Enhanced repellent and anti-nutritional activities of polymeric nanoparticles containing essential oils against red flour beetle, Tribolium castaneum.

Encapsulation of essential oils (EOs) is an important strategy that can be applied to intensify the stability and efficiency of these compounds in integrated pest management. The present study aimed to investigate the sub-lethal activity of polymer-based EOs nanoparticles against red flour beetle, Tribolium castaneum adults as an important critical pest of stored products. Chitosan nanoparticles (CSNPs) containing garlic and cinnamon essential oils (GEO and CEO) prepared using the ionic cross-link technique. Stability of nano-formulations evaluated over temperature and storage time. The fumigant effect (LC10, LC20, LC30) and contact toxicity (LC10, LC15, LC25) determined. In addition, the contact toxicities of EOs and their nanoparticles on nutritional indices evaluated. An olfactometer used to assess the repellent activity of EOs and EOs loaded in CSNPs (EOs@CSNPs) in sub-lethal fumigant concentrations. Characterization results showed GEO loaded in CSNPs has particle size of 231.14 ± 7.55 nm, polydispersity index (PDI) value of 0.15 ± 0.02, encapsulation efficiency (EE) percentage of 76.77 ± 0.20 and zeta potential of - 18.82 ± 0.90 mV, in which these values for the CEO loaded in CSNPs (CEO@CSNPs) changed to 303.46 ± 0.00 nm, 0.20 ± 0.05, 86.81 ± 0.00% and - 20.16 ± 0.35 mV, respectively. A lower PDI value for both CSNPs showed an appropriate NPs size distribution. Furthermore, NPs size and encapsulation efficiency did not change in various temperatures and during four months which confirm good stability of the EOs@CSNPs. In LC30 of GEO@CSNPs, the maximum repellency was determined as 66.66 ± 3.33. Among nutritional indices, in LC25 of GEO@CSNPs, the relative growth rate (RGR) (0.011 ± 0.003 mg.mg-1.day-1), relative consumption rate (RCR) (0.075 ± 0.004 mg.mg-1.day-1) and feeding deterrence index (FDI) (54.662 ± 1.616%) were more affected, so GEO@CSNPs was more effective than CEO@CSNPs. The results of repellent and anti-dietary activities of EOs and EOs@CSNPs confirmed the higher repellency and adverse effectivity on nutritional indices of Tribolium castaneum pest treated with EOs@CSNPs compared to free EOs. In conclusion, the NPs form of GEO and CEO can be a novel and efficient carrier for improving the repellent and anti-nutritional activities of EOs.

Synergism of (E)-ß-farnesene and Its Analogue to Insecticides against the Green Peach Aphid Myzus persicae.

With high aphid-repellent activity but low stability, (E)-ß-farnesene (EßF), the major component of the aphid alarm pheromone, can be used as a synergist to insecticides. Some EßF analogues possess both good aphid-repellent activity and stability, but the synergistic effect and related mechanism are still unclear. Therefore, this study investigated the synergistic effect and underlying mechanism of the EßF and its analogue against the aphid Myzus persicae. The results indicated that EßF and the analogue showed significantly synergistic effects to different insecticides, with synergism ratios from 1.524 to 3.446. Mechanistic studies revealed that EßF and the analogue exhibited effective repellent activity, significantly upregulated target OBP genes by 161 to 731%, increased aphid mobility, and thereby enhanced contact with insecticides. This research suggests that the EßF analogue represents a novel synergist for insecticides, with the potential for further application in aphid control owing to its enhanced bioactivity and the possibility of reducing insecticide doses.

Repellency, Toxicity, and Chemical Composition of Plant Essential Oils from Myrtaceae against Asian Citrus Psyllid, Diaphorina citri Kuwayama (Hemiptera Liviidae).

