Mode of action of triflumezopyrim: A novel mesoionic insecticide which inhibits the nicotinic acetylcholine receptor

Recent developments in fluorine-containing pesticides

To ensure ongoing sustainability, the modern agrochemical industry is faced with enormous challenges. These arise from provision of high-quality food to increasing water use and environmental impact as well as a growing world population. The loss of previous agrochemicals due to consumer perception, changing grower needs and ever-changing regulatory requirements is higher than the number of active ingredients that are being introduced into the crop protection market. Therefore, the development of novel agrochemicals is essential to provide improved efficacy and environmental profiles. In this context, the introduction of fluorine atoms and fluorine-containing motifs into a molecule is an important method to influence its physicochemical properties. These include, for example, small difluoro- and trifluoromethyl, or trifluoromethoxy groups at aryl or heterocyclic aryl moieties but also fragments like 2,2,2-trifluoroethoxycarbonyl, trifluoromethylsulfonyl, trifluoroacetyl, as well as the so far unusal rest like heptafluoro-iso-propyl. This review gives an overview of recent developments of fluorine-containing pesticides launched over the past 7 years and describes a selection of current fluorine-containing development candidates. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Potency and Target Surface Interaction of Diazinoyl Nicotinic Insecticides

Structure-activity relationships of diazinoyl nicotinic insecticides (diazinoyl isomers and 5- or 6-substituted pyrazin-2-oyl analogues) are considered in terms of affinity to the insect nicotinic acetylcholine receptor (nAChR) and insecticidal activity against the imidacloprid-resistant brown planthopper. Among the test compounds, 3-(6-chloropyridin-3-ylmethyl)-2-(pyrazinoyl)iminothiazoline shows the highest potency in nAChR affinity and insecticidal activity. Aplysia californica acetylcholine binding protein (AChBP) mutants (Y55W + Q57R and Y55W + Q57T) are utilized to compare molecular recognition of nicotinic insecticides with diverse pharmacophores. N-nitro- or N-cyanoimine imidacloprid or acetamiprid, respectively, exhibits a high affinity to these AChBP mutants at a similar potency level. Intriguingly, the pyrazin-2-oyl analogue has a higher affinity to AChBP Y55W + Q57R than that to Y55W + Q57T, thereby indicating that pyrazine nitrogen atoms contact Arg57 guanidinium and Trp55 indole NH. Furthermore, nicotine prefers AChBP Y55W + Q57T over Y55W + Q57R, conceivably suggesting that the protonated nicotine is repulsed by Arg57 guanidinium, consistent with its inferior potency to insect nAChR.

Design, Synthesis, and Insecticidal Activity of Pyridino[1,2-a]pyrimidines Containing Indole Moeites at the 1-Position

Research on mesoionic structures in pesticide design has gained significant attention in recent years. However, the 1-position of pyridino[1,2-a]pyrimidine is usually designed with 2-chlorothiazole, 2-chloropyridine, or cyano moieties commonly found in neonicotinoid insecticides. In order to enrich the available pharmacophore library, here, we disclose a series of new pyridino[1,2-a]pyrimidine mesoionics bearing indole-containing substituents at the 1-position. Most of these target compounds are confirmed to have good insecticidal activity against aphids through bioevaluation. In addition, a three-dimensional structure-activity relationship model is established to allow access to optimal compound F45 with an LC50 value of 2.97 mg/L. This value is comparable to the property achieved by the positive control triflumezopyrim (LC50 = 2.94 mg/L). Proteomics and molecular docking analysis suggest that compound F45 has the potential to modulate the functioning of the aphid nervous system through its interaction with neuronal nicotinic acetylcholine receptors. This study expands the existing pharmacophore library for the future development of new mesoionic insecticides based on 1-position modifications of the pyridino[1,2-a]pyrimidine scaffold.

MicroRNA PC-5p-3991_515 mediates triflumezopyrim susceptibility in the small brown planthopper through regulating the post-transcriptional expression of P450 CYP417A2

BACKGROUND

Cytochrome P450 monooxygenases (P450s) are recognized as a major contributor to metabolic resistance in insects to most insecticides, through gene overexpressions and protein mutations. MicroRNA (miRNA), an important post-transcriptional regulator, has been reported to promote insecticide resistance by mediating the expression of detoxification enzyme genes.

RESULTS

In the present study, we reported that a novel microRNA PC-5p-3991_515 was involved in the post-transcriptional regulation of CYP417A2 and mediated the triflumezopyrim susceptibility in the small brown planthopper (SBPH), Laodelphax striatellus (Fallén). The tissue expression profiles showed that CYP417A2 was highly expressed in fat body. CYP417A2 was significantly up-regulated at 12, 36, 60, 84 and 108 h after the triflumezopyrim treatment. RNA interference (RNAi) against CYP417A2 significantly increased triflumezopyrim susceptibility in SBPH. According to the prediction by miRanda and TargetScan software, three miRNAs were indicated to bind to CYP417A2. However, when oversupply of agomir, only two miRNAs, PC-3p-625_4405 and PC-5p-3991_515, significantly increased the susceptibility to triflumezopyrim and decreased CYP417A2 levels. Furthermore, PC-5p-3991_515 was confirmed to be involved in the post-transcriptional regulation of CYP417A2 by dual luciferase reporter assay. Meanwhile, PC-5p-3991_515 was co-localized with CYP417A2 in the midgut in situ hybridization.

CONCLUSION

Our findings revealed that the novel microRNA, PC-5p-3991_515, post-transcriptionally regulated CYP417A2 expression, which then mediated the triflumezopyrim susceptibility in SBPH. © 2023 Society of Chemical Industry.

Discovery and biological characterization of a novel mesoionic insecticide fenmezoditiaz

BACKGROUND

Many piercing-sucking insects have developed resistance or cross-resistance to many insecticides targeting insect neural nicotinic acetylcholine receptor (nAChR). Here we are aiming to present the discovery of a novel mesoionic insecticide, fenmezoditiaz, by BASF through structure-based drug design (SBDD) approaches. It has recently been added to the Insecticide Resistance Action Committee mode of classification (IRAC 4E). It is being developed for plant protection against piercing-sucking pests, especially rice hopper complex.

RESULTS

The soluble acetylcholine binding protein (AChBP) from the sea slug Aplysia californica was modified using site-directed mutagenesis and based on putative aphid nAChR subunit sequences to create soluble insect-like AChBPs. Among them, insect-like β1 AChBP and native aphid membrane preparation showed the highest correlated biochemical affinity toward structurally diverse ligands. This mutant AChBP was used to understand how insect nAChRs structurally interact with mesoionics, which was then utilized to design novel mesoionics including fenmezoditiaz. It is an excellent systemic insecticide with diverse application methods and has a broad insecticidal spectrum, especially against piercing/sucking insects. It lacks cross-resistance for neonicotinoid resistant plant hoppers. Field-collected brown plant hopper populations from Asian countries showed high susceptibility.

CONCLUSIONS

Fenmezoditiaz is a systemic insecticide with a broad spectrum, lack of cross-resistance and it could be an additional tool for integrated pest management and insecticide resistance management, especially for the rice hopper complex. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Design, Synthesis, and Biological Evaluation of Pyrido [1,2-α] Pyrimidinone Mesoionic Derivatives Bearing Propenylbenzene as the Vector Control Insecticide

A series of novel pyrido [1,2-α] pyrimidinone mesoionic derivatives bearing a propenylbenzene group at the 1-position were synthesized on the basis of the structure of mesoionic insecticides triflumezopyrim and dicloromezotiaz via a rationally conceived pharmacophore model and evaluated for their insecticidal activities against three insect vectors. The bioassay results showed that some compounds exerted remarkable insecticidal activities against M. domestica, Ae. albopictus, and B. germanica. Particularly, compound 26l displayed outstanding insecticidal activity against Ae. Albopictus, with an LC50 value of 0.45 μg/mL, far superior to that of imidacloprid (LC50 = 1.82 μg/mL) and equivalent to that of triflumezopyrim (0.35 μg/mL). Meanwhile, compound 34l presented a broad insecticidal spectrum, with LC50 values of 1.51 μg/g sugar, 0.52 μg/mL and 0.14 μg/adult, which were about 2.88, 3.50, and 1.50 times better than that of imidacloprid (LC50 = 4.35 μg/g sugar, 1.82 μg/mL and 0.21 μg/adult against M. domestica, Ae. albopictus, and B. germanica, respectively) and equivalent to that of triflumezopyrim against M. domestica (1.13 μg/g sugar) and Ae. albopictus (0.35 μg/mL) but lower than the potency against B. germanica (0.06 μg/g sugar). The molecular docking study by energy minimizations revealed that introducing propenylbenzene at the 1-position of compounds 26l and 34l could embed into the binding pocket of nicotinic acetylcholine receptors and form pi-alkyl interaction with LEU306. These results demonstrated that compounds 26l and 34l could be promising candidates for vector control insecticides, which deserved further investigation.

