Fecundity decline is male derived following transfluthrin exposures in a field strain of Aedes albopictus (Diptera: Culicidae).

Volatile pyrethroids are effective in reducing mosquito populations and repelling vectors away from hosts. However, many gaps in knowledge exist for the sublethal impacts of volatile pyrethroids on mosquitoes. To that end, transfluthrin exposures were conducted on a field strain of Aedes albopictus (Skuse) held as a laboratory colony. Dose-response analysis was conducted on both sexes at either 1-4 days old or 5-10 days old. Resultant concentration data were used to evaluate the LC20 and LC50 values in various mate pairings of treatments and controls in which either the male or female was from a selectively treated group and mated with a counterpart that was treated independently. Blood feeding proportion, delayed mortality after a 24-h recovery period, egg collection totals, and F1 larval survival were determined following transfluthrin treatment in the F0, but outcomes were not significant. In contrast, sterility was predicated on male treatment, with treated females resulting in higher overall egg viability. Treated males in the mating pair resulted in significantly lower egg viability and accelerated larval hatch in the F1. Additionally, the presence of sperm in female spermathecae was significantly diminished in test groups containing treated male mosquitoes. Male sublethal effects may be a critical determinant of a mixed population's reproductive success.

Differences in excito-repellent behavior between susceptible and permethrin-resistant strains of Aedes aegypti upon exposure to insecticide-treated fabric.

BACKGROUND: An important component of the biological activity of pyrethroids, when used in disease vector control, is excito-repellency. In this study, behavioral differences between insecticide susceptible (Orlando) and pyrethroid resistant (Puerto Rican) strains of Aedes aegypti were explored in a round glass arena using fabrics treated with permethrin, etofenprox, deltamethrin, or DDT. Repellency was evaluated across several variables, including the time to first flight (TFF), number of landings (NOL), total flight time (TFT), and maximum surface contact (MSC), all by video analysis. RESULTS: Results from the Orlando strain indicated they were less likely than the Puerto Rican strain to tolerate tarsal contact with the treated fabrics. All four response variables indicated that the mosquito flight and landing behavior was most affected by pyrethroid resistance [knockdown resistance (kdr)] status. In other experiments, mosquitoes were surgically altered, with antennae ablated bilaterally, and these mosquitoes were more likely to stay on the treated surfaces for longer periods of time, irrespective of any chemical exposure. There were also differences in the responses to antennal ablation between the two strains of mosquitoes, indicating that resistance factors, probably kdr, influence the reactivity of mosquitoes to pyrethroid and DDT treatments, and that it was not completely negated by antennal ablation. CONCLUSIONS: These findings confirm the role of antennal olfactory components in the expression of excito-repellent behaviors, and also support the hypothesis that excito-repellency from pyrethroid/DDT exposure is probably due to a combination of sublethal neurotoxic excitation and interactions with the olfactory system. © 2023 Society of Chemical Industry.

Repellency and Toxicity of Vapor-Active Benzaldehydes against Aedes aegypti.

Chemical screening efforts recently found that 3-phenoxybenzaldehyde, a breakdown product of alpha-cyano pyrethroids, was a potent spatial repellent against Aedes aegypti mosquitoes in a glass tube repellency assay. In order to characterize this molecule further and identify structure-activity relationships, a set of 12 benzaldehyde analogues were screened for their repellency and toxicity in vapor phase exposures at 100 µg/cm2. Dose-response analyses were performed for the most active compounds in order to better characterize their repellent potency and toxicity compared to those of other commercially available toxicants. The three most toxic compounds (LC50 values) were 3-chlorobenzaldehyde (CBA) (37 µg/cm2), biphenyl-3-carboxaldehyde (BCA) (48 µg/cm2), and 3-vinylbenzaldehyde (66 µg/cm2), which makes them less toxic than bioallethrin (6.1 µg/cm2) but more toxic than sandalwood oil (77 µg/cm2), a repellent/toxic plant essential oil. The most repellent analogues with EC50 values below 30 µg/cm2 were 3-phenoxybenzaldehyde (6.3 µg/cm2), isophthalaldehyde (23 µg/cm2), BCA (17 µg/cm2), and CBA (22 µg/cm2), which makes them about as active as N,N-diethyl-3-methylbenzamide (25.4 µg/cm2). We further investigated the activity of a select group of these benzaldehydes to block the firing of the central nervous system of A. aegypti larvae. Compounds most capable of repelling and killing mosquitoes in the vapor phase were also those most capable of blocking nerve firing in the larval mosquito nervous system. The results demonstrate that benzaldehyde analogues are viable candidate repellent and insecticidal molecules and may lead to the development of future repellent and vapor toxic vector control tools.

