Meta-diamide insecticides acting on distinct sites of RDL GABA receptor from those for conventional noncompetitive antagonists

The Frequency and Spread of a GABA-Gated Chloride Channel Target-Site Mutation and Its Impact on the Efficacy of Ethiprole Against Neotropical Brown Stink Bug, Euschistus heros (Hemiptera: Pentatomidae)

The Neotropical brown stink bug (NBSB), Euschistus heros, is the most prevalent sucking soybean pest in Brazil, and control of it largely relies on the application of synthetic insecticides such as ethiprole, a phenylpyrazole insecticide targeting GABA-gated chloride channels encoded by the Rdl (resistant to dieldrin) gene. This study monitored 41 NBSB populations collected between 2021 and 2024 and revealed, for the first time, the presence of a mutation, A301S, in NBSB RDL receptors commonly known to confer target-site resistance to channel blockers such as phenylpyrazoles. Laboratory contact bioassays with ethiprole at 150 g a.i./ha (ethiprole label dose) revealed that most populations were quite susceptible, despite rather high resistance allele frequencies in some populations. Genotyping results confirmed that susceptible and A301S heterozygous genotypes largely dominate in frequency compared to homozygous resistant individuals, which exhibited high survivorship (84%) when exposed to discriminating rates of ethiprole in laboratory bioassays, while susceptible and heterozygote individuals showed lower survival rates (13% and 34%, respectively), suggesting an incompletely recessive trait conferring ethiprole resistance. Furthermore, we developed a TaqMan assay for molecular genotyping to monitor the spread of resistance allele frequency and to inform resistance management strategies for sustainable NBSB control using highly effective phenylpyrazole insecticides such as ethiprole.

Design, Synthesis, and Insecticidal Activity of Novel m-Diamide Compounds Containing Sulfur

To find efficient insecticides with novel structures, 16 novel m-diamide compounds containing sulfur were designed and synthesized based on the principle of biologically active factor splicing with cyproflanilide as the lead compound. Preliminary biological assay data showed that some of the target compounds exhibited insecticidal activity above 90% against Mythimna separata at 0.01 mg/L and Aphis craccivora at 100 mg/L, which were better than cyproflanilide. Meanwhile, most compounds still maintain 100% mortality at 1 mg/L for Plutella xylostella. In particular, compounds 5c, 5e, 5f, and 5g revealed excellent activities against P. xylostella, M. separata, and A. craccivora, exhibiting efficient and broad-spectrum insecticidal activity. Additionally, the interaction between the metabolites of cyproflanilide, compound 5g, and γ-aminobutyric acid receptor protein was studied by molecular docking method. The results showed that they could be stably bound to the potential active cavity in the protein to form a protein-ligand complex, thus showing excellent insecticidal activity, which is consistent with the data on the excellent insecticidal activity of compounds. This work showed that sulfur-containing compounds might provide certain clues for structure optimization for the development of new insecticides.

Risk assessment of broflanilide resistance in Panonychus citri (McGregor): Cross-resistance, inheritance and relative fitness

Panonychus citri (McGregor) is an important economic pest in the orange orchard of the world, which has developed varying degrees of resistance to many acaricides. Broflanilide is a novel γ-aminobutyric acid (GABA) receptor allosteric modulator with high insecticidal activity against a broad spectrum of insects. However, the risk of resistance to broflanilide in P. citri has not been studied. In this study, the BR strain selected from susceptible strain of P. citri with broflanilide for 44 generations, developed 32.5-fold resistance to broflanilide, and did not exhibit cross-resistance to fipronil, fluxametamide, abamectin, pyridaben, and cyflumetofen. Broflanilide resistance in the BR strain of P. citri was autosomal, incomplete dominant and polygenic. The duration of larval and deutonymph, total life span and the number of eggs were significantly increased in the BR strain compared to the SS strain. The higher relative fitness (Rf) value (1.289) indicated that there was a fitness advantage in the BR strain. Glutathione S-transferase and esterase activities in the BR strain were significantly increased compared with the SS strain. These findings provide valuable information for developing resistance management strategies to delay broflanilide resistance and maintain sustainable control of P. citri.

Novel Insecticidal Benzo[4,5]imidazo[1,2-b]pyrazole Derivatives Idenatified Through Ring-Closure Scaffold Hopping on Fipronil

A series of innovative benzo[4,5]imidazo[1,2-b]pyrazole scaffold containing compounds were rationally designed through a ring-closure scaffold hopping strategy and synthetized with an intermediate derivatization approach. Physicochemical properties analysis indicated the potential pesticide-likeness of the target compounds. The optimal target compound A14 showed relatively good insecticidal activity against P. xylostella, with an LC50 value of 37.58 mg/L, and demonstrated lower acute fish toxicity compared to fipronil. Docking binding mode analysis demonstrated that compound A14 bound to GABAR through a H-bond between the amide group and the residue of 6'Thr. The differences in binding modes between benzo[4,5]imidazo[1,2-b]pyrazole target compounds and fipronil may be a key factor for the reduced insecticidal activities. The elucidated binding mode and SAR profile lay a foundation for the further structural optimization of insecticidal benzo[4,5]imidazo[1,2-b]pyrazole derivatives.

The bioefficacy of a novel VECTRON™ T500 indoor residual spray formulation in an experimental huts trial against Anopheles gambiae s.l. populations

BACKGROUND

The emergence of insecticide resistance and its spread through populations of malaria vectors has decreased the number of insecticides available for control. Insecticide resistance has been observed in vector populations across sub-Saharan Africa in malaria endemic areas. Therefore, new compounds with different modes of action are needed that can be used in the management of resistance. The current study assessed the bioefficacy of the new indoor residual spray formulation, VECTRON™ T500 against Klypson 500 WG and water against laboratory reared and wild populations of Anopheles gambiae s.l.. The comparative experimental hut trial was implemented between June 2022 and December 2022 to determine the efficacy of VECTRON™ T500, containing the active ingredient (ai) broflanilide as a 50 % wettable powder (WP). The efficacy of VECTRON™ T500 was compared with a positive control, Klypson 500 WG, a wettable granule (WG) formulation that contains 50 % clothianidin. Cement and mud walls were sprayed with VECTRON™ T500, Klypson 500 WG with water sprayed as a negative control. The two insecticides and negative control were evaluated monthly for six months against laboratory and the field-derived An. gambiae s.l. using the standard WHO cone bioassays. Each wall had two cones on each day of testing. VECTRON™ T500 was sprayed on both surface types at a rate of 100mg ai/m2 whilst Klypson 500 WG was applied at a rate of 300 mg ai/m2. For both wall surface types, the vector from the laboratory-reared and the wild populations exhibited a low knockdown effect to both VECTRON™ T500 and Klypson 500 WG. A total of 3,840 mosquitoes were used of which 2,880 (75 %) were susceptible colony of An. gambiae s.s. and 960 (25 %) were wild An. gambiae s.l.. The VECTRON™ T500 induced a mortality of 97.8 %-98.1 % in the laboratory population and 83.2-95.0 % wild population mosquitoes on cement and mud-walled huts respectively while Klypson 500 WG ranged from 89.6-99.0 % for wild and 99.0-99.3 % for the laboratory population mosquitoes on cement and mud walls respectively. The knockdown due to VECTRON™ T500 was 7.08 % in the sixth month, while for Klypson 500 WG was 16.04-17.50 %. The monthly wall cone bioassay mortality with VECTRON™ T500 remained over 80 % for 6 months post-spraying for both laboratory and wild populations. The findings of this study have shown VECTRON™ T500 to have extended efficacy against malaria vector mosquitoes when applied to cement and mud walls. The evaluated new IRS formulation, VECTRON™ T500, performed equally with the positive control, Klypson 500 WG, regarding its impact on vector mortality.

N318L Blocks the Interaction of Fluralaner but Not Broflanilide or Fipronil with the Insect GABA Receptor In Vivo

Fluralaner is a novel insecticide targeting the ionotropic GABA receptor (GABAR) subunit, RDL. A recent study revealed that N316L, a substitution of asparagine (N) with leucine (L), in the second transmembrane (M2)-spanning region reduced the antagonist action of fluralaner on the housefly Musca domestica RDL (MdRDL) in vitro. To verify the impact of N316L in vivo, the corresponding mutation (N318L) in the fruitfly Drosophila melanogaster RDL (DmRDL) was constructed using CRISPR/Cas9 genome editing. The homozygous DmRDLN318L mutant showed a 9.87-fold resistance to fluralaner compared with w1118 while still being highly sensitive to broflanilide and fipronil, which is consistent with those findings observed in the electrophysiology assays of the homomeric DmRDLWT or DmRDLN318L channel. Moreover, DmRDLN318L led to malformed ovaries, stunted eggs, and sterility in homozygous females. These results highlighted N318 as a molecular site for fluralaner in vivo and in vitro and might elucidate the resistance mechanisms of insects against fluralaner.

Sublethal and transgenerational effects of broflanilide on the citrus red mite, Panonychus citri

BACKGROUND

The citrus red mite, Panonychus citri is a serious pest of the citrus industry and has developed resistance to many acaricides. Broflanilide is a novel meta-diamide insecticide that binds to a new site on the γ -aminobutyric acid receptor with high potency against pests. However, little information has been reported about its effect on the citrus red mite.

