Molecular modeling of sulfoxaflor and neonicotinoid binding in insect nicotinic acetylcholine receptors: impact of the Myzus β1 R81T mutation

Mutation V65I in the β1 Subunit of the Nicotinic Acetylcholine Receptor Confers Neonicotinoid and Sulfoxaflor Resistance in Insects

Neonicotinoids have been widely used to control pests with remarkable effectiveness. Excessive insecticides have led to serious insect resistance. Mutations of the nicotinic acetylcholine receptor (nAChR) are one of the reasons for neonicotinoid resistance conferred in various agricultural pests. Two mutations, V65I and V104I, were found in the nAChR β1 subunit of two neonicotinoid-resistant aphid populations. However, the specific functions of the two mutations remain unclear. In this study, we cloned and identified four nAChR subunits (α1, α2, α8, and β1) of thrips and found them to be highly homologous to the nAChR subunits of other insects. Subsequently, we successfully expressed two subtypes nAChR (α1/α2/α8/β1 and α1/α8/β1) by coinjecting three cofactors for the first time in thrips, and α1/α8/β1 showed abundant current rapidly. Acetylcholine, neonicotinoids, and sulfoxaflor exhibited different activation capacities for the two subtypes of nAChRs. Finally, V65I was found to significantly reduce the binding ability of nAChR to neonicotinoids and sulfoxaflor through electrophysiology and computer simulations. V104I caused a decrease in agonist affinity (pEC50) but an increase in the efficacy (Imax) of nAChR against neonicotinoids and reduced the binding ability of nAChR to sulfoxaflor. This study provides theoretical and technical support for studying the molecular mechanisms of neonicotinoid resistance in pests.

Discovery of piperonyl-tethered sulfoximines as novel low bee-toxicity aphicides targeting Amelα1/ratβ2 complex

Nicotinic acetylcholine receptor (nAChR) is recognized as a significant insecticide target for neonicotinoids and some agonists. In this study, the nAChR α1 subunit from Apis mellifera was first found to be narrowly tuned to different bee toxicity insecticides, namely, sulfoxaflor (SFX) and flupyradifurone (FPF). Hence, novel sulfoximine derivatives 7a-h were rationally designed and synthesized by introducing a benzo[d][1,3]dioxole moiety into a unique sulfoximine skeleton based on the binding cavity characteristics of Amelα1/ratβ2. The two electrode voltage clamp responses of 7a-h were obviously lower than that of SFX, indicating their potentially low bee toxicity. Besides, representative compounds 7b and 7g exhibited low bee toxicity (LD50 > 11.0 μg/bee at 48 h) revealed by acute contact toxicity bioassays. Molecular modelling results indicated that Ile152, Ala151, and Val160 from honeybee subunit Amelα1 and Lys144 and Trp80 from aphid subunit Mpα1 may be crucial for bee toxicity and aphicidal activity, respectively. These results clarify the toxic mechanism of agonist insecticides on nontargeted pollinators and reveal novel scaffold sulfoximine aphicidal candidates with low bee toxicity. These results will provide a new perspective on the rational design and highly effective development of novel eco-friendly insecticides based on the structure of the nAChR subunit.

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.

Defining the Biologically Plausible Taxonomic Domain of Applicability of an Adverse Outcome Pathway: A Case Study Linking Nicotinic Acetylcholine Receptor Activation to Colony Death

For the majority of developed adverse outcome pathways (AOPs), the taxonomic domain of applicability (tDOA) is typically narrowly defined with a single or a handful of species. Defining the tDOA of an AOP is critical for use in regulatory decision-making, particularly when considering protection of untested species. Structural and functional conservation are two elements that can be considered when defining the tDOA. Publicly accessible bioinformatics approaches, such as the Sequence Alignment to Predict Across Species Susceptibility (SeqAPASS) tool, take advantage of existing and growing databases of protein sequence and structural information to provide lines of evidence toward structural conservation of key events (KEs) and KE relationships (KERs) of an AOP. It is anticipated that SeqAPASS results could readily be combined with data derived from empirical toxicity studies to provide evidence of both structural and functional conservation, to define the tDOA for KEs, KERs, and AOPs. Such data could be incorporated in the AOP-Wiki as lines of evidence toward biological plausibility for the tDOA. We present a case study describing the process of using bioinformatics to define the tDOA of an AOP using an AOP linking the activation of the nicotinic acetylcholine receptor to colony death/failure in Apis mellifera. Although the AOP was developed to gain a particular biological understanding relative to A. mellifera health, applicability to other Apis bees, as well as non-Apis bees, has yet to be defined. The present study demonstrates how bioinformatics can be utilized to rapidly take advantage of existing protein sequence and structural knowledge to enhance and inform the tDOA of KEs, KERs, and AOPs, focusing on providing evidence of structural conservation across species. Environ Toxicol Chem 2023;42:71-87. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.