Structural Exploration of Novel Pyrethroid Esters and Amides for Repellent and Insecticidal Activity against Mosquitoes.

The emergence of pyrethroid-resistant mosquitoes is a worldwide problem that necessitates further research into the development of new repellents and insecticides. This study explored the modification of existing pyrethroid acids to identify structural motifs that might not be affected by kdr active site mutations that elicit pyrethroid resistance. Because synthetic pyrethroids almost always contain activity-dependent chiral centers, we chose to focus our efforts on exploring alkoxy moieties of esters obtained with 1R-trans-permethrinic and related acids, which we showed in previous studies to have repellent and/or repellent synergistic properties. To this end, compounds were synthesized and screened for spatially acting repellency and insecticidal activity against the susceptible, Orlando, and pyrethroid-resistant, Puerto Rico, strains of Aedes aegypti mosquito. Screening utilized a high-throughput benchtop glass tube assay, and the compounds screened included a mixture of branched, unbranched, aliphatic, halogenated, cyclic, non-cyclic, and heteroatom-containing esters. Structure-activity relationships indicate that n-propyl, n-butyl, n-pentyl, cyclobutyl, and cyclopentyl substituents exhibited the most promising repellent activity with minimal kdr cross resistance. Preliminary testing showed that these small alcohol esters can be synergistic with phenyl amides and pyrethroid acids. Further derivatization of pyrethroid acids offer an interesting route to future active compounds, and while mosquitoes were the focus of this work, pyrethroid acids and esters have potential for use in reducing pest populations and damage in cropping systems as well.

Fir (Abies balsamea) (Pinales: Pinaceae) needle essential oil enhances the knockdown activity of select insecticides.

Because of the increased interest in plant essential oils (PEO) for both home pest control and personal bite protection, the ability of fir needle (Abies balsamea) oil to synergize the 1-h knockdown and 24-h toxicity of 9 different synthetic insecticides was evaluated. Fir needle oil strongly synergized knockdown of the neonicotinoids, clothianidin, and thiamethoxam (between 16- and 24-fold), as well as natural pyrethrins (12-fold), but had less effect with organophosphates and fipronil. For 24-h mortality, only pirimiphos-methyl was strongly synergized by fir needle oil pretreatment (18-fold). Chemical analysis and testing identified delta-3-carene is the most bioactive constituent, producing synergism similar to that of the whole oil. In fact, this constituent synergized the 24-h mortality of clothianidin to a higher degree than fir needle oil itself (4.9-fold vs. 2.4-fold). Synergism is unlikely to be mediated by effects on the nervous system, as fir needle oil caused no change in mosquito central nervous system firing at 100 ppm and did not synergize an inactive concentration of natural pyrethrins (10 nM). To better understand fir needle oil effects, we evaluated the ability of pretreatment with this oil to impact Aedes aegypti monooxygenase degradation of a model substrate, 7-ethoxycoumarin. Interestingly, both fir needle oil and delta-3-carene caused a significant increase in metabolic degradation of 7-ethoxycoumarin, perhaps indicating they upregulate oxidative metabolic processes. Such an action would explain why fir needle oil enhances knockdown, but not 24-h mortality for most of the insecticides studied here, whereas increased bioactivation would explain the synergism of pirimiphos-methyl toxicity.

Sodium channel-directed alkaloids synergize the mosquitocidal and neurophysiological effects of natural pyrethrins.