The knockout of the nicotinic acetylcholine receptor subunit gene α1 (nAChR α1) through CRISPR/Cas9 technology exposes its involvement in the resistance of Spodoptera exigua to insecticides

Insect nicotinic acetylcholine receptors (nAChRs) are the directed targets of many insecticides. However, there have been no reports on the molecular characterization of the nAChR gene family or the causal association between nAChR α1 and resistance to insecticides in S. exigua, which is a significant agricultural pest. In this study, we identified a total of 9 candidate nAChR subunits in S. exigua, namely nAChR α1-α7 and nAChR β1-β2. For functional validation roles of Seα1 in insecticide resistance of S. exigua, we introduced a ∼ 1041-bp deletion of the Seα1 gene in a homozygous mutant strain (Seα1-KO) by CRISPR/Cas9 genome editing system, resulting in a premature truncation of the Seα1 protein and the subsequent loss of functional transmembrane (TM) 3 and TM4 elements. Compared with WH-S strain (wild-type strain), the Seα1-KO strain exhibited 2.62-folds resistant to trifluoropyrimidine, 8.3-folds resistant to dimehypo, and 5.28-folds resistant to dinotefuran, but no significant change in susceptibility to emamectin benzoate, spinetoram, lambda-cyhalothrin, permethrin and chlorpyrifos. Thus, this study has laid a solid foundation for investigating the role of nAChRs in S. exigua, and provides evidence for the crucial involvement of the α1 subunit in the mechanism of trifluoropyrimidine, dimehypo, and dinotefuran in S. exigua. Moreover, it provides a reference for the value of Seα1 subunit and its homologues in other species as insecticide targets.

Field evaluation of a six-rotor unmanned agricultural aerial sprayer: effects of application parameters on spray deposition and control efficacy against rice planthopper

BACKGROUND

Unmanned Aerial Spraying System (UASS) has emerged as an advanced, precise, and efficient tool for pesticide application in numerous nations in recent years. Despite this, there is a noticeable gap in research advocating viable, quantifiable methodologies for application parameter optimization. This investigation was primarily oriented toward identifying optimal UASS application parameters. It did so by exploring the effects of varying spray volumes and flight parameters on spray performance in a comprehensive manner, and by assessing the biological potency of aerial insecticide application against Rice Planthopper (RPH) using the optimal parameters, aided by two types of nozzles in rice field settings.

RESULTS

Increased spray volume increased the spray deposition. Working height impacted the distribution of spray deposition, with a higher working height leading to superior distribution uniformity. Both spray volume and working height were observed to influence spray deposition and its percentage in tandem. Upon factor analysis, the optimal parameters determined for rice at the heading stage were an application volume of 15.0 L·ha-1 , a working height of 2.0 m, and a driving speed of 5.0 m·s-1 . Under these parameters, the air-induction twin flat fan nozzle IDKT120-015 demonstrated approximately 5% higher spray deposition than the flat fan nozzle SX11001VS, albeit with inferior distribution uniformity. Both nozzle types achieved over 93.0% control efficacy against RPH using triflumezopyrim, persisting for up to 40 days post-treatment.

CONCLUSION

This study furnishes invaluable insights and data for controlling rice planthopper via UASS pesticide application, contributing to the progress of modern intensive and sustainable agriculture. © 2023 Society of Chemical Industry.

Functional impact of subunit composition and compensation on Drosophila melanogaster nicotinic receptors-targets of neonicotinoids

Neonicotinoid insecticides target insect nicotinic acetylcholine receptors (nAChRs) and their adverse effects on non-target insects are of serious concern. We recently found that cofactor TMX3 enables robust functional expression of insect nAChRs in Xenopus laevis oocytes and showed that neonicotinoids (imidacloprid, thiacloprid, and clothianidin) exhibited agonist actions on some nAChRs of the fruit fly (Drosophila melanogaster), honeybee (Apis mellifera) and bumblebee (Bombus terrestris) with more potent actions on the pollinator nAChRs. However, other subunits from the nAChR family remain to be explored. We show that the Dα3 subunit co-exists with Dα1, Dα2, Dβ1, and Dβ2 subunits in the same neurons of adult D. melanogaster, thereby expanding the possible nAChR subtypes in these cells alone from 4 to 12. The presence of Dα1 and Dα2 subunits reduced the affinity of imidacloprid, thiacloprid, and clothianidin for nAChRs expressed in Xenopus laevis oocytes, whereas the Dα3 subunit enhanced it. RNAi targeting Dα1, Dα2 or Dα3 in adults reduced expression of targeted subunits but commonly enhanced Dβ3 expression. Also, Dα1 RNAi enhanced Dα7 expression, Dα2 RNAi reduced Dα1, Dα6, and Dα7 expression and Dα3 RNAi reduced Dα1 expression while enhancing Dα2 expression, respectively. In most cases, RNAi treatment of either Dα1 or Dα2 reduced neonicotinoid toxicity in larvae, but Dα2 RNAi enhanced neonicotinoid sensitivity in adults reflecting the affinity-reducing effect of Dα2. Substituting each of Dα1, Dα2, and Dα3 subunits by Dα4 or Dβ3 subunit mostly increased neonicotinoid affinity and reduced efficacy. These results are important because they indicate that neonicotinoid actions involve the integrated activity of multiple nAChR subunit combinations and counsel caution in interpreting neonicotinoid actions simply in terms of toxicity.

Resistance selection of triflumezopyrim in Laodelphax striatellus (fallén): Resistance risk, cross-resistance and metabolic mechanism

The risk assessment and resistance mechanisms of insecticide resistance are critical for resistance management strategy before a new insecticide is widely used. Triflumezopyrim (TFM) is the first commercialized mesoionic insecticide, which can inhibit nicotinic acetylcholine receptor with high-performance against the small brown planthopper (SBPH), Laodelphax striatellus (Fallén). In our study, the resistance of SBPH to TFM increased 26.29-fold, and the actual heritability of resistance was 0.09 after 21 generations of continuous selection by TFM. After five generations of constant feeding under insecticide-free conditions from F16 generation, the resistance level decreased 2.05-fold, and the average resistance decline rate per generation was 0.01, but there were no statistical decline. The TFM resistant strains had no cross-resistance to imidacloprid, nitenpyram, thiamethoxam, dinotefuran, flonicamid, pymetrozine, and chlorfenapyr. The third and fifth nymphal stage duration, pre-adult stage, adult preoviposition period, longevity, emergence rate, and hatchability of the resistant strain were significantly lower than those of the susceptible strain, while the female-male ratio was considerably increased. The fitness cost was 0.89. Further, cytochrome P450 monooxygenase (P450) and carboxylesterase (CarE) activities were markedly increased, but only the enzyme inhibitor piperonyl butoxide (PBO) had a significant synergistic effect on the resistant strain. The expression of CYP303A1, CYP4CE2, and CYP419A1v2 of P450 genes was significantly increased. SBPH has a certain risk of resistance to TFM with continuous application. The TFM resistance may be due to the increased activity of P450 enzyme regulated by the overexpression of P450 genes.

(−)-Adaline from the Adalia Genus of Ladybirds Is a Potent Antagonist of Insect and Specific Mammalian Nicotinic Acetylcholine Receptors

Ladybird beetles (Coleoptera: Coccinellidae) possess strong chemical defences that are secreted in response to stress and are also found on the coating of eggs, which are rich in alkaloids that are responsible for their toxicity to other species. Recent studies have shown that alkaloids from several species of ladybird beetle can target nicotinic acetylcholine receptors (nAChRs) acting as receptor antagonists. Here, we have explored the actions of (−)-adaline, found in the 2-spot (Adalia bipunctata) and 10-spot (Adalia decempunctata) ladybirds, on both mammalian (α1β1γδ, α7, α4β2, α3β4) and insect nAChRs using patch-clamp of TE671 cells and locust brain neurons natively expressing nAChRs, as well as two-electrode voltage clamp of Xenopus laevis oocytes recombinantly expressing nAChRs. All nAChR subtypes were antagonised by (−)-adaline in a time-dependent, voltage-dependent and non-competitive manner with the lowest IC₅₀s at rat α3β4 (0.10 μM) and locust neuron (1.28 μM) nAChRs, at a holding potential of −75 mV. The data imply that (−)-adaline acts as an open channel blocker of nAChRs.

