Field-evolved resistance to chlorpyrifos by Spodoptera frugiperda (Lepidoptera: Noctuidae): Inheritance mode, cross-resistance patterns, and synergism.

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 LD50 values of chlorpyrifos for the resistant (Clorp-R) and susceptible (Sus) FAW genotypes were 24.26 and 0.023 µg per larva, respectively, representing a resistance ratio > 1050-fold. The LD50 values of chlorpyrifos against heterozygotes were 3.34 and 4.00 µg 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. © 2021 Society of Chemical Industry.

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.

Field-evolved resistance to chitin synthesis inhibitor insecticides by soybean looper, Chrysodeixis includens (Lepidoptera: Noctuidae), in Brazil.

Soybean looper (SBL), Chrysodeixis includens (Walker), is an economically important soybean and cotton pest in Brazil. Here, we selected an SBL strain resistant to teflubenzuron using F2 screening, estimated the resistance allele frequency, characterized the inheritance of resistance, investigated fitness costs, evaluated patterns of cross-resistance, and determined the magnitude of resistance. The teflubenzuron-resistant strain (Teflu-R) was selected from field-collected populations with an estimated allele frequency of 0.1700. Estimated LC50 values were 0.010 and 363.61 µg a.i. cm-2 for the susceptible (Sus) and Teflu-R strains, respectively, representing a 36,361-fold resistance ratio (RR). The LC50 values of reciprocal crosses were 1.02 and 0.59 µg a.i. cm-2, suggesting that resistance is autosomally inherited. The low survival of reciprocal crosses (16 and 20%) on teflubenzuron-sprayed leaves indicates incomplete recessive resistance. The number of segregations influencing resistance was 2.72, suggesting a polygenic effect. The Teflu-R strain showed longer development periods as well as lower survival and population growth than the Sus strain, revealing fitness costs. The Teflu-R strain also showed high cross-resistancesto other chitin inhibitor insecticides, such as novaluron (RR = 6147-fold) and lufenuron (RR = 953-fold), but low cross-resistance to methoxyfenozide, flubendiamide, and indoxacarb (RR < 3.45-fold). On discriminatory concentrations of teflubenzuron and novaluron, populations of SBL showed survival rates from 15 to 52%, indicating field resistance to these insecticides. Our findings indicated that resistance to teflubenzuron in SBL is autosomal, recessive, polygenic, and associated with fitness cost. We also found a high cross-resistance to other benzoylphenylureas and a high frequency of resistance to this mode-of-action in SBL in Brazil.

Susceptibility of Euschistus heros and Dichelops furcatus (Hemiptera: Pentatomidae) to Selected Insecticides in Brazil.

Euschistus heros (Fabricius, 1798) and Dichelops furcatus (Fabricius, 1775) are key pests of agricultural crops in Brazil. Chemical insecticides are the main control tactic used against these species. Here, we investigated the susceptibility of E. heros and D. furcatus from distinct regions to the mains insecticides used to stink bugs control in Brazil. Field populations of these species were collected throughout the 2017-2019 crop seasons and insects were exposed to insecticides in dip-test bioassays using fresh green bean pods. Populations of E. heros exhibited low variation in the susceptibility to acephate (LC50 = 172.2 to 1,008 µg a.i. per ml), and thiamethoxam (LC50 = 28.8 to 433.9 µg a.i. per ml); resistance ratios were less than 5.9- and 15.1-fold, respectively. In contrast, these populations had higher variation in the susceptibility to bifenthrin (LC50 = 26.7 to 636.1 µg a.i. per ml) and lambda-cyhalothrin (LC50 = 10.0 to 636.1 µg a.i. per ml); resistance ratios reaching 23.8- and 63.6-fold, respectively. Susceptibility monitoring data indicated a higher susceptibility of E. heros to the manufacturers field-recommended rates of acephate, lambda-cyhalothrin + thiamethoxam, and bifenthrin + acetamiprid than lambda-cyhalothrin. Populations of D. furcatus exhibited low variation in the susceptibility to acephate (LC50 = 219.2 to 614.1 µg a.i. per ml), bifenthrin (LC50 = 62.8 to 197.4 µg a.i. per ml), and lambda-cyhalothrin (LC50 = 189.5 to 2,538 µg a.i. per ml); resistance ratios were less than 13.4-fold. In summary, populations of E. heros are less susceptible to pyrethroids, while populations of D. furcatus have similar susceptibility to the insecticides evaluated.