We explored the potential of two sodium channel activators, veratrine and aconitine, as both insecticides and synergists of natural pyrethrins (NP) on Aedes aegypti adults and larvae. Aconitine was more toxic than veratrine, with an LD50 of 157 ng/mg compared to 376 ng/mg, on the pyrethroid-susceptible Orlando strain, but only aconitine showed significant resistance in the pyrethroid-resistant Puerto Rico strain (RR = 14.6 in topical application and 8.8 in larval bioassay). When applied in mixtures with piperonyl butoxide (PBO) and NP, large synergism values were obtained on the Orlando strain. Aconitine + PBO mixture synergized NP 21.8-fold via topical adult application and 10.2-fold in larval bioassays, whereas veratrine + PBO synergized NP 5.3-fold via topical application and 30.5-fold in larval bioassays. Less synergism of NP was observed on the resistant Puerto Rico strain, with acontine + PBO synergizing NP only 4.1-fold in topical application (8-fold in larval bioassays) and veratrine + PBO synergizing NP 9.5-fold in topical application (13.3-fold in larval bioassays). When alkaloids were applied directly to the mosquito larval nervous system, veratrine was nearly equipotent on both strains, while aconitine was less active on pyrethroid-resistant nerve preparations (no block at 10 µM compared to block at 1 µM on the susceptible strain). The nerve blocking effect of NP was significantly synergized by both compounds on the pyrethroid-susceptible strain by about 10-fold, however only veratrine synergized NP block on the pyrethroid-resistant strain, also showing 10-fold synergism). These results highlight the potential of site II sodium channel activators as insecticides and their ability to synergize pyrethroids, which may extend the commercial lifetime of these chemistries so essential to public health vector control.

Mode of action and toxicological effects of the sesquiterpenoid, nootkatone, in insects.

Nootkatone, a sesquiterpenoid isolated from Alaskan yellow cedar (Cupressus nootkatensis), is known to possess insect repellent and acaricidal properties and has recently been registered for commercial use by the Environmental Protection Agency. Previous studies failed to elucidate the mechanism of action of nootkatone, but we found a molecular overlay of picrotoxinin and nootkatone indicated a high degree of structural and electrostatic similarity. We therefore tested the hypothesis that nootkatone was a GABA-gated chloride channel antagonist, similar to picrotoxinin. The KD50 and LD50 of nootkatone on the insecticide-susceptible strain of Drosophila melanogaster (CSOR) showed resistance ratios of 8 and 11, respectively, compared to the cyclodiene-resistant strain of RDL1675, indicating significant cross-resistance. Nootkatone reversed GABA-mediated block of the larval CSOR central nervous system; nerve firing of 78 ± 17% of baseline in the CSOR strain was significantly different from 24 ± 11% of baseline firing in the RDL1675 strain (p = 0.035). This finding indicated that the resistance was expressed within the nervous system. Patch clamp recordings on D. melanogaster central neurons mirrored extracellular recordings where nootkatone inhibited GABA-stimulated currents by 44 ± 9% at 100 µM, whereas chloride current was inhibited 4.5-fold less at 100 µM in RDL1675. Taken together, these data suggest nootkatone toxicity in D. melanogaster is mediated through GABA receptor antagonism.

Honey Bee (Apis mellifera) Exposure to Pesticide Residues in Nectar and Pollen in Urban and Suburban Environments from Four Regions of the United States.