RESULTS

Broflanilide exhibited higher toxicity to female adults and eggs of a laboratory strain of P. citri The median lethal concentration (LC50), 9.769 mg/L and 4.576 mg/L, respectively) than other commonly used acaricides and was also toxic to two P. citri field strains. Broflanilide treatment with LC10, LC20, and LC30 significantly decreased the fecundity and longevity of female adults of F0 P. citri compared with the control. The duration of larva, protonymph, deutonymph and adult, and total life span in the F1 generation were significantly reduced after treatment of F0 with broflanilide. Population parameters, including the intrinsic rate of increase (r) and finite rate of increase (λ), were significantly increased, and the mean generation time (T) of F1 progeny was significantly reduced in the LC20 treatment. The predicted population size of F1 increased when parental female adults were treated with sublethal concentrations.

CONCLUSION

Broflanilide had high acaricidal activity toward P. citri, and exposure to a sublethal concentration significantly inhibited the population growth of F0. The transgenerational hormesis effect is likely to cause population expansion of F1. More attention should be paid when broflanilide is applied to control P. citri in citrus orchards. © 2024 Society of Chemical Industry.

Effects of RDL GABA Receptor Point Mutants on Susceptibility to Meta-Diamide and Isoxazoline Insecticides in Drosophila melanogaster

Ionotropic γ-aminobutyric acid (GABA) receptors in insects, specifically those composed of the RDL (resistant to dieldrin) subunit, serve as important targets for commonly used synthetic insecticides. These insecticides belong to various chemical classes, such as phenylpyrazoles, cyclodienes, meta-diamides, and isoxazolines, with the latter two potentially binding to the transmembrane inter-subunit pocket. However, the specific amino acid residues that contribute to the high sensitivity of insect RDL receptors to these novel insecticides remain elusive. In this study, we investigated the susceptibility of seven distinct Drosophila melanogaster Rdl point mutants against four meta-diamide and isoxazoline insecticides: isocycloseram, fluxametamide, fluralaner, and broflanilide. Our findings indicate that, despite exhibiting increased sensitivity to fluralaner in vitro, the RdlI276C mutant showed resistance to isocycloseram and fluxametamide. Similarly, the double-points mutant RdlI276F+G279S also showed decreased sensitivity to the tested isoxazolines. On the other hand, the RdlG335M mutant displayed high levels of resistance to all tested insecticides. Molecular modeling and docking simulations further supported these findings, highlighting similar binding poses for these insecticides. In summary, our research provides robust in vivo evidence supporting the idea that the inter-subunit amino acids within transmembrane M1 and M3 domains form the binding site crucial for meta-diamide and isoxazoline insecticide interactions. This study highlights the complex interplay between mutations and insecticide susceptibility, paving the way for more targeted pest control strategies.

Mechanistic insights into the selectivity of bicyclophosphorothionate antagonists for housefly versus rat GABA receptors

BACKGROUND

γ-Aminobutyric acid (GABA) receptors (GABARs) are validated targets of insecticides. Bicyclophosphorothionates are a group of insecticidal compounds that act as noncompetitive antagonists of GABARs. We previously reported that the analogs exhibit various degrees of selectivity for housefly versus rat GABARs, depending on substitutions at the 3- and 4-positions. We here sought to elucidate the unsolved mechanisms of the receptor selectivity using quantitative structure-activity relationship (QSAR), molecular docking, and molecular dynamics approaches.

RESULTS

Three-dimensional (3D)-QSAR studies using Topomer comparative molecular field analysis quantitatively demonstrated how the introduction of a small alkyl group at the 3-position of bicyclophosphorothionates contributes to the housefly versus rat GABAR selectivity. To investigate the molecular mechanisms of the selective action, bicyclophosphorothionates were docked into housefly Resistance to dieldrin (RDL) GABAR and rat α1β2γ2 GABAR homology models built using the published 3D-structures of human GABARs as templates. The results of molecular docking and molecular dynamics simulations revealed that the 2'Ala and 6'Thr residues of the RDL subunit within the channel are the key amino acids for binding to the housefly GABARs, whereas the 2'Ser residue of γ2 subunit plays a crucial role in binding to rat GABARs.

CONCLUSION

We revealed the molecular mechanisms underlying the selective antagonistic action of bicyclophosphorothionates on housefly versus rat GABARs. The information presented should help design and develop novel, safe GABAR-targeting insecticides. © 2023 Society of Chemical Industry.

Knock-in mutagenesis in Drosophila Rdl underscores the critical role of the conserved M3 glycine in mediating the actions of broflanilide and isocycloseram on GABA receptors

γ-Aminobutyric acid receptors (GABARs) are crucial targets for pest control chemicals, including meta-diamide and isoxazoline insecticides, which act as negative allosteric modulators of insect GABARs. Previous cell-based assays have indicated that amino acid residues in the transmembrane cavity between adjacent subunits of Drosophila RDL GABAR (i.e., Ile276, Leu280, and Gly335) are involved in mediating the action of meta-diamides. In this study, to confirm this result at the organismal level, we employed CRISPR/Cas9-mediated genome editing, generated six transgenic Drosophila strains carrying substitutions in these amino acid residues, and investigated their sensitivity to broflanilide and isocycloseram. Flies homozygous for the I276F mutation did not exhibit any change in sensitivity to the tested insecticides compared to the control flies. Conversely, I276C homozygosity was lethal, and heterozygous flies exhibited ∼2-fold lower sensitivity to broflanilide than the control flies. Flies homozygous for the L280C mutation survived into adulthood but exhibited infertility. Both heterozygous and homozygous L280C flies exhibited ∼3- and ∼20-fold lower sensitivities to broflanilide and isocycloseram, respectively, than the control flies. The reduction in sensitivity to isocycloseram in L280C flies diminished to ∼3-fold when treated with piperonyl butoxide. Flies homozygous for the G335A mutation reached the adult stage. However, they were sterile, had small bodies, and exhibited reduced locomotion, indicating the critical role of Gly335 in RDL function. These flies exhibited markedly increased tolerance to topically applied broflanilide and isocycloseram, demonstrating that the conserved Gly335 is the target of the insecticidal actions of broflanilide and isocycloseram. Considering the significant fitness costs, the Gly335 mutation may not pose a serious risk for the development of resistance in field populations of insect pests. However, more careful studies using insect pests are needed to investigate whether our perspective applies to resistance development under field conditions.

G3'MTMD3 in the insect GABA receptor subunit, RDL, confers resistance to broflanilide and fluralaner

Meta-diamides (e.g. broflanilide) and isoxazolines (e.g. fluralaner) are novel insecticides that target the resistant to dieldrin (RDL) subunit of insect γ-aminobutyric acid receptors (GABARs). In this study, we used in silico analysis to identify residues that are critical for the interaction between RDL and these insecticides. Substitution of glycine at the third position (G3') in the third transmembrane domain (TMD3) with methionine (G3'M TMD3), which is present in vertebrate GABARs, had the strongest effect on fluralaner binding. This was confirmed by expression of RDL from the rice stem borer, Chilo suppressalis (CsRDL) in oocytes of the African clawed frog, Xenopus laevis, where the G3'MTMD3 mutation almost abolished the antagonistic action of fluralaner. Subsequently, G3'MTMD3 was introduced into the Rdl gene of the fruit fly, Drosophila melanogaster, using the CRISPR/Cas9 system. Larvae of heterozygous lines bearing G3'MTMD3 did not show significant resistance to avermectin, fipronil, broflanilide, and fluralaner. However, larvae homozygous for G3'MTMD3 were highly resistant to broflanilide and fluralaner whilst still being sensitive to fipronil and avermectin. Also, homozygous lines showed severely impaired locomotivity and did not survive to the pupal stage, indicating a significant fitness cost associated with the G3'MTMD3. Moreover, the M3'GTMD3 in the mouse Mus musculus α1β2 GABAR increased sensitivity to fluralaner. Taken together, these results provide convincing in vitro and in vivo evidence for both broflanilide and fluralaner acting on the same amino acid site, as well as insights into potential mechanisms leading to target-site resistance to these insecticides. In addition, our findings could guide further modification of isoxazolines to achieve higher selectivity for the control of insect pests with minimal effects on mammals.

Chronic toxicity of broflanilide in Daphnia magna: changes in molting, behavior, and gene expression

Broflanilide is a novel pesticide used in agriculture that binds to unique receptors on pests; however, the widespread use of broflanilide has led to toxicity in Daphnia magna. At present, little information on the potential threats broflanilide imposes on D. magna is available. Therefore, the present study examined the chronic toxicity of broflanilide in D. magna by comparing changes in molting, neurotransmitter function, and behavior. The results showed that broflanilide caused chronic toxicity in D. magna at a concentration of 8.45 μg/L, and growth, development, reproduction, and the development of offspring were affected. In addition, broflanilide affected the molting of D. magna by significantly inhibiting the expression of chitinase, ecdysteroid, and related genes. Broflanilide also affected the expression of γ-glutamic acid, glutamine, gamma-aminobutyric acid, 5-hydroxytryptamine, 5-hydroxytryptophan, dopa, and dopamine. Furthermore, the swimming distance and speed of D. magna were reduced. Taken together, the results demonstrate the chronic toxicity and exposure risk of broflanilide in D. magna.

