Laboratory and field survival of Spodoptera frugiperda (Lepidoptera: Noctuidae) on Bt and non-Bt maize and its susceptibility to insecticides

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.

Characterization of Indoxacarb Resistance in the Fall Armyworm: Selection, Inheritance, Cross-Resistance, Possible Biochemical Mechanisms, and Fitness Costs

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a voracious insect pest that is difficult to control due to resistance to insecticides and Bt proteins. We assessed cross-resistance, resistance mechanism, and fitness costs based on the life history traits of S. frugiperda. We established an S. frugiperda strain selected for resistance to indoxacarb (Ind-SEL) from a field-collected population and an unselected strain, Ind-UNSEL. Results indicated that after 24 generations of selection, the resistance to indoxacarb was increased by 472.67-fold as compared to the Ind-UNSEL. There was high cross-resistance to deltamethrin (31.23-fold) with very low or negligible cross-resistance to chlorantraniliprole, emamectin benzoate, and/or methoxyfenozide in the Ind-SEL population. Butoxide synergist increased susceptibility to indoxacarb, indicating that P450 enzymes may be involved in indoxacarb resistance. Significantly longer developmental time of larvae extended pupal duration, shorter adult longevity, and lower fecundity were observed in Ind-SEL as compared with the Ind-UNSEL population. The Net reproductive rate (R₀) was the only growth parameter that differs between crosses of Ind-SEL♂ × Ind-UNSEL♀ (176 ± 46) and Ind-SEL♀ × Ind-UNSEL♂ (328 ± 57). On the other hand, all population growth parameters differ between Ind-SEL and Ind-UNSEL strains. Our work contributes to the growing body of research that demonstrates the importance of strain genetics in fitness cost experiments and helps resistance management programs make decisions.

Assessing fitness costs of the resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to pyramided Cry1 and Cry2 insecticidal proteins on different host plants

Fall armyworm (FAW), Spodoptera frugiperda (Smith), is one of the major pests targeted by transgenic crops expressing insecticidal proteins from Bacillus thuringiensis (Bt) Berliner. However, FAW presents a high capacity to develop resistance to Bt protein-expressing crop lines, as reported in Brazil, Argentina, Puerto Rico and the southeastern U.S. Here, FAW genotypes resistant to pyramided maize events expressing Cry1F/Cry1A.105/Cry2Ab2 (P-R genotype) and Cry1A.105/Cry2Ab2 (Y-R genotype) from Brazil were used to investigate the interactions between non-Bt hosts (non-Bt maize, non-Bt cotton, millet and sorghum) and fitness costs. We also tested a FAW genotype susceptible to Bt maize and F₁ hybrids of the resistant and susceptible genotypes (heterozygotes). Recessive fitness costs (i.e., costs affecting the resistant insects) were observed for pupal and neonate to adult survival of the P-R genotype on non-Bt cotton; larval developmental time of the P-R genotype on millet and sorghum; larval and neonate-to-adult developmental time of the Y-R genotype on non-Bt cotton and sorghum; the fecundity of the Y-R genotype on non-Bt cotton; and mean generation time of both resistant genotypes. However, on non-Bt cotton and non-Bt maize, the P-R genotype had a higher fitness (i.e., fitness benefits), displaying greater fecundity and rates of population increases than the Sus genotype. Non-recessive fitness costs (i.e., costs affecting heterozygotes) were found for fecundity and population increases on millet and sorghum. These findings suggest that, regardless of the disadvantages of the resistant genotypes in some hosts, the resistance of FAW to Cry1 and Cry2 Bt proteins is not linked with substantial fitness costs, and may persist in field conditions once present.

