Dose Effects of Flubendiamide and Thiodicarb against Spodoptera Species Developing on Bt and Non-Bt Soybean.

An increase in Spodoptera species was reported in Bt soybean fields expressing Cry1Ac insecticidal proteins in Brazil, requiring additional management with chemical insecticides. Here, we evaluated the dose effects of flubendiamide and thiodicarb on Spodoptera cosmioides (Walker, 1858), Spodoptera eridania (Stoll, 1782), Spodoptera albula (Walker, 1857) and Spodoptera frugiperda (J. E. Smith, 1797) (Lepidoptera: Noctuidae) that survived on MON 87751 × MON 87708 × MON 87701 × MON 89788, expressing Cry1A.105, Cry2Ab2 and Cry1Ac; MON 87701 × MON 89788 soybean, expressing Cry1Ac; and non-Bt soybean. On unsprayed Cry1A.105/Cry2Ab2/Cry1Ac soybean, only S. frugiperda showed ~60% mortality after 10 d, whereas S. cosmioides, S. eridania and S. albula showed >81% mortality. The surviving larvae of all species on this Bt soybean showed >80% mortality when exposed to the field label dose of flubendiamide (70 mL/ha) or thiodicarb (400 g/ha) or at 50% of these doses. In contrast, all four species had <25% and <19% mortality on Cry1Ac and non-Bt soybean, respectively. The surviving S. cosmioides, S. eridania and S. albula on these soybean types presented >83% mortality after exposure to both dose levels of flubendiamide and thiodicarb. Some S. frugiperda larvae surviving on Cry1Ac and non-Bt soybean sprayed with a 50% dose of either insecticide developed into adults. However, the L1 larvae developing on Cry1Ac soybean leaves sprayed with flubendiamide and the L2 larvae on this soybean sprayed with thiodicarb had a prolonged immature stage, and the females displayed lower fecundity, which are likely to impact S. frugiperda population growth on soybean.

Characterizing the lethal and sub-lethal effects of genetically modified soybean expressing Cry1A.105, Cry2Ab2, and Cry1Ac insecticidal proteins against Spodoptera species (Lepidoptera: Noctuidae) in Brazil.

BACKGROUND: MON 87701 × MON 89788 × MON 87751 × MON 87708 soybean, that expresses Cry1A.105, Cry2Ab2, and Cry1Ac insecticidal proteins and confers tolerance to glyphosate and dicamba, is a potential tool for managing Spodoptera species in soybean fields in Brazil. In this study, we characterized the lethal and sub-lethal effects of Cry1A.105/Cry2Ab2/Cry1Ac soybean against Spodoptera species and genotypes of Spodoptera frugiperda resistant and susceptible to Cry1 and Cry2 proteins. These evaluations were also conducted with MON 87701 × MON 89788 soybean, which expresses Cry1Ac protein. RESULTS: Cry1A.105/Cry2Ab2/Cry1Ac soybean caused high lethality in neonates of Spodoptera cosmioides and Spodoptera albula. However, it showed low lethality in S. frugiperda genotypes homozygous for resistance to Cry1 and Cry2 proteins but reduced their population growth potential. No relevant lethal effects of Cry1Ac soybean were detected in the Spodoptera species and genotypes evaluated. Spodoptera frugiperda genotypes heterozygous for Cry1 and Cry2 resistance were controlled by Cry1A.105/Cry2Ab2/Cry1Ac soybean, with no insects developing into adults. This Bt soybean also caused intermediate mortality of neonates of Spodoptera eridania (60%-83%) but no surviving larvae developed to adulthood, resulting in population suppression. CONCLUSIONS: Cry1A.105/Cry2Ab2/Cry1Ac soybean caused high mortality of S. cosmioides, S. albula, and S. frugiperda genotypes susceptible to Cry1 and Cry2 and heterozygous for Cry1 and Cry2 resistance. This Bt soybean also suppressed population growth of S. eridania but had minimal impact on S. frugiperda homozygous for resistance to Cry1 and Cry2 proteins. Cry1Ac soybean had minimal impact on all Spodoptera species and genotypes evaluated. © 2022 Society of Chemical Industry.