Toxicity and Sublethal Effects of Diamide Insecticides on Key Non-Target Natural Predators, the Larvae of Coccinella septempunctata L. (Coleoptera: Coccinellidae)

Coccinella septempunctata (ladybird) is an extremely important natural predator that feeds on aphids. An assessment of the toxicity of pesticides on environmental organisms is an essential component of Integrated Pest Management (IPM) strategies. This study evaluated diamide insecticides' toxicity at lethal and 30% lethal doses (LR₃₀) against C. septempunctata larvae. The pre-imaginal median lethal doses (LR₅₀) of chlorantraniliprole 10% SC, tetrachlorantraniliprole 10% SC, and broflanilide 10% SC were calculated to be 42.078, 289.516, and 0.0943 g active ingredient (a.i.)/ha, respectively. The mortality tests demonstrated that chlorantraniliprole and tetrachlorantraniliprole are comparatively less toxic to C. septempunctata than broflanilide, which were detected to be highly toxic to C. septempunctata. The mortality rates of the groups treated with the three diamide insecticides tended to stabilize after 96 h, extending to the pre-imaginal stage. Furthermore, when compared to broflanilide, which had a much higher potential risk, the hazard quotient (HQ) values indicated that chlorantraniliprole and tetrachlorantraniliprole have a lower risk potential for C. septempunctata in farmland and off farmland. The LR₃₀ dose induces abnormalities in the development phase 4th-instar larvae weight, pupal weight, and adult weight of treated C. septempunctata. The study emphasizes the importance of assessing the adverse effects of diamide insecticides on natural predator species that serve as biological control agents in agricultural IPM strategies.

Baseline Susceptibility, Cross-Resistance, and Sublethal Effects of Broflanilide, a Novel Meta-Diamide Pesticide, in Spodoptera litura

Spodoptera litura is a damaging and notorious insect pest of agricultural crops that has developed resistance to various insecticides. Broflanilide is a novel pesticide with a unique mode of action that displays high efficiency against lepidopterous larvae. We here determined the baseline susceptibility of a laboratory strain of S. litura to broflanilide and 10 other popular insecticides. Furthermore, we measured susceptibility and cross-resistance using three common insecticides in 11 field-collected S. litura populations. Broflanilide caused the highest toxicity among all tested insecticides, with the laboratory strain and all field-collected populations showing high susceptibility. Moreover, no cross-resistance was detected between broflanilide and the other tested insecticides. We subsequently evaluated the sublethal effects of broflanilide and found that treatment with the 25% lethal concentration (LC25) prolonged the development duration in the larvae, reduced the pupation rate and pupae weight, and decreased egg hatchability. Finally, the activities of three detoxifying enzymes were measured in S. litura after treatment with the LC25 dose. The results suggested that enhanced cytochrome P450 monooxygenase (P450) activity could be involved in broflanilide detoxification. Overall, these findings demonstrate the strong toxicity and significant sublethal effects of broflanilide in S. litura and suggest that increased P450 activity may be associated with broflanilide detoxification.

Investigating discriminating concentrations for monitoring susceptibility to broflanilide and cross resistance to other insecticide classes in Anopheles gambiae sensu lato, using the new WHO bottle bioassay method

BACKGROUND

Broflanilide is a new insecticide being developed for malaria vector control. As new insecticide chemistries become available, strategies to preserve the susceptibility of local malaria vectors and extend their useful life need to be considered before large scale deployment. This requires the development of appropriate testing procedures and identification of suitable discriminating concentrations for monitoring susceptibility in wild vector populations to facilitate decision making by control programmes.

METHODS

Dose-response WHO bottle bioassays were conducted using the insecticide-susceptible Anopheles gambiae s.s. Kisumu strain to determine a discriminating concentration of broflanilide. Bioassays were performed without the adjuvant Mero® and with two concentrations of Mero® (500 ppm and 800 ppm) to investigate its impact on the discriminating concentration of the insecticide. Probit analysis was used to determine the lethal doses at 50% (LC50) and 99% (LC99) at 24-, 48- and 72-hours post-exposure. Cross-resistance to broflanilide and pyrethroids, DDT, dieldrin and carbamates, was investigated using An. gambiae s.l. Covè and An. coluzzii Akron strains. The susceptibility of wild pyrethroid-resistant mosquitoes from communities in Southern Benin to broflanilide was assessed using the estimated discriminating concentrations.

RESULTS

Broflanilide induced a dose-dependent and delayed mortality effect. Mortality rates in bottles treated without Mero® were <80% using the range of broflanilide doses tested (0-100 μg/bottle) leading to high and unreliable estimates of LC99 values. The discriminating concentrations defined as 2XLC99 at 72h post exposure were estimated to be 2.2 μg/bottle with 800 ppm of Mero® and 6.0 μg/bottle with 500 ppm of Mero®. Very low resistance ratios (0.6-1.2) were determined with the insecticide resistant An. gambiae s.l. Covè and An. coluzzii Akron strains suggesting the absence of cross-resistance via the mechanisms of resistance to pyrethroids, DDT, dieldrin and carbamates they possess. Bottle bioassays performed with broflanilide at both discriminating concentrations of 6 μg/bottle with 500 ppm of Mero® and 2.2 μg/bottle with 800 ppm of Mero®, showed susceptibility of wild highly pyrethroid-resistant An. gambiae s.l. from villages in Southern Benin.

CONCLUSION

We determined discriminating concentrations for monitoring susceptibility to broflanilide in bottle bioassays, using susceptible An. gambiae vectors. Using the estimated discriminating concentrations, we showed that wild pyrethroid-resistant populations of An. gambiae s.l. from southern Benin were fully susceptible to the insecticide. Broflanilide also shows potential to be highly effective against An. gambiae s.l. vector populations that have developed resistance to other public health insecticides.

Activity, selection response and molecular mode of action of the isoxazoline afoxolaner in Tetranychus urticae

BACKGROUND

Afoxolaner is a novel representative of the isoxazolines, a class of ectoparasiticides which has been commercialized for the control of tick and flea infestations in dogs. In this study, the biological efficacy of afoxolaner against the two-spotted spider mite Tetranychus urticae was evaluated. Furthermore, as isoxazolines are known inhibitors of γ-aminobutyric acid-gated chloride channels (GABACls), the molecular mode of action of afoxolaner on T. urticae GABACls (TuRdls) was studied using functional expression in Xenopus oocytes followed by two-electrode voltage-clamp (TEVC) electrophysiology, and results were compared with inhibition by fluralaner, fipronil and endosulfan. To examine the influence of known GABACl resistance mutations, H301A, I305T and A350T substitutions in TuRdl1 and a S301A substitution in TuRdl2 were introduced.

RESULTS

Bioasassays revealed excellent efficacy of afoxolaner against all developmental stages and no cross-resistance was found in a panel of strains resistant to most currently used acaricides. Laboratory selection over a period of 3 years did not result in resistance. TEVC revealed clear antagonistic activity of afoxolaner and fluralaner for all homomeric TuRdl1/2/3 channels. The introduction of single, double or triple mutations to TuRdl1 and TuRdl2 did not lower channel sensitivity. By contrast, both endosulfan and fipronil had minimal antagonistic activities against TuRdl1/2/3, and channels carrying single mutations, whereas the sensitivity of double and triple TuRdl1 mutants was significantly increased.

CONCLUSIONS

Our results demonstrate that afoxolaner is a potent antagonist of GABACls of T. urticae and has a powerful mode of action to control spider mites. © 2022 Society of Chemical Industry.

Multi-centre discriminating concentration determination of broflanilide and potential for cross-resistance to other public health insecticides in Anopheles vector populations

Novel insecticides are urgently needed to control insecticide-resistant populations of Anopheles malaria vectors. Broflanilide acts as a non-competitive antagonist of the gamma-aminobutyric acid receptor and has shown prolonged effectiveness as an indoor residual spraying product (VECTRON T500) in experimental hut trials against pyrethroid-resistant vector populations. This multi-centre study expanded upon initial discriminating concentration testing of broflanilide, using six Anopheles insectary colonies (An. gambiae Kisumu KCMUCo, An. gambiae Kisumu NIMR, An. arabiensis KGB, An. arabiensis SENN, An. coluzzii N'Gousso and An. stephensi SK), representing major malaria vector species, to facilitate prospective susceptibility monitoring of this new insecticide; and investigated the potential for cross-resistance to broflanilide via the A296S mutation associated with dieldrin resistance (rdl). Across all vector species tested, the discriminating concentration for broflanilide ranged between LC99 × 2 = 1.126-54.00 μg/ml or LC95 × 3 = 0.7437-17.82 μg/ml. Lower concentrations of broflanilide were required to induce complete mortality of An. arabiensis SENN (dieldrin-resistant), compared to its susceptible counterpart, An. arabiensis KGB, and there was no association between the presence of the rdl mechanism of resistance and survival in broflanilide bioassays, demonstrating a lack of cross-resistance to broflanilide. Study findings provide a benchmark for broflanilide susceptibility monitoring as part of ongoing VECTRON T500 community trials in Tanzania and Benin.