Applying a Bioisosteric Replacement Strategy in the Discovery and Optimization of Mesoionic Pyrido[1,2-a]pyrimidinone Insecticides: A Review

Mesoionic pyrido[1,2-a]pyrimidinones are a unique class of heterocyclic compounds. Compounds from this class with a n-propyl group substituted at the 1 position of the mesoionic core were discovered with interesting insecticidal activity in our screen. In this overview, we showcase how a bioisosteric replacement strategy was applied during the discovery and optimization of this class of compounds. Through exploring various substituents at the 1 position, evaluating a variety of mesoionic bicyclic ring scaffolds, and examining substituents on the phenyl group at the 3 position of the mesoionic core as well as substituents on the mesoionic ring skeleton, many compounds were discovered with excellent hopper activity or potent activity against a wide range of Lepidoptera. Ultimately, dicloromezotiaz was identified for commercial development to control a broad spectrum of lepidopteran pests.

Risk Assessment of Triflumezopyrim and Imidacloprid in Rice through an Evaluation of Residual Data

Triflumezopyrim, a novel mesoionic insecticide used to control planthoppers, is a potential substitute for imidacloprid. In this study, triflumezopyrim and imidacloprid residues in rice were determined using a quick, easy, cheap, effective, rugged, and safe procedure combined with ultra-high-performance liquid chromatography–tandem mass spectrometry. The limit of quantification of both triflumezopyrim and imidacloprid was 0.01 mg kg⁻¹, and the average recovery values were 94–104% and 91–106%, with relative standard deviations (RSDs) of 1.1–1.4% and 2.1–3.4% (n = 5), respectively. The consumer protection level was assessed by calculating the theoretical maximum daily intake using the reported maximum residue limits of triflumezopyrim and imidacloprid. The established method was successfully applied to 200 commercial rice samples collected from four provinces in China, and their potential public health risks were assessed using triflumezopyrim and imidacloprid residues. The risk associated with triflumezopyrim and imidacloprid dietary intake was assessed by calculating the national estimated short-term intake and the acute reference dose percentage (%ARfD). The results show that the theoretical maximum daily intake (NEDI) values of triflumezopyrim and imidacloprid in different age and gender groups were 0.219–0.543 and 0.377–0.935 μg kg⁻¹ d⁻¹ bw, and the risk quotient (RQ) values were 0.188–0.467% and 0.365–0.906%, respectively. The acute reference dose (%ARfD) of triflumezopyrim and imidaclopridin ranged from 0.615 to 0.998% and from 0.481 to 0.780%, respectively.

Green Design, Synthesis, and Molecular Docking Study of Novel Quinoxaline Derivatives with Insecticidal Potential against Aphis craccivora

An efficient and environmentally friendly method was established for designing novel 3-amino-1,4-dihydroquinoxaline-2-carbonitrile (1) via the reaction of bromomalononitrile and benzene-1,2-diamine under microwave irradiation in an excellent yield (93%). This targeted amino derivative was utilized for the construction of a series of Schiff bases (8-13). A new series of thiazolidinone derivatives (15-20) were synthesized in high yields (89-96%) via treatment of thioglycolic acid with Schiff bases (8-13) under microwave irradiation in high yields (89-96%). Moreover, new pyrimidine derivatives (26-30 and 35-38) were prepared by treatment of compound 1 with arylidenes (21-25) and/or alkylidenemalononitriles (31-34) using piperidine as a basic catalyst under microwave conditions. Based on elemental analyses and spectral data, the structures of the new assembled compounds were determined. The newly synthesized quinoxaline derivatives were screened and studied as an insecticidal agent against Aphis craccivora. The obtained results indicate that compound 16 is the most toxicological agent against nymphs of cowpea aphids (Aphis craccivora) compared to the other synthesized pyrimidine and thiazolidinone derivatives. The molecular docking study of the new quinoxaline derivatives registered that compound 16 had the highest binding score (-10.54 kcal/mol) and the thiazolidinone moiety formed hydrogen bonds with Trp143.

Effects of nicotinic acetylcholine receptor subunit deletion mutants on insecticide susceptibility and fitness in Drosophila melanogaster

BACKGROUND

Nicotinic acetylcholine receptors (nAChRs) are major excitatory neurotransmitter receptors in insects and also the target site for many insecticides. Unfortunately, the effectiveness of these insecticides is diminishing as a consequence of the evolution of insecticide resistance. Further exploration of insecticide targets is important to sustainable pest management.

RESULTS

In order to validate the role of nAChR subunits in insecticide susceptibility and test whether the subunit's absence imposes the fitness cost on insects, we determined the susceptibility of eight nAChR subunit deletion mutants of Drosophila melanogaster to nine insecticides. These findings highlighted the specific resistance of the Dα6 deletion mutant to spinosyns. Although triflumezopyrim, dinotefuran and imidacloprid are competitive modulators of nAChRs, differences in susceptibility of the insect with different deletion mutants suggested that the target sites of these three insecticides do not overlap completely. Mutants showed decreased susceptibility to insecticides, accompanied by a reduction in fitness. The number of eggs produced by Dα1^attP , Dα2^attP , Dβ2^attP and Dβ3^attP females was significantly lesser than that of the vas-Cas9 strain as the control. In addition, adults of Dα2^attP , Dα3^attP and Dα7^attP strains showed lower climbing performance. Meanwhile, males of Dα3^attP , Dα5^attP , Dβ2^attP and Dβ3^attP , and females of Dβ2^attP showed significantly shorter longevity than those of the vas-Cas9 strain.

CONCLUSION

This study provides new insights into the interactions of different insecticides with different nAChRs subunit in D. melanogaster as a research model, it could help better understand such interaction in agricultural pests whose genetic manipulations for toxicological research are often challenging. © 2022 Society of Chemical Industry.

Discovery of Pyrido[1,2-a]pyrimidine Mesoionic Compounds Containing Benzo[b]thiophene Moiety as Potential Pesticide Candidates

The increasing evolution of insect resistance has made it challenging for traditional insecticides to control the bean aphid (Aphis craccivora Koch). To address this pending issue, a range of pyrido[1,2-a]pyrimidine mesoionic compounds containing benzo[b]thiophene were designed and synthesized. The biological activity test results of the target compounds indicated that they had moderate to outstanding insecticidal activity against the bean aphid (Aphis craccivora) and moderate insecticidal activity against the white-backed planthopper (Sogatella furcifera). Compound L14 exhibited significant insecticidal activity against A. craccivora, with an LC₅₀ value of 1.82 μg/mL, which was superior to triflumezopyrim (LC₅₀ = 4.76 μg/mL). The results of enzyme activity assay showed that compound L14 had a definite inhibitory effect on ATPase. Moreover, the proteomics and docking findings of compound L14 suggested that it may act on the central nervous system of aphids and interact with nicotinic acetylcholine receptors. Therefore, compound L14 is a potentially novel insecticide candidate for further utilization.

Risk assessment and resistance inheritance of triflumezopyrim resistance in Laodelphax striatellus

BACKGROUND

Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae) is one of the most important insect pests of rice, and it has been able to develop strong resistance to many insecticides. Triflumezopyrim, a new type of mesoionic insecticide developed by Corteva Agriscience, showed high biological activity in controlling piercing-sucking insect pests such as planthopper and leafhopper.

RESULTS

In this study, we continuously selected a susceptible laboratory stain (Unsel) of L. striatellus for 16 generations by exposing it to triflumezopyrim in the laboratory. A 45.1-fold triflumezopyrim-resistant strain (Tri-sel) was established, in which cross-resistance to nitenpyram and acetamiprid was not detected. The realized heritability (h² ) of the Tri-sel strain was estimated at 0.13. The mortalities of the testing F₁ (the offspring of a cross between Unsel and Tri-sel strains) suggested that the resistance of L. striatellus to triflumezopyrim was autosomal and incompletely dominant, as well as a polygenic inheritance. In addition, the results of synergist experiment showed that P450s potentially contributed to the triflumezopyrim resistance. The activities of detoxification enzymes in the Unsel and Tri-sel strains indicated that the activity of P450s in the Tri-sel strain was significantly increased, consistent with the results of synergist experiments. Furthermore, 12 P450 genes demonstrated significant up-regulation.