Impact of the nicotinic acetylcholine receptor mutation R81T on the response of European Myzus persicae populations to imidacloprid and sulfoxaflor in laboratory and in the field

Sulfoxaflor (Isoclast™ active) is a sulfoximine insecticide that is active on a broad range of sap-feeding insects, including species that exhibit reduced susceptibility to currently available insecticides. Colonies of Myzus persicae (green peach aphid) were established from aphids collected in the field from peach (Prunus persica) and nectarine (Prunus persica var. nucipersica) orchards in France, Italy and Spain. The presence of the nicotinic acetylcholine receptor (nAChR) point mutation R81T was determined for all the colonies. Eight of the 35 colonies collected were susceptible relative to R81T (i.e., R81T absent), three of the colonies were found to be homozygous for R81T while 24 colonies had R81T present in some proportion (heterozygous). Sulfoxaflor and imidacloprid were tested in the laboratory against these M. persicae field colonies, which exhibited a wide range of susceptibilities (sulfoxaflor RR = 0.6 to 61, imidacloprid RR = 0.7 to 986) (resistance ratios, RR) to both insecticides. Although sulfoxaflor was consistently more active than imidacloprid against these field collected M. persicae, there was a statistically significant correlation across all colonies between the RRs for imidacloprid and sulfoxaflor (Pearson's r = 0.939, p < 0.0001). However, when a larger group of the colonies from Spain possessing R81T were analyzed, there was no correlation observed for the RRs between imidacloprid and sulfoxaflor (r = 0.2901, p = 0.3604). Thus, consistent with prior studies, the presence of R81T by itself is not well correlated with altered susceptibility to sulfoxaflor. In field trials, sulfoxaflor (24 and 36 gai/ha) was highly effective (~avg. 88-96% control) against M. persicae, demonstrating similar levels of efficacy as flonicamid (60-70 gai/ha) and spirotetramat (100-180 gai/ha) at 13-15 days after application, in contrast to imidacloprid (110-190 gai/ha) and acetamiprid (50-75 gai/ha) with lower levels of efficacy (~avg. 62-67% control). Consequently, sulfoxaflor is an effective tool for use in insect pest management programs for M. persicae. However, it is recommended that sulfoxaflor be used in the context of an insecticide resistance management program as advocated by the Insecticide Resistance Action Committee involving rotation with insecticides possessing other modes of action (i.e., avoiding rotation with other Group 4 insecticides) to minimize the chances for resistance development and to extend its future utility.

MicroRNA-263b confers imidacloprid resistance in Sitobion miscanthi (Takahashi) by regulating the expression of the nAChRβ1 subunit

The Chinese wheat aphid Sitobion miscanthi (CWA) is an important harmful pest in wheat fields. Imidacloprid plays a critical role in controlling pests with sucking mouthparts. However, imidacloprid-resistant pests have been observed after insecticide overuse. Point mutations and low expression levels of the nicotinic acetylcholine receptor β1 (nAchRβ1) subunit are the main imidacloprid-resistant mechanisms. However, the regulatory mechanism underlying nAChRβ1 subunit expression is poorly understood. In this study, a target of miR-263b was isolated from the 5'UTR of the nAchRβ1 subunit in the CWA. Low expression levels were found in the imidacloprid-resistant strain CWA. Luciferase reporter assays showed that miR-263b could combine with the 5'UTR of the nAChRβ1 subunit and suppress its expression by binding to a site in the CWA. Aphids treated with the miR-263b agomir exhibited a significantly reduced abundance of the nAchRβ1 subunit and increased imidacloprid resistance. In contrast, aphids treated with the miR-263b antagomir exhibited significantly increased nAchRβ1 subunit abundance and decreased imidacloprid resistance. These results provide a basis for an improved understanding of the posttranscriptional regulatory mechanism of the nAChRβ1 subunit and further elucidate the function of miRNAs in regulating susceptibility to imidacloprid in the CWA. These results provide a better understanding of the mechanisms of posttranscriptional regulation of nAChRβ1 and will be helpful for further studies on the role of miRNAs in the regulation of nAChRβ1 subunit resistance in homopteran pests.

CYP4CJ6-mediated resistance to two neonicotinoid insecticides in Sitobion miscanthi (Takahashi)

The wheat aphid Sitobion miscanthi (CWA) is an important harmful pest in wheat fields. Insecticide application is the main method to effectively control wheat aphids. However, CWA has developed resistance to some insecticides due to its extensive application, and understanding resistance mechanisms is crucial for the management of CWA. In our study, a new P450 gene, CYP4CJ6, was identified from CWA and showed a positive response to imidacloprid and thiamethoxam. Transcription of CYP4CJ6 was significantly induced by both imidacloprid and thiamethoxam, and overexpression of CYP4CJ6 in the imidacloprid-resistant strain was also observed. The sensitivity of CWA to these two insecticides was increased after the knockdown of CYP4CJ6. These results indicated that CYP4CJ6 could be associated with CWA resistance to imidacloprid and thiamethoxam. Subsequently, the posttranscriptional regulatory mechanism was assessed, and miR-316 was confirmed to participate in the posttranscriptional regulation of CYP4CJ6. These results are crucial for clarifying the roles of P450 in the resistance of CWA to insecticides.

Chronic exposure to insecticides impairs honeybee optomotor behaviour

Honeybees use wide-field visual motion information to calculate the distance they have flown from the hive, and this information is communicated to conspecifics during the waggle dance. Seed treatment insecticides, including neonicotinoids and novel insecticides like sulfoxaflor, display detrimental effects on wild and managed bees, even when present at sublethal quantities. These effects include deficits in flight navigation and homing ability, and decreased survival of exposed worker bees. Neonicotinoid insecticides disrupt visual motion detection in the locust, resulting in impaired escape behaviors, but it had not previously been shown whether seed treatment insecticides disrupt wide-field motion detection in the honeybee. Here, we show that sublethal exposure to two commonly used insecticides, imidacloprid (a neonicotinoid) and sulfoxaflor, results in impaired optomotor behavior in the honeybee. This behavioral effect correlates with altered stress and detoxification gene expression in the brain. Exposure to sulfoxaflor led to sparse increases in neuronal apoptosis, localized primarily in the optic lobes, however there was no effect of imidacloprid. We propose that exposure to cholinergic insecticides disrupts the honeybee's ability to accurately encode wide-field visual motion, resulting in impaired optomotor behaviors. These findings provide a novel explanation for previously described effects of neonicotinoid insecticides on navigation and link these effects to sulfoxaflor for which there is a gap in scientific knowledge.