VECTRON™ T500, a new broflanilide insecticide for indoor residual spraying, provides prolonged control of pyrethroid-resistant malaria vectors

Background

Broflanilide is a newly discovered insecticide with a novel mode of action targeting insect γ-aminobutyric acid receptors. The efficacy of VECTRON™ T500, a wettable powder formulation of broflanilide, was assessed for IRS against wild pyrethroid-resistant malaria vectors in experimental huts in Benin.

Methods

VECTRON™ T500 was evaluated at 100 mg/m2 in mud and cement-walled experimental huts against wild pyrethroid-resistant Anopheles gambiae sensu lato (s.l.) in Covè, southern Benin, over 18 months. A direct comparison was made with Actellic® 300CS, a WHO-recommended micro-encapsulated formulation of pirimiphos-methyl, applied at 1000 mg/m2. The vector population at Covè was investigated for susceptibility to broflanilide and other classes of insecticides used for vector control. Monthly wall cone bioassays were performed to assess the residual efficacy of VECTRON™ T500 using insecticide susceptible An. gambiae Kisumu and pyrethroid-resistant An. gambiae s.l. Covè strains. The study complied with OECD principles of good laboratory practice.

Results

The vector population at Covè was resistant to pyrethroids and organochlorines but susceptible to broflanilide and pirimiphos-methyl. A total of 23,171 free-flying wild pyrethroid-resistant female An. gambiae s.l. were collected in the experimental huts over 12 months. VECTRON™ T500 induced 56%-60% mortality in wild vector mosquitoes in both cement and mud-walled huts. Mortality with VECTRON™ T500 was 62%-73% in the first three months and remained > 50% for 9 months on both substrate-types. By comparison, mortality with Actellic® 300CS was very high in the first three months (72%-95%) but declined sharply to < 40% after 4 months. Using a non-inferiority margin defined by the World Health Organization, overall mortality achieved with VECTRON™ T500 was non-inferior to that observed in huts treated with Actellic® 300CS with both cement and mud wall substrates. Monthly in situ wall cone bioassay mortality with VECTRON™ T500 also remained over 80% for 18 months but dropped below 80% with Actellic® 300CS at 6–7 months post spraying.

Conclusion

VECTRON™ T500 shows potential to provide substantial and prolonged control of malaria transmitted by pyrethroid-resistant mosquito vectors when applied for IRS. Its addition to the current list of WHO-approved IRS insecticides will provide a suitable option to facilitate rotation of IRS products with different modes of action.

Establishment of Toxicity and Susceptibility Baseline of Broflanilide for Aphis gossypii Glove

The Aphis gossypii is an important pest that can damage cotton plants and can cause a huge economic loss worldwide. Chemical control is a main method to manage this pest, but the cotton aphid resistance to insecticides has become a severe problem in the management of the cotton aphid. It is important to introduce a novel insecticide for rotational application with other insecticides. Broflanilide, as a meta-diamide insecticide with a special mode of action, showed high efficiency against lepidopterous larvae. However, we found that broflanilide possessed high insecticidal activity against the sap-sucking pest A. gossypii. The susceptibility of A. gossypii to broflanilide from 20 field populations in main cotton planting areas of China in 2021 was determined by the leaf-dipping method. LC₅₀ values of broflanilide to A. gossypii ranged from 0.20 μg mL^-1 to 1.48 μg mL^-1. The susceptible baseline of A. gossypii to broflanilide was established with the LC₅₀ value of 0.41 μg mL^-1 and might be used to calculate the resistance ratio (RR) of cotton aphid population in broflanilide resistance monitoring. The RR value of field populations in China was from 0.49 to 3.61 in 2021. It suggested that the broflanilide may be a potential agent in the resistance management of A. gossypii to insecticides. These results are significantly useful for the rational chemical control of cotton aphids.

Baseline susceptibility of Helicoverpa armigera, Plutella xylostella, and Spodoptera frugiperda to the meta-diamide insecticide broflanilide

Broflanilide is a novel meta-diamide insecticide that acts as a γ-aminobutyric acid-gated chloride channel allosteric modulator. With its unique mode of action, broflanilide has no known cross-resistance with existing insecticides and is expected to be an effective tool for the management of insecticide resistance. Establishing the baseline susceptibility to this insecticide is an essential step for developing and implementing effective resistance management strategies. Here we evaluated the baseline susceptibility to broflanilide for 3 cosmopolitan lepidopteran pest species, Helicoverpa armigera, Plutella xylostella, and Spodoptera frugiperda. Broflanilide exhibited high activity against populations sampled in the major distribution range of these pests in China, with median lethal concentrations (LC₅₀ ) ranging between 0.209 and 0.684, 0.076 and 0.336, and 0.075 and 0.219 mg/L for H. armigera, P. xylostella, and S. frugiperda, respectively. Among-population variability in susceptibility to broflanilide was moderate for H. armigera (3.3-fold), P. xylostella (4.4-fold), and S. frugiperda (2.9-fold). The recommended diagnostic concentrations for H. armigera, P. xylostella, and S. frugiperda were 8, 4, and 2 mg/L, respectively. Little or no cross-resistance to broflanilide was detected in 3 diamide-resistant strains of P. xylostella and 1 spinosyns-resistant strain of S. frugiperda. Our results provide critical information for the development of effective resistance management programs to sustain efficacy of broflanilide against these key lepidopteran pests.

The mode of action of isocycloseram: A novel isoxazoline insecticide

Isocycloseram is a novel isoxazoline insecticide and acaricide with activity against lepidopteran, hemipteran, coleopteran, thysanopteran and dipteran pest species. Isocycloseram selectively targets the invertebrate Rdl GABA receptor at a site that is distinct to fiproles and organochlorines. The widely distributed cyclodiene resistance mutation, A301S, does not affect sensitivity to isocycloseram, either in vitro or in vivo, demonstrating the suitability of isocylsoseram to control pest infestations with this resistance mechanism. Detailed studies demonstrated that the binding sites relevant to the insecticidal activity of avermectins and isocycloseram are distinct. Isocycloseram was shown to compete for binding with metadiamide insecticides related to broflanilide. In addition, a G335M mutation in the third transmembrane domain of the Rdl GABA receptor, impaired the ability of both isocycloseram and metadiamides to block the GABA mediated response. As such the Insecticides Resistance Action Committee (IRAC) has classified isocycloseram in Group 30 "GABA-Gated Chloride Channel Allosteric Modulators".

Structural insights into the interaction between gabazine (SR-95531) and Laodelphax striatellus GABA receptors

γ-Aminobutyric acid receptors (GABARs) mediate fast inhibitory neurotransmission and are targets for insecticides. GABARs are composed of five subunits, the composition of which dictates the pharmacological characteristics of GABARs. Both competitive and noncompetitive GABAR antagonists can be used as insecticides. Gabazine is a potent competitive antagonist of mammalian α1β2γ2 GABARs; however, it is less potent against insect GABARs. To explore how gabazine interacts with GABARs, we examined whether the sensitivity of the small brown planthopper (Laodelphax striatellus) RDL GABAR (LsRDLR) to gabazine is increased when its amino acid residues are substituted with α1β2γ2 GABAR residues. In the results, two of the generated mutants showed enhanced gabazine sensitivity. Docking simulations of gabazine using LsRDLR homology models and an α1β2γ2 GABAR cryo-EM structure revealed that the accommodation of gabazine into the "aromatic box" in the orthosteric site lowered the binding energy. This information may help in designing GABAR-targeting insecticides with novel modes of action.

5-(4-Pyridinyl)-3-isothiazolols as Competitive Antagonists of Insect GABA Receptors: Design, Synthesis, and a New Mechanism Leading to Insecticidal Effects

Ionotropic γ-aminobutyric acid (GABA) receptors (iGABARs) are validated targets of drugs and insecticides. Our previous studies showed that the competitive antagonists of insect iGABARs exhibit insecticidal activities and that the 3-isothiazolol scaffold is used as a lead for developing novel iGABAR antagonists. Here, we designed a novel series of 4-aryl-5-(4-pyridinyl)-3-isothiazolol (4-API) analogs that have various aromatic substituents at the 4-position. Two-electrode voltage clamp experiments showed that all synthesized 4-APIs exhibited antagonistic activity against Musca domestica and Spodoptera litura iGABARs (RDL) expressed in oocytes of Xenopus laevis at 100 μM. Of the 4-APIs, the 4-(1,1'-biphenylyl) analog was the most potent antagonist with IC₅₀s of 7.1 and 9.9 μM against M. domestica and S. litura RDL receptors, respectively. This analog also showed a certain insecticidal activity against S. litura larvae, with >75% mortality at 100 μg/g diet. Molecular docking studies with a M. domestica iGABAR model indicated that the π-π stacking interactions formed between the pyridinyl ring and Y252 and between the 4-substituted aromatic group and Y107 might be important for antagonism by the 4-(1,1'-biphenylyl) analog. Our studies provide important information for designing novel iGABAR antagonists and suggest that the 4-APIs acting on iGABARs are promising insecticide leads for further studies.