CONCLUSIONS

L. striatellus has a certain risk of resistance to triflumezopyrim after continuous selection. Triflumezopyrim resistance did not result in cross-resistance to neonicotinoid insecticides. The up-regulation of multiple P450 genes may mediate triflumezopyrim resistance in L. striatellus. © 2022 Society of Chemical Industry.

Resistance of Sogatella furcifera to triflumezopyrim mediated with the overexpression of CYPSF01 which was regulated by nuclear receptor USP

Sogatella furcifera is one of the main agricultural pests in many Asian countries, bringing about enormous injury. A triflumezopyrim-resistant (Tri) strain of S. furcifera was established through continuous screening in laboratory. The determination of synergist and enzyme activity indicated that P450s, especially for the upregulation expression of CYPSF01, played a key role in the increased resistance, confirmed by RNAi, and the recombinant protein of CYPSF01 and NADPH-P450 reductase was able to degrade triflumezopyrim. CYPSF01 had an obviously co-expression relationship with nuclear receptor ultraspiracle (USP), which were all significantly up-regulated when exposed to triflumezopyrim. Further, a USP-binding motif MA0534.1 was enriched from the upregulated peaks by Assay for Transposase Accessible Chromatin (ATAC-seq) analysis, which exited in the peaks located on the promoter of CYPSF01; the yeast one-hybrid experiments confirmed that USP could bind to the CYPSF01 promoter. And the USP interference significantly down-regulated CYPSF01 expression, and resulted in the significantly increasing sensitivity to triflumezopyrim, its mortality rate increased 28.37%. Therefore, the overexpression of USP could cause to the overexpression of CYPSF01, ultimately resulting in the resistance to triflumezopyrim in S. furcifera.

Design, Synthesis, and Insecticidal Activity of Novel Pyrido[1, 2-a]pyrimidinone Mesoionic Compounds Containing an Indole Moiety as Potential Acetylcholine Receptor Insecticides

In this study, a series of novel mesoionic pyrido[1,2-a]pyrimidinone compounds containing a natural skeleton indole were designed and synthesized, and the insecticidal activities of the target compounds were tested. The results showed that the target compounds had good to excellent insecticidal activities against white-backed planthoppers (Sogatella furcifera) and bean aphids (Aphis craccivora). Among them, compound 7 showed outstanding insecticidal activities against both S. furcifera and A. craccivora, with LC₅₀ values of 0.86 and 0.85 μg/mL, respectively. The insecticidal activity against bean aphids (A. craccivora) was superior to that of triflumezopyrim (LC₅₀ = 3.67 μg/mL). Proteomics and quantitative real-time polymerase chain reaction (qRT-PCR) results revealed that compound 7 may interact with α1 and α7 nAChR subunits of S. furcifera. The results of enzyme activities indicated that compound 7 was an inhibitor of AChE in S. furcifera. This study provides new ideas for the discovery of new mesoionic pyrido[1,2-a]pyrimidinone insecticides.

Residue dynamics and bio-efficacy of triflumezopyrim against Nilaparvata lugens and non-targeted effect on natural enemies in a rice ecosystem

Triflumezopyrim (TMP), a mesoionic insecticide, is commonly used for controlling planthoppers in rice. However, the relationship between the TMP residue and toxicity against brown planthoppers (BPHs) has not been studied in detail. We are reporting the dissipation of TMP from rice plant and soil under field conditions. The median lethal dose and median lethal concentration were 0.036 ng per insect and 0.525 mg L-1, respectively. TMP at recommended dose (25 g a.i. ha-1) recorded 1.25 live BPH per hill as against 25.5 per hill in control at 14 days after treatment. TMP was considered to be harmless to the natural enemies, namely, Cyrtorhinus lividipennis and Lycosa pseudoannulata in the rice ecosystem. The residue of TMP from rice plant and soil was estimated using the QuEChERS method using three different doses (12.5, 25, and 50 g a.i. ha-1). The limit of quantitation (LOQ) of TMP in plant and soil was 5 µg kg-1 and 1 µg kg-1, respectively. The maximum content of TMP in soil was less than 1% that of plant content on day 1. The dissipation pattern of TMP both from plant and soil was better explained by the first-order double-exponential decay model (FODED) as compared to the first-order kinetic model. Overall, the half-lives of TMP were ranged from 2.21 to 3.02 days in plant tissues and 3.78 to 4.79 days in soil as per the FODED model. Based on the persistence and toxicity of TMP, we could conclude that TMP will be effective against BPH up to 7-10 days after application. Triflumezopyrim with reasonable persistence and high efficacy could be recommended as an alternate pesticide in BPH management in rice.

Structural Biology-Guided Design, Synthesis, and Biological Evaluation of Novel Insect Nicotinic Acetylcholine Receptor Orthosteric Modulators

The development of novel and safe insecticides remains an important need for a growing world population to protect crops and animal and human health. New chemotypes modulating the insect nicotinic acetylcholine receptors have been recently brought to the agricultural market, yet with limited understanding of their molecular interactions at their target receptor. Herein, we disclose the first crystal structures of these insecticides, namely, sulfoxaflor, flupyradifurone, triflumezopyrim, flupyrimin, and the experimental compound, dicloromezotiaz, in a double-mutated acetylcholine-binding protein which mimics the insect-ion-channel orthosteric site. Enabled by these findings, we discovered novel pharmacophores with a related mode of action, and we describe herein their design, synthesis, and biological evaluation.

Nicotinic acetylcholine receptor modulator insecticides act on diverse receptor subtypes with distinct subunit compositions

Insect nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels mainly expressed in the central nervous system of insects. They are the directed targets of many insecticides, including neonicotinoids, which are the most widely used insecticides in the world. However, the development of resistance in pests and the negative impacts on bee pollinators affect the application of insecticides and have created a demand for alternatives. Thus, it is very important to understand the mode of action of these insecticides, which is not fully understood at the molecular level. In this study, we systematically examined the susceptibility of ten Drosophila melanogaster nAChR subunit mutants to eleven insecticides acting on nAChRs. Our results showed that there are several subtypes of nAChRs with distinct subunit compositions that are responsible for the toxicity of different insecticides. At least three of them are the major molecular targets of seven structurally similar neonicotinoids in vivo. Moreover, spinosyns may act exclusively on the α6 homomeric pentamers but not any other nAChRs. Behavioral assays using thermogenetic tools further confirmed the bioassay results and supported the idea that receptor activation rather than inhibition leads to the insecticidal effects of neonicotinoids. The present findings reveal native nAChR subunit interactions with various insecticides and have important implications for the management of resistance and the development of novel insecticides targeting these important ion channels.

Neonicotinoids and spinosyns account for approximately 24% and 3% of the world market value of insecticides, respectively. However, the negative effects of neonicotinoids on pollinators have led to the development of novel insecticides, such as sulfoxaflor, flupyradifurone and triflumezopyrim. Although all act via insect nicotinic acetylcholine receptors, their modes of action are not fully understood. Our work shows that these insecticides act on diverse receptor subtypes with distinct subunit compositions. This finding could lead to the development of more selective insecticides to control pests with minimal effects on beneficial insects.

Discovery of Pyrido[1,2-a]pyrimidinone Mesoionic Compounds Incorporating a Dithioacetal Moiety as Novel Potential Insecticidal Agents

A series of novel mesoionic pyrido[1,2-a]pyrimidinone compounds incorporating a dithioacetal skeleton were designed and synthesized for use as insecticidal agents. The biological activity of the title compounds indicated good to excellent insecticidal activities against bean aphids (Aphis craccivora) and white-backed planthoppers (Sogatella furcifera). Compound 34 showed excellent insecticidal activity against bean aphids (A. craccivora) with an LC₅₀ value of 2.80 μg/mL, exceeding the insecticidal activity of trifluoropyrimidine (LC₅₀ = 4.20 μg/mL). Proteomics and molecular docking results indicated that compound 34 could act on nicotinic acetylcholine receptors. This study provides support for the application of mesoionic pyrido[1,2-a]pyrimidinone compounds containing dithioacetal as novel insecticidal agents.

Resistance of Nilaparvata lugens (Hemiptera: Delphacidae) to triflumezopyrim: inheritance and fitness costs

BACKGROUND

Triflumezopyrim, a novel commercialized mesoionic chemical insecticide, has been confirmed as a promising insecticide for efficiently controlling the brown planthopper, Nilaparvata lugens (Stål). Here, a laboratory triflumezopyrim-resistant (TR) strain and an isogenic susceptible (TS) strain were established to characterize the inheritance and fitness costs of triflumezopyrim resistance in N. lugens.