Pesticide resistance in arthropods: Ecology matters too

Pesticide resistance development is an example of rapid contemporary evolution that poses immense challenges for agriculture. It typically evolves due to the strong directional selection that pesticide treatments exert on herbivorous arthropods. However, recent research suggests that some species are more prone to evolve pesticide resistance than others due to their evolutionary history and standing genetic variation. Generalist species might develop pesticide resistance especially rapidly due to pre‐adaptation to handle a wide array of plant allelochemicals. Moreover, research has shown that adaptation to novel host plants could lead to increased pesticide resistance. Exploring such cross‐resistance between host plant range evolution and pesticide resistance development from an ecological perspective is needed to understand its causes and consequences better. Much research has, however, been devoted to the molecular mechanisms underlying pesticide resistance while both the ecological contexts that could facilitate resistance evolution and the ecological consequences of cross‐resistance have been under‐studied. Here, we take an eco‐evolutionary approach and discuss circumstances that may facilitate cross‐resistance in arthropods and the consequences cross‐resistance may have for plant–arthropod interactions in both target and non‐target species and species interactions. Furthermore, we suggest future research avenues and practical implications of an increased ecological understanding of pesticide resistance evolution.

Systematic identification and characterization of differentially expressed microRNAs under tetraniliprole exposure in the fall armyworm, Spodoptera frugiperda

The fall armyworm, Spodoptera frugiperda, is a worldwide agricultural pest and causes huge losses of crop production each year. Tetraniliprole is a novel diamide insecticide with high efficacy against even the insecticide resistant pests of Lepidoptera, Coleoptera, and Diptera. MicroRNAs (miRNAs) are small noncoding RNAs that regulate gene expression at the posttranscriptional level and play an important regulatory role in the insecticide resistance in insects. However, the effects of miRNAs on the tetraniliprole tolerance in S. frugiperda are poorly understood. In the present research, the miRNAs response to tetraniliprole application in S. frugiperda were systematically investigated by high-throughput sequencing. A total of thirty differentially expressed miRNAs were identified under tetraniliprole treatment in S. frugiperda. The functions of the target genes of these differentially expressed miRNAs were further predicted by Gene Ontology terms and Kyoto Encyclopedia of Genes and Genomes database pathway, and the most significantly enriched pathway was MAPK signaling pathway. The expression changes of six differentially expressed miRNAs were validated by quantitative real-time polymerase chain reaction. Furthermore, miR-278-5p had the highest expression in the hemolymph and malpighian tubule and the lowest expression in the gut. Oversupply of miR-278-5p significantly increased the mortality of S. frugiperda following exposure to tetraniliprole. These results will provide the basis for understanding the regulatory roles of miRNAs regarding to tetraniliprole tolerance in S. frugiperda.

Fitness Cost of Chlorpyrifos Resistance in Spodoptera frugiperda (Lepidoptera: Noctuidae) on Different Host Plants

Spodoptera frugiperda (J. E. Smith, 1797) is a polyphagous pest of global relevance due to the damage it inflicts on agricultural crops. In South American countries, this species is one of the principal pests of maize and cotton. Currently, S. frugiperda is also emerging as an important pest of soybeans and winter cereals in Brazil. Chemical control is one of the main control tactics against S. frugiperda, even though resistance against numerous modes of action insecticides has been reported. To support insect resistance management programs, we evaluated the fitness costs of resistance of S. frugiperda to the acetylcholinesterase inhibitor chlorpyrifos. Fitness costs were quantified by comparing biological parameters of chlorpyrifos-resistant and -susceptible S. frugiperda and their F1 hybrids (heterozygotes) on non-Bt cotton, non-Bt maize, non-Bt soybean, and oats. The results revealed that the chlorpyrifos-resistant genotype showed lower pupa-to-adult and egg-to-adult survivorship and reduced larval weights on oats; longer neonate-to-pupa and egg-to-adult developmental periods, and lower pupal weights and fecundity on maize; lower pupal weights on soybean; and reduced fecundity on cotton compared with the chlorpyrifos-susceptible genotype. Fitness costs also affected fertility life table parameters of the resistant genotype, increasing the mean length of a generation on cotton and maize and reducing the potential for population growth on all hosts. These findings suggest fitness costs at the individual and population levels of chlorpyrifos resistance in S. frugiperda, indicating that removal of the selective agent from the environment would result in reduced resistance and opportunities for the restoration of susceptibility.