Regional pest suppression associated with adoption of Cry1Ac soybean benefits pest management in tropical agriculture.

BACKGROUND: Bacillus thuringiensis (Bt) crops have been adopted worldwide, providing high-level protection from insect pests. Furthermore, Bt crops preserve natural enemies, promote higher yield, and economically benefit farmers. Although regional pest suppression by widespread Bt crop adoption has been observed in temperate regions, this possibility remains uncertain in tropical areas due to the high diversity of alternative hosts and mild winters. RESULTS: Evidence of regional reduction in insecticide use across areas was observed in Brazil where Cry1Ac soybean has been grown since 2013, with up to 50% reduction in the number of insecticide sprays for managing lepidopteran pests on non-Bt soybean observed at specific locations from 2012 to 2019. Pest monitoring data from four mesoregions across 5 years of commercial plantings of Cry1Ac soybean from December 2014 to July 2019 showed reduced numbers of Chrysodeixis includens moths captured in pheromone traps across years at all locations. The number of Helicoverpa spp. moths captured also was reduced at three locations. CONCLUSION: We provide evidence for regional suppression of lepidopteran pests and reduced insecticide use with the widespread adoption of Cry1Ac soybean in Brazil, bringing economic, social and environmental benefits. © 2022 Society of Chemical Industry.

MON 95379 Bt maize as a new tool to manage sugarcane borer (Diatraea saccharalis) in South America.

BACKGROUND: The sugarcane borer (SCB), Diatraea saccharalis (Lepidoptera: Crambidae), is a key pest of maize in Argentina, and genetically modified maize, producing Bacillus thuringiensis (Bt) proteins, has revolutionized the management of this insect in South America. However, field-evolved resistance to some Bt technologies has been observed in SCB in Argentina. Here we assessed a new Bt technology, MON 95379, in the laboratory, greenhouse and field for efficacy against SCB. RESULTS: In a laboratory leaf disc bioassay, both MON 95379 (producing Cry1B.868 and Cry1Da_7) and Cry1B.868_single maize (producing only Cry1B.868) resulted in 100% mortality of SCB. The level of Cry1B.868 in the Cry1B.868_single maize is comparable to that in MON 95379 maize. However, the Cry1Da_7 protein does not have high efficacy against SCB, as evidenced by < 20% mortality on Cry1Da_7_single leaf tissue. Total (100%) mortality of SCB in a Cry1B.868_single tissue dilution bioassay indicated that Cry1B.868_single maize meets the criteria to be classified as a high dose. Similar median lethal concentration (LC50 ) values were observed for MON 89034-R and susceptible SCB strains exposed to Cry1B.868 protein. MON 95379 also controlled SCB strains resistant to MON 89034 (Cry1A.105/Cry2Ab2) and Cry1Ab. Under field conditions in Brazil and Argentina, MON 95379 maize plants were consistently protected from SCB damage. CONCLUSION: MON 95379 maize will bring value to maize growers in South America by effectively managing SCB even in locations where resistance to other Bt-containing maize technologies has been reported. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Resistance status of lepidopteran soybean pests following large-scale use of MON 87701 × MON 89788 soybean in Brazil.

Widespread adoption of MON 87701 × MON 89788 soybean, expressing Cry1Ac Bt protein and glyphosate tolerance, has been observed in Brazil. A proactive program was implemented to phenotypically and genotypically monitor Cry1Ac resistance in Chrysodeixis includens (Walker). Recent cases of unexpected injury in MON 87701 × MON 89788 soybean were investigated and a large-scale sampling of larvae on commercial soybean fields was performed to assess the efficacy of this technology and the distribution of lepidopteran pests in Brazil. No significant shift in C. includens susceptibility to Cry1Ac was observed eight years after commercial introduction of this technology in Brazil. F2 screen results confirmed that the frequency of Cry1Ac resistance alleles remains low and stable in C. includens. Unexpected injury caused by Rachiplusia nu (Guenée) and Crocidosema aporema (Walsingham) in MON 87701 × MON 89788 soybean was detected during the 2020/21 season, and studies confirmed a genetically based alteration in their susceptibility to Cry1Ac. MON 87701 × MON 89788 soybean remains effective against Anticarsia gemmatalis (Hübner), C. includens, Chloridea virescents (Fabricius) and Helicoverpa armigera (Hübner) in Brazil. However, there is evidence of field-evolved resistance to MON 87701 × MON 89788 soybean by the secondary soybean pests R. nu and C. aporema.