Diaphorina citri Kuwayama (D. citri) is one of the major pests in the citrus industry, which spreads Citrus Huanglongbing disease. It has developed resistance to chemical insecticides. Therefore, searching for greener solutions for pest management is critically important. The main aim of this study was to evaluate the repellent and insecticidal efficacy of essential oils (EOs) from four species of Myrtaceae plants: Psidium guajava (PG), Eucalyptus robusta (ER), Eucalyptus tereticornis (ET), and Baeckea frutescens (BF) against D. citri and to analyze their chemical compositions. GC-MS analysis was performed, and the results indicated that the EOs of PG, ER, ET, and BF were rich in terpenoids, ketones, esters, and alcohol compounds. The repellent rate of all four EOs showed that it decreased with exposure time but increased with the concentration of EOs from 80.50% to 100.00% after treating D. citri for 6 h with four EOs at 100% concentration and decreased to 67.71% to 85.49% after 24 h of exposure. Among the compounds from the EOs tested, eucalyptol had the strongest repellent activity, with a 24 h repellency rate of 100%. The contact toxicity bioassay results showed that all EOs have insecticidal toxicity to D. citri; the LC50 for nymphs was 36.47-93.15 mL/L, and for adults, it was 60.72-111.00 mL/L. These results show that when PG is used as the reference material, the ER, ET, and BF EOs have strong biological activity against D. citri, which provides a scientific basis for the further development of plant-derived agrochemicals.

Chemical Composition, Repellent, and Oviposition Deterrent Potential of Wild Plant Essential Oils against Three Mosquito Species.

In this study, the chemical composition, repellent, and oviposition deterrent effects of five plant essential oils (EOs) extracted from Lantana camara (Verbenaceae), Schinus terebinthifolia (Anacardiaceae), Callistemon viminalis (Myrtaceae), Helichrysum odoratissimum (Asteraceae), and Hyptis suaveolens (Lamiaceae) were evaluated against Aedes aegypti, Anopheles gambiae, and Culex quinquefasciatus. When tested at 33.3 µg/cm2, L. camara, S. terebinthifolia, C. viminalis, and H. odoratissimum were effective repellents against Ae. aegypti (89%, 91%, 90%, and 51% repellency, respectively), but they were less repellent against An. gambiae (66%, 86%, 59%, and 49% repellency, respectively). Interestingly, L. camara, S. terebinthifolia, C. viminalis, and H. odoratissimum exhibited 100% repellency against Cx. quinquefasciatus at 33.3 µg/cm2. In time-span bioassays performed at 333 µg/cm2, the EO of L. camara exhibited 100% repellence against Ae. aegypti and An. gambiae for up to 15 min and against Cx. quinquefasciatus for 75 min. The oviposition bioassays revealed that L. camara exhibited the highest activity, showing 85%, 59%, and 89% oviposition deterrence against Ae. aegypti, An. gambiae, and Cx. quinquefasciatus, respectively. The major compounds of L. camara, S. terebinthifolia, and C. viminalis were trans-ß-caryophyllene (16.7%), α-pinene (15.5%), and 1,8-cineole (38.1%), respectively. In conclusion, the L. camara and S. terebinthifolia EOs have the potential to be natural mosquito repellents.

Bioactivities and Synergistic Effect of Elsholtzia ciliata Essential Oil and Its Main Components against Lasioderma serricorne.

Investigations have shown that storage bugs seriously harm grains during storage. In the interim, essential oils (EOs) have been proven to be a good botanical pesticide. The anti-Lasioderma serricorne properties of Elsholtzia ciliata essential oil, which was obtained by steam distillation, were evaluated using DL-limonene, carvone, and their two optical isomer components using contact, repelling, and fumigation techniques. Simultaneously, the fumigation, contact, and repellent activities of carvone and its two optical isomers mixed with DL-limonene against L. serruricorne were evaluated. The results showed that E. ciliata, its main components (R-carvone, DL-limonene), and S-carvone exhibited both fumigations (LC50 = 14.47, 4.42, 20.9 and 3.78 mg/L) and contact (LD50 = 7.31, 4.03, 28.62 and 5.63 µg/adult) activity against L.serricorne. A binary mixture (1:1) of R-carvone and DL-limonene displayed an obvious synergistic effect. A binary mixture (1:1) of carvone and its two optical isomers exhibited an obvious synergistic effect, too. Furthermore, the repellent activity of the EO, carvone, and its two optical isomers, DL-limonene, and a combination of them varied. To stop insect damage during storage, E. ciliata and its components can be utilized as bio-insecticides.