The risk of honey bee (Apis mellifera L.) exposure to pesticide residues while foraging for nectar and pollen is commonly explored in the context of agroecosystems. However, pesticides are also used in urban and suburban areas for vegetation management, vector control, and the management of ornamental plants in public and private landscapes. The extent to which pesticides pose a health risk to honey bees in these settings remains unclear. We addressed this at a landscape scale by conducting pesticide residue screening analyses on 768 nectar and 862 pollen samples collected monthly over 2 years from honey bee colonies located in urban and suburban areas in eight medium to large cities in California, Florida, Michigan, and Texas (USA). A risk assessment was performed using the US Environmental Protection Agency's BeeREX model whenever an oral toxicity value was available for a compound. Chemical analyses detected 17 pesticides in nectar and 60 in pollen samples during the survey. Approximately 73% of all samples contained no detectable pesticide residues. Although the number of detections varied among the sampled regions, fewer pesticides were detected in nectar than in pollen. Per BeeREX, four insecticides showed a potential acute risk to honey bees: imidacloprid, chlorpyrifos, and esfenvalerate in nectar, and deltamethrin in nectar and pollen. In general, exposure of honey bees to pesticides via nectar and pollen collection was low in urban and suburban areas across the United States, and no seasonal or spatial trends were evident. Our data suggest that honey bees are exposed to fewer pesticides in developed areas than in agricultural ones. Environ Toxicol Chem 2022;41:991-1003. © 2022 SETAC.

Studies on the Volatiles Composition of Stored Sheep Wool, and Attractancy toward Aedes aegypti Mosquitoes.

To discover new natural materials for insect management, commercially available stored sheep wool was investigated for attractancy to female adult Aedes aegypti mosquitoes. The volatiles from sheep wool were collected by various techniques of headspace (HS) extractions and hydrodistillation. These extracts were analyzed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-flame ionization detector (GC-FID) coupled with GC-MS. Fifty-two volatile compounds were detected, many of them known for their mosquito attractant activity. Seven compounds were not previously reported in sheep products. The volatile composition of the extracts varied significantly across collections, depending on the extraction techniques or types of fibers applied. Two types of bioassay were conducted to study attractancy of the sheep wool volatiles to mosquitoes: laboratory bioassays using glass tubes, and semi-field bioassays using large, screened outdoor cages. In bioassays with glass tubes, the sheep wool hydrodistillate and its main component, thialdine, did not show any significant attractant activity against female adult Ae. aegypti mosquitoes. Semi-field bioassays in two large screened outdoor cages, each equipped with a U.S. Centers for Disease Control (CDC) trap and the various bait setups with Vortex apparatus, revealed that vibrating wool improved mosquito catches compared to the setups without wool or with wool but not vibrating. Sheep wool, when vibrated, may release intensively volatile compounds, which could serve as olfactory cues, and play significant role in making the bait attractive to mosquitoes. Sheep wool is a readily available, affordable, and environment-friendly material. It should have the potential to be used as a mosquito management and surveillance component in dynamic bait setups.

Nutritional status significantly affects toxicological endpoints in the CDC bottle bioassay.

BACKGROUND: The CDC Bottle Bioassay serves as an inexpensive and effective way to screen field-caught mosquitoes against a wide variety of insecticidal active ingredients and commercial formulations, with the goal of detecting resistant individuals. For this study, we assessed how sucrose-water (10% w/v) feeding status impacted the response of Aedes aegypti mosquitoes to select insecticides. RESULTS: Starvation for 24 or 48 h decreased permethrin and malathion mean survival time by about 40%, with little difference in the two starvation times. Similar findings were also observed in a pyrethroid-resistant Puerto Rico strain challenged with permethrin, but these effects were less pronounced. To test the impact of mosquito weight, we measured weight under different 48-h nutritional conditions and found that sugar-water-fed and sugar-only-fed individuals were approximately the same weight (ANOVA, Bonferroni post-test P value >0.999) and that individuals fed water only were considerably lighter than either sugar-water-fed (ANOVA, Bonferroni post-test P value = 0.034) or sugar-only-fed individuals (ANOVA, Bonferroni post-test P value = 0.027) but equal in weight to starved mosquitoes (ANOVA, Bonferroni post-test P value >0.99). Of the nutritional challenges, water-only-fed mosquitoes were the most insecticide tolerant (ANOVA, Bonferroni post-test P values to all other treatments <0.01). CONCLUSIONS: The results indicate insect nutritional status is an important experimental variable, particularly the hydration status of mosquitoes shortly before insecticide exposure. Moreover, as significant differences were observed between starved and component-fed (water, sugar, or sugar and water) mosquitoes in a pyrethroid-resistant strain, some caution is appropriate when interpreting resistance/susceptibility diagnoses with this bioassay. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Testing new compounds for efficacy against Varroa destructor and safety to honey bees (Apis mellifera).

