Bulk segregant mapping and transcriptome analyses reveal the molecular mechanisms of spinetoram resistance in Spodoptera frugiperda.

The evolution of resistance to insecticides poses a significant threat to pest management programs. Understanding the molecular mechanisms underlying insecticide resistance is essential to design sustainable pest control and resistance management programs. The fall armyworm, Spodoptera frugiperda, is an important insect pest of many crops and has a remarkable ability to evolve resistance to insecticides. In this study, we employed bulk segregant analysis (BSA) combined with DNA and RNA sequencing to characterize the molecular basis of spinetoram resistance in S. frugiperda. Analysis of genomic data derived from spinetoram selected and unselected bulks and the spinetoram-resistant and susceptible parental strains led to the identification of a three-nucleotide deletion in the gene encoding the nicotinic acetylcholine receptor α6 subunit (nAChR α6). Transcriptome profiling identified the upregulation of few genes encoding detoxification enzymes associated with spinetoram resistance. Thus, spinetoram resistance in S. frugiperda appears to be mediated mainly by target site insensitivity with a minor role of detoxification enzymes. Our findings provide insight into the mechanisms underpinning resistance to spinetoram in S. frugiperda and will inform the development of strategies to control this highly damaging, globally distributed crop pest.

Inheritance patterns, cross-resistance and synergism in Spodoptera frugiperda (Lepidoptera: Noctuidae) resistant to emamectin benzoate.

BACKGROUND: Fall armyworm, Spodoptera frugiperda (J. E. Smith), is a relevant global pest due to severe damage caused on agricultural crops and its capacity to evolve resistance to insecticides. Here, we selected a strain of S. frugiperda resistant to emamectin benzoate under laboratory conditions to understand the inheritance patterns, cross-resistance and synergism involved in the resistance. RESULTS: The emamectin benzoate-resistant (Ben-R) strain was isolated by using F2 screen in a field population collected in Lucas do Rio Verde, Mato Grasso state, Brazil. After ten generations of selection pressure with emamectin benzoate, the estimated LC50 of the Ben-R strain was 678.38 µg a.i. mL-1 whereas that of the susceptible (Sus) strain was 0.29 µg a.i.mL-1 , resulting in a resistance ratio (RR) of ~ 2340-fold. The LC50 values of the offspring from reciprocal crosses of Sus and Ben-R strains were 93.37 and 105.32 µg a.i. mL-1 , suggesting that resistance is an autosomal incompletely dominant trait. The high survival of heterozygous and Ben-R strains (>92%) on non-Bt maize sprayed with the field rate of emamectin benzoate confirmed that resistance is functionally dominant. The minimum number of segregations influencing resistance was 3.55, suggesting a polygenic effect. Low cross-resistance was detected between emamectin benzoate and the insecticides methomyl, chlorpyrifos, lambda-cyhalothrin, spinetoram, indoxacarb and chlorantraniliprole (RR <5.75-fold). There was no effect of synergists piperonyl butoxide, diethyl maleate and S, S, S-tributyl phosphorotrithiotate on the Ben-R strain, suggesting a minor role of metabolic resistance. CONCLUSIONS: Our results showed a high risk of resistance evolution of S. frugiperda to emamectin benzoate, based on incompletely dominant inheritance. Rotation of insecticides with different modes of action can be one of the resistance management strategies to be implemented to delay the evolution of resistance of S. frugiperda to emamectin benzoate in Brazil.

Baseline Response, Monitoring, and Cross-Resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to Sodium Channel Blocker Insecticides in Brazil.

Spodoptera frugiperda (J.E. Smith) is one of the key cross-crop pests in Brazilian agroecosystems. Field-evolved resistance of S. frugiperda to some conventional insecticides and Bt proteins has already been reported. Thus, the use of insecticides with new mode of action such as sodium channel blockers (indoxacarb and metaflumizone) could be an important tool in insecticide resistance management (IRM) programs. To implement a proactive IRM, we conducted baseline response and monitoring to indoxacarb and metaflumizone in 87 field populations of S. frugiperda collected from major maize-growing regions of Brazil from 2017 to 2020, estimated the frequency of resistance alleles to indoxacarb, and evaluated cross-resistance of indoxacarb and metaflumizone to some selected insecticides and Bt proteins. Low variation in susceptibility to indoxacarb (4.6-fold) and metaflumizone (2.6-fold) was detected in populations of S. frugiperda in 2017. The frequency of the resistance allele to indoxacarb was 0.0452 (0.0382-0.0527 CI 95%), by using F2 screen method. The mean survival at diagnostic concentration, based on CL99, varied from 0.2 to 12.2% for indoxacarb and from 0.0 to 12.7% for metaflumizone, confirming high susceptibility of S. frugiperda to these insecticides in Brazil. No cross-resistance was detected between sodium channel blocker insecticides and other insecticides (organophosphate, pyrethroid, benzoylurea, spinosyn, and diamide) and Bt proteins. These findings showed that sodium channel blocker insecticides are important candidates to be exploited in IRM strategies of S. frugiperda in Brazil.

Resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to spinetoram: inheritance and cross-resistance to spinosad.

BACKGROUND: The use of spinosyn insecticides is one of the major control strategies of the fall armyworm, Spodoptera frugiperda (J. E. Smith) in Brazil. In this study, we selected a spinetoram-resistant strain from a field-population of S. frugiperda to characterize the inheritance of the resistance and cross-resistance relationship between spinosyn insecticides. RESULTS: The values of LC50 (95% CI) obtained from concentration-response bioassays were 0.63 (0.55-0.73) µg spinetoram mL-1 for the susceptible strain (SUS), and 1170.96 (1041.61-1323.89) µg spinetoram mL-1 for the strain resistant to spinetoram (SPT-R). These values resulted in a resistance ratio of 1844-fold. The SPT-R strain showed cross-resistance with spinosad (resistance ratio = 1196-fold). The reciprocal crosses showed LC50 values of 3.91 (2.97-5.84) and 5.37 (4.52-6.52) µg spinetoram mL-1 , suggesting that the resistance of S. frugiperda to spinetoram is autosomal and incompletely recessive. The backcrosses of the F1 progeny with the SPT-R strain suggest a resistance with polygenic effect. Estimates of the effective number of loci with equal contributions to the resistance effect were from 1.18 to 1.76, suggesting that resistance to spinetoram is associated with a few genes. CONCLUSION: The inheritance pattern of resistance of S. frugiperda to spinetoram was characterized as autosomal, incompletely recessive and polygenic. Cross-resistance between spinosyns was confirmed in S. frugiperda. The importance of this information for implementing insect resistance management strategies is discussed in this paper. © 2020 Society of Chemical Industry.

Geographical distribution of Vip3Aa20 resistance allele frequencies in Spodoptera frugiperda (Lepidoptera: Noctuidae) populations in Brazil.

BACKGROUND: The use of Bt plants has been the main strategy for controlling the fall armyworm Spodoptera frugiperda (J. E. Smith) in Brazil. However, many resistance cases were already registered. The resistance of S. frugiperda to the Vip3Aa20 protein was recently characterized under laboratory conditions but it is still efficient under field conditions. Here, resistance monitoring studies were conducted using phenotypic (purified protein and Bt maize leaves) and genotypic (F1 and F2 screen) methods to support insect resistance management (IRM) programs and preserve Vip3Aa20 technology on maize. RESULTS: Phenotypic monitoring with purified protein showed two populations significantly different from the susceptible strain on the second crop season in 2016. This number increased for the first and second crop seasons in 2017 in several regions. The genotypic monitoring estimated a mean frequency of the resistance allele of 0.0027 for the F1 screen and 0.0033 for the F2 screen. Three new resistant strains to Vip3Aa20 were selected from F2 screen assays. Complementation tests on these new resistant strains were positive with the previous resistant strain. CONCLUSION: Here we showed that the resistance allele of S. frugiperda to Vip3Aa20 protein is widely distributed in maize-producing regions in Brazil and its frequency increases throughout crop seasons. © 2019 Society of Chemical Industry.

Selection and characterization of the inheritance of resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to chlorantraniliprole and cross-resistance to other diamide insecticides.

BACKGROUND: Understanding the genetic basis of insect resistance to insecticides can help to implement insecticide resistance management (IRM) strategies. In this study, we selected a strain of Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) resistant to chlorantraniliprole using the F2 screen method, characterized the inheritance of resistance and evaluated patterns of cross-resistance to other diamide insecticides. RESULTS: The chlorantraniliprole-resistant strain (Chlorant-R) was selected from a field-collected population with an estimated allele frequency of 0.1316. The estimated median lethal concentration (LC50 ) values were 0.011 and 2.610 µg a.i. cm-2 for the susceptible (Sus) and Chlorant-R strains, respectively, resulting in a resistance ratio (RR) of 237-fold. The LC50 values of the reciprocal crosses were 0.155 and 0.164 µg a.i. cm-2 , indicating that resistance is autosomally inherited. Resistance was characterized as incompletely recessive and monogenic at concentrations close to the recommended field rates of chlorantraniliprole. Survival of the resistant strain and heterozygous larvae in maize plants treated at the field rate was ∼ 60 (similar to the susceptible strain on untreated maize) and 15%, respectively. The Chlorant-R strain showed low cross-resistance to cyantraniliprole (RR ∼ 27-fold) and very high cross-resistance to flubendiamide (RR > 42 000-fold). CONCLUSIONS: The frequency of chlorantraniliprole resistance allele was moderately high in a field-collected population of S. frugiperda. The inheritance of chlorantraniliprole resistance was characterized as autosomal, incompletely recessive and monogenic. S. frugiperda showed cross-resistance to other diamide insecticides. These results provide essential information for the implementation of IRM programs to preserve the useful life of diamide insecticides for controlling S. frugiperda in Brazil. © 2019 Society of Chemical Industry.