Xenopus Oocytes: A Tool to Decipher Molecular Specificity of Insecticides towards Mammalian and Insect GABA—A Receptors

The number of insect GABA receptors (GABAr) available for expression studies has been recently increased by the cloning of the Acyrthosiphon pisum (pea aphid) RDL subunits. This large number of cloned RDL subunits from pest and beneficial insects opens the door to parallel pharmacological studies on the sensitivity of these different insect GABAr to various agonists or antagonists. The resulting analysis of the molecular basis of the species-specific GABAr responses to insecticides is necessary not only to depict and understand species toxicity, but also to help at the early identification of unacceptable toxicity of insecticides toward beneficial insects such as Apis mellifera (honeybees). Using heterologous expression in Xenopus laevis oocytes, and two-electrode voltage-clamp recording to assess the properties of the GABAr, we performed a comparative analysis of the pharmacological sensitivity of RDL subunits from A. pisum, A. mellifera and Varroa destructor GABAr to three pesticides (fipronil, picrotoxin and dieldrin). These data were compared to similar characterizations performed on two Homo sapiens GABA-A receptors (α₂β₂γ₂ and α₂β₂γ₂). Our results underline a global conservation of the pharmacological profiles of these receptors, with some interesting species specificities, nonetheless, and suggest that this approach can be useful for the early identification of poorly specific molecules.

Novel meta-diamide insecticide, broflanilide, suppresses the population of common cutworm Spodoptera litura through its lethal and sublethal effects

BACKGROUND

Broflanilide has been registered in China for the control of Lepidoptera and Coleoptera pests, and is widely used to control the target pests at lethal and sublethal levels. The lethal and sublethal effects of broflanilide on the common cutworm (CCW) Spodoptera litura Fabricius, a representative Lepidopteran pest in agricultural crops, were examined to explore its ecological influence on pests.

RESULTS

In F₀ , broflanilide had little influence on the hatchability of eggs, but significantly reduced the neonate survival rate. The lethal activity of broflanilide towards third-instar larvae and adults was 0.13 mg kg^-1 (LD₅₀ ) and 3.59 mg L^-1 (LC₅₀ ) respectively at 48 h. After being treated with a sublethal dose (LD₁₀ and LD₃₀ ) of broflanilide, the duration of third- to sixth-instar larvae and the mean fecundity of reproductive females were significantly increased, but pupation rate, weight of pupae and life-cycle rate were significantly decreased. In F₁ , the duration of F₁ larvae and the doubling time were prolonged, whereas the rates of pupation and the life cycle were decreased by 14.92% and 18.00%, respectively. The intrinsic rate of increase, finite rate of increase and net reproductive rate in the sublethal group were lower than in the control group. The relative fitness of F₁ treated by LD₁₀ and LD₃₀ was 0.81 and 0.66, respectively.

CONSLUSION

Broflanilide not only has highly lethal activity, but also suppresses the population growth and progeny of CCW, as a critical factor for guidelines of its usage in the field.

Insect RDL Receptor Models for Virtual Screening: Impact of the Template Conformational State in Pentameric Ligand-Gated Ion Channels

The RDL receptor is one of the most relevant protein targets for insecticide molecules. It belongs to the pentameric ligand-gated ion channel (pLGIC) family. Given that the experimental structures of pLGICs are difficult to obtain, homology modeling has been extensively used for these proteins, particularly for the RDL receptor. However, no detailed assessments of the usefulness of homology models for virtual screening (VS) have been carried out for pLGICs. The aim of this study was to evaluate which are the determinant factors for a good VS performance using RDL homology models, specially analyzing the impact of the template conformational state. Fifteen RDL homology models were obtained based on different pLGIC templates representing the closed, open, and desensitized states. A retrospective VS process was performed on each model, and their performance in the prioritization of active ligands was assessed. In addition, the three best-performing models among each of the conformations were subjected to molecular dynamics simulations (MDS) in complex with a representative active ligand. The models showed variations in their VS performance parameters that were related to the structural properties of the binding site. VS performance tended to improve in more constricted binding cavities. The best performance was obtained with a model based on a template in the closed conformation. MDS confirmed that the closed model was the one that best represented the interactions with an active ligand. These results imply that different templates should be evaluated and the structural variations between their channel conformational states should be specially examined, providing guidelines for the application of homology modeling for VS in other proteins of the pLGIC family.

Toxicological analysis of stilbenes against the fall armyworm, Spodoptera frugiperda

The fall armyworm (FAW), Spodoptera frugiperda, is a global pest of multiple economically important row crops and the development of resistance to commercially available insecticidal classes has inhibited FAW control. Thus, there is a need to identify chemical scaffolds that can provide inspiration for the development of novel insecticides for FAW management. This study aimed to assess the sensitivity of central neurons and susceptibility of FAW to chloride channel modulators to establish a platform for repurposing existing insecticides or designing new chemicals capable of controlling FAW. Potency of select chloride channel modulators were initially studied against FAW central neuron firing rate and rank order of potency was determined to be fipronil > lindane > Z-stilbene > DIDS > GABA > E-stilbene. Toxicity bioassays identified fipronil and lindane as the two most toxic modulators studied with topical LD₅₀'s of 41 and 75 ng/mg of caterpillar, respectively. Interestingly, Z-stilbene was toxic at 300 ng/mg of caterpillar, but no toxicity was observed with DIDS or E-stilbene. The significant shift in potency between stilbene isomers indicates structure-activity relationships between stilbene chemistry and the binding site in FAW may exist. The data presented in this study defines the potency of select chloride channel modulators to FAW neural activity and survivorship to establish a platform for development of novel chemical agents to control FAW populations. Although stilbenes may hold promise for insecticide development, the low toxicity of the scaffolds tested in this study dampen enthusiasm for their development into FAW specific insecticides.

Influence of three insecticides targeting GABA receptor on fall armyworm Spodoptera frugiperda: Analyses from individual, biochemical and molecular levels

The fall armyworm (FAW) Spodoptera frugiperda (Lepidoptera: Noctuidae) is a severe agricultural pest, which has invaded into China in 2019 and caused heavy damage to maize. The γ-aminobutyric acid receptor (GABAR)-targeted insecticides including broflanilide, fluralaner and fipronil exhibit high toxicity towards lepidopteran pests. However, whether they could be used for control of FAW and their possible mode of action in FAW remain unclear. In this study, broflanilide, fluralaner and fipronil exhibited high oral toxicity in FAW larvae with median lethal dose (LD₅₀) values of 0.677, 0.711, and 23.577 mg kg^-1 (active ingredient/ artificial food), respectively. In the electrophysiological assay, fluralaner and fipronil could strongly inhibit GABA-induced currents of homomeric FAW resistance to dieldrin 1 (RDL1) receptor with median inhibitory concentration (IC₅₀) values of 5.018 nM (95% confidence interval (CI) 2.864-8.789) and 8.595 nM (95% CI 5.105-14.47), respectively, whereas broflanilide could not. In addition, the cytochrome P450 (P450), glutathione-S-transferase (GST) and carboxylesterase (CarE) activities were positively response to broflanilide, P450 and GST to fluralaner, and GST and CarE to fipronil, respectively, compared with those of control. In conclusion, we firstly reported a notable insecticidal activity of three representative GABAR-targeted insecticides to FAW in vivo, and in vitro using electrophysiological assay. The GST is the primary detoxification enzyme for three tested insecticides. Our results would guide the rotational use of GABAR-targeted insecticides in field.

Application of computational methods in the analysis of pesticide target-site and resistance mechanisms

Meta-diamide insecticides including broflanilide have a high insecticidal activity by acting on RDL GABA receptors. Both membrane potential assays and docking studies suggest that the target site of meta-diamides is different from that of conventional noncompetitive inhibitors, such as fipronil. In fact, meta-diamides are effective against cyclodiene- and fipronil-resistant pests that carry target-site mutations. Dinotefuran uniquely possesses a tetrahydrofuran ring, whereas other neonicotinoids possess aromatic rings. Moreover, dinotefuran has been reported to be effective against imidacloprid-resistant strains. A docking study predicted the weak binding of dinotefuran to cytochrome P450s which are associated with imidacloprid resistance. Metabolic assays revealed that dinotefuran was not metabolized by these cytochrome P450s. These findings suggest that the lack of metabolic activity of P450s against dinotefuran causes a low level of cross-resistance.

Mechanisms underlying the selectivity of meta-diamides between insect resistance to dieldrin (RDL) and human γ-aminobutyric acid (GABA) and glycine receptors

BACKGROUND

Meta-diamides [3-benzamido-N-(4-(perfluoropropan-2-yl)phenyl)benzamides] show high insecticide activity by acting as antagonists to the insect resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors. In contrast, low-level antagonist activities of meta-diamides have been demonstrated against the human GABA type A receptor (GABA_A R) α1β2γ2S, mammalian GABA_A R α1β3γ2S, and the human glycine receptor (GlyR) α1β. Glycine residue 336 in the membrane-spanning region M3 of the Drosophila RDL GABA receptor is essential for its high sensitivity to meta-diamide 7, [3-benzamido-N-(2-bromo-4-(perfluoropropan-2-yl)-6-(trifluoromethyl)phenyl)-2-fluorobenzamide].