BACKGROUND: Varroa destructor is among the greatest threats to honey bee health worldwide. Acaricides used to control Varroa are becoming increasingly ineffective due to resistance issues, prompting the need for new compounds that can be used for control purposes. Ideally, such compounds would exhibit high toxicity to Varroa while maintaining relatively low toxicity to bees and beekeepers. We characterized the lethal concentrations (LC50 ) of amitraz, matrine, FlyNap®, the experimental carbamates 2-((2-ethylbutyl)thio)phenyl methylcarbamate (1) and 2-(2-ethylbutoxy)phenyl methylcarbamate (2), and dimethoate (positive control) for Varroa using a glass vial assay. The test compounds also were applied to honey bees using an acute contact toxicity assay to determine the adult bee LD50 for each compound. RESULTS: Amitraz was the most toxic compound to Varroa, but carbamate 2 was nearly as active (within 2-fold) and the most selective due to its lower bee toxicity, demonstrating its promise as a Varroa control. While carbamate 1 was less toxic to honey bees than was amitraz, it was also 4.7-fold less toxic to the mites. Both matrine and FlyNap® were relatively ineffective at killing Varroa and were moderately toxic to honey bees. CONCLUSION: Additional testing is required to determine if carbamate 2 can be used as an effective Varroa control. As new chemical treatments are identified, it will be necessary to determine how they can be utilized best alongside other control techniques as part of an integrated pest management program. © 2021 Society of Chemical Industry.

Recording central neurophysiological output from mosquito larvae for neuropharmacological and insecticide resistance studies.

Resistance to currently utilized chemical insecticidal agents represents a significant threat to public health and food security worldwide. Better understanding the neurophysiological effects of available and candidate insecticidal molecules is valuable for characterizing the mechanisms of insecticide resistance, as well as the design and study of novel control chemistries. In this paper, we describe a method of recording nerve firing from the central nervous system of Aedes aegypti fourth instar larvae. In short, mosquito larvae were immobilized by placing small pins through the head and siphon of the larvae in a wax dish, ventral side down. A single, longitudinal, dorsal incision from the distal abdomen to the pronotum of the larva was made, the alimentary canal removed, and the ventral nerve cord severed between the second and third abdominal ganglia. A recording suction electrode was connected directly to axons within the severed end of the connective in a novel way to record nerve firing in the ventral nerve cord at a high signal-to-noise ratio with conventional electrophysiological equipment. Using this novel method, we report the effects of four neuroactive compounds using this method: octopamine, pilocarpine, nicotine, and γ-aminobutyric acid (GABA). The utility of this recording technique for elucidating target site mechanisms involved in insecticide resistance is demonstrated with p,p'-dichlorodiphenyltrichlorethane (DDT) and its difluoro analog (DFDT).

Toxicity and mode of action of the aporphine plant alkaloid liriodenine on the insect GABA receptor.

Liriodenine is a biologically active plant alkaloid with multiple effects on mammals, fungi, and bacteria, but has never been evaluated for insecticidal activity. Accordingly, liriodenine was applied topically in ethanolic solutions to adult female Anopheles gambiae, and found to be mildly toxic. Its lethality was synergized in mixtures with dimethyl sulfoxide and piperonyl butoxide. Recordings from the ventral nerve cord of larval Drosophila melanogaster showed that liriodenine was neuroexcitatory and reversed the inhibitory effect of 1 mM GABA at effective concentrations of 20-30 µM. GABA antagonism on the larval nervous system was equally expressed on both susceptible and cyclodiene-resistant rdl preparations. Acutely isolated neurons from Periplaneta americana were studied under patch clamp and inhibition of GABA-induced currents with an IC50 value of about 1 µM were observed. In contrast, bicuculline did not reverse the effects of GABA on cockroach neurons, as expected. In silico molecular models suggested reasonable structural concordance of liriodenine and bicuculline and isosteric hydrogen bond acceptor sites. This study is the first assessing of the toxicology of liriodenine on insects and implicates the GABA receptor as one likely neuronal target, where liriodenine might be considered an active chemical analog of bicuculline.

