISA with manufacturer supplied analytical standards. Applicability of negative and positive controls (C− and C+) was examined by Shewhart Control Charts for bias and Control Chart of the Range of Duplicates for precision. Linear regression (up to 5 ng ml−1 Cry1Ab concentration) of the commercial ELISA kit was compared to sigmoid calibration (up to 60 ng ml−1 Cry1Ab concentration). For immunoassay optimization process, possible matrix effects in different liquid and solid vertebrate tissues were examined by determination of the limit of detection values in these matrices.

Insect Resistance to Bacillus thuringiensis Toxin Cry2Ab Is Conferred by Mutations in an ABC Transporter Subfamily A Protein.

The use of conventional chemical insecticides and bacterial toxins to control lepidopteran pests of global agriculture has imposed significant selection pressure leading to the rapid evolution of insecticide resistance. Transgenic crops (e.g., cotton) expressing the Bt Cry toxins are now used world wide to control these pests, including the highly polyphagous and invasive cotton bollworm Helicoverpa armigera. Since 2004, the Cry2Ab toxin has become widely used for controlling H. armigera, often used in combination with Cry1Ac to delay resistance evolution. Isolation of H. armigera and H. punctigera individuals heterozygous for Cry2Ab resistance in 2002 and 2004, respectively, allowed aspects of Cry2Ab resistance (level, fitness costs, genetic dominance, complementation tests) to be characterised in both species. However, the gene identity and genetic changes conferring this resistance were unknown, as was the detailed Cry2Ab mode of action. No cross-resistance to Cry1Ac was observed in mutant lines. Biphasic linkage analysis of a Cry2Ab-resistant H. armigera family followed by exon-primed intron-crossing (EPIC) marker mapping and candidate gene sequencing identified three independent resistance-associated INDEL mutations in an ATP-Binding Cassette (ABC) transporter gene we named HaABCA2. A deletion mutation was also identified in the H. punctigera homolog from the resistant line. All mutations truncate the ABCA2 protein. Isolation of further Cry2Ab resistance alleles in the same gene from field H. armigera populations indicates unequal resistance allele frequencies and the potential for Bt resistance evolution. Identification of the gene involved in resistance as an ABC transporter of the A subfamily adds to the body of evidence on the crucial role this gene family plays in the mode of action of the Bt Cry toxins. The structural differences between the ABCA2, and that of the C subfamily required for Cry1Ac toxicity, indicate differences in the detailed mode-of-action of the two Bt Cry toxins.

ABCs of Insect Resistance to Bt.

ABCs of Insect Resistance to Bt.

Relevance of a new scientific publication (Trtikova et al., 2015) on previous EFSA GMO Panel conclusions on the risk assessment of maize MON 810 and other Cry1Ab‐expressing Bt‐maize events

Relevance of a new scientific publication (Trtikova et al., 2015) on previous EFSA GMO Panel conclusions on the risk assessment of maize MON 810 and other Cry1Ab‐expressing Bt‐maize events

Nanobody-based electrochemical immunoassay for Bacillus thuringiensis Cry1Ab toxin by detecting the enzymatic formation of polyaniline

We describe an electrochemical immunoassay for the Cry1Ab toxin that is produced by Bacillus thuringiensis. It is making use of a nanobody (a heavy-chain only antibody) that was selected from an immune phage displayed library. A biotinylated primary nanobody and a HRP-conjugated secondary nanobody were applied in a sandwich immunoassay where horseradish peroxidase (HRP) is used to produce polyaniline (PANI) from aniline. PANI can be easily detected by differential pulse voltammetry at a working voltage as low as 40 mV (vs. Ag/AgCl) which makes the assay fairly selective. This immunoassay for Cry1Ab has an analytical range from 0.1 to 1000 ng∙mL-1 and a 0.07 ng∙mL-1 lower limit of detection. The average recoveries of the toxin from spiked samples are in the range from 102 to 114 %, with a relative standard deviation of <7.5 %. The results demonstrated that the assay represented an attractive alternative to existing immunoassays in enabling affordable, sensitive, robust and specific determination of this toxin.