Large-scale assessment of lepidopteran soybean pests and efficacy of Cry1Ac soybean in Brazil.

The soybean technology MON 87701 × MON 89788, expressing Cry1Ac and conferring tolerance to glyphosate, has been widely adopted in Brazil since 2013. However, pest shifts or resistance evolution could reduce the benefits of this technology. To assess Cry1Ac soybean performance and understand the composition of lepidopteran pest species attacking soybeans, we implemented large-scale sampling of larvae on commercial soybean fields during the 2019 and 2020 crop seasons to compare with data collected prior to the introduction of Cry1Ac soybeans. Chrysodeixis includens was the main lepidopteran pest in non-Bt fields. More than 98% of larvae found in Cry1Ac soybean were Spodoptera spp., although the numbers of Spodoptera were similar between Cry1Ac soybean and non-Bt fields. Cry1Ac soybean provided a high level of protection against Anticarsia gemmatalis, C. includens, Chloridea virescens and Helicoverpa spp. Significant reductions in insecticide sprays for lepidopteran control in soybean were observed from 2012 to 2019. Our study showed that C. includens and A. gemmatalis continue to be primary lepidopteran pests of soybean in Brazil and that Cry1Ac soybean continues to effectively manage the target lepidopteran pests. However, there was an increase in the relative abundance of non-target Spodoptera spp. larvae in both non-Bt and Cry1Ac soybeans.

A new generation of Bt maize for control of fall armyworm (Spodoptera frugiperda).

BACKGROUND: The pyramided genetically modified maize (Zea mays [L.]) event MON 95379, expressing the Cry1B.868 and Cry1Da_7 proteins, was designed to protect against larval feeding damage by the fall armyworm, Spodoptera frugiperda (FAW). Here, we conducted laboratory, greenhouse, and field studies to assess the dose and field efficacy of MON 95379 against FAW and inform the development of insect resistance management plans. RESULTS: The Cry1B.868 and Cry1Da_7 proteins were active against susceptible FAW neonates in diet-incorporation bioassays: median lethal concentration [LC50 ] (95% CI) = 62.8 (42.6-87.6) µg/ml diet for Cry1B.868 and 9.4 (5.3-18.6) µg/ml diet for Cry1Da_7. In laboratory leaf disc bioassays, MON 95379 maize and experimental maize lines expressing the individual components were effective in controlling susceptible FAW. In whole-plant assays, MON 95379 controlled FAW resistant to the Cry1A.105 and Cry2Ab2 proteins. Likewise, under field conditions, MON 95379 maize expressing Cry1B.868 and Cry1Da_7 was highly effective at protecting plants against the larval feeding of FAW. CONCLUSIONS: The expression of Cry1B.868 and Cry1Da_7 in MON 95379 consistently protected maize plants against larval feeding by FAW and represents an alternative to manage trait resistance issues in South America. © 2021 Bayer Crop Science-US. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Host plant use of Helicoverpa spp. (Lepidoptera: Noctuidae) in the Brazilian agricultural landscape.