Enhanced pyrethroid potency in Drosophila melanogaster expressing voltage-gated potassium channel mutants: Insecticidal activity and neuronal action.

Previous studies have shown that blockers of voltage-gated potassium (Kv) channels, such as 4-aminopyridine (4-AP) and 2-methoxy-N-((1-phenylcyclopentyl)methyl)benzamide (2-MPB) synergized pyrethroid toxicity as well, or better than, piperonyl butoxide. The present study assessed the involvement of different Kv channels as possible pyrethroid synergist targets in Drosophila melanogaster. Three Kv1 mutants (Sh5, Sh133, and ShM) and one Kv2 mutant (Shab3) were tested. All Kv1 mutant flies showed increased sensitivity to permethrin in topical and glass contact toxicity assays, of 2- to 11-fold. Central nervous system (CNS) recordings of larval D. melanogaster showed a similar pattern of increased sensitivity. Potentiated effects were also observed with deltamethrin on the mutants Sh5 (30- to 35-fold) and Sh133 (33- to 47-fold), but the mutant ShM showed little change in sensitivity. In contrast, the Shab3 strain showed toxicity and physiological effects of both pyrethroids that were similar to the susceptible OR strain. Thus, some K+ channel mutations mimicked the synergistic effect of channel blockers. Additional studies showed that Shab3 had the highest sensitivity to 4-AP in topical assays, and the Shaker-null mutants, ShM and Sh133 showed greater sensitivity to 2-MPB in CNS recordings of larval D. melanogaster. These results suggest that Kv1 channels are a useful synergist target for pyrethroids, as assessed both in whole insects and at the level of the nervous system. Thus, Kv1-targeting compounds can potentially serve as insect control tools to reduce pyrethroid use via synergistic action.

Resistance-Breaking Insecticidal Activity of New Spatial Insecticides against Aedes aegypti.

The use of N-aryl amide derivatives as spatially acting insecticides remains relatively unexplored. To expand this knowledge, we synthesized eighty-nine N-aryl amide analogues and screened them for mortality against an insecticide-susceptible strain of Aedes aegypti mosquitoes, Orlando (OR), using a vapor exposure glass tube assay. Of the screened compounds, twenty-two produced >92% mortality at 24 h and warranted further investigation to determine LC50 values. Fifteen of these analogues had LC50 values within 2 orders of magnitude of transfluthrin, and of significant interest, N-(2,6-dichloro-4-(trifluoromethyl)phenyl)-2,2,3,3,3-pentafluoropropanamide (compound 70) was nearly as potent as transfluthrin and exhibited greater toxicity than metofluthrin when screened against OR A. aegypti. Compounds exhibiting potent toxicity against OR A. aegypti or whose structure-activity relationship potentially offered beneficial insights into structure optimization were screened against the insecticide-resistant, Puerto Rico (PR), strain of A. Aegypti, and it was discovered that not only did these N-arylamides typically show little resistance, some such as N-(2,6-dichloropyridin-4-yl)-2,2,3,3,4,4,4-heptafluorobutanamide (compound 36) and 2,2,3,3,4,4,4-heptafluoro-N-(3,4,5-trifluorophenyl)butanamide (compound 40) were actually more potent against the PR mosquitoes. Due to this promising insecticidal activity, five compounds were administered orally to mice to determine acute oral rodent toxicity. All five compounds were found to have mouse oral toxicity LD50 values well above the minimum safe level as set by the Innovative Vector Control Consortium (50 mg/kg). In addition to the promising biological activity documented here, we report the structure-activity relationship analysis used to guide the derivatization approach taken and to further inform future efforts in the development of N-arylamides as potential resistance-breaking, spatially acting insecticides.

Co-Toxicity Factor Analysis Reveals Numerous Plant Essential Oils Are Synergists of Natural Pyrethrins against Aedes aegypti Mosquitoes.