RESULTS

After 29 generations of successive selection with triflumezopyrim, the TR strain developed a 155.23-fold higher resistance level than the TS strain. The median lethal concentration (LC₅₀ ) values from progenies (F₁ RS and F₁ SR) of reciprocal crosses between TR and TS strains suggested that triflumezopyrim resistance in N. lugens was autosomal and codominant. Chi-square analyses of self-bred and backcrossed progenies suggested that the resistance results from a polygenic effect. Compared to the TS strain, the TR strain showed a lower relative fitness (0.62) with a significantly decreased female adult period, longevity, total fecundity, egg hatchability, intrinsic rate of increase (r), finite rate of increase (λ), net reproductive rate (R₀ ), and prolonged pre-adult period and total preoviposition period (TPOP).

CONCLUSION

The inheritance mode of triflumezopyrim resistance in N. lugens was characterized as autosomal, codominant and polygenic. The resistance had a fitness cost, which may be an important factor limiting the evolution of resistance. These findings provide valuable information for optimizing resistance management strategies to delay triflumezopyrim resistance development and maintain sustainable control of N. lugens. © 2021 Society of Chemical Industry.

Insecticide Resistance Monitoring in Field Populations of the Whitebacked Planthopper Sogatella furcifera (Horvath) in China, 2019-2020

Simple Summary

The whitebacked planthopper (WBPH), Sogatella furcifera (Horváth), is one of the most destructive pests that seriously threatens the high-quality and safe production of rice. Overuse of chemical insecticides has led to varying levels of resistance to insecticides in the field population of S. furcifera. In this study, we measured the susceptibility of 18 populations to 10 insecticides by the rice-seedling dip method. Enzyme assays were performed to measure the levels of esterase (EST), glutathione S- transferase (GST) and cytochrome P450 monooxygenase (P450). A risk of cross-resistance between some insecticides were found by pairwise correlation, and EST may be contributed to the resistance to nitenpyram, thiamethoxam and clothianidin in S. furcifera. Overall, our findings will help inform the effective insecticide resistance management strategies to delay the development of insecticide resistance in S. furcifera.

Abstract

Monitoring is an important component of insecticide resistance management. In this study, resistance monitoring was conducted on 18 field populations in China. The results showed that S. furcifera developed high levels of resistance to chlorpyrifos and buprofezin, and S. furcifera showed low to moderate levels of resistance to imidacloprid, thiamethoxam, dinotefuran, clothianidin, sulfoxaflor, isoprocarb and ethofenprox. Sogatella furcifera remained susceptible or low levels of resistance to nitenpyram. LC₅₀ values of nitenpyram and dinotefuran, imidacloprid, thiamethoxam, clothianidin and chlorpyrifos exhibited significant correlations, as did those between dinotefuran and thiamethoxam, clothianidin, sulfoxaflor, imidacloprid, isoprocarb and buprofezin. Similarly, significant correlations were observed between thiamethoxam and clothianidin, sulfoxaflor and imidacloprid. In addition, the activity of EST in field populations of S. furcifera were significantly correlated with the LC₅₀ values of nitenpyram, thiamethoxam and clothianidin. These results will help inform effective insecticide resistance management strategies to delay the development of insecticide resistance in S. furcifera.

Determination of Residual Triflumezopyrim Insecticide in Agricultural Products through a Modified QuEChERS Method

A rapid and simple analytical method for triflumezopyrim, a new class of mesoionic insecticides and commercialized molecules from DuPont, was developed with a modified QuEChERS method. The pH adjustment was used to improve the extraction efficiency of acetonitrile solvent, and dispersive solid-phase extraction was employed for the clean-up process. The five selected food commodities were used to verify the present optimized method, which displayed good linearity with an excellent correlation coefficient (R² = 0.9992–0.9998) in the 0.003–0.30 mg/kg calibration range. The method limits of detection (LOD) and quantification (LOQ) were determined to be a value of 0.003 and 0.01 mg/kg, respectively. The mean recovery for the triflumezopyrim was in the 89.7–104.3% range. The relative standard deviations were ≤9.8% for intra- (n = 5) and inter-day (n = 15) precisions at concentrations of 0.01, 0.1, and 0.5 mg/kg in the five representative samples. The matrix effect has been calculated to confirm the effect during ionization of the analyte in the UPLC-MS/MS. The matrix effects of the instrumental analysis showed that triflumezopyrim was less susceptible to matrices. The proposed analytical method in this study has effectively improved the accuracy, selectivity, and sensitivity for the determination of triflumezopyrim in agricultural commodities; therefore, it can serve as a reference method for the establishment of maximum residue limits (MRLs).

Deciphering the Flupyrimin Binding Surface on the Insect Nicotinic Acetylcholine Receptor

A novel insecticide flupyrimin (FLP) with a trifluoroacetyl pharmacophore acts as an antagonist at the insect nicotinic acetylcholine receptor (nAChR). This investigation examines a hypothesis that the FLP C(O)CF₃ moiety is primarily recognized by the β subunit-face in the ligand-binding pocket (interface between α and β subunits) of the insect nAChR. Accordingly, we evaluate the atomic interaction between a fluorine atom of FLP and the partnering amino acid side chain on the β subunit employing a recombinant hybrid nAChR consisting of aphid Mpα2 and rat Rβ2 subunits (with a mutation at T77 on the Rβ2). The H-donating T77R, T77K, T77N, or T77Q nAChR enhances the FLP binding potency relative to that of the wild-type receptor, whereas the affinity of neonicotinoid imidaclprid (IMI) with a nitroguanidine pharmacophore remains unchanged. These results facilitate the establishment of the unique FLP molecular recognition at the Mpα2/Mpβ1 interface structural model, thereby underscoring a distinction in its binding mechanism from IMI.

Discovery of novel iminosydnone compounds with insecticidal activities based on the binding mode of triflumezopyrim

Triflumezopyrim (TFM) is a new mesoionic insecticide developed by DuPont. Like other neonicotinoid insecticides, it binds to the orthosteric site of the nicotinic acetylcholine receptor (nAChR), but the binding mode has not been reported. Nicotinic acetylcholine binding proteins (nAChBPs) are ideal alternative structure for nAChRs. In this study, molecular docking, molecular dynamics (MD) simulations, binding free energy calculation, and per-residue binding free energy decomposition were used to study the binding modes of TFM and other 12 mesoionic insecticides. By comparing the binding free energy and the insecticidal activity, it was found that the sub-pocket around the benzyl group of the mesoionic insecticide is the key area for maintaining its activity, which is composed of A: Val116, A: Met124, A: Ile126, B: Trp155 and B: Val156. In order to verify the druggability of the sub-pocket, a series of iminosydnone compounds were designed and synthesized based on the structure of the sub-pocket. The lethality rate of compound 1 against Mythimna separata were 100% at 500 mg/L. Our research provides a basis for designing new mesoionic insecticides based on structure.

Selective actions of insecticides on desensitizing and non-desensitizing nicotinic acetylcholine receptors in cockroach (Periplaneta americana) neurons

BACKGROUND

Insect desensitizing nicotinic acetylcholine (nAChD) receptors are desensitized by low concentrations of agonists, including neonicotinoid insecticides, but are essentially insensitive to spinosyns, while non-desensitizing nicotinic acetylcholine (nAChN) receptors are selectively activated by spinosyns and relatively insensitive to neonicotinoids.

RESULTS

The single-electrode voltage-clamp technique was used to measure the actions of newer nicotinic insecticides dinotefuran, sulfoxaflor, triflumezopyrim, spinetoram and GS-ω/k-hexatoxin-Hv1a on cockroach neuronal nAChD and nAChN currents. Like imidacloprid and clothianidin, newer orthosteric nicotinic agonist insecticides dinotefuran and sulfoxaflor act by desensitizing nAChD receptors. The mesoionic insecticide triflumezopyrim selectively inhibited nAChD current with an half maximal inhibitory concentration (IC₅₀ ) of 1.2 nmol L^-1 , with no activation. Unlike other Group 4 insecticides, it did not activate nAChN current, but inhibited it with an IC₅₀ of 3.8 μmol L^-1 , indicating that the compound is a true antagonist. Spinosad and the spinosyn-derived insecticide spinetoram potently and selectively activated nAChN receptors. GS-ω/k-hexatoxin-Hv1a had no effect on nAChN currents and it had a complex action on nAChD currents, inhibiting at sub-nanomolar concentrations and causing some activation and enhancement of ACh-evoked currents at 30 nmol L^-1 and above. Some cells express GS-ω/k-hexatoxin-Hv1a-resistant nAChD receptors.