Integrated approach for the analysis of neonicotinoids in fruits and food matrices

We searched for five neonicotenoids (namely acetamiprid, clothianidin, imidacloprid, thiacloprid and thiamethoxam) in 67 samples of fruits, leaves, pollen and honey via HPLC-MS by employing QueChERs for extraction and purification. Clothianidin was never detected, while imidacloprid was identified in apple (9.2 µg/kg) and pollen (18-28 µg/Kg), thiacloprid in peaches (21-35 µg/kg) and acetamiprid was identified in the hazel leaves (1266 µg/kg), honey (13-26 µg /Kg) and pollen (11-24 µg/kg). Since the levels found of acetamiprid in hazel, honey and pollen were concerning, we accomplished a study to identify and characterize the possible transformation products via a laboratory simulation. The methodology exploited the analysis by HPLC-HRMS and its application in all matrices. We identify twelve transformation products, whose formation involved dimerization, hydroxylation, oxidation, demethylation and cleavage of the molecule. Three of them were also detected in hazel leaves.

Neonicotinoid’s resistance monitoring, diagnostic mechanisms and cytochrome P450 expression in green peach aphid [Myzus persicae (Sulzer) (Hemiptera: Aphididae)]

Green peach aphid [Myzus persicae (Sulzer) (Hemiptera: Aphididae)] is a significant pest with a known history of insecticide resistance. Neonicotinoids could manage this pest; however, their frequent use led to the evolution of resistance in field populations of M. persicae. Toxicity data for neonicotinoid insecticides synergized with pipernyl butoxide (PBO) in a field population (FP) were collected and compared to a laboratory susceptible clone (SC) of aphids. The enhanced expression of metabolic resistance-related cytochrome P450 gene CYP6CY3 and an arginine-threonine substitution were detected in FP, causing a single point mutation (R81T) at β1 subunit of nicotinic acetylcholine receptor (nAChR) within D loop. High level of resistance to imidacloprid was developed in FP with 101-fold resistance ratio and moderate resistance level (10.9-fold) to acetamiprid. The results of PBO synergized bioassay suggested that cytochrome P450 enzymes were involved in the resistance to neonicotinoids. The mRNA transcriptional level of CYP6CY3 gene was significantly higher (3.74 fold) in FP compared to SC. The R81T mutation associated with neonicotinoid resistance had 26% resistant allele frequency in FP. Both P450 enzymes and R81T mutation of nAChR were found in field-evolved neonicotinoid resistance. It is concluded that field-evolved resistance in green peach aphid could be managed by using appropriate synergists such as PBO.

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.

Sulfoxaflor - A sulfoximine insecticide: Review and analysis of mode of action, resistance and cross-resistance

The sulfoximines, as exemplified by sulfoxaflor (Isoclast™active), are a relatively new class of nicotinic acetylcholine receptor (nAChR) competitive modulator (Insecticide Resistance Action Committee [IRAC] Group 4C) insecticides that provide control of a wide range of sap-feeding insect pests. The sulfoximine chemistry and sulfoxaflor exhibits distinct interactions with metabolic enzymes and nAChRs compared to other IRAC Group 4 insecticides such as the neonicotinoids (Group 4A). These distinctions translate to notable differences in the frequency and degree of cross-resistance between sulfoxaflor and other insecticides. Most insect strains exhibiting resistance to a variety of insecticides, including neonicotinoids, exhibited little to no cross-resistance to sulfoxaflor. To date, only two laboratory-based studies involving four strains (Koo et al. 2014, Chen et al. 2017) have observed substantial cross-resistance (>100 fold) to sulfoxaflor in neonicotinoid resistant insects. Where higher levels of cross-resistance to sulfoxaflor are observed the magnitude of that resistance is far less than that of the selecting neonicotinoid. Importantly, there is no correlation between presence of resistance to neonicotinoids (i.e., imidacloprid, acetamiprid) and cross-resistance to sulfoxaflor. This phenomenon is consistent with and can be attributed to the unique and differentiated chemical class represented by sulfoxalfor. Recent studies have demonstrated that high levels of resistance (resistance ratio = 124-366) to sulfoxaflor can be selected for in the laboratory which thus far appear to be associated with enhanced metabolism by specific cytochrome P450s, although other resistance mechanisms have not yet been excluded. One hypothesis is that sulfoxaflor selects for and is susceptible to a subset of P450s with different substrate specificity. A range of chemoinformatic, molecular modeling, metabolism and target-site studies have been published. These studies point to distinctions in the chemistry of sulfoxaflor, and its metabolism by enzymes associated with resistance to other insecticides, as well as its interaction with insect nicotinic acetylcholine receptors, further supporting the subgrouping of sulfoxaflor (Group 4C) separate from that of other Group 4 insecticides. Herein is an expansion of an earlier review (Sparks et al. 2013), providing an update that considers prior and current studies focused on the mode of action of sulfoxaflor, along with an analysis of the presently available resistance / cross-resistance studies, and implications and recommendations regarding resistance management.