RESULTS

We examined the effects of an equivalent mutation (M288G) in spontaneously opened human GABA_A R β3 homomers using membrane potential assay. Picrotoxin and fipronil blocked spontaneously opened human GABA_A Rs β3 and β3-M286G in a concentration-dependent manner. In contrast, meta-diamide 7 did not block spontaneously opened GABA_A R β3 homomers, although meta-diamide 7 blocked spontaneously opened GABA_A R β3-M286G homomers. In addition, inhibitory potency of meta-diamide 7 for GABA-induced membrane potential change in cells expressing GABA_A R α1β3-M286G was much higher than that in cells expressing GABA_A R α1β3. In the same way, the equivalent mutation (A288G) in GlyR α1 increased the inhibitory potency of meta-diamide 7 for GlyRs α1 and α1β.

CONCLUSION

Studies substituting an equivalent mutation (M288G) in spontaneously opening human GABA_A R β3 homomers and human GABA_A Rs α1β3 heteromers suggest that M286 in human GABA_A R β3 is important for the low sensitivity to meta-diamide 7. In this study, we summarize the mechanisms underlying the selectivity of meta-diamides between insect RDL and human GABA and glycine receptors. © 2020 Society of Chemical Industry.

Important amino acids for function of the insect Rdl GABA receptor

BACKGROUND

Insect Rdl GABA receptor is an important insecticide target. To design a novel insecticide, studies on the structures of homologous pentameric ligand-gated ion channels provide information about important amino acids that are necessary for the function of insect Rdl GABA receptors.

RESULTS

L9'A, T12'A, T13'A, T13'S, M15'S, and M15'N mutations in the Drosophila Rdl GABA receptor subunit caused the protein to spontaneously adopt the open state conformation. In contrast, the S16'A, S16'T, S17'A, and S17'H mutant homomers showed the same levels of agonist and antagonist sensitivity as the wild-type receptor. The G336M mutation in the Drosophila Rdl GABA receptor abolished the agonist activities of ivermectin and milbemectin, but the F339M mutation did not. Additionally, the F339M mutation caused spontaneous opening of the receptor. In the Drosophila Rdl model, the hydrophobic girdle plays an important role in stabilization of the closed state. Mutations which decrease hydrophobic interactions resulted in spontaneous opening, supporting the importance of the hydrophobic girdle for keeping the channel closed. Through a mutational study of transmembrane 3 (TM3) cytoplasmic domain and Rdl GABA receptor modeling, hydrophobic interactions between TM3 and TM4 and intersubunit interaction were demonstrated to be important for channel gating. Alternatively, the intrasubunit interaction between TM2 and TM3 domains were less important for channel gating in case of Drosophila Rdl GABA receptor.

CONCLUSION

This study demonstrates important amino acids critical to the function of the Drosophila Rdl GABA receptor based on the mutational studies and Drosophila Rdl GABA receptor modeling approach. © 2020 Society of Chemical Industry.

Effect of broflanilide on the phytophagous mite Tetranychus urticae and the predatory mite Typhlodromips swirskii

BACKGROUND

Two-spotted spider mite (TSSM), Tetranychus urticae Koch, is one of the most serious pests of agricultural crops. Broflanilide exhibits high lethality against various pests and has been marketed worldwide under the Vedira and Tenebenal brands in 2020. Nevertheless, little information has been reported about its effects on agricultural mites.

RESULTS

Broflanilide displayed higher toxicity to TSSM eggs (24 h LC₅₀ , 1.015 mg L^-1 ) and adult females (24 h LC₅₀ , 2.062 mg L^-1 ) than commercial acaricides, including cyflumetofen, bifenazate, and profenofos. In contrast, the adverse effects of broflanilide on the predatory mite, Typhlodromips swirskii Athias-Henriot, was lower than those of fenpyroximate and abamectin. In the sublethal effect study, while adult females were treated with broflanilide, the number of eggs and longevity were reduced in LC₁₀ and LC₃₀ treatments; when eggs were treated with broflanilide, the egg duration and deutonymph duration were prolonged in LC₃₀ treatment. A significant decrease in the total life span and duration and fecundity of adult females was observed in LC₁₀ and LC₃₀ treatments. Furthermore, the number of eggs per adult female was significantly reduced from 103.48 ± 3.69 in the control group to 69.42 ± 2.22 and 48.33 ± 1.75 in LC₁₀ and LC₃₀ treatments, respectively. In the greenhouse bioassay, broflanilide 5% suspension concentrate (MCI-8007) showed excellent acaricidal activity to TSSM, with 99.22% corrected control, compared with the MCI-8007 untreated group.

CONCLUSION

Our results indicated that broflanilide has a high acaricidal activity to TSSM and significant inhibition to fecundity of adult female, and could be considered as a potential alternative for TSSM management. © 2021 Society of Chemical Industry.

Ion channels and G protein-coupled receptors as targets for invertebrate pest control: from past challenges to practical insecticides

In the late 1970s, we discovered that toxic bicyclic phosphates inhibit the generation of miniature inhibitory junction potentials, implying their antagonism of γ-aminobutyric acid (GABA) receptors (GABARs; GABA-gated chloride channels). This unique mode of action provided a strong incentive for our research on GABARs in later years. Furthermore, minor structural changes conferred insect GABAR selectivity to this class of compounds, convincing us of the possibility of GABARs as targets for insecticides. Forty years later, third-generation insecticides acting as allosteric modulator antagonists at a distinctive site of action in insect GABARs were developed. G protein-coupled receptors (GPCRs) are also promising targets for pest control. We characterized phenolamine receptors functionally and pharmacologically. Of the tested receptors, β-adrenergic-like octopamine receptors were revealed to be the most sensitive to the acaricide/insecticide amitraz. Given the presence of multiple sites of action, ion channels and GPCRs remain potential targets for invertebrate pest control.

Efficacy of broflanilide (VECTRON T500), a new meta-diamide insecticide, for indoor residual spraying against pyrethroid-resistant malaria vectors

The rotational use of insecticides with different modes of action for indoor residual spraying (IRS) is recommended for improving malaria vector control and managing insecticide resistance. Insecticides with new chemistries are urgently needed. Broflanilide is a newly discovered insecticide under consideration. We investigated the efficacy of a wettable powder (WP) formulation of broflanilide (VECTRON T500) for IRS on mud and cement wall substrates in laboratory and experimental hut studies against pyrethroid-resistant malaria vectors in Benin, in comparison with pirimiphos-methyl CS (Actellic 300CS). There was no evidence of cross-resistance to pyrethroids and broflanilide in CDC bottle bioassays. In laboratory cone bioassays, broflanilide WP-treated substrates killed > 80% of susceptible and pyrethroid-resistant An. gambiae sl for 6–14 months. At application rates of 100 mg/m² and 150 mg/m², mortality of wild pyrethroid-resistant An. gambiae sl entering experimental huts in Covè, Benin treated with VECTRON T500 was similar to pirimiphos-methyl CS (57–66% vs. 56%, P > 0.05). Throughout the 6-month hut trial, monthly wall cone bioassay mortality on VECTRON T500 treated hut walls remained > 80%. IRS with broflanilide shows potential to significantly improve the control of malaria transmitted by pyrethroid-resistant mosquito vectors and could thus be a crucial addition to the current portfolio of IRS insecticides.

Broflanilide effectively controls Helicoverpa armigera and Spodoptera exigua exhibiting diverse susceptibilities to chlorantraniliprole and emamectin benzoate

BACKGROUND

The cotton bollworm, Helicoverpa armigera (Hübner), and the beet armyworm, Spodoptera exigua (Hübner), are two major polyphagous lepidopteran pests of cultivated crops. They develop various levels of resistance to many frequently applied broad-spectrum insecticides. Here, the larval susceptibilities of a laboratory population and six field-collected populations per pest from Hunan Province, China to three insecticides were determined using a standard leaf-dipping method in the laboratory. Field-plot trials were conducted to verify the control efficacies of broflanilide 100 g L^-1 suspension concentrate (SC), chlorantraniliprole 5% SC and emamectin benzoate 5.7% water-dispersible granule, against H. armigera and S. exigua larvae using foliar sprays in 2013 and 2019.

RESULTS

Variations among H. armigera and S. exigua field populations in their susceptibility levels to the three insecticides were observed in Linli, Yueyang and Changsha counties from 2013 to 2019. They were still high susceptibility to broflanilide only, but developed low and/or moderate levels of resistance to chlorantraniliprole and emamectin benzoate. In addition, broflanilide at 25 g ha^-1 provided an excellent control efficacy of 81.92% to 96.46% against these pests during the 3-14-days period after treatment in both years, whereas chlorantraniliprole at 50 g ha^-1 and emamectin benzoate at 4.5 g ha^-1 exhibited significantly decreased efficacies, which were consistent with their corresponding toxicity levels.

CONCLUSIONS

These results suggest that broflanilide should be an important new tool for the effective control of diamide- and avermectin-resistant H. armigera and S. exigua. © 2020 Society of Chemical Industry.