CONCLUSIONS

Nicotinic acetylcholine receptor competitive modulators (IRAC Group 4) and nicotinic acetylcholine receptor allosteric modulators, site II (hexatoxins, IRAC Group 32) are selective for nAChD receptors, while nicotinic acetylcholine receptor allosteric modulators, site I (spinosyns, IRAC Group 5) are selective for nAChN receptors. It is proposed that IRAC Groups 5 and 32 be re-named non-desensitizing nicotinic acetylcholine receptor allosteric modulators and desensitizing nicotinic acetylcholine receptor allosteric modulators, respectively. © 2021 Society of Chemical Industry.

Novel trifluoromethyl sydnone derivatives: Design, synthesis and fungicidal activity

Crop pathogens reduce the yield and quality of agricultural production. The development of new fungicides will help to sustain this protection and overcome fungicide resistance. Sydnone is a kind of mesoionic, which has a wide range of biological activities. The application of sydnones in agriculture is less, and the study of these compounds will lead to the discovery of new active compounds. In this study, we designed and synthesized a series of noval sydnone mesoionic derivatives by active substructure splicing. All compounds were characterized using ¹H and ¹³C NMR spectroscopy. Among them, trifluoromethyl compound D17 showed good bioactivity against Pseudoperonospora cubensis (EC₅₀ = 49 mg L^-1) in vivo, the activity was similar to that of the control Kresoxim-methyl (EC₅₀ = 44 mg L^-1). However, the target of these compounds should not only be tyrosinase, and the mode of action needs to be further studied. In addition, the structure-activity relationship indicated that the trifluoromethyl group was more beneficial for antifungal activity. This is the first report that fluorine-containing N(3)-benzyl sydnone compounds have good fungicidal activity. These results will provide a basis for the development of sydnone mesoionic as new lead fungicidal agents.

Seed dressing with triflumezopyrim controls brown planthopper populations by inhibiting feeding behavior, fecundity and enhancing rice plant resistance

BACKGROUND

Triflumezopyrim (TFM), a novel mesoionic insecticide, has high efficiency at a low dosage, and is mainly used to control hopper species. A previous study demonstrated that seed dressing with TFM effectively controlled rice planthopper populations in mechanically transplanted rice fields; however, mode of action for control was unclear.

RESULTS

The study shows that seed dressing with TFM resulted in elevated levels of oxalic acid, flavonoids, phenolic substances, callose and other compounds associated with Nilaparvata lugens resistance in rice plants, and low TFM residue content in rice plant stem and grain. Host choice behavioral experiments showed that N. lugens females prefer feeding on untreated rice plants. Electrical penetration graph (EPG) data showed that seed dressing with TFM at medium and high doses significantly prolonged the non-probing period and inhibited phloem ingestion in N. lugens females. These changes led to a significant decrease in female secretion of honeydew, expression of genes encoding vitellogenin and juvenile hormone acid methyltransferase, body weight and longevity, and significantly influenced several physiological parameters resulting in impaired oocyte growth, fecundity and population. Field survey data showed that seed dressing with TFM was efficacious and relatively durable in protecting rice plants from infestation by planthoppers.

CONCLUSION

This study revealed that seed dressing with TFM enhances rice plant resistance to N. lugens by limiting phloem ingestion and increasing the N. lugens non-probing period; this leads to reduced fecundity of females and lowers N. lugens numbers in the field. © 2021 Society of Chemical Industry.

Oxidative stress and DNA damage in earthworm (Eisenia fetida) induced by triflumezopyrim exposure

Triflumezopyrim is an excellent pesticide for preventing and controlling rice planthoppers. It is widely used in the production of field rice and mainly through spraying with some inadvertent loss of pesticide to the soil. The future may bring development and utilization of seeds containing triflumezopyrim which will certainly impact earthworms. To evaluate the toxic effects of triflumezopyrim on earthworms, reactive oxygen species (ROS), superoxide dismutase (SOD) and catalase (CAT) activities, malondialdehyde content (MDA), glutathione S-transferase (GST), and DNA oxidative damage (8-hydroxy-2'-deoxyguanosine, 8-OHdG) were measured after 7, 14, 21, and 28 d analyzing the effects of 6 concentrations (0, 0.5, 1, 2.5, 5 and 10 mg/kg) of triflumezopyrim, respectively. ROS content did not change significantly in the early stages but showed a significant dose-effect relationship in the late stages. Antioxidant enzymes were activated in most treatment groups, and catalase activity was the same as that in the control group at 28 d. GST activity showed a trend of increasing first and then decreasing whereas MDA content had no obvious change in trend. 8-OHdG showed significant positive correlation with the concentration of triflumezopyrim at 28 d, indicating that the degree of DNA oxidative damage increased accordingly. The results showed that chronic exposure of triflumezopyrim may cause both oxidative stress and DNA damage in earthworms and alter the activity of antioxidant enzymes.

Current susceptibilities of brown planthopper Nilaparvata lugens to triflumezopyrim and other frequently used insecticides in China

The brown planthopper is a notorious rice pest in many areas of Asia. The evolution of insecticide resistance in Nilaparvata lugens has become a serious problem in the effective control of this pest in the paddy field. In this article, the current susceptibility of N. lugens field populations to novel mesoionic insecticide triflumezopyrim and major classes of chemical insecticides was determined and compared. The monitoring results indicated that field populations of N. lugens had developed low resistance to triflumezopyrim (resistance ratio, RR: 1.3-7.3-fold) during 2015-2018 in China, and the median lethal concentration values varied from 0.05 to 0.29 mg/L. Additionally, during 2017 to 2018, field populations of N. lugens showed high resistance levels to thiamethoxam (RR: 456.1-1025.6-fold), imidacloprid (RR: 2195.3-6899.0-fold) and buprofezin (RR: 1241.5-4521.7-fold), moderate to high resistance levels to dinotefuran (RR: 97.6-320.1-fold), clothianidin (RR: 69.4-230.1-fold) and isoprocarb (RR: 44.1-108.0-fold), and low to moderate levels of resistance to chlorpyrifos (RR: 12.0-29.7-fold) and nitenpyram (RR: 6.9-24.1-fold). In contrast, N. lugens just showed low resistance to sulfoxaflor (RR: 3.3-8.5-fold) and etofenprox (RR: 5.0-9.1-fold) in the field. Additionally, the P450 gene CYP6ER1 was found to be significantly overexpressed in all five field populations of N. lugens collected in 2018 when compared with a laboratory susceptible strain. Our findings will provide useful information to delay the evolution of insecticide resistance in N. lugens.

Status and outlook for acaricide and insecticide discovery

To guarantee sustainability and progress, the agrochemical industry is faced with several major challenges. Currently, loss of active ingredients due to consumer perception, changing grower needs and ever-changing regulatory requirements is far higher than the number being introduced into the market. Therefore, there is a need to develop new products that can provide improved efficacy, selectivity and favorable environmental profiles. Strategies to achieve these goals are the search for acaricides and insecticides with new modes of action, or the discovery of novel molecules with activity on the most attractive target sites having resistance breaking properties against pest species. In this context, the introduction of halogen atoms or asymmetric centers into an active ingredient remains an important tool to modulate their properties, but so too is the pro-pesticide concept. This review gives an overview of agrochemicals launched over the past 8 years, reflects new insights into known mechanisms of action, and describes the status and outlook for acaricide and insecticide discovery.

Neonicotinoid Insecticides: Molecular Targets, Resistance, and Toxicity

Neonicotinoids have been used to protect crops and animals from insect pests since the 1990s, but there are concerns regarding their adverse effects on nontarget organisms, notably on bees. Enhanced resistance to neonicotinoids in pests is becoming well documented. We address the current understanding of neonicotinoid target site interactions, selectivity, and metabolism not only in pests but also in beneficial insects such as bees. The findings are relevant to the management of both neonicotinoids and the new generation of pesticides targeting insect nicotinic acetylcholine receptors.

Recent Research Progress in and Perspectives of Mesoionic Insecticides: Nicotinic Acetylcholine Receptor Inhibitors

Triflumezopyrim exemplifies a new class of mesoionic insecticides and has attracted increasing attention as a result of its unique structure, high level of insecticidal activity, new mechanisms of action, low toxicity toward non-target organisms, and environmental friendliness. It inhibits the nicotinic acetylcholine receptor and has high potency against sucking pests, including the brown planthopper (Nilaparvata lugens), which has developed serious resistance to conventional neonicotinoids and low cross-resistance to some newly developed neonicotinoids. This review focuses on the discovery, synthesis, structure-activity relationships, and mechanism of action of mesoionic insecticides. Finally, potential directions for the development of mesoionic insecticides are discussed.