Intercropped Bt and non-Bt corn with ruzigrass (Urochloa ruziziensis) as a tool to resistance management of Spodoptera frugiperda (JE Smith, 1797) (Lepidoptera: Noctuidae)

BACKGROUND

Corn intercropped with ruzigrass is common in Brazil, and it can improve the physicochemical features of soils, increase water retention, and suppress the emergence of resistant weeds and soil nematodes. As both corn and ruzigrass are hosts to Spodoptera frugiperda (the main corn pest in South America), the cultivation of both these plants in the same place at one time motivates investigation into the pest population dynamics. We hypothesize that the intercropping system influences S. frugiperda pressure and leaf injury in corn. Considering that if corn hybrids with high dose toxin expression are used, ruzigrass may be a potential host to susceptible S. frugiperda larvae, consequently an alternative refuge and be included as a tool for the resistant management of Bt corn. To test these hypotheses about the use of ruzigrass as an alternative host of S. frugiperda in an intercropping system with corn, we conducted field trials in three seasons to verify S. frugiperda population dynamics and leaf injury to crop systems. In addition to phenotypic evaluation, we also characterized molecularly S. frugiperda strains collected in corn and ruzigrass to identify strain differences.

RESULTS

The insects collected in both corn and ruzigrass were identified as corn strains. Ruzigrass was used as a S. frugiperda host during all crop cycles. The intercropped system did not increase the S. frugiperda population or leaf injury on Bt corn.

CONCLUSION

The results suggest that the intercropped system is not prejudicial to Bt corn cultivation since high dose concept applies to all larvae instars. The results also suggest that ruzigrass may be used as a promising alternative refuge in Bt corn agroecosystems, if compliance with management strategies is followed. © 2021 Society of Chemical Industry.

ABC transporter mutations in Cry1F-resistant fall armyworm (Spodoptera frugiperda) do not result in altered susceptibility to selected small molecule pesticides

BACKGROUND

Transgenic crops producing Cry and Vip3 insecticidal proteins from the bacterium Bacillus thuringiensis provide effective control of the fall armyworm, Spodoptera frugiperda J. E. Smith. However, cases of practical S. frugiperda resistance to transgenic corn producing Cry1F, Cry1Ab and Cry1A.105 proteins have been reported in the Western hemisphere. Importantly, S. frugiperda resistance to Cry1F corn in Puerto Rico was previously associated with lower susceptibility to synthetic pesticides. When characterized, resistance to transgenic corn in S. frugiperda involved alterations in an ABC transporter subfamily C2 (SfABCC2) gene. The main goal of this work was to test the role of mutations in SfABCC2 that result in resistance to Cry1F in susceptibility to synthetic and semisynthetic small molecule pesticides.

RESULTS

Marginal but significantly reduced susceptibility to bifenthrin and increased susceptibility to spinetoram was detected in a Cry1F-resitant S. frugiperda strain from Puerto Rico carrying a frameshift mutation in the SfABCC2 gene. Gene editing by CRISPR/Cas9 created a SfABCC2 knockout in a laboratory reference S. frugiperda strain. When compared to the parental reference, the knockout strain displayed 25-fold resistance to Cry1F but no alteration in susceptibility to small molecule pesticides.

CONCLUSION

These results support that resistance to Cry1F due to mutations in the SfABCC2 gene do not affect susceptibility to the tested small molecule pesticides.

Cross-crop resistance of Spodoptera frugiperda selected on Bt maize to genetically-modified soybean expressing Cry1Ac and Cry1F proteins in Brazil

Spodoptera frugiperda is one of the main pests of maize and cotton in Brazil and has increased its occurrence on soybean. Field-evolved resistance of this species to Cry1 Bacillus thuringiensis (Bt) proteins expressed in maize has been characterized in Brazil, Argentina, Puerto Rico and southeastern U.S. Here, we conducted studies to evaluate the survival and development of S. frugiperda strains that are susceptible, selected for resistance to Bt-maize single (Cry1F) or pyramided (Cry1F/Cry1A.105/Cry2Ab2) events and F1 hybrids of the selected and susceptible strains (heterozygotes) on DAS-444Ø6-6 × DAS-81419-2 soybean with tolerance to 2,4-D, glyphosate and ammonium glufosinate herbicides (event DAS-444Ø6-6) and insect-resistant due to expression of Cry1Ac and Cry1F Bt proteins (event DAS-81419-2). Susceptible insects of S. frugiperda did not survive on Cry1Ac/Cry1F-soybean. However, homozygous-resistant and heterozygous insects were able to survive and emerge as fertile adults when fed on Cry1Ac/Cry1F-soybean, suggesting that the resistance is partially recessive. Life history studies revealed that homozygous-resistant insects had similar development, reproductive performance, net reproductive rate, intrinsic and finite rates of population increase on Cry1Ac/Cry1F-soybean and non-Bt soybean. In contrast, heterozygotes had their fertility life table parameters significantly reduced on Cry1Ac/Cry1F-soybean. Therefore, the selection of S. frugiperda for resistance to single and pyramided Bt maize can result in cross-crop resistance to DAS-444Ø6-6 × DAS-81419-2 soybean. The importance of these results to integrated pest management (IPM) and insect resistance management (IRM) programs is discussed.