Residues Analysis and Dissipation Dynamics of Broflanilide in Rice and Its Related Environmental Samples

Herein, we present a method for the quantitative analysis of broflanilide residues in water, soil, and rice samples from a paddy field in Jiangxi Province, China. The quick, easy, cheap, effective, rugged, and safe (QuEChERS) method was optimized for the extraction and purification of broflanilide residues. Residual broflanilide concentrations in different matrices were then determined by high-performance liquid chromatography (HPLC). The calibration curve of broflanilide showed good linearity in all matrices for concentrations between 0.005 and 1 mg·L⁻¹, with a correlation coefficient greater than 0.99. The matrix effect varied from −69% to −54%, indicating matrix suppression. The average recoveries ranged between 85.82% and 97.46%, with relative standard deviations of 3.29%–8.15%. The limits of detection ranged from 0.16 to 1.67 μg·kg⁻¹, and the limits of quantification were in the range of 0.54 to 5.48 μg·kg⁻¹. Dissipation dynamic tests indicated broflanilide half-lives of 0.46–2.46, 2.09–5.34, and 1.31–3.32 days in soil, water, and rice straw, respectively. Broflanilide was dissipated more rapidly in water than in soil and rice straw. More than 90% of broflanilide residues dissipated within 14 days. The final residues of broflanilide in rice were all below LOQ at harvest.

Design, Synthesis, and Insecticidal Activity of 5,5-Disubstituted 4,5-Dihydropyrazolo[1,5-a]quinazolines as Novel Antagonists of GABA Receptors

To control the development of resistance to conventional insecticides acting as γ-aminobutyric acid (GABA) receptor antagonists (e.g., fipronil), new GABAergic 5,5-disubstituted 4,5-dihydropyrazolo[1,5-a]quinazolines were designed via a scaffold-hopping strategy and synthesized with a facile method. Among the 50 target compounds obtained, compounds 5a, 5b, 7a, and 7g showed excellent insecticidal activities against a susceptible strain of Plutella xylostella (LC₅₀ values ranging from 1.03 to 1.44 μg/mL), which were superior to that of fipronil (LC₅₀ = 3.02 μg/mL). Remarkably, the insecticidal activity of compound 5a was 64-fold better than that of fipronil against the field population of fipronil-resistant P. xylostella. Electrophysiological studies against the housefly GABA receptor heterologously expressed in Xenopus oocytes indicated that compound 5a could act as a potent GABA receptor antagonist, and IC₅₀ was calculated to be 32.5 nM. Molecular docking showed that the binding poses of compound 5a with the housefly GABA receptor can be different compared to fipronil, which explains the effectiveness of compound 5a against fipronil-resistant insects. These findings have suggested compound 5a as a lead compound for a novel GABA receptor antagonist controlling field-resistant insects and provided a basis for further design, structural modification, and development of 4,5-dihydropyrazolo[1,5-a]quinazoline motifs as new insecticidal GABA receptor antagonists.

Exploring the Interaction Mechanism of Desmethyl-broflanilide in Insect GABA Receptors and Screening Potential Antagonists by In Silico Simulations

Broflanilide, a novel insecticide, is classified as a negative allosteric modulator (NAM) of insect γ-aminobutyric acid (GABA) receptors (GABARs) as desmethyl-broflanilide (DMBF) allosterically inhibits the GABA-induced responses. The G277M mutation of the Drosophila melanogaster GABAR subunit has been reported to abolish the inhibitory activity of DMBF. The binding mode of DMBF in insect GABARs needs to be clarified to understand the underlying mechanism of this mutation and to develop novel, efficient NAMs of insect GABARs. Here, we found that a hydrogen bond formed between DMBF and G277 of the D. melanogaster GABAR model might be the key interaction for the antagonism of DMBF by in silico simulations. The volume increase induced by the G277M mutation blocks the entrance of the binding pocket, making it difficult for DMBF to enter the binding pocket and thereby decreasing its activity. The following virtual screening and bioassay results identified a novel NAM candidate of insect GABARs. Overall, we reported a possible binding mode of DMBF in insect GABARs and proposed the insensitivity mechanism of the G277M mutant GABAR to DMBF using molecular simulations. The identified NAM candidates might provide more alternatives or potentials for the design of GABAR-targeting insecticides.

2-Isoxazolines: A Synthetic and Medicinal Overview

Isoxazolines are nitrogen- and oxygen-containing five-membered heterocyclic scaffolds with extensive biological activities. This framework can be readily obtained in good to excellent yields through 1,3-dipolar cycloaddition between nitrones with alkynes or allenes, aryl/alkyl halides, alkynes, and oxaziridines under mild conditions. This scaffold has been an emerging area of interest for many researchers given their wide range of bioactivities. Herein we review synthetic strategies toward isoxazolines and the role these efforts have had in enhancing the biological activity of natural products and synthetic compounds such as antitubercular agents, COX-1 inhibitors, COX-2 inhibitors (e. g., valdecoxib), nicotinic receptor modulators, and MIF inhibitors. With a focus on efforts from 2010 onward, this review provides in-depth coverage of the design and biological evaluation of isoxazoline systems and their impact on various pathologies.

Tenebenal: a meta-diamide with potential for use as a novel mode of action insecticide for public health

Background

There is an urgent need for insecticides with novel modes of action against mosquito vectors. Broflanilide is a meta-diamide, discovered and named Tenebenal™ by Mitsui Chemicals Agro, Inc., which has been identified as a candidate insecticide for use in public health products.

Methods

To evaluate its potential for use in public health, Tenebenal™ was screened using an array of methodologies against Anopheles and Aedes strains. Initially it was assessed for intrinsic efficacy by topical application. Tarsal contact bioassays were then conducted to further investigate its efficacy, as well as its potency and speed of action. The potential of the compound for use in indoor residual spray (IRS) applications was investigated by testing the residual efficacy of a prototype IRS formulation on a range of typical house building substrates, and its potential for use in long-lasting insecticidal nets (LLIN) was tested using dipped net samples. Finally, bioassays using well-characterized insecticide-resistant mosquito strains and an in silico screen for mutations in the insecticide’s target site were performed to assess the risk of cross-resistance to Tenebenal™.

Results

Tenebenal™ was effective as a tarsal contact insecticide against both Aedes and Anopheles mosquitoes, with no apparent cross-resistance caused by mechanisms that have evolved to insecticides currently used in vector control. Topical application showed potent intrinsic activity against a Kisumu reference strain and an insecticide-resistant strain of Anopheles gambiae. Applied to filter paper in a WHO tube bioassay, Tenebenal™ was effective in killing 100% of susceptible and resistant strains of An. gambiae and Aedes aegypti at a concentration of 0.01%. The discriminating concentration of 11.91 µg/bottle shows it to be very potent relative to chemistries previously identified as having potential for vector control. Mortality occurs within 24 h of exposure, 80% of this mortality occurring within the first 10 h, a speed of kill somewhat slower than seen with pyrethroids due to the mode of action. The potential of Tenebenal™ for development in LLIN and IRS products was demonstrated. At least 12 months residual efficacy of a prototype IRS formulation applied at concentrations up to 200 mg of AI/sq m was demonstrated on a range of representative wall substrates, and up to 18 months on more inert substrates. A dipped net with an application rate of around 2 g/sq m Tenebenal™ killed 100% of exposed mosquitoes within a 3-min exposure in a WHO cone test.

Conclusions

Tenebenal™ is a potent insecticide against adult Aedes and Anopheles mosquitoes, including strains resistant to classes of insecticide currently used in vector control. The compound has shown great potential in laboratory assessment and warrants further investigation into development for the control of pyrethroid-resistant mosquitoes.

Reverse genetics reveals contrary effects of two Rdl-homologous GABA receptors of Helicoverpa armigera on the toxicity of cyclodiene insecticides

The resistance to dieldrin gene (Rdl) encodes a subunit of the insect γ-amino butyric acid (GABA) receptor, and the encoded Rdl subunit is a major target site for cyclodiene and phenylpyrazole insecticides. Since the substitution of a single amino acid (Ala to Ser/Gly at position 302) of the Drosophila melanogaster Rdl gene was first identified to confer high level resistance to dieldrin, mutations at the equivalent positions have been reported to confer resistance to dieldrin and/or fipronil in a wide range of different insects. In the cotton bollworm Helicoverpa armigera, there are two Rdl homologs (HaRdl-1 and HaRdl-2) in close proximity on the Z chromosome, which as wild-type sequences, encode alanine and serine respectively at amino acid position 302. In the present study, we used the CRISPR/Cas9 gene editing approach to knock out HaRdl-1 and HaRdl-2 and establish two homozygous knockout strains (ΔRdl-1 and ΔRdl-2). The ΔRdl-1 strain showed low levels of resistance (8.0- to 9.3-fold) to three cyclodiene insecticides (endosulfan, aldrin and dieldrin) compared with the background SCD strain. In contrast, toxicity of the three cyclodiene insecticides to the ΔRdl-2 strain increased significantly (3.6- to 6.3-fold) when compared with the SCD strain. Genetic analysis indicated the obtained resistance to endosulfan and dieldrin in the ΔRdl-1 strain was sex-linked, which is consistent with the fact that HaRdl-1 locus is located on the Z chromosome. The above results demonstrate that both HaRdl-1 and HaRdl-2 are important determinants for the susceptibility of H. armigera SCD strain to the three cyclodiene insecticides, but have opposite effects. It was also found that HaRdl-1 and HaRdl-2 are involved, to some extent, in mediating sensitivity of H. armigera to avermectin and fipronil respectively. We speculate that the HaRdl-1 and HaRdl-2 subunits have different pharmacological properties, which contribute to the differential sensitivities of H. armigera to the tested cyclodienes and other insecticides.