Exposure Level of Neonicotinoid Insecticides in the Food Chain and the Evaluation of Their Human Health Impact and Environmental Risk: An Overview

Neonicotinoid insecticides (neonics) were the most rapidly growing class of insecticides over the past few decades, and are used mainly for vegetables, fruits, and grains. Although neonics exhibit lower toxicity in mammals and humans compared to traditional insecticides, increasing numbers of studies are demonstrating that neonics may accumulate in the food chain and environmental media. Long-term exposure to neonics may raise potential risks to animals and even to humans. The present report reviews the development, application, and prohibition of neonics in the farmland ecosystem, and summarizes the exposure level and harmful effects of these insecticides in the food chain. In addition, the present review analyzes and summarizes the evaluation of the human health impact and environmental risk of the neonics, and overviews the unresolved problems and future research directions in this field. The aim of the present report was to review the exposure level, potential toxicity, human health impact, and environmental risk assessment of neonics in various media in order to provide reliable technical support for strengthening the environmental and food safety supervision and green pesticide designing.

Toxicity and Sublethal Effect of Triflumezopyrim Against Red Imported Fire Ant (Hymenoptera: Formicidae)

The use of insecticide remains the frontline method in controlling red imported fire ant (Solenopsis invicta Buren) (Hymenoptera: Formicidae), one of the worst invasive ants in the world. Neonicotinoids are effective ingredients in toxic baits for suppressing S. invicta population. To search for new and effective insecticides, the toxicity and sublethal effects of triflumezopyrim, a novel neonicotinoid analog, were evaluated against S. invicta. No high mortality of ants was observed after they fed on sugar water containing 120 μg/ml triflumezopyrim for 72 h; however, 100% mortality was achieved after ants fed on sugar water containing 10 μg/ml triflumezopyrim for 2 wk. Furthermore, at 10 μg/ml, triflumezopyrim did not inhibit ant food consumption within the 7-d treatment period. These results indicate that triflumezopyrim is a slow acting toxin and may be qualified as bait toxin for managing red imported fire ants. At 1 μg/ml, triflumezopyrim did not cause any significant effect on colony growth within 56 d and did not inhibit the food consumption during the whole trial period. At 10 μg/ml, triflumezopyrim displayed a significant reduction of aggressiveness during confrontation with native ants, resulting in higher mortality than the ants in the control. However, at 1 μg/ml, triflumezopyrim did not show any significant impact on both aggressiveness and mortality of the red imported fire ants.

Uptake and Translocation of Triflumezopyrim in Rice Plants

A new type of mesoionic insecticide triflumezopyrim is mainly used to control rice planthoppers, leafhoppers, etc. In order to study the uptake and translocation characteristics of this new insecticide in rice (Oryza sativa), a method for the detection of triflumezopyrim in rice, soil, and water was established using liquid-liquid extraction and QuEChERS sample pretreatment combined with liquid chromatography-triple quadrupole tandem mass spectrometry. The distribution of triflumezopyrim in rice was investigated after hydroponic treatment and foliar treatment at the concentrations of 2.5 and 5 mg·L^-1 within the ranges of 24, 48, and 72 h. The results showed that triflumezopyrim could be absorbed by roots and form a systematic distribution in rice by hydroponic treatment; meanwhile, it could also be absorbed by leaves and transported to the bottom leaves under foliar treatment, but no triflumezopyrim was detected in the roots. Thus, triflumezopyrim exhibited high acropetal translocation within the rice plant. This study provides an important scientific basis for the development of an application strategy of triflumezopyrim to control planthoppers and leafhoppers as well as for the residue detection method and safety evaluation.

Baseline determination, susceptibility monitoring and risk assessment to triflumezopyrim in Nilaparvata lugens (Stål)

Triflumezopyrim, a novel mesoionic chemical insecticide, is promoted as a powerful tool for control of susceptible and resistant hopper species in rice throughout Asia. For a newly commercialized insecticide it is important to establish susceptibility baseline, conduct susceptibility monitoring, and assess the risk of resistance via artificial selection to provide foundational information on designing resistance management strategy. The susceptibility baseline of triflumezopyrim was established for three rice planthopper species, Nilarpavata lugens (Stål), Sogatella furcifera (Horváth) and Laodelphax striatellus (Fallén). The LD₅₀ of triflumezopyrim was 0.026, 0.032 and 0.094 ng/individual for the adults of the susceptible strains of S. furcifera, L. striatellus and N. lugens, respectively, determined by a topical application method. Using a rice stem (seedling) dipping method, the LC₅₀ was determined as 0.042, 0.024 and 0.150 mg/L for the nymphs (3rd instar) of the three hopper species, respectively. In the meanwhile, the LC₅₀ of Pyraxalt™ (triflumezopyrim 10% SC) was 0.064 mg/L for the N. lugens susceptible strain. Furthermore, the susceptibility of triflumezopyrim and other five neonicotinoid insecticides were monitored for N. lugens field populations collected from major rice production areas in China in 2015-2019. All monitored populations were susceptible to triflumezopyrim (0.5 to 3.9-fold resistance ratio), and showed no cross-resistance to the other five neonicotinoids. These results suggested that triflumezopyrim is a good option to control resistant N. lugens. In addition, a field-collected population of N. lugens was artificially selected with triflumezopyrim for 20 generations and resulted in 3.5-fold increase in LC₅₀ from F₀ and 6.0-fold increase from that of the susceptible strain. The realized heritability (h²) of resistance was estimated as 0.0451 by using threshold trait analysis. With this h² value, the projected triflumezopyrim resistance development (a 10-fold increase in LC₅₀) would be expected after 30.3 or 24.0 generations if 80% or 90% of the population was killed at each generation.

Assessing the acute toxicity of insecticides to the buff-tailed bumblebee (Bombus terrestris audax)

The buff-tailed bumblebee, Bombus terrestris audax is an important pollinator within both landscape ecosystems and agricultural crops. During their lifetime bumblebees are regularly challenged by various environmental stressors including insecticides. Historically the honey bee (Apis mellifera spp.) has been used as an 'indicator' species for 'standard' ecotoxicological testing, but it has been suggested that it is not always a good proxy for other eusocial or solitary bees. To investigate this, the susceptibility of B. terrestris to selected pesticides within the neonicotinoid, pyrethroid and organophosphate classes was examined using acute insecticide bioassays. Acute oral and topical LD50 values for B. terrestris against these insecticides were broadly consistent with published results for A. mellifera. For the neonicotinoids, imidacloprid was highly toxic, but thiacloprid and acetamiprid were practically non-toxic. For pyrethroids, deltamethrin was highly toxic, but tau-fluvalinate only slightly toxic. For the organophosphates, chlorpyrifos was highly toxic, but coumaphos practically non-toxic. Bioassays using insecticides with common synergists enhanced the sensitivity of B. terrestris to several insecticides, suggesting detoxification enzymes may provide a level of protection against these compounds. The sensitivity of B. terrestris to compounds within three different insecticide classes is similar to that reported for honey bees, with marked variation in sensitivity to different insecticides within the same insecticide class observed in both species. This finding highlights the need to consider each compound within an insecticide class in isolation rather than extrapolating between different insecticides in the same class or sharing the same mode of action.

Actions on mammalian and insect nicotinic acetylcholine receptors of harmonine-containing alkaloid extracts from the harlequin ladybird Harmonia axyridis

The harlequin ladybird, Harmonia axyridis (H. axyridis), possesses a strong chemical defence that has contributed to its invasive success. Ladybird beetle defensive chemicals, secreted in response to stress and also found on the coating of laid eggs, are rich in alkaloids that are thought to be responsible for this beetle's toxicity to other species. Recent studies have shown that alkaloids from several species of ladybird beetle can target nicotinic acetylcholine receptors (nAChRs) acting as receptor antagonists, hence we have explored the actions of alkaloids of the ladybird H. axyridis on both mammalian and insect nAChRs. Electrophysiological studies on native and functionally expressed recombinant nAChRs were used to establish whether an alkaloid extract from H. axyridis (HAE) targeted nAChRs and whether any selectivity exists for insect over mammalian receptors of this type. HAE was found to be an inhibitor of all nAChRs tested with the voltage-dependence of inhibition and the effect on ACh EC₅₀ differing between nAChR subtypes. Our finding that an HAE fraction consisting almost entirely of harmonine had a strong inhibitory effect points to this alkaloid as a key component of nAChR inhibitory actions. Comparison of HAE inhibition between the mammalian and insect nAChRs investigated indicates some preference for the insect nAChR supporting the view that investigation of ladybird alkaloids shows promise as a method for identifying natural product leads for future insecticide development.