Effects of natural Ionones and derived novel analogues with simplified structures on behavioral responses of whiteflies.

BACKGROUND: Whiteflies are major pests in agriculture, causing damage to crops and transmitting plant viruses. Using Volatile Organic Compounds (VOCs) as semiochemicals offers a sustainable approach for combating whiteflies. One such group of compounds, represented by ß-ionone, has been found to possess repellent/attractant properties. To further explore the behavioral effects of these compounds on whiteflies, we selected five natural ionone compounds and synthesized six novel analogues to examine the impact of structural variations on whitefly behavior. RESULTS: Our results demonstrated that ß-ionone and its analogues have a significant impact on the behavior of whiteflies. Among them, 0.01% pseudo ionone solution exhibited an attractant effect on whiteflies. Notably, the application of 1% ß-ionone and 0.1% ß-ionol solution demonstrated a notable repellent effect and oviposition deterrent effect on whitefly. We also found that the novel ionone analogue (±)1A exhibited a strong repellent effect. Both ß-ionol and compound (±)1A possess high logP values and low saturation vapor pressures, which contribute to enhanced lipophilicity, making them more likely to penetrate insect antennae and prolong their presence in the air. CONCLUSION: The newly discovered ionone analogue (±)1A and ß-ionol exhibit improved repellent effects, while pseudo ionone shows an attractant effect. These three compounds hold promising potential for development as novel biological control agents. Our work highlights the efficacy of VOCs as a protection method against whiteflies. These findings indicate that our new technology for a 'push-pull' control method of B. tabaci can offer a novel tool for integrated pest management (IPM). © 2024 Society of Chemical Industry.

Insecticidal action, repellency, and toxicity mechanism of the essential oil of Lippia turbinata against the stored product pest Rhipibruchus picturatus (F.).

The use of essential oils (EOs) in the development of alternative management methods for bruchid control under storage conditions aroused great interest because they have proven to be effective, less toxic, and less persistent in the ecosystem than synthetic pesticides. In this sense, leaves of Lippia turbinata (Griseb.) Moldenke EO were studied in the present work. The monoterpene limonene and the monoterpenoid eucalyptol were its main constituents. EO showed a potent insecticidal activity, both in contact and fumigant conditions, against Rhipibruchus picturatus (F.) which is one of the main pests of Prosopis alba pods in stored conditions. Moreover, the EO produces repellency in these insects. Additionally, the toxicity mechanism of action was studied. In this regard, the EO inhibits the acetylcholinesterase enzyme in in vitro assays, alters the activity of the antioxidant enzymes superoxide dismutase and catalase, and produces an increase in the lipid peroxidation reactions. This is the first report of the use of the L. turbinata EO against R. picturatus insect pest. The data obtained demonstrate its potential for developing more efficient and natural storage pest control strategies.

The Repellent Capacity against Sitophilus zeamais (Coleoptera: Curculionidae) and In Vitro Inhibition of the Acetylcholinesterase Enzyme of 11 Essential Oils from Six Plants of the Caribbean Region of Colombia.