Chronic Responses of Daphnia magna Under Dietary Exposure to Leaves of a Transgenic (Event MON810) Bt–Maize Hybrid and its Conventional Near-Isoline

Insect resistance is the second most common trait globally in cultivated genetically modified (GM) plants. Resistance is usually obtained by introducing into the plant’s genome genes from the bacterium Bacillus thuringiensis (Bt) coding for insecticidal proteins (Cry proteins or toxins) that target insect pests. The aim of this study was to examine the hypothesis that a chronic, high-dose dietary exposure to leaves of a Bt–maize hybrid (GM event MON810, expressing a transgenic or recombinant Cry1Ab toxin), exerted no adverse effects on fitness parameters of the aquatic nontarget organism Daphnia magna (water flea) when compared to an identical control diet based on leaves of the non-GM near-isoline. Cry1Ab was immunologically detected and quantified in GM maize leaf material used for Daphnia feed. A 69-kD protein near Bt’s active core-toxin size and a 34-kD protein were identified. The D. magna bioassay showed a resource allocation to production of resting eggs and early fecundity in D. magna fed GM maize, with adverse effects for body size and fecundity later in life. This is the first study to examine GM-plant leaf material in the D. magna model, and provides of negative fitness effects of a MON810 maize hybrid in a nontarget model organism under chronic, high dietary exposure. Based upon these results, it is postulated that the observed transgenic proteins exert a nontarget effect in D. magna and/or unintended changes were produced in the maize genome/metabolome by the transformation process, producing a nutritional difference between GM-maize and non-GM near-isoline.

Resistance to Bacillus thuringiensis Toxin Cry2Ab in Trichoplusia ni Is Conferred by a Novel Genetic Mechanism.

The resistance to the Bacillus thuringiensis (Bt) toxin Cry2Ab in a greenhouse-originated Trichoplusia ni strain resistant to both Bt toxins Cry1Ac and Cry2Ab was characterized. Biological assays determined that the Cry2Ab resistance in the T. ni strain was a monogenic recessive trait independent of Cry1Ac resistance, and there existed no significant cross-resistance between Cry1Ac and Cry2Ab in T. ni. From the dual-toxin-resistant T. ni strain, a strain resistant to Cry2Ab only was isolated, and the Cry2Ab resistance trait was introgressed into a susceptible laboratory strain to facilitate comparative analysis of the Cry2Ab resistance with the susceptible T. ni strain. Results from biochemical analysis showed no significant difference between the Cry2Ab-resistant and -susceptible T. ni larvae in midgut proteases, including caseinolytic proteolytic activity and zymogram profile and serine protease activities, in midgut aminopeptidase and alkaline phosphatase activity, and in midgut esterases and hemolymph plasma melanization activity. For analysis of genetic linkage of Cry2Ab resistance with potential Cry toxin receptor genes, molecular markers for the midgut cadherin, alkaline phosphatase (ALP), and aminopeptidase N (APN) genes were identified between the original greenhouse-derived dual-toxin-resistant and the susceptible laboratory T. ni strains. Genetic linkage analysis showed that the Cry2Ab resistance in T. ni was not genetically associated with the midgut genes coding for the cadherin, ALP, and 6 APNs (APN1 to APN6) nor associated with the ABC transporter gene ABCC2. Therefore, the Cry2Ab resistance in T. ni is conferred by a novel but unknown genetic mechanism.

Inhibition of Helicoverpa zea (Lepidoptera: Noctuidae) Growth by Transgenic Corn Expressing Bt Toxins and Development of Resistance to Cry1Ab.

Transgenic corn, Zea mays L., that expresses the Bacillus thuringiensis (Bt) toxin Cry1Ab is only moderately toxic to Helicoverpa zea (Boddie) and has been planted commercially since 1996. Growth and development of H. zea was monitored to determine potential changes in susceptibility to this toxin over time. Small plots of corn hybrids expressing Cry1F, Cry1F × Cry1Ab, Cry1Ab × Cry3Bb1, Cry1A.105 × Cry2Ab2 × Cry3Bb1, Cry1A.105 × Cry2Ab2, and Vip3Aa20 × Cry1Ab × mCry3A were planted in both 2012 and 2013 inNorth and South Carolina with paired non-Bt hybrids from the same genetic background. H. zea larvae were sampled on three time periods from ears and the following factors were measured: kernel area injured (cm(2)) by H. zea larvae, larval number per ear, larval weight, larval length, and larval head width. Pupae were sampled on a single time period and the following factors recorded: number per ear, weight, time to eclosion, and the number that eclosed. There was no reduction in larval weight, number of insect entering the pupal stadium, pupal weight, time to eclosion, and number of pupae able to successfully eclose to adulthood in the hybrid expressing Cry1Ab compared with a non-Bt paired hybrid. As Cry1Ab affected these in 1996, H. zea may be developing resistance to Cry1Ab in corn, although these results are not comprehensive, given the limited sampling period, size, and geography. We also found that the negative impacts on larval growth and development were greater in corn hybrids with pyramided traits compared with single traits.

Evidence of field-evolved resistance of Spodoptera frugiperda to Bt corn expressing Cry1F in Brazil that is still sensitive to modified Bt toxins.