Role of nicotinic acetylcholine receptor subunits in the mode of action of neonicotinoid, sulfoximine and spinosyn insecticides in Drosophila melanogaster

Insecticides remain valuable tools for the control of insect pests that significantly impact human health and agriculture. A deeper understanding of insecticide targets is important in maintaining this control over pests. Our study systematically investigates the nicotinic acetylcholine receptor (nAChR) gene family, in order to identify the receptor subunits critical to the insect response to insecticides from three distinct chemical classes (neonicotinoids, spinosyns and sulfoximines). Applying the CRISPR/Cas9 gene editing technology in D. melanogaster, we were able to generate and maintain homozygous mutants for eight nAChR subunit genes. A ninth gene (Dβ1) was investigated using somatic CRISPR in neural cells to overcome the low viability of the homozygous germline knockout mutant. These findings highlight the specificity of the spinosyn class insecticide, spinosad, to receptors containing the Dα6 subunit. By way of contrast, neonicotinoids are likely to target multiple receptor subtypes, beyond those receptor subunit combinations previously identified. Significant differences in the impacts of specific nAChR subunit deletions on the resistance level of flies to neonicotinoids imidacloprid and nitenpyram indicate that the receptor subtypes they target do not completely overlap. While an R81T mutation in β1 subunits has revealed residues co-ordinating binding of sulfoximines and neonicotinoids differ, the resistance profiles of a deletion of Dβ1 examined here provide new insights into the mode of action of sulfoxaflor (sulfoximine) and identify Dβ1 as a key component of nAChRs targeted by both these insecticide classes. A comparison of resistance phenotypes found in this study to resistance reported in insect pests reveals a strong conservation of subunit targets across many different insect species and that mutations have been identified in most of the receptor subunits that our findings would predict to have the potential to confer resistance.

An update of the Worldwide Integrated Assessment (WIA) on systemic insecticides. Part 3: alternatives to systemic insecticides

Over-reliance on pesticides for pest control is inflicting serious damage to the environmental services that underpin agricultural productivity. The widespread use of systemic insecticides, neonicotinoids, and the phenylpyrazole fipronil in particular is assessed here in terms of their actual use in pest management, effects on crop yields, and the development of pest resistance to these compounds in many crops after two decades of usage. Resistance can only be overcome in the longterm by implementing methods that are not exclusively based on synthetic pesticides. A diverse range of pest management tactics is already available, all of which can achieve efficient pest control below the economic injury level while maintaining the productivity of the crops. A novel insurance method against crop failure is shown here as an example of alternative methods that can protect farmer's crops and their livelihoods without having to use insecticides. Finally, some concluding remarks about the need for a new framework for a truly sustainable agriculture that relies mainly on natural ecosystem services instead of chemicals are included; this reinforcing the previous WIA conclusions (van der Sluijs et al. Environ Sci Pollut Res 22:148-154, 2015).

Unraveling individual host-guest interactions in molecular recognition from first principles quantum mechanics: Insights into the nature of nicotinic acetylcholine receptor agonist binding

Drug binding to a protein target is governed by a complex pattern of noncovalent interactions between the ligand and the residues in the protein's binding pocket. Here we introduce a generally applicable, parameter-free, computational method that allows for the identification, quantification, and analysis of the key ligand-residue interactions responsible for molecular recognition. Our strategy relies on Local Energy Decomposition analysis at the "gold-standard" coupled cluster DLPNO-CCSD(T) level. In the study case shown in this paper, nicotine and imidacloprid binding to the nicotinic acetylcholine receptor, our approach provides new insights into how individual amino acids in the active site determine sensitivity and selectivity of the ligands, extending and refining classical pharmacophore hypotheses. By inference, the method is applicable to any kind of host/guest interactions with potential applications in industrial biocatalysis and protein engineering.

In Silico Studies of Lamiaceae Diterpenes with Bioinsecticide Potential against Aphis gossypii and Drosophila melanogaster

Background: The growing demand for agricultural products has led to the misuse/overuse of insecticides; resulting in the use of higher concentrations and the need for ever more toxic products. Ecologically, bioinsecticides are considered better and safer than synthetic insecticides; they must be toxic to the target organism, yet with low or no toxicity to non-target organisms. Many plant extracts have seen their high insecticide potential confirmed under laboratory conditions, and in the search for plant compounds with bioinsecticidal activity, the Lamiaceae family has yielded satisfactory results. Objective: The aim of our study was to develop computer-assisted predictions for compounds with known insecticidal activity against Aphis gossypii and Drosophila melanogaster. Results and conclusion: Structure analysis revealed ent-kaurane, kaurene, and clerodane diterpenes as the most active, showing excellent results. We also found that the interactions formed by these compounds were more stable, or presented similar stability to the commercialized insecticides tested. Overall, we concluded that the compounds bistenuifolin L (1836) and bistenuifolin K (1931), were potentially active against A. gossypii enzymes; and salvisplendin C (1086) and salvixalapadiene (1195), are potentially active against D. melanogaster. We observed and highlight that the diterpenes bistenuifolin L (1836), bistenuifolin K (1931), salvisplendin C (1086), and salvixalapadiene (1195), present a high probability of activity and low toxicity against the species studied.

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.