The repellent capacity against Sitophilus zeamais and the in vitro inhibition on AChE of 11 essential oils, isolated from six plants of the northern region of Colombia, were assessed using a modified tunnel-type device and the Ellman colorimetric method, respectively. The results were as follows: (i) the degree of repellency (DR) of the EOs against S. zeamais was 20-68% (2 h) and 28-74% (4 h); (ii) the IC50 values on AChE were 5-36 µg/mL; likewise, the %inh. on AChE (1 µg/cm3 per EO) did not show any effect in 91% of the EO tested; (iii) six EOs (Bursera graveolens-bark, B. graveolens-leaves, B. simaruba-bark, Peperomia pellucida-leaves, Piper holtonii (1b*)-leaves, and P. reticulatum-leaves) exhibited a DR (53-74%) ≥ C+ (chlorpyrifos-61%), while all EOs were less active (8-60-fold) on AChE compared to chlorpyrifos (IC50 of 0.59 µg/mL). Based on the ANOVA/linear regression and multivariate analysis of data, some differences/similarities could be established, as well as identifying the most active EOs (five: B. simaruba-bark, Pep. Pellucida-leaves, P. holtonii (1b*)-leaves, B. graveolens-bark, and B. graveolens-leaves). Finally, these EOs were constituted by spathulenol (24%)/ß-selinene (18%)/caryophyllene oxide (10%)-B. simaruba; carotol (44%)/dillapiole (21%)-Pep. pellucida; dillapiole (81% confirmed by 1H-/13C-NMR)-P. holtonii; mint furanone derivative (14%)/mint furanone (14%)-B. graveolens-bark; limonene (17%)/carvone (10%)-B. graveolens-leaves.

Essential oils extracted from Citrus macroptera and Homalomena aromatica (Spreng.) Schott. exhibit repellent activities against Aedes aegypti (Diptera: Culicidae).

BACKGROUND OBJECTIVES: Mosquitoes alone transmit diseases to around 700 million individuals annually, killing approximately 0.7 million people every year worldwide. Considering the potential health risks linked with synthetic repellents, it has become vital to identify eco-friendly, natural repellents for mosquito control as well as to understand the underlying mechanism for mosquito repellent activity. To address this, objectives were set to extract essential oils from Citrus macroptera peel and Homalomena aromatica (Spreng.) Schott. rhizomes, evaluate their mosquito repellent activity against Aedes aegypti, and further explore their mosquito odorant receptor inhibition potential. METHODS: The oils were extracted using Clevenger's apparatus, and properties like specific gravity, refractive index, and boiling point were evaluated and characterised using Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectroscopy (GC-MS). Aedes aegypti mosquito eggs collected from the Indian Council of Medical Research (ICMR), Dibrugarh, were reared in the Department of Pharmaceutical Sciences, Research Laboratory, to obtain adult Aedes aegypti mosquitoes for the mosquito repellent activity evaluation of the essential oils using the Human Bait technique'. Molecular docking studies were performed for the oil components against mosquito odorant binding proteins. Further, toxicity studies of these two oils were evaluated against human dermal fibroblast adult (HDFa) cells. RESULTS: The results revealed the presence of limonene (86.76%) and linalool (52.35%), respectively, in Citrus macroptera and Homalomena aromatica oils. It was found that the combination of the oils in a ratio of 1:1 showed mosquito repellent activity for up to 6.33 ± 0.23 h. Molecular docking studies showed the presence of major oil components having mosquito odorant receptor blocking potential comparable to N, N-diethyl-meta-toluamide (DEET), indicating a rationale for extended mosquito repellent action. Further, both of these oils were found to be non-cytotoxic against HDFa cells after 24 h. INTERPRETATION CONCLUSION: The encouraging mosquito repellent activity of these two oils as compared to synthetic mosquito repellent DEET might pave the way for the development of novel herbal mosquito repellent formulations containing these essential oils.

Chemical Composition of Essential Oils from Natural Populations of Artemisia scoparia Collected at Different Altitudes: Antibacterial, Mosquito Repellent, and Larvicidal Effects.

The current study aimed to evaluate the presence of chemical variations in essential oils (EOs) extracted from Artemisia scoparia growing at different altitudes and to reveal their antibacterial, mosquito larvicidal, and repellent activity. