A novel class of insecticidal alkylsulfones are potent inhibitors of vesicular acetylcholine transport.

Pyridine alkylsulfone derivatives typified by oxazosulfyl (Sumitomo Chemical Company Ltd.) and compound A2 (Syngenta) represent a new class of insecticides, with potent activity against several insect orders. Whilst the MOA of this class has been attributed to interaction with the voltage-gated sodium channel (VGSC), here we present strong evidence that their toxicity to insects is mediated primarily through inhibition of the vesicular acetylcholine transporter (VAChT). Alkylsulfone intoxication in insects is characterised by (i) a reduction in cholinergic synaptic transmission efficiency demonstrated by a depression of cercal afferent activity in giant-interneurone preparations of American cockroach (Periplaneta americana), (ii) selective block of cholinergic-transmission dependent post-synaptic potentials in the Drosophila giant-fibre pathway and (iii) abolition of miniature excitatory post-synaptic currents (mEPSCs) in an identified synapse in Drosophila larvae. Ligand-binding studies using a tritiated example compound ([3H]-A1) revealed a single saturable binding-site, with low nanomolar Kd value, in membrane fractions of green bottle fly (Lucilia sericata). Binding is inhibited by vesamicol and by several examples of a previously identified class of insecticidal compounds known to target VAChT, the spiroindolines. Displacement of this binding by analogues of the radioligand reveals a strong correlation with insecticidal potency. No specific binding was detected in untransformed PC12 cells but a PC12 line stably expressing Drosophila VAChT showed similar affinity for [3H]-A1 as that seen in fly head membrane preparations. Previously identified VAChT point mutations confer resistance to the spiroindoline class of insecticides in Drosophila by Gal-4/UAS directed expression in cholinergic neurones and by CRISPR gene-editing of VAChT, but none of these flies show detectable cross-resistance to this new chemical class. Oxazosulfyl was previously shown to stabilise voltage-gated sodium channels in their slow-inactivated conformation with an IC50 value of 12.3µM but inhibits binding of [3H]-A1 with approximately 5000 times greater potency. We believe this chemistry class represents a novel mode-of-action with high potential for invertebrate selectivity.

Fate of synthetic chemicals in the agronomic insect pest Spodoptera littoralis: experimental feeding-contact assay and toxicokinetic model.

Insecticides prevent or reduce insect crop damage, maintaining crop quality and quantity. Physiological traits, such as an insect's feeding behavior, influence the way insecticides are absorbed and processed in the body (toxicokinetics), which can be exploited to improve species selectivity. To fully understand the uptake of insecticides, it is essential to study their total uptake and toxicokinetics independent of their toxic effects on insects. We studied the toxicokinetics (TK) of insecticidally inactive test compounds incorporating agro-like structural motifs in larvae of the Egyptian cotton leafworm (Spodoptera littoralis, Lepidoptera), and their distribution across all biological matrices, using laboratory experiments and modeling. We measured Spodoptera larval behavior and temporal changes of whole-body concentrations of test compounds during feeding on treated soybean leaf disks and throughout a subsequent depuration period. Differences in the distribution of the total quantities of compounds were found between the biological matrices leaf, larva, and feces. Rate constants for uptake and elimination of test compounds were derived by calibrating a toxicokinetic model to the whole-body concentrations. Uptake and elimination rate constants depended on the physicochemical properties of the test compounds. Increasing hydrophobicity increased the bioaccumulation potential of test compounds. Incomplete quantities in larval matrices indicated that some compounds may undergo biotransformation. As fecal excretion was a major elimination pathway, the variable time of release and number of feces pellets led to a high variability in the body burden. We provide quantitative models to predict the toxicokinetics and bioaccumulation potential of inactive insecticide analogs (parent compounds) in Spodoptera.

Effects of Zwitterionic buffers on sorption of ferrous iron at goethite and its oxidation by CCl4.

A major factor which controls sorption and oxidation of Fe(II) at the mineral-water interface is pH, hence buffers are commonly used to control pH in experimental studies. Here, we examined the effects of widely used organic buffers (3-morpholinopropane-1-sulfonic acid (MOPS) and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES)) on Fe(II) uptake and oxidation by CCl(4) in aqueous suspensions of goethite. Significant sorption of these zwitterionic buffers occurred only at Fe(II)-loaded goethite but not at native goethite. The addition of MOPS and HEPES caused substantial release of Fe(II) from goethite, retarded the oxidation of surface-bound Fe(II) by CCl(4) and changed the reaction pathway as indicated by lower yields of CHCl(3). To explore electrostatic and steric contributions of MOPS and HEPES to the observed phenomena we studied sorption and competitive effects of model sorbates (Ca(2+), sulfonates) which suggest the formation of a complex between surface-bound Fe(II) and MOPS or HEPES. Our study shows for the first time that these frequently used zwitterionic organic buffers may interfere significantly with the surface chemistry and thus with redox reactions of Fe(II) at goethite. Hence, kinetic or mechanistic information obtained in such systems requires careful interpretation.

Spiro N-methoxy piperidine ring containing aryldiones for the control of sucking insects and mites: discovery of spiropidion.

BACKGROUND: Crop protection solutions for the control of key economic sucking pests derive essentially from neuronal and muscular acting chemistries, wherein neonicotinoid uses largely dominated for the last two decades. Anticipating likely resistance development of some of those arthropod species to this particular class, we intensified research activities on a non-neuronal site of action targeting insect growth and development some 10 years ago. RESULTS: Our innovation path featured reactivation of a scarcely used and simple building block from the 1960s, namely N-methoxy-4-piperidone 3. Its judicious incorporation into the 2-aryl-1,3-dione scaffold of IRAC group 23 inhibitors of fatty acid biosynthesis resulted in novel tetramic acid derivatives acting on acetyl-coenzyme A carboxylase (ACCase). The optimization campaign focused on modulation of the aryl substitution pattern and understanding substituent options at the lactam nitrogen position of those spiroheterocyclic pyrrolidine-dione derivatives towards an effective control of sucking insects and mites. This work gratifyingly culminated in the discovery of spiro N-methoxy piperidine containing proinsecticide spiropidion 1. Following in planta release, its insecticidally active dione metabolite 2 is translaminar and two-way systemic (both xylem and phloem mobile) for a full plant protection against arthropod pests. CONCLUSION: Owing to such unique plant systemic properties, growing shoots and roots actually not directly exposed to spiropidion-based chemistry after foliar application nevertheless benefit from its long-lasting efficacy. Spiropidion is for use in field crops, speciality crops and vegetables controlling a broad range of sucking pests. In light of other performance and safety profiles of spiropidion, an IPM fit may be expected. © 2020 Society of Chemical Industry.

How To Design for a Tailored Subcellular Distribution of Systemic Agrochemicals in Plant Tissues.

Foliar-applied systemic agrochemicals require entrance into the plant vascular system or into specific subcellular compartments to reach their target in planta or to be imbibed by piercing/sucking pests. An inappropriate subcellular localization, like accumulation of aphicides in vacuoles, might lower the compound's efficiency due to reduced exposure to the target. Permeabilities and mass distributions of 16 compounds covering a broad range of properties were measured across a pH gradient in a PAMPA ("parallel artificial membrane permeability assay") system, providing experimental evidence for ion trapping of acids and bases in basic and acidic compartments, respectively. The results validated a predictive model which was then expanded to simulate a standardized plant cell (cytosol and vacuole) with a vascular system (phloem and xylem). This approach underlined that the absolute mass distribution across aqueous phases is defined by membrane retention, whereas the relative mass distribution is determined by the species (neutral, acidic, basic) of compounds. These processes depend largely on p Ka and log  Kow of the test compounds, which subsequently determine the partitioning of the substances in plant cell compartments. The validated model can be used as a tool in agrochemistry research to tailor the subcellular distribution by chemistry design and to interpret biology results.

How active ingredient localisation in plant tissues determines the targeted pest spectrum of different chemistries.

BACKGROUND: The efficacies of four commercial insecticides and of two research compounds were tested against aphids (Aphis craccivora and Myzus persicae), whiteflies (Bemisia tabaci) and red-spotted spider mites (Tetranychus urticae) in intrinsic (oral administration), curative (direct contact spray) and translaminar (arthropods infested on untreated leaf underside) assays. With a new translaminar model, the transport across the leaf cuticle and tissues and the electrochemical distribution of test compounds in cellular compartments and apoplast were calculated. RESULTS: The comparison of both information sets revealed that the intracellular localisation of active ingredients determines the performance of test compounds against different target pests because of different feeding behaviours: mites feed on mesophyll, and aphids and whiteflies mostly in the vascular system. Polar compounds have a slow adsorption into leaf cells and thus a favourable distribution into apoplast and xylem sap. Slightly lipophilic bases get trapped in vacuoles, which is a less suited place to control hemipteran pests but appropriate to control mites. Non-favourable cellular localisation led to a strong reduction in translaminar efficacy against phloem feeders. CONCLUSION: Prediction and optimisation of intracellular localisation of pesticides add valuable new information for targeted bioavailability and can indicate directions for improved pesticide design.

Translocation and translaminar bioavailability of two neonicotinoid insecticides after foliar application to cabbage and cotton.

A laboratory study was undertaken to investigate the leaf systemic properties and the translaminar aphicidal activity of two commercialised neonicotinoid (chloronicotinyl) insecticides. For that purpose [14C]imidacloprid was subjected to uptake and translocation studies in cabbage and cotton after foliar application. Foliar penetration and short-term translocation patterns of imidacloprid were similar in both plant species. Nevertheless imidacloprid penetrated twice as much into cabbage leaves as it did into cotton leaves. It showed a comparable translaminar behaviour and was entirely translocated acropetally, indicating its well-known xylem mobility. The translaminar and acropetal movement of imidacloprid and acetamiprid were quantified by simple laboratory bioassays using the green peach aphid, Myzus persicae (Sulzer), and the cotton aphid, Aphis gossypii (Glover), as typical homopteran pests for cabbage and cotton, respectively. A single dose (7.5 micrograms AI per leaf) applied to the upper leaf surface of cabbage and cotton was tested against aphids feeding on the lower leaf surface both close to and distant from the site of application 1, 5 and 12 days after treatment. The translaminar residual activity of imidacloprid on cabbage leaves was superior to that of acetamiprid, whereas its translaminar efficacy against A gossypii on cotton was inferior to that of acetamiprid. However, oral ingestion bioassays using an artificial double membrane feeding system revealed no significant differences in intrinsic activity between the two neonicotinoids tested.

Characterization of the diffusion of non-electrolytes across plant cuticles: properties of the lipophilic pathway.

Systemic crop protection products are commonly sprayed onto foliage, whereupon the active substances must penetrate into the leaves in order to become biologically active. Penetration of the plant cuticle is the rate-limiting step. The diffusion of organic non-electrolytes within cuticles is a purely physical process that can be described and analysed in the same way as is done for diffusion in synthetic polymer membranes. Solute mobilities in cuticles vary considerably between plant species. For a given species they decrease with increasing solute size, and this size selectivity holds for all of the plant species investigated so far. Wax extraction from leaf cuticles increases the mobility of solutes tremendously, but size selectivity is not affected. Furthermore, diffusion within plant cuticles is extremely temperature dependent. An analogous increase in solute mobility can be achieved by using accelerators, which enhance the fluidity of the polymer matrix and of the waxes. The effects of temperature and plasticizers on the diffusion of non-electrolytes in wax and the cutin matrix have been used to characterize the nature of the lipophilic pathway. The 'free volume' theory can be used to explain the influence of the size and shape of the solute, and its dependence on temperature. The physico-chemical nature of the diffusion pathway has been shown, by thermodynamic analysis, to be identical for a wide range of solute lipophilicities. This approach also explains the mode of action and the intrinsic activity of plasticizers.