Evolutionary trade-offs of insecticide resistance - The fitness costs associated with target-site mutations in the nAChR of Drosophila melanogaster

The evolution of resistance to drugs and pesticides poses a major threat to human health and food security. Neonicotinoids are highly effective insecticides used to control agricultural pests. They target the insect nicotinic acetylcholine receptor and mutations of the receptor that confer resistance have been slow to develop, with only one field‐evolved mutation being reported to date. This is an arginine‐to‐threonine substitution at position 81 of the nAChR_β1 subunit in neonicotinoid‐resistant aphids. To validate the role of R81T in neonicotinoid resistance and to test whether it may confer any significant fitness costs to insects, CRISPR/Cas9 was used to introduce an analogous mutation in the genome of Drosophila melanogaster. Flies carrying R81T showed an increased tolerance (resistance) to neonicotinoid insecticides, accompanied by a significant reduction in fitness. In comparison, flies carrying a deletion of the whole nAChR_α6 subunit, the target site of spinosyns, showed an increased tolerance to this class of insecticides but presented almost no fitness deficits.

Sulfoxaflor efficacy in the laboratory against imidacloprid-resistant and susceptible populations of the green peach aphid, Myzus persicae: Impact of the R81T mutation in the nicotinic acetylcholine receptor

A key to effective insect pest management and insecticide resistance management is to provide growers with a range of new tools as potential alternatives to existing compounds or approaches. Sulfoxaflor (Isoclast™ active) is a new sulfoximine insecticide which is active on a broad range of sap-feeding insects, including species that have reduced susceptibility to currently used insecticides, such as imidacloprid from the neonicotinoid class. Sulfoxaflor (SFX) and imidacloprid (IMI) were tested in laboratory bioassays to compare the susceptibility of field populations of green peach aphid, Myzus persicae (Sulzer), exhibiting varying degrees of resistance involving an alteration (R81T) to the insect nicotinic acetylcholine receptor. The LC₅₀ values for M. persicae exposed to SFX ranged from 0.09 to 1.31 (mg litre^-1), whereas when the same populations were exposed to IMI the LC₅₀ values ranged from 0.6 to 76.2 (mg litre^-1). M. persicae were significantly more sensitive to SFX as compared to IMI for nine of the 13 populations tested. For M. persicae populations confirmed to be homozygous susceptible (ss) or heterozygous rs) for the R81T point mutation, there was no significant differences in the observed LC₅₀ values for either SFX or IMI relative to the susceptible reference population (15LP1). However, in all M persicae populations that were homozygous (rr) for the R81T point mutation, susceptibility was significantly less to IMI as compared to the reference population with resistance ratios ranging from 22.1 to 63.5-fold. In contrast, only one homozygous resistant population (15MP9) exhibited a statistically significant change in susceptibility (RR = 10-fold) to SFX as compared to the reference population, which was far less than the 56-fold observed for imidacloprid in that same population. Thus, this study indicates there is no specific correlation between the laboratory efficacy of SFX and IMI in field collected populations in Spain displaying varying degrees of resistance to IMI. Furthermore, the presence of target site resistance in M. persicae to IMI, in the form of the R81T mutation, does not a priori translate to a reduction in sensitivity to sulfoxaflor. Consequently, SFX can be an effective tool for use in insect pest management programs for green peach aphid. These data also serve as a baseline reference for green peach aphid sensitivity to SFX prior to commercial uses in Spain.

Neonicotinoid and sulfoximine pesticides differentially impair insect escape behavior and motion detection

Novel insecticides are developed and implemented in agriculture without a broad understanding of their sublethal effects. Although all act via nicotinic acetylcholine receptors, neonicotinoids and the novel sulfoxaflor insecticide exhibit differences in relative toxicity. In this study comparing the effects of these insecticides on visual motion detection and escape behavior, we show that sulfoxaflor displays decreased sublethal toxicity despite similar lethal endpoints of these insecticides. Imidacloprid reduces putative neural population code variability when responding to approaching objects, suggesting that neonics may constrain the tuning of visual sensory circuits. We suggest that neuroethologic methods are powerful tools to link toxic effects across levels of biological organization and further our understanding of how neural populations operate in complex sensory environments.

Insect nervous systems offer unique advantages for studying interactions between sensory systems and behavior, given their complexity with high tractability. By examining the neural coding of salient environmental stimuli and resulting behavioral output in the context of environmental stressors, we gain an understanding of the effects of these stressors on brain and behavior and provide insight into normal function. The implication of neonicotinoid (neonic) pesticides in contributing to declines of nontarget species, such as bees, has motivated the development of new compounds that can potentially mitigate putative resistance in target species and declines of nontarget species. We used a neuroethologic approach, including behavioral assays and multineuronal recording techniques, to investigate effects of imidacloprid (IMD) and the novel insecticide sulfoxaflor (SFX) on visual motion-detection circuits and related escape behavior in the tractable locust system. Despite similar LD₅₀ values, IMD and SFX evoked different behavioral and physiological effects. IMD significantly attenuated collision avoidance behaviors and impaired responses of neural populations, including decreases in spontaneous firing and neural habituation. In contrast, SFX displayed no effect at a comparable sublethal dose. These results show that neonics affect population responses and habituation of a visual motion detection system. We propose that differences in the sublethal effects of SFX reflect a different mode of action than that of IMD. More broadly, we suggest that neuroethologic assays for comparative neurotoxicology are valuable tools for fully addressing current issues regarding the proximal effects of environmental toxicity in nontarget species.