Optimized residue analysis method for broflanilide and its metabolites in agricultural produce using the QuEChERS method and LC-MS/MS

Since broflanilide is a newly developed pesticide, analytical methods are required to determine the corresponding pesticide residues in diverse crops and foods. In this study, a pesticide residue analysis method was optimized for the detection and quantification of broflanilide and its two metabolites, DM-8007 and S(PFH-OH)-8007, in brown rice, soybean, apple, green pepper, mandarin, and kimchi cabbage. Residue samples were extracted from the produce using QuEChERS acetate and citrate buffering methods and were purified by dispersive solid-phase extraction (d-SPE) using six different adsorbent compositions with varying amounts of primary secondary amine (PSA), C18, and graphitized carbon black. All the sample preparation methods gave low-to-medium matrix effects, as confirmed by liquid chromatography-tandem mass spectrometry using standard solutions and matrix-matched standards. In particular, the use of the citrate buffering method, in combination with purification by d-SPE using 25 mg of PSA and a mixture of other adsorbents, consistently gave low matrix effects that in the range from -18.3 to 18.8%. Pesticide recoveries within the valid recovery range 70-120% were obtained both with and without d-SPE purification using 25 mg of PSA and other adsorbents. Thus, the developed residue analysis method is viable for the determination of broflanilide and its metabolites in various crops.

Evolution of the Insecticide Target Rdl in African Anopheles Is Driven by Interspecific and Interkaryotypic Introgression

The evolution of insecticide resistance mechanisms in natural populations of Anopheles malaria vectors is a major public health concern across Africa. Using genome sequence data, we study the evolution of resistance mutations in the resistance to dieldrin locus (Rdl), a GABA receptor targeted by several insecticides, but most notably by the long-discontinued cyclodiene, dieldrin. The two Rdl resistance mutations (296G and 296S) spread across West and Central African Anopheles via two independent hard selective sweeps that included likely compensatory nearby mutations, and were followed by a rare combination of introgression across species (from A. gambiae and A. arabiensis to A. coluzzii) and across nonconcordant karyotypes of the 2La chromosomal inversion. Rdl resistance evolved in the 1950s as the first known adaptation to a large-scale insecticide-based intervention, but the evolutionary lessons from this system highlight contemporary and future dangers for management strategies designed to combat development of resistance in malaria vectors.

Insights into the synergistic mechanism of target resistance: A case study of N. lugens RDL-GABA receptors and fipronil

It is known that a single mutation exerts moderate resistance to pesticide, while double mutations (DM) cause severe resistance problem through synergistic effect, and even result in failure application of pesticides. However, little is known about how double mutations would synergistically cause much high resistance level. In this work, computational studies were performed on the interaction of fipronil with N. lugens RDL-GABA receptors, to see how single and double mutations changed receptor structure properties and then conferred distinct resistance levels. The A2'S mutation displayed relative weak influence on receptor structure properties. The R0'Q mutation, which has not been detected in the absence of A2'S, however could deeply alter the electrostatic potential around the inner pore region and significantly narrow the bottom region around -2'Pro. For the DM system, the synergistic effect of two mutations lead to similar pore diameters to the WT system, except for the slightly reduced middle part. Docking study and binding free energy calculation revealed that fipronil displayed binding potencies in the order of WT > A2'S > R0'Q > DM systems, coinciding well with the reported fipronil sensitivity trends and resistance levels.

Acute toxicity, bioconcentration, elimination, action mode and detoxification metabolism of broflanilide in zebrafish, Danio rerio

Broflanilide, a novel meta-diamide insecticide, shows high insecticidal activity against agricultural pests and is scheduled to be launched onto the market in 2020. However, little information about its potential toxicological effects on fish has been reported. In this study, broflanilide showed low toxicity to the zebrafish, Danio rerio, with LC₅₀ > 10 mg L^-1 at 96 h and also did not inhibit GABA-induced currents of the heteromeric Drα1β2Sγ2 GABA receptor. Broflanilide showed medium bioconcentration level with a bioconcentration factor at steady state (BCF_ss) of 10.02 and 69.40 in D. rerio at 2.00 mg L^-1 and 0.20 mg L^-1, respectively. In the elimination process, the concentration of broflanilide rapidly decreased within two days and slowly dropped below the limit of quantification after ten days. In the 2.00 mg L^-1 broflanilide treatment, CYP450 activity was significantly increased up to 3.11-fold during eight days. Glutathione-S- transferase (GST) activity significantly increased by 91.44 % within four days. In conclusion, the acute toxicity of broflanilide was low, but it might induce chronic toxicity, affecting metabolism. To our knowledge, this is the first report of the toxicological effects of broflanilide on an aquatic organism, which has the potential to guide the use of broflanilide in the field.

Potential of Competitive Antagonists of Insect Ionotropic γ-Aminobutyric Acid Receptors as Insecticides

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) represent an important insecticide target. Currently used GABAR-targeting insecticides are non-competitive antagonists (NCAs) of these receptors. Recent studies have demonstrated that competitive antagonists (CAs) of GABARs have functions of inhibiting insect GABARs similar to NCAs and that they also exhibit insecticidal activity. CAs have different binding sites and different mechanisms of action compared to those of NCAs. Therefore, GABAR CAs should have the potential to be developed into novel insecticides, which could be used to overcome the developed resistance of insect pests to conventional NCA insecticides. Although research on insect GABAR CAs has lagged behind that on mammalian GABAR CAs, research on the CAs of insect ionotropic GABARs has made great progress in recent years, and several series of heterocyclic compounds, such as 3-isoxazolols and 6-iminopyridazines, have been identified as insect GABAR CAs. In this review, we briefly summarize the design strategies, structures, and biological activities of the novel GABAR CAs that have been found in the past decade. Updated information about GABAR CAs may benefit the design and development of novel GABAR-targeting insecticides.

Sigillins from Springtails Are Potent Natural Insecticides

Sigillins are highly chlorinated natural products from the springtail Ceratophysella sigillata (Collembola) that are used to deter arthropod predators. We report here the isolation and structure elucidation of sigillin F, a hydrogenated benzopyranone compound bearing two trichloromethyl groups, and the synthesis of trideoxysigillin (8), a non-natural compound representing the basic scaffold of the sigillins. Sigillins A and F showed insecticidal activity toward various insects, similar to the commercial insecticide imidacloprid, whereas 8 was inactive. The highest mortality was observed for the aphids Megoura viciae and Myzus persicae, but other insect species were also susceptible. Sigillins act as noncompetitive antagonists of the GABA receptor. This mode of action is identical to that of known insecticides with high chlorine content such as dieldrin or endosulfan. The high content of sigillins in C. sigillata, more than 4 mM in concentration, indicates self-resistance. Strikingly, the Collembola and humans have both arrived at the same target with related types of compounds to combat insects.

Effects of intersubunit amino acid substitutions on GABA receptor sensitivity to the ectoparasiticide fluralaner

The isoxazoline ectoparasiticide fluralaner exerts antiparasitic effects by inhibiting the function of γ-aminobutyric acid (GABA) receptors (GABARs). The present study was conducted to identify the amino acid residues that contribute to the high sensitivity of insect GABARs to fluralaner. We generated housefly (Musca domestica) GABARs with amino acid substitutions in the first through third α-helical transmembrane segments (TM1-TM3) of the RDL subunit using site-directed mutagenesis and examined the effects of the substitutions on the sensitivity of GABARs expressed in Xenopus oocytes to fluralaner using two-electrode voltage clamp electrophysiology. The Q271L substitution in TM1 caused a significant reduction in the sensitivity to fluralaner. Although the I274A and I274F substitutions in TM1 did not affect fluralaner sensitivity, the I274C substitution significantly enhanced the sensitivity to fluralaner. In contrast, the L278C substitution in TM1 reduced fluralaner sensitivity. Substitutions of Gly333 in TM3 led to substantial reductions in the sensitivity to fluralaner. These findings indicate that Gln271, Ile274, Leu278, and Gly333, which are situated in the outer half of the transmembrane subunit interface, are closely related to the antagonism of GABARs by fluralaner.

Neurotoxicity and mode of action of fluralaner on honeybee Apis mellifera L

BACKGROUND

Fluralaner, a novel pesticide that targets the γ-aminobutyric acid (GABA) receptor (GABAR) subunit of resistant to dieldrin (RDL), exhibits strong potential to be an insecticide to control agricultural insect pests. However, the risk and action of fluralaner to economic insects, e.g., honeybee Apis mellifera Linnaeus, remains unclear.

RESULTS

In this study, both oral and contact toxicity of fluralaner to honeybee were found to be 0.13 μg adult^-1 . Abamectin, dieldrin, ethiprole, α-endosulfan, fipronil and fluralaner strongly inhibited the GABA-induced current in A. mellifera RDL (AmRDL), expressed in Xenopus laevis oocytes, with median inhibitory concentration (IC₅₀ ) values of 7.99, 868.1, 27.10, 412.0, 11.21 and 13.59 nM, respectively. The binding free energy and electrophysiological response of AmRDL and insecticides were opposite. The correlation values between toxicity (to A. mellifera) and binding free energy/electrophysiological inhibition (to AmRDL) were at a moderate level.

CONCLUSION

In conclusion, we report for the first time the notable risk of fluralaner to honeybee in vivo and compared the actions of GABAR-targeted insecticides on the AmRDL receptor. © 2019 Society of Chemical Industry.