Brazil ranked second only to the United States in hectares planted to genetically modified crops in 2013. Recently corn producers in the Cerrado region reported that the control of Spodoptera frugiperda with Bt corn expressing Cry1Fa has decreased, forcing them to use chemicals to reduce the damage caused by this insect pest. A colony of S. frugiperda was established from individuals collected in 2013 from Cry1Fa corn plants (SfBt) in Brazil and shown to have at least more than ten-fold higher resistance levels compared with a susceptible colony (Sflab). Laboratory assays on corn leaves showed that in contrast to SfLab population, the SfBt larvae were able to survive by feeding on Cry1Fa corn leaves. The SfBt population was maintained without selection for eight generations and shown to maintain high levels of resistance to Cry1Fa toxin. SfBt showed higher cross-resistance to Cry1Aa than to Cry1Ab or Cry1Ac toxins. As previously reported, Cry1A toxins competed the binding of Cry1Fa to brush border membrane vesicles (BBMV) from SfLab insects, explaining cross-resistance to Cry1A toxins. In contrast Cry2A toxins did not compete Cry1Fa binding to SfLab-BBMV and no cross-resistance to Cry2A was observed, although Cry2A toxins show low toxicity to S. frugiperda. Bioassays with Cry1AbMod and Cry1AcMod show that they are highly active against both the SfLab and the SfBt populations. The bioassay data reported here show that insects collected from Cry1Fa corn in the Cerrado region were resistant to Cry1Fa suggesting that resistance contributed to field failures of Cry1Fa corn to control S. frugiperda.

Identification of a New cry1I-Type Gene as a Candidate for Gene Pyramiding in Corn To Control Ostrinia Species Larvae.

Pyramiding of diverse cry toxin genes from Bacillus thuringiensis with different modes of action is a desirable strategy to delay the evolution of resistance in the European corn borer (Ostrinia nubilalis). Considering the dependency of susceptibility to Cry toxins on toxin binding to receptors in the midgut of target pests, a diverse mode of action is commonly defined as recognition of unique binding sites in the target insect. In this study, we present a novel cry1Ie toxin gene (cry1Ie2) as a candidate for pyramiding with Cry1Ab or Cry1Fa in corn to control Ostrinia species larvae. The new toxin gene encodes an 81-kDa protein that is processed to a protease-resistant core form of approximately 55 kDa by trypsin digestion. The purified protoxin displayed high toxicity to Ostrinia furnacalis and O. nubilalis larvae but low to no activity against Spodoptera or heliothine species or the coleopteran Tenebrio molitor. Results of binding assays with (125)I-labeled Cry1Ab toxin and brush border membrane vesicles from O. nubilalis larvae demonstrated that Cry1Ie2 does not recognize the Cry1Ab binding sites in that insect. Reciprocal competition binding assays with biotin-labeled Cry1Ie2 confirmed the lack of shared sites with Cry1Ab or Cry1Fa in O. nubilalis brush border membrane vesicles. These data support Cry1Ie2 as a good candidate for pyramiding with Cry1Ab or Cry1Fa in corn to increase the control of O. nubilalis and reduce the risk of resistance evolution.

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

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. 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. 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.

Composition of the Putative Prepore Complex of Bacillus thuringiensis Cry1Ab Toxin.

Prepore formation is hypothesized to be an obligate step in the insertion of Cry1Ab toxin into insect brush border membrane vesicles. We examined the architecture of the putative prepore when isolated using the published protocols [1] [2]. Our results demonstrate that the putative prepore form of Cry1Ab is a combination of receptor proteins attached to the toxin, when purified. The results also suggest that this prepore form as prepared by the methods published is different from other membrane-extracted oligomeric forms of Cry toxins and prepore of other toxins in general. While most other known prepores are composed of multimers of a single protein, the Cry1Ab prepore, as generated, is a protein-receptor complex oligomer and monomers of Cry toxins.

Toxicity and Cross-Resistance of Insecticides to Cry2Ab-Resistant and Cry2Ab-Susceptible Helicoverpa armigera and Helicoverpa punctigera (Lepidoptera: Noctuidae).