Sulfoxaflor Applied via Drip Irrigation Effectively Controls Cotton Aphid (Aphis gossypii Glover)

Aphis gossypii Glover is a major pest of cotton and can severely affect cotton yield and lint quality. In this study, the efficacy of sulfoxaflor applied via drip irrigation and foliar spray on controlling cotton aphids was evaluated in 2016 and 2017 in Xinjiang, China. The distribution of sulfoxaflor in cotton roots, stems, leaves, and aphids, as well as its effects on two natural enemies of aphids, were also investigated. Results showed that sulfoxaflor applied through drip irrigation mainly concentrated in leaves and provided effective control of cotton aphids for 40 days, compared to 20 days when applied through foliar spray. Furthermore, drip application resulted in much lower sulfoxaflor concentrations in aphids than foliar spray. As a result, ladybird beetle and lacewing populations were higher in drip applied plants than in foliar sprayed plants. Additionally, the cost of drip irrigation was lower than foliar spray as cotton plants are commonly irrigated via drip irrigation in Xinjiang. Our results showed that application of sulfoxaflor through drip irrigation is an effective way of controlling cotton aphids in Xinjiang due to a prolonged control period, safety to two natural enemies, and lower cost of application.

Binding of Sulfoxaflor to Aplysia californica-AChBP: Computational Insights from Multiscale Approaches

Structural features and binding properties of sulfoxaflor (SFX) with Ac-AChBP, the surrogate of the insect nAChR ligand binding domain (LBD), are reported herein using various complementary molecular modeling approaches (QM, molecular docking, molecular dynamics, and QM/QM'). The different SFX stereoisomers show distinct behaviors in terms of binding and interactions with Ac-AChBP. Molecular docking and Molecular Dynamics (MD) simulations highlight the specific intermolecular contacts involved in the binding of the different SFX isomers and the relative contribution of the SFX functional groups. QM/QM' calculations provide further insights and a significant refinement of the geometric and energetic contributions of the various residues leading to a preference for the SS and RR stereoisomers. Notable differences in terms of binding interactions are pointed out for the four stereoisomers. The results point out the induced fit of the Ac-AChBP binding site according to the SFX stereoisomer. In this process, the water molecules-mediated contacts play a key role, their energetic contribution being among the most important for the various stereoisomers. In all cases, the interaction with Trp147 is the major binding component, through CH···π and π···π interactions. This study provides a rationale for the binding of SFX to insect nAChR, in particular with respect to the new class of sulfoximine-based insect nAChR competitive modulators, and points out the requirements of various levels of theory for an accurate description of ligand-receptor interactions.

Lethal Toxicity and Sublethal Metabolic Interference Effects of Sulfoxaflor on the Earthworm ( Eisenia fetida)

Testing for effects of pesticides on nontarget organisms is an integral part of ecological risk assessment. In the present study, the acute toxicity of sulfoxaflor to earthworms was evaluated using an artificial soil toxicity test, and sublethal effects were assessed through oxidative stress and metabolomics. Sulfoxaflor is a supertoxic pollutant to earthworms that easily bioaccumulates in earthworms, and contains LC₂, LC₁₀, and LC₅₀ values of 0.08 (0.04-0.13), 0.19 (0.11-0.25), and 0.54 (0.45-0.65) mg/kg, respectively. Sublethal doses of sulfoxaflor resulted in oxidative damage to earthworms in which antioxidant enzymatic activities including SOD, CAT, and GST were significantly inhibited and MDA content accumulated. Metabolomics analysis suggested that the energy metabolism and the urea cycle in earthworms were significantly activated, while nucleotide metabolism was depressed, which could cause DNA damage. The results suggest earthworms have the potential to be a new entry point for sulfoxaflor into the wildlife food chain. Since earthworms significantly contribute to soil function and ecosystems, the high safety risks of sulfoxaflor to the earthworm could extend to the environment. In view of these findings, more attention should be given to the risks sulfoxaflor poses to the environment through its effects on earthworms.

Current status of chirality in agrochemicals

The agrochemical industry is searching continuously for new pesticides to develop products that have optimal efficacy, lower application rates in the field, increased selectivity, favourable toxicological and environmental safety, enhanced user friendliness and better economic viability. One strategy by which to achieve these ambitious goals makes use of the unique properties of molecules containing asymmetric centres. In the past, many natural products and their congeners have been a source of inspiration in the design of new active ingredients, and the molecular structures of the resulting compounds have become increasingly complex; some 30% contain fragments with asymmetric centres. However, despite enormous progress in catalytic asymmetric processes over the past decade, few agrochemicals are produced in an enantiomerically pure or enriched form on an industrial scale. Since 2007, ∼ 43% of the 44 products launched (insecticides, acaricides, fungicides, nematicides and herbicides) contain one or more asymmetric centres in the molecule (∼ 47%) and most have been launched as racemic mixtures of enantiomers or diastereomers. This review provides an overview of the current status of chiral agrochemicals launched over the past 10 years and describes the inherently connected challenges of modern agricultural chemistry by managing important aspects resulting from the stereochemistry of these innovative products. © 2018 Society of Chemical Industry.

Toxicity of seven insecticides to different developmental stages of the whitefly Bemisia tabaci MED (Hemiptera: Aleyrodidae) in multiple field populations of China

Chemical control is important in the management of the tobacco whitefly, Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). Susceptibility of B. tabaci to insecticides may vary among different developmental stages and geographical populations. In this study, we examined toxicity of seven commonly-used insecticides to B. tabaci MED in four field populations from China. Avermectin has high level of toxicity to all stages of B. tabaci MED in all four populations. Cyantraniliprole and sulfoxaflor have high toxicity to adults. Spirotetramat, cyantraniliprole and flonicamid have high toxicity to nymphs but not adults. Acetamiprid, cyantraniliprole and sulfoxaflor have high toxicity to eggs. However, the relative toxicity of B. tabaci MED to these chemicals varied across different populations, with little consistency in population differences across developmental stages. Our findings together with some instances where LC₉₅ values were higher than field recommended dosages indicate field-evolved resistance to insecticides (such as thiamethoxam and sulfoxaflor) and stage-specific mechanisms that will influence effective control of B. tabaci MED by insecticides.

Characterisation of imidacloprid resistance in the bird cherry-oat aphid, Rhopalosiphum padi, a serious pest on wheat crops

BACKGROUND

Rhopalosiphum padi is a destructive insect pest of wheat worldwide. Studies have shown that R. padi has developed resistance to different insecticides, including imidacloprid. We studied the mechanisms conferring resistance to imidacloprid at the biochemical and molecular levels.

RESULTS

An R. padi imidacloprid-resistant (IM-R) strain and a susceptible (SS) strain were established. Fitness analysis using life-tables showed that the IM-R strain had obvious disadvantages in several parameters, indicating reduced fitness. Profiles of cross-resistance of IM-R and SS to seven insecticides were detected. Both synergistic and enzyme activity data suggested that P450 plays a role in resistance. Furthermore, the mRNA expression levels of cytochrome P450 (CYP) genes CYP6CY3-1 and CYP6CY3-2 were significantly increased in the IM-R strain. No target-site mutation within the nicotinic acetylcholine receptor (nAChR) subunits was detected in the IM-R strain. Interestingly, the expression levels of the nAChR α1, α2, α3, α7-2, and β1 subunit genes were significantly decreased, suggesting that down-regulation of these subunits may be involved in resistance.

CONCLUSION

Multiple mechanisms confer imidacloprid resistance in R. padi. © 2017 Society of Chemical Industry.

Characterization of a nicotinic acetylcholine receptor binding site for sulfoxaflor, a new sulfoximine insecticide for the control of sap-feeding insect pests

Sulfoxaflor (SFX, Isoclast™ Active) is a recently developed sulfoximine insecticide that is highly effective against sap-feeding insect pests. SFX has been shown to act through an interaction with insect nicotinic acetylcholine receptors (nAChRs). SFX was previously found to interact weakly with the binding site for the neonicotinoid imidacloprid. However, radioligand displacement studies characterizing the binding site of the insecticide SFX itself have not been conducted. In this study, we report the characterization of a high affinity [³H]SFX Myzus persicae (green peach aphid, GPA) binding site with relatively low abundance. Through the evaluation of a set of SFX analogs, we have demonstrated that displacement of [³H]SFX shows an excellent correlation with GPA toxicity, and thus is toxicologically relevant. Comparison with the previously described methyl-SFX binding site information reveals differences with the SFX binding site that are discussed herein. [³H]SFX therefore represents a new tool for the characterization of insect nAChRs.

The R81T mutation in the nicotinic acetylcholine receptor of Aphis gossypii is associated with neonicotinoid insecticide resistance with differential effects for cyano- and nitro-substituted neonicotinoids

The cotton aphid, Aphis gossypii Glover, is one of the most agriculturally important insect pests. Neonicotinoid insecticides and sulfoxaflor have generally shown excellent control of A. gossypii, but these aphids have recently developed resistance against neonicotinoid insecticides. We previously characterized a field-collected A. gossypii Kushima clone that showed higher resistance to nitro-substituted neonicotinoids, such as imidacloprid, than to cyano-substituted neonicotinoids, such as acetamiprid. This Kushima clone harbors the R81T mutation in the nicotinic acetylcholine receptor (nAChR) β1 subunit; this mutation is the source of neonicotinoid insecticide resistance. In the present study, electrophysiological analyses and molecular modeling were employed to investigate the differential effects of the R81T mutation on cyano- and nitro-substituted neonicotinoids and sulfoxaflor. We isolated full-length coding sequences of A. gossypii nAChR α1, α2, and β1 subunits. When co-expressed in Xenopus laevis oocytes with chicken β2 nAChR, A. gossypii α1 evoked inward currents in a concentration-dependent manner in response to acetylcholine (ACh) and showed sensitivity to neonicotinoid and sulfoxaflor. Additionally, the chicken β2 T77R+E79V (equivalent double mutant of R81T) mutation resulted in a lower effect to cyano-substituted neonicotinoids and sulfoxaflor than to nitro-substituted neonicotinoids. Electrophysiological data and nAChR homology modeling analysis suggested that the Kushima clone exhibited different levels of resistance to cyano- and nitro-substituted neonicotinoid insecticides.