With insecticide-resistant mosquito populations becoming an ever-growing concern, new vector control technologies are needed. With the lack of new chemical classes of insecticides to control mosquito populations, the development of novel synergists may improve the performance of available insecticides. We screened a set of 20 plant essential oils alone and in combination with natural pyrethrins against Aedes aegypti (Orlando) female adult mosquitoes to assess their ability to synergize this natural insecticide. A co-toxicity factor analysis was used to identify whether plant oils modulated the toxicity of natural pyrethrins antagonistically, additively, or synergistically. Both knockdown at 1 h and mortality at 24 h were monitored. A majority of oils increased the toxicity of natural pyrethrins, either via an additive or synergistic profile. Many oils produced synergism at 2 µg/insect, whereas others were synergistic only at the higher dose of 10 µg/insect. Amyris, cardamom, cedarwood, and nutmeg East Indies (E.I.) oils were the most active oils for increasing the mortality of natural pyrethrins at 24 h with co-toxicity factors greater than 50 at either or both doses. A number of oils also synergized the 1 h knockdown of natural pyrethrins. Of these, fir needle oil and cypress oils were the most successful at improving the speed-of-action of natural pyrethrins at both doses, with co-toxicity factors of 130 and 62, respectively. To further assess the co-toxicity factor method, we applied selected plant essential oils with variable doses of natural pyrethrins to calculate synergism ratios. Only the oils that produced synergistic co-toxicity factors produced statistically significant synergism ratios. This analysis demonstrated that the degree of co-toxicity factor correlated well with the degree of synergism ratio observed (Pearson correlation coefficient r = 0.94 at 2 µg/insect; r = 0.64 at 10 µg/insect) and that the co-toxicity factor is a useful tool in screening for synergistic activity.

Development of an a priori computational approach for brain uptake of compounds in an insect model system.

Delivery of compounds to the brain is critical for the development of effective treatment therapies of multiple central nervous system diseases. Recently a novel insect-based brain uptake model was published utilizing a locust brain ex vivo system. The goal of our study was to develop a priori, in silico cheminformatic models to describe brain uptake in this insect model, as well as evaluate the predictive ability. The machine learning program Orange® was used to evaluate several machine learning (ML) models on a published data set of 25 known drugs, with in vitro data generated by a single laboratory group to reduce inherent inter-laboratory variability. The ML models included in this study were linear regression (LR), support vector machines (SVN), k-nearest neighbor (kNN) and neural nets (NN). The quantitative structure-property relationship models were able to correlate experimental logCtot (concentration of compound in brain) and predicted brain uptake of r2 > 0.5, with the descriptors log(P*MW-0.5) and hydrogen bond donor used in LR, SVN and KNN, while log(P*MW-0.5) and total polar surface area (TPSA) descriptors used in the NN models. Our results indicate that the locust insect model is amenable to data mining chemoinformatics and in silico model development in CNS drug discovery pipelines.

Heterocyclic Amine-Induced Feeding Deterrence and Antennal Response of Honey Bees.

The productivity and survival of managed honey bee colonies is negatively impacted by a diverse array of interacting factors, including exposure to agrochemicals, such as pesticides. This study investigated the use of volatile heterocyclic amine (HCA) compounds as potential short-term repellents that could be employed as feeding deterrents to reduce the exposure of bees to pesticide-treated plants. Parent and substituted HCAs were screened for efficacy relative to the repellent N,N-diethyl-meta-toluamide (DEET) in laboratory and field experiments. Additionally, electroantennogram (EAG) recordings were conducted to determine the level of antennal response in bees. In video-tracking recordings, bees were observed to spend significantly less time with an HCA-treated food source than an untreated source. In a high-tunnel experiment, the HCA piperidine was incorporated in a feeding station and found to significantly reduce bee visitations relative to an untreated feeder. In field experiments, bee visitations were significantly reduced on melon flowers (Cucumis melo L.) and flowering knapweed (Centaurea stoebe L.) that were sprayed with a piperidine solution, relative to untreated plants. In EAG recordings, the HCAs elicited antennal responses that were significantly different from control or vehicle responses. Overall, this study provides evidence that HCAs can deter individual bees from food sources and suggests that this deterrence is the result of antennal olfactory detection. These findings warrant further study into structure-activity relationships that could lead to the development of short-term repellent compounds that are effective deterrents to reduce the contact of bees to pesticide-treated plants.