BACKGROUND: The Old-World bollworm, Helicoverpa armigera (Hübner), was recently documented attacking cotton and soybean plants in Brazil; however, restricted basic knowledge on host plant interactions and landscape use in Brazil have limited the effectiveness of control measures. In this study, we evaluated the suitability of different crops commonly cultivated in Brazil as hosts for H. armigera and H. zea, and examined their contribution to the establishment and size of H. armigera and H. zea field populations. We also estimated the proportions of H. armigera and H. zea moths that used cotton, noncotton C3 plants, and C4 plants as hosts in four regions in Brazil through the length of the cropping season. RESULTS: Viability of H. armigera larvae was highest on cotton (46.1%), followed by millet (39.5%), sorghum (31.2%), soybean (24.2%), and maize (21.1%). Noncotton C3 hosts served as the major source of H. armigera moths in all regions evaluated, and C4 hosts were a source of H. armigera mainly in regions where winter maize is typically cultivated. H. armigera moths that used cotton plants as natal hosts were observed during the reproductive stage of the crop mainly in the state of Bahia. Only C4 host plants were a consistent source of H. zea moths, primarily when maize was in the reproductive stage. H. armigera individuals were the main species infesting cotton and soybean fields while H. zea individuals were the main species infesting maize ears. CONCLUSIONS: Regional differences in the host use and population dynamics of H. armigera among the regions evaluated may be attributed to variation in alternative host utilization (crops, noncrops, and weeds) and the possible occurrence of facultative diapause and or migration.

Performance of Genetically Modified Soybean Expressing the Cry1A.105, Cry2Ab2, and Cry1Ac Proteins Against Key Lepidopteran Pests in Brazil.

The pyramided genetically modified (GM) soybean [Glycine max L. (Merr.)] MON87751 × MON87708 × MON87701 × MON89788, expressing Cry1A.105, Cry2Ab2, and Cry1Ac from Bacillus thuringiensis Berliner, was approved for commercial use in Brazil. We conducted laboratory, greenhouse, and field studies to assess the efficacy of this Bt soybean against key soybean lepidopteran pests. Neonates of Anticarsia gemmatalis (Hübner) (Lepidoptera: Erebidae), Chrysodeixis includens (Walker), and Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) were exposed to Bt proteins in diet-overlay bioassays. MON87751 × MON87708 × MON87701 × MON89788 soybean and individual components were evaluated in laboratory (leaf disc), greenhouse (high artificial infestations), and in field conditions (natural infestations). Neonates of A. gemmatalis, C. includens, and H. armigera were highly susceptible to Cry1A.105 (LC50 from 0.79 to 48.22 ng/cm2), Cry2Ab2 (LC50 from 1.24 to 8.36 ng/cm2), and Cry1Ac (LC50 from 0.15 to 5.07 ng/cm2) in diet-overlay bioassays. In laboratory leaf disc bioassays and greenhouse trials, MON87751 × MON87708 × MON87701 × MON89788 soybean as well as the individual components were highly effective in controlling A. gemmatalis, C. includens, and H. armigera. Similarly, under field conditions, the pyramided genotypes expressing Cry1A.105, Cry2Ab2, and Cry1Ac were highly effective at protecting soybean against C. includens. We concluded that the individual Bt proteins expressed by GM soybean MON87751 × MON87708 × MON87701 × MON89788 killed all or nearly all the susceptible A. gemmatalis, C. includens, and H. armigera, fulfilling one important criterion for successfully delaying resistance to pyramided Bt crops.

Lack of relevant cross-resistance to Bt insecticide XenTari in strains of Spodoptera frugiperda (J. E. Smith) resistant to Bt maize.

The resistance of fall armyworm, Spodoptera frugiperda, has been characterized to Cry and Vip3A proteins of Bacillus thuringiensis (Bt) expressed in maize in Brazil. Here, we investigate the cross-resistance to Bt-insecticide XenTari in selected fall armyworm strains resistant to Bt maize varieties. The LC50 of XenTari in neonates of resistant strains ranged from 0.28 to 0.68 µg a.i./cm2, while for the susceptible reference strain (Sus), LC50 was 0.21 µg a.i./cm2. This indicated a resistance ratio lower than 3.2-fold. A similar variation in susceptibility was detected in EC50 values, which ranged from 0.04 to 0.13 µg a.i./cm2, demonstrating a maximum resistance ratio of 4.3-fold relative to the Sus strain (EC50 = 0.03 µg a.i./cm2). In the F1 progeny from reciprocal crosses, the LC50 ranged from 0.28 to 0.64 µg a.i./cm2 and EC50 from 0.03 to 0.18 µg a.i./cm2, similar to the values verified in parental resistant strains and representing a maximum resistance ratio of 3.0 and 6.0-fold, respectively. We also determined that susceptibility of third instar larvae to XenTari decreased when compared to neonates, however the variation remained similar. For third instar larvae from resistant strains, LC50 of XenTari ranged from 10.79 to 39.85 µg a.i./cm2, while for the Sus strain, LC50 was 9.25 µg a.i./cm2 (resistance ratio inferior to 4.3-fold). At the same stage, in heterozygous strains the LC50 ranged from 14.75 to 58.47 µg a.i./cm2 (resistance ratio inferior to 6.3-fold). Our data demonstrate a lack of significant cross-resistance to Bt-based insecticide XenTari in fall armyworm strains with resistance to Bt maize varieties.

Inheritance and fitness costs of Spodoptera frugiperda (Lepidoptera: Noctuidae) resistance to spinosad in Brazil.

BACKGROUND: Spodoptera frugiperda is a pest of economically important crops in South America. In Brazil, this species is considered the most destructive pest of maize. Use of spinosyn insecticides in insect resistance management (IRM) has been one strategy to control this pest. In this study, we selected a strain of S. frugiperda resistant to spinosad and evaluated the inheritance and fitness costs of the resistance. RESULTS: Estimated LC50 (concentration required to kill 50% of larvae) values were 0.011 and 9.80 µg cm-2 for the spinosad-susceptible (Sus) and -resistant (Spin-res) strains, respectively. This represents an 890-fold resistance ratio. LC50 values for reciprocal crosses were 0.18 and 0.14 µg cm-2 , indicating that resistance to spinosad is an autosomal incompletely recessive trait. Backcrosses of the F1 progeny from reciprocal crosses with the parental Spin-res strain showed a polygenic effect. The estimated minimum number of independent segregations was ∼ 2.45, indicating that resistance to spinosad is associated with multiple genes. In greenhouse assays, third-instar larvae from the Spin-res strain showed >92% survival on spinosad-treated maize. By contrast Sus and reciprocal crosses exhibited 0% and <5% survival, respectively, indicating that resistance is recessive. Life history studies to investigate the fitness cost of resistance revealed a 41% reduction in the rate of survival to adulthood and a 49% lower reproductive rate in the Spin-res strain compared with the Sus strain. CONCLUSIONS: The autosomal, incompletely recessive and polygenic resistance to spinosad in S. frugiperda and the fitness costs associated with this resistance can be exploited in IRM strategies to preserve the lifetime of spinosad for control of S. frugiperda in Brazil. © 2017 Society of Chemical Industry.

Effective dominance of resistance of Spodoptera frugiperda to Bt maize and cotton varieties: implications for resistance management.

The resistance of fall armyworm (FAW), Spodoptera frugiperda, has been characterized to some Cry and Vip3A proteins of Bacillus thuringiensis (Bt) expressed in transgenic maize in Brazil. Here we evaluated the effective dominance of resistance based on the survival of neonates from selected Bt-resistant, heterozygous, and susceptible (Sus) strains of FAW on different Bt maize and cotton varieties. High survival of strains resistant to the Cry1F (HX-R), Cry1A.105/Cry2Ab (VT-R) and Cry1A.105/Cry2Ab/Cry1F (PW-R) proteins was detected on Herculex, YieldGard VT PRO and PowerCore maize. Our Vip3A-resistant strain (Vip-R) exhibited high survival on Herculex, Agrisure Viptera and Agrisure Viptera 3 maize. However, the heterozygous from HX-R × Sus, VT-R × Sus, PW-R × Sus and Vip-R × Sus had complete mortality on YieldGard VT PRO, PowerCore, Agrisure Viptera, and Agrisure Viptera 3, whereas the HX-R × Sus and Vip-R × Sus strains survived on Herculex maize. On Bt cotton, the HX-R, VT-R and PW-R strains exhibited high survival on Bollgard II. All resistant strains survived on WideStrike, but only PW-R and Vip-R × Sus survived on TwinLink. Our study provides useful data to aid in the understanding of the effectiveness of the refuge strategy for Insect Resistance Management of Bt plants.

Frequency of Cry1F resistance alleles in Spodoptera frugiperda (Lepidoptera: Noctuidae) in Brazil.

BACKGROUND: The frequency of resistance alleles is a major factor influencing the rate of resistance evolution. Here, we adapted the F2 screen procedure for Spodoptera frugiperda (J. E. Smith) with a discriminating concentration assay, and extended associated statistical methods to estimate the frequency of resistance to Cry1F protein in S. frugiperda in Brazil when resistance was not rare. RESULTS: We show that F2 screen is efficient even when the resistance frequency is 0.250. It was possible to screen 517 isoparental lines from 12 populations sampled in five states of Brazil during the first half of 2012. Western Bahia had the highest allele frequency of Cry1F resistance, 0.192, with a 95% confidence interval (CI) between 0.163 and 0.220. All other states had a similar and lower frequency varying from 0.042 in Paraná to 0.080 in Mato Grosso do Sul. CONCLUSION: The high frequency in western Bahia may be related to year-round availability of maize, the high population density of S. frugiperda, the lack of refuges and the high adoption rate of Cry1F maize. Cry1F resistance alleles were not rare and occurred at frequencies that have already compromised the useful life of TC1507 maize in western Bahia. © 2016 Society of Chemical Industry.

Selection and characterization of resistance to the Vip3Aa20 protein from Bacillus thuringiensis in Spodoptera frugiperda.

BACKGROUND: Spodoptera frugiperda is one the main target pests of maize events expressing Vip3Aa20 protein from Bacillus thuringiensis (Bt) in Brazil. In this study, we selected a resistant strain of S. frugiperda on Bt maize expressing Vip3Aa20 protein and characterized the inheritance and fitness costs of the resistance. RESULTS: The resistance ratio of the Vip3Aa20-resistant strain of S. frugiperda was >3200-fold. Neonates of the Vip3Aa20-resistant strain were able to survive and emerge as fertile adults on Vip3Aa20 maize, while larvae from susceptible and heterozygous strains did not survive. The inheritance of Vip3Aa20 resistance was autosomal recessive and monogenic. Life history studies to investigate fitness cost revealed an 11% reduction in the survival rate until adult stage and a ∼50% lower reproductive rate of the Vip3Aa20-resistant strain compared with susceptible and heterozygous strains. CONCLUSION: This is the first characterization of S. frugiperda resistance to Vip3Aa protein. Our results provide useful information for resistance management programs designed to prevent or delay resistance evolution to Vip3Aa proteins in S. frugiperda. © 2016 Society of Chemical Industry.

Near-Isogenic Cry1F-Resistant Strain of Spodoptera frugiperda (Lepidoptera: Noctuidae) to Investigate Fitness Cost Associated With Resistance in Brazil.

Field-evolved resistance to Cry1F maize in Spodoptera frugiperda (J.E. Smith) populations in Brazil was reported in 2014. In this study, to investigate fitness costs, we constructed a near-isogenic S. frugiperda-resistant strain (R-Cry1F) using Cry1F-resistant and Cry1F-susceptible strains sharing a close genetic background. A near-isogenic R-Cry1F strain was obtained by eight repeated backcrossings, each followed by sib-mating and selection among resistant and susceptible strains. Fitness cost parameters were evaluated by comparing the biological performance of resistant, susceptible, and heterozygous strains on artificial diet. Fitness parameters monitored included development time and survival rates of egg, larval, pupal, and egg-to-adult periods; sex ratio; adult longevity; timing of preoviposition, oviposition, and postoviposition; fecundity; and fertility. A fertility life table was also calculated. The near-isogenic R-Cry1F strain showed lower survival rate of eggs (32%), when compared with Sus and reciprocal crosses (41 and 55%, respectively). The number of R-Cry1F insects that completed the life cycle was reduced to ∼25%, compared with the Sus strain with ∼32% reaching the adult stage. The mean generation time (T) of R-Cry1F strain was ∼2 d shorter than R-Cry1F♂×Sus♀ and Sus strains. The reproductive parameters of R-Cry1F strain were similar to the Sus strain. However, fewer females were produced by R-Cry1F strain than R-Cry1F♀×Sus♂ and more females than R-Cry1F♂×Sus♀. In summary, no relevant fitness costs are observed in a near-isogenic Cry1F-resistant strain of S. frugiperda, indicating stability of resistance to Cry1F protein in Brazilian populations of this species in the absence of selection pressure.