Detection of a ryanodine receptor target-site mutation in diamide insecticide resistant fall armyworm, Spodoptera frugiperda

BACKGROUND

Fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), a major lepidopteran pest in Latin and North America, has very recently invaded the continents of Africa and Asia. FAW has evolved resistance to different insecticides and transgenic corn expressing Bacillus thuringiensis (Bt) toxins. Here, we investigated the extent and mechanisms of resistance to diamide insecticides in a Brazilian field-collected FAW strain selected using chlorantraniliprole.

RESULTS

Continuous laboratory selection of a field-collected FAW strain with chlorantraniliprole resulted in resistance ratios of 225-fold and > 5400-fold against chlorantraniliprole and flubendiamide, respectively, when compared with a susceptible strain. Pre-exposure to different synergists known to inhibit detoxification enzymes did not result in significantly increased larval toxicity, suggesting a minor role for metabolic resistance. Sequencing of the FAW ryanodine receptor (RyR) C-terminal domains II to VI revealed a single nucleotide polymorphism, resulting in a I4734M mutation recently said to confer target-site resistance to diamides in lepidopteran pests. Genotyping by pyrosequencing of field-collected FAW larvae sampled in the 2018 crop season suggests a low resistance allele frequency. Furthermore, we developed a fluorescent polymerase chain reaction (PCR)-based allelic discrimination assay for rapid genotyping of field-collected FAW samples, because diamides are increasingly used in Bt-/non-Bt corn.

CONCLUSIONS

Recently, the identified RyR mutation has been shown to confer field resistance in other lepidopteran pests such as diamondback moth, tomato leafminer and striped rice stem borer. The developed PCR-based allelic discrimination assay will help to monitor the frequency and future spread of diamide resistance allele in FAW field populations and help to implement appropriate resistance management measures. © 2019 Society of Chemical Industry.

Performance of Field-Collected Spodoptera frugiperda (Lepidoptera: Noctuidae) Strains Exposed to Different Transgenic and Refuge Maize Hybrids in Argentina

Spodoptera frugiperda (J. E. Smith) is one of the major pests of maize in Argentina. The main tool for its control is the use of genetically modified maize hybrids expressing Bacillus thuringiensis (Bt) insecticidal proteins. Maize growers in Argentina initially controlled this pest with Bt maize that expressed a single Bt protein (Cry1Ab or Cry1F). Currently it is necessary to plant maize cultivars that produce two Bt proteins to achieve the satisfactory control. Recently, Cry1F field-evolved resistant populations of this species were confirmed in Argentina. The objective of this study was to evaluate the performance of S. frugiperda field-collected strains on different Bt and non-Bt maize hybrids. Strains were collected from non-Bt maize (T1), Agrisure TDMax (T2), Agrisure Viptera (T3), Agrisure Viptera 3110 (T4), Genuity VT Triple Pro (T5), and Power Core (T6). Three experiments were performed to 1) determine the survivorship and reproduction of field-collected larvae (F0) from Bt maize hybrids, 2) evaluate Cry1F resistance using an F1 screen, and 3) assess the performance of F1 strains on different maize hybrids. In the F0, the survivorship from larva to adult ranged from 0 to 63%. We obtained adults from only the T1, T2, T5, and T6 strains with no significant differences in the reproductive parameters. Continuously rearing F1 larvae on their collected hosts affected larval duration, which was significantly shorter for a known-laboratory Bt-susceptible strain than the field-collected strains. Our results support the existence of Cry1F-resistance alleles in S. frugiperda field populations in Argentina.