Susceptibility of Australian Myzus persicae (Hemiptera: Aphididae) to Three Recently Registered Insecticides: Spirotetramat, Cyantraniliprole, and Sulfoxaflor

The green peach aphid, Myzus persicae (Sulzer), is a significant agricultural pest that has developed resistance to a large number of insecticides globally. Within Australia, resistance has previously been confirmed for multiple chemical groups, including pyrethroids, carbamates, organophosphates, and neonicotinoids. In this study, we use leaf-dip and topical bioassays to investigate susceptibility and potential cross-resistance of 12 field-collected populations of Australian M. persicae to three recently registered insecticides: sulfoxaflor, spirotetramat, and cyantraniliprole. Despite all 12 populations carrying known resistance mechanisms to carbamates, organophosphates, and pyrethroids, and two populations also exhibiting low-level metabolic resistance to neonicotinoids, we found little evidence of variation in susceptibility to sulfoxafor, spirotetramat, or cyantraniliprole. This provides further evidence that cross-resistance to spirotetramat, cyantraniliprole, and sulfoxaflor in M. persicae is not conferred by the commonly occurring resistance mechanisms MACE, super-kdr, amplification of the E4 esterase gene, or enhanced expression and copy number of the P450 gene, CYP6CY3. Importantly, this study also established toxicity baseline data that will be important for future monitoring of insecticide responses of M. persicae from both broadacre and horticultural crops.

Toxicity and sublethal effects of sulfoxaflor on the red imported fire ant, Solenopsis invicta

To understand whether sulfoxaflor, a novel neonicotinoid, poses unacceptable risks to the environment, it is important to assess its effects on nontarget insects. Therefore, the effects of short-term exposure (28 days) of free-feeding sublethal concentrations (1-2μg/ml) of sulfoxaflor to the red imported fire ant, Solenopsis invicta, were investigated. The following parameters were evaluated to determine the impact of exposure: colony growth, food consumption (sugar water and locusts), and interspecific interactions. Sulfoxaflor exposure produced significant negative effects on S. invicta colony growth, with cumulative colony weight losses of 83.36% and 100.00% after treatment with 1μg/ml and 2μg/ml, respectively. The consumption of sugar water (containing sulfoxaflor) of surviving colonies decreased with increasing sulfoxaflor concentration. Moreover, the consumption of locusts decreased after treatment with 2μg/ml, but not 1μg/ml, sulfoxaflor. Sulfoxaflor treatment for 14 days led to reduced aggressiveness of S. invicta workers in interspecific confrontations (S. invicta vs. unexposed Pheidole fervida), and their probability of survival of aggressive encounters was reduced significantly to 48% of that of control ants. Our results indicate that sublethal concentrations of sulfoxaflor are likely to have a negative impact on ants.

Studies toward understanding the SAR around the sulfoximine moiety of the sap-feeding insecticide sulfoxaflor

BACKGROUND

The discovery of sulfoxaflor (Isoclast™ active) stemmed from a novel scaffold-based approach toward identifying bioactive molecules. It exhibits broad-spectrum control of many sap-feeding insect pests, including aphids, whiteflies, hoppers and Lygus. Systematic modifications of the substituents flanking each side of the sulfoximine moiety were carried out to determine whether these changes would improve potency.

RESULTS

Structure-activity relationship (SAR) studies showed that, with respect to the methylene linker, both mono- and disubstitution with alkyl groups of varying sizes as well as cyclic analogs exhibited excellent control of cotton aphids. However, against green peach aphids a decrease in activity was observed with substituents larger than ethyl as well as larger cycloalkyl groups. At the terminal tail there appeared to be a narrow steric tolerance as well, with linear groups or small rings more active against green peach aphids than bulkier groups.

CONCLUSION

A novel series of compounds exploring the substituents flanking the sulfoximine moiety of sulfoxaflor were prepared and tested for bioactivity against cotton aphids and green peach aphids. SAR studies indicated that a decrease in green peach aphid potency was observed at the methylene linker as well as at the terminal tail with bulkier substituents. A quantitative structure-activity relationship analysis of the compounds revealed significant correlation of activity with two molecular descriptors, vol (volume of a molecule) and GCUT_SMR_3 (molar refractivity). This predictive model helps to explain the observed activity with the various substituents. © 2016 Society of Chemical Industry.

Relative Toxicity of Two Aphicides to Hippodamia convergens (Coleoptera: Coccinellidae): Implications for Integrated Management of Sugarcane Aphid, Melanaphis sacchari (Hemiptera: Aphididae)

Flupyradifurone and sulfoxaflor present novel insecticide chemistries with particular efficacy against aphids, and the recent emergence of sugarcane aphid, Melanaphis sacchari (Zehntner), as a pest of sorghum in the United States has resulted in their widespread use. We examined their toxicity to Hippodamia convergens Guerin-Meneville, an important aphid biocontrol agent. We exposed beetles to topical applications of the field rate (FR) of these insecticides, fed them contaminated food (eggs of Ephestia kuehniella Zeller), and gave first-instar larvae 24-h exposures to leaf residues. More than half of fourth-instar larvae receiving topical applications of sulfoxaflor at FR survived, whereas flupyradifurone at 0.1× FR caused 90% mortality. Adults survived topical treatments better than larvae and without measurable mortality, except flupyradifurone at FR, which killed more than 80% of beetles. Survivors of all treatments had fertility similar to controls, whether treated as larvae or adults. Ingestion of contaminated food caused significant mortality in all treatments (15-40% for adults and 55-85% for larvae), with no significant differences between insecticides at FR. Leaf residues of sulfoxaflor at 1.0 and 2.0× FR caused approximately 60 and 80% mortality of first instars, respectively, whereas flupyradifurone at 0.1 and 1.0× FR caused > 90% mortality. Although sulfoxaflor was less toxic to H. convergens than flupyradifurone, the tested FR of flupyradifurone has now been reduced by half. We conclude that neither insecticide appears as toxic as other nicotinic acetylcholine receptor agonists, and that both materials are compatible with integrated pest management programs for M. sacchari.