Dominance of Cry1F resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) on TC1507 Bt maize in Brazil.

BACKGROUND: Dominance of resistance has been one of the major parameters affecting the rate of evolution of resistance to Bt crops. High dose is the capacity of Bt crops to kill heterozygous insects and has been an essential component of the most successful strategy to manage resistance to these crops. Experiments were conducted to evaluate directly and indirectly whether the TC1507 event is high dose to Spodoptera frugiperda (JE Smith). RESULTS: About 8% of heterozygote neonate larvae were able to survive, complete larval development and emerge as normal adults on TC1507 leaves, while susceptible larvae could not survive for 5 days. The estimated dominance of resistance was 0.15 ± 0.09 and significantly higher than zero; therefore, the resistance to Cry1F expressed in TC1507 was not completely recessive. A 25-fold dilution of TC1507 maize leaf tissue in an artificial diet was able to cause a maximum mortality of only 37%, with growth inhibition of 82% at 7 days after larval infestation. CONCLUSION: Resistance to Cry1F in TC1507 maize is incompletely recessive in S. frugiperda. TC1507 maize is not high dose for S. frugiperda. Additional or alternative resistance management strategies, such as the replacement of single-trait Bt maize with pyramided Bt maize, which produces multiple proteins targeting the same insect pests, should be implemented wherever this technology is in use and S. frugiperda is the major pest.

Geographical and Temporal Variability in Susceptibility to Cry1F Toxin From Bacillus thuringiensis in Spodoptera frugiperda (Lepidoptera: Noctuidae) Populations in Brazil.

The genetically modified maize TC1507 event with the cry1F gene (Cry1F maize) has been used to control Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) in Brazil since the 2009-2010 cropping season. As part of the insect resistance management program, we conducted studies to determine the baseline susceptibility to Cry1F before the widespread planting of Cry1F maize. Subsequently, we evaluated the geographical and temporal variability of susceptibility to this toxin in populations of S. frugiperda collected from major maize-growing regions in Brazil. The baseline susceptibility to Cry1F was determined using a diet-overlay bioassay for a susceptible reference population and four field populations of S. frugiperda. We then monitored the susceptibility to Cry1F in 43 populations of S. frugiperda sampled in nine States of Brazil between 2011 and 2013. In the baseline study, the MIC50 (the concentration that inhibits molting to second instars in 50% of individuals) ranged from 3.59 to 72.47 ng Cry1F toxin per centimeter square. Based on the upper limit of the MIC99 value of the joint analysis from the baseline susceptibility data, the concentrations of 200 and 2,000 ng of Cry1F toxin per centimeter square were defined as diagnostic concentrations for potentially resistant individuals, and these were used to monitor the susceptibility of S. frugiperda to Cry1F. Survival at 2,000 ng Cry1F toxin per centimeter square increased significantly throughout the cropping seasons in S. frugiperda populations from São Paulo, Santa Catarina, Rio Grande do Sul, Bahia, Mato Grosso, Goiás, Mato Grosso do Sul, and Paraná. The highest survival (>50%) was reached in populations collected from Bahia, Mato Grosso, Goiás, Mato Grosso do Sul, and Paraná during the 2012-2013 cropping season. Therefore, a significant decrease in susceptibility to Cry1F was detected in S. frugiperda throughout cropping seasons, especially in regions with intensive maize production in Brazil, according to field-evolved resistance that has been demonstrated in Brazilian populations.