Development of novel pesticides in the 21st century

General trends and strategies for novel pesticides are summarized. Global pesticide sales and pesticide discovery research are also briefly reviewed. At least 105 chemical pesticides have been launched during the past decade or are under development: 43 fungicides, 34 insecticides/acaricides, 6 nematicides, 21 herbicides, and 1 herbicide safener. Most of them are safe to humans and environmentally friendly. The most developed fungicides are SDHI (succinate dehydrogenase inhibitors), DMI (demethylation inhibitors), QoI (quinone outside inhibitors), and QiI (quinone inside inhibitors). Due to the development of resistance to fungicides with existing modes of action, many fungicides possessing various novel modes of action have been launched or are under development. The trend of insecticide development is changing from organophosphorus, carbamate, and synthetic pyrethroids to nicotinic and diamide insecticides. During the past decade, compounds possessing a variety of novel modes of action have also been launched or are under development. Flupyradifurone and flupyrimin, exhibiting extremely low honeybee toxicity, have been developed and subjected to practical use. Herbicides possessing varied modes of action, such as acetolactate synthase, p-hydroxyphenylpyruvate dioxygenase, protoporphyrinogen oxidase, and very-long-chain fatty acid elongase inhibition, have been developed, but no herbicides possessing a novel mode action have commercialized in nearly 30 years. It is of interest that cyclopyrimorate, which was recently launched, and tetflupyrolimet, which is under development, have novel modes action: homogentisate solanesyltransferase (HST) and dihydroorotate dehydrogenase (DHODH) inhibition, respectively. The development of useful acaricides and nematicides is also progressing. Some natural product origin pesticides are getting attention.

The population growth, development and metabolic enzymes of the white-backed planthopper, Sogatella furcifera (Hemiptera: Delphacidae) under the sublethal dose of triflumezopyrim

Triflumezopyrim, a new nicotinic acetylcholine receptor (nAChR) inhibition, can effectively control piercing-sucking insect pests such as white-backed planthopper (Sogatella furcifera). At present, there has been no reports on the effects of triflumezopyrim on the population growth and development of S. furcifera. In this experiment, an age-stage two-sex life table was used to evaluate the impact of triflumezopyrim on the biological parameters of S. furcifera. The results showed that the adult preoviposition period (APOP) and total preoviposition period (TPOP) of the F₁ generation were significantly higher than those of the F₀ and F₄ generations, on the contrary the average fecundity, intrinsic rate of increase (r) and finite rate of increase (λ) of the F₄ generation were higher than those of the F₀ and F₁ generations. The results of synergists and enzyme activities indicated that the CarE and P450 activities in the F₄ generation were significantly higher than those in the F₀ generation (P < 0.05). The protein contents of vitellogenin (Vg) and vitellogenin receptor (VgR) and relative expression quality of VgR in the F₄ female adults were also significantly higher than those in the F₀ generation (P < 0.05). These results showed that triflumezopyrim at a low concentration could promote the growth and reproduction of S. furcifera, and that may provide a reference for the rational use of triflumezopyrim in the future.

Sublethal Effects of Triflumezopyrim on Biological Traits and Detoxification Enzyme Activities in the Small Brown Planthopper Laodelphax striatellus (Hemiptera: Delphacidae)

The small brown planthopper [Laodelphax striatellus (Fallén) (Hemiptera, Delphacidae)] is one of the most destructive insect pests of rice and has developed strong resistance to several kinds of chemical insecticides. Triflumezopyrim, a novel mesoionic insecticide developed by Corteva Agriscience (formerly DuPont Crop Protection), has efficient biological activity in controlling sucking insects, such as the planthopper. However, the effects of triflumezopyrim on the growth and reproduction of L. striatellus have not been reported. In this study, an F₅ generation was obtained by conducting five rounds of insecticide screening on a sensitive L. striatellus strain (F₀ generation). An age-stage life table procedure was used to evaluate the effects of a sublethal concentration (LC₅₀) of triflumezopyrim on the biological parameters of L. striatellus. Compared with those of the F₀ generation, the intrinsic rate of increase (r), the finite rate (λ), and the net reproductive rate (R₀) of the F₅ generation were significantly decreased; nevertheless, the average duration of life (T) was not significantly affected. The results of detoxification enzyme activity assays indicated that the glutathione S-transferase and cytochrome P450 monooxygenase (P450) activities in the F₅ generation were significantly higher than those in the F₀ generation. The contents of vitellogenin (Vg) and vitellogenin receptor (VgR) were also detected, and the results indicated that the contents of Vg and VgR in the F₅ generation were significantly decreased compared to those in the F₀ generation. Furthermore, we detected the relative expression of ecdysone receptor (EcR), Vg, and VgR in the F₀ and F₅ generations and found that the relative expression levels of Vg and VgR in the F₅ generation female adults were obviously lower than those in the F₀ generation (P < 0.05), whereas the relative expression of EcR was slightly increased, although the difference was not significant (P > 0.05). Based on these results, a sublethal concentration (median lethal concentration, LC₅₀) of triflumezopyrim may inhibit the generational growth and reproduction of L. striatellus. Moreover, our results may provide a reference for further studies of the suitability and resistance mechanisms of L. striatellus subjected to a sublethal dose of triflumezopyrim.

Effects of maximum residue limit of triflumezopyrim exposure on fitness of the red imported fire ant Solenopsis invicta

The impact of exposure to free feeding concentrations of triflumezopyrim to the red imported fire ant, Solenopsis invicta, in maximum residue tolerances for 56 days was investigated to understand whether triflumezopyrim, a novel neonicotinoid, poses unacceptable risks to the environment. Our results demonstrated that neither 0.5 μg/ml nor 0.2 μg/ml triflumezopyrim have a significant impact on the growth of the S. invicta colony and their food consumption (sugar water and locusts) during the length of treatment. While both 0.5 μg/ml and 0.2 μg/ml triflumezopyrim improved the grasping ability of S. invicta, and 0.5 μg/ml not 0.2 μg/ml triflumezopyrim increased their rate of locomotion. In addition, although 0.5 μg/ml and 0.2 μg/ml triflumezopyrim increased their individual aggressiveness index, the probability of the survival of S. invicta was not impacted by triflumezopyrim treatments in aggressive group encounters. This study suggests that triflumezopyrim did not have a negative impact on the fitness of S. invicta at 0.5 μg/ml and 0.2 μg/ml exposures.

First Discovery of Novel Pyrido[1,2-a]pyrimidinone Mesoionic Compounds as Antibacterial Agents

Plant bacterial diseases cause tremendous decreases in crop yield and quality, and there is a lack of highly effective and low-risk antibacterial agents. A series of novel pyrido[1,2-a]pyrimidinone mesoionic compounds containing vanillin moieties were synthesized, and the application of these mesoionic compounds as plant antibacterial agents was reported here for the first time. The bioassay results revealed that the mesoionic compounds had good antibacterial activity. Of these compounds, compound 11 showed excellent in vitro activity against Xanthomonas oryzae pv. oryzae, with an EC₅₀ value of 1.1 μg/mL, which was substantially better than that of bismerthiazol (92.7 μg/mL) and thiodiazole copper (105.4 μg/mL). Moreover, greenhouse condition trials indicated that the protective and curative activities of compound 11 against rice bacterial leaf blight were 75.12 and 72.04%, respectively, which were better than those of bismerthiazol (62.24 and 50.83%, respectively) and thiodiazole copper (53.35 and 65.04%, respectively). These results provide a basis for the application of mesoionic vanillin moieties as new antibacterial agents.

Crystal structure of 2-oxo-1-(pyrimidin-5-ylmethyl)-3-(3-(trifluoromethyl)phenyl)-1,2-dihydro-5l4-pyrido[1,2-a]pyrimidin-4-olate, C20H13F3N4O2

C20H13F3N4O2, triclinic, P1̄ (no. 2), a = 11.3525(10) Å, b = 12.7530(11) Å, c = 13.1926(12) Å, α = 83.894(1)°, β = 69.314(1)°, γ = 84.720(1)°, V = 1773.7(3) Å3, Z = 4, R gt(F) = 0.0477, wR ref(F 2) = 0.1433, T = 296(2) K.