Since 2004-2005 cotton expressing Cry1Ac and Cry2Ab insecticidal proteins from the bacterium Bacillus thuringiensis has been commercially available in Australia to manage the target pests Helicoverpa armigera (Hübner) and Helicoverpa punctigera (Wallengren). In both target species, the frequency of alleles conferring resistance to Cry2Ab is unexpectedly high in field populations. A significant challenge for managing these pests would occur if resistance to Cry2Ab toxins inadvertently selected for resistance to other insecticides used to control them. Dose-response bioassays were performed to measure the toxicity of currently registered insecticide sprays on isogenic strains of Cry2Ab-resistant and Cry2Ab-susceptible H. armigera and H. punctigera. Within-species comparisons of Cry2Ab-resistant and Cry2Ab-susceptible strains of H. armigera and H. punctigera indicate no cross-resistance with pyrethroid insecticides. Additionally, Cry2Ab-resistant strains were not cross-resistant to the following selective insecticides: indoxacarb, chlorantraniliprole, and avermectins. In both H. armigera and H. punctigera, Cry2Ab-resistant colonies exhibited a small, but significant, degree of enhanced susceptibility in response to chlorpyrifos and methomyl. We report higher tolerance to conventional insecticides in H. armigera compared with H. punctigera. Our results indicate that there is no significant interplay between Cry2Ab resistance frequencies in H. armigera and H. punctigera and frequencies of resistance to a range of insecticide sprays currently registered for cotton. Therefore, we conclude that any increases in frequencies of the common Cry2Ab resistance phenotypes identified in Australian populations of Helicoverpa spp. are unlikely to increase resistance risk for the indoxacarb, chlorantraniliprole, or avermectin classes of insecticide.

Bacillus thuringiensis Cry1AbMod toxin counters tolerance associated with low cadherin expression but not that associated with low alkaline phosphatase expression in Manduca sexta

To exert their toxic effect, Bacillus thuringiensis Cry1Ab toxin undergoes a sequential binding mechanism with different larval gut proteins including glycosyl-phosphatidyl-inositol anchored proteins like aminopeptidase-N (APN) or alkaline-phosphatase (ALP) and a transmembrane cadherin to form pre-pore structures that insert into the membrane. Cadherin binding induces oligomerization of the toxin by facilitating removal of the N-terminal region, while APN/ALP binding helps in oligomer membrane insertion. Cry1AbMod toxin was engineered to lack N-terminal region of the toxin and shown to counter resistance linked to cadherin mutations. In this manuscript we determined the toxicity of Cry1AbMod to Manduca sexta larvae silenced in the expression of cadherin, ALP or APN receptors. As previously reported Cry1Ab toxicity relied principally in ALP and cadherin in comparison to APN. Our data shows that Cry1AbMod counters resistance associated with low cadherin expression but was not effective against ALP silenced larvae. These results show that Cry1AbMod could be effective against resistance insects linked to mutations on binding molecules involved in toxin oligomerization but not against resistant insects linked to mutations on binding molecules involved in oligomer membrane insertion.

Inheritance patterns, dominance and cross-resistance of Cry1Ab- and Cry1Ac-selected Ostrinia furnacalis (Guenée).

Two colonies of Asian corn borer, Ostrinia furnacalis (Guenée), artificially selected from a Bt-susceptible colony (ACB-BtS) for resistance to Cry1Ab (ACB-AbR) and Cry1Ac (ACB-AcR) toxins, were used to analyze inheritance patterns of resistance to Cry1 toxins. ACB-AbR and ACB-AcR evolved significant levels of resistance, with resistance ratios (RR) of 39-fold and 78.8-fold to Cry1Ab and Cry1Ac, respectively. The susceptibility of ACB-AbR larvae to Cry1Ac and Cry1F toxins, which had not previously been exposed, were significantly reduced, being >113-fold and 48-fold, respectively. Similarly, susceptibility of ACB-AcR larvae to Cry1Ab and Cry1F were also significantly reduced (RR > nine-fold, RR > 18-fold, respectively), indicating cross-resistance among Cry1Ab, Cry1Ac, and Cry1F toxins. However, ACB-AbR and ACB-AcR larvae were equally susceptible to Cry1Ie as were ACB-BtS larvae, indicating no cross-resistance between Cry1Ie and Cry1Ab or Cry1Ac toxins; this may provide considerable benefits in preventing or delaying the evolution of resistance in ACB to Cry1Ab and Cry1Ac toxins. Backcrossing studies indicated that resistance to Cry1Ab toxin was polygenic in ACB-AbR, but monogenic in ACB-AcR, whilst resistance to Cry1Ac toxin was primarily monogenic in both ACB-AbR and ACB-AcR, but polygenic as resistance increased.

Spodoptera frugiperda (J.E. Smith) with field-evolved resistance to Bt maize are susceptible to Bt pesticides

Field-evolved resistance to maize event TC1507 expressing the Cry1Fa toxin from Bacillus thuringiensis (Bt) was detected in populations of Spodoptera frugiperda from Puerto Rico. We tested for cross-resistance to purified Cry1A toxins and commercial Bt pesticides in susceptible (Benzon) and TC1507-resistant (456) strains of S. frugiperda. Larvae from the 456 strain exhibited cross-resistance to Cry1Ab and Cry1Ac toxins, while no differences in susceptibility to XenTari WG and DiPel ES pesticides were detected. These data support cross-resistance to toxins that share binding sites with Cry1Fa and no cross-resistance to Bt pesticides in S. frugiperda with field-evolved resistance to Bt maize.