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.
BACKGROUND: Fall armyworm (FAW), Spodoptera frugiperda (Smith), is an economically important pest worldwide. In this study, we selected a genotype of FAW resistant to chlorpyrifos from a field-collected population, characterized the genetic basis of resistance, and evaluated cross-resistance and mechanisms of resistance using synergists. RESULTS:The LD 50 values of chlorpyrifos for the resistant (Clorp-R) and susceptible (Sus) FAW genotypes were 24.26 and 0.023 ∼g per larva, respectively, representing a resistance ratio > 1050-fold. The LD 50 values of chlorpyrifos against heterozygotes were 3.34 and 4.00 ∼g per larva, suggesting that resistance is autosomally inherited. The chlorpyrifos resistance in FAW was influenced by few genes, with the minimum numbers of segregations being 1.74 and 1.88. On chlorpyrifos-sprayed plants and leaves, Clorp-R and heterozygote genotypes showed >95% and >52% survival, respectively, whereas the Sus genotype had no survival, indicating that the resistance is incompletely dominant at the field rate of chlorpyrifos. The Clorp-R genotype presented some cross-resistance to acephate, but low cross-resistance to thiodicarb, methomyl, chlorfenapyr, flubendiamide, methoxyfenozide, spinetoram, and teflubenzuron. The synergists piperonyl butoxide, diethyl maleate, and S,S,S-tributyl phosphorotrithiotate did not have relevant effects on the Clorp-R genotype, suggesting a minor role for metabolic resistance.CONCLUSIONS: The inheritance of chlorpyrifos resistance in FAW was characterized as autosomal, incompletely dominant, and polygenic, with metabolic resistance playing a small role in the detoxification of chlorpyrifos. Low cross-resistance between chlorpyrifos and other mode of action (MoA) insecticides occurs in FAW, highlighting the importance of considering the rotation of MoA as a strategy to delay resistance.
Genetically modified (GM) soybeans expressing Cry1A.105/Cry2Ab2/Cry1Ac (event MON 87701 × MON 89788 × MON 87751 × MON 87708) and Cry1Ac (event MON 87701 × MON 89788) from Bacillus thuringiensis Berliner (Bt) are valuable technologies for managing key lepidopteran pests of soybean in South America, but do not provide stand-alone protection against Spodoptera species. Here, we evaluated the use of these Bt soybeans and their integration with insecticides for managing Spodoptera species. Cry1A.105/Cry2Ab2/Cry1Ac soybean provided reasonable levels of protection against S. cosmioides, S. albula, and S. eridania. However, S. frugiperda had higher survival on this Bt soybean, and Cry1Ac soybean showed low lethality against all species evaluated. Spodoptera larvae that survived on Bt and non-Bt soybean showed comparable susceptibility to flubendiamide and thiodicarb in diet-overlay bioassays. Regardless of soybean plant type, the field doses of flubendiamide and thiodicarb were effective in controlling surviving Spodoptera larvae. We conclude that Cry1A.105/Cry2Ab2/Cry1Ac soybean is effective in controlling S. cosmioides and S. albula, and also has reasonable control of S. eridania, but not S. frugiperda. Cry1Ac soybean provided poor control of all Spodoptera species. Nonetheless, Spodoptera larvae surviving on both Bt and non-Bt soybean were controlled by flubendiamide and thiodicarb. Thus, integrated control tactics would provide effective management of Spodoptera species in Bt soybean fields in South America.
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