Synergistic effects of potassium channel blockers and pyrethroids: mosquitocidal activity and neuronal mode of action†.

BACKGROUND: The purpose of this research was to explore the possibility of co-applying pyrethroids (agonists of voltage-sensitive sodium channels) with potassium channel blockers in order to potentiate the neurological effects of pyrethroids on Anopheles gambiae. We hypothesized that the toxicity of pyrethroids caused by persistent sodium currents would be augmented by blockage of outward potassium current flow, which normally repolarizes the membrane potential during a nerve membrane action potential. RESULTS: Topical treatments with LD10 s (10% mortality doses) of synergists were given with pyrethroids. 2S-65465 (2S) showed the best synergism of permethrin (8.6-fold) and deltamethrin (7.2-fold), whereas piperonyl butoxide and 4-aminopyridine only showed 2.2- to 3.4-fold synergism with these pyrethroids. In electrophysiological recordings of Periplaneta americana giant axons, 2S (10 µm) and 4-AP (30 µm) caused multiple spikes after a single stimulation. Permethrin at 10 µm showed significant summating depolarization (4.5 ± 1.1 mV) after a train of ten stimuli were applied at 5 Hz, and deltamethrin at 0.03 µm showed significant membrane depolarization of 2.9 ± 0.4 mV without stimuli. 2S at 0.3 µm and 4-AP at 1-3 µm significantly synergized the effects of 3 µm permethrin and 0.01 µm deltamethrin. CONCLUSIONS: Co-application of potassium channel blockers 2S and 4-AP with pyrethroids can synergize the mosquitocidal activities on An. gambiae, and these activities are correlated with synergistic effects at the level of the nerve membrane. If deployed in the field, this approach can potentially reduce the amount of chemicals needed for effective control of mosquitoes. © 2020 Society of Chemical Industry.

Bioactivities and modes of action of VUAA1.

BACKGROUND: The compound 2-((4-ethyl-5-(pyridin-3-yl)-4H-1,2,4-triazol-3-yl)thio)-N-(4-ethylphenyl) acetamide (VUAA1) is reported to be an odorant receptor co-receptor (Orco) agonist in insects with potential use as an insect repellent. For this study, the biological activity of VUAA1 was investigated in several bioassays with Aedes aegypti, including adult contact, spatial repellency, and larval repellency assays, as well as topical, injection, and feeding toxicity assays. Neurophysiological action was further explored by analysis of fruit fly central nervous system firing, cockroach axon recordings, patch clamp analysis of Kv2 potassium channel, and acetylcholinesterase inhibition studies. Finally, the metabolic impact on the toxicity of VUAA1 was explored by applying it in combination with established metabolic synergists. RESULTS: In repellency and bite protection screens, VUAA1 showed little activity against adult mosquitoes, apparently due to its low volatility, since its effectiveness was increased by heating or mixing with transfluthrin acid and citronella oil. It did produce measurable repellency of mosquito larvae that was more potent than N,N-diethyl-m-toluamide (DEET). Overall, VUAA1 showed low acute toxicity to both insects and mice, and it was weakly synergized by triphenyl phosphate. There was no observed cross-resistance in a pyrethroid-resistant strain of Anopheles gambiae. VUAA1 showed a two-phase effect on the central nervous system, with neuroexcitation at 1 µmol L-1 and an inhibitory effect at 100 µmol L-1 that may relate to block of Kv2 potassium channels. CONCLUSIONS: VUAA1 presented low toxicity, similar to other insect repellents. Its limited solubility, low volatility, and resulting poor adult repellency without additional adjuvants may restrict the utility of VUAA1 in typical public health applications. © 2020 Society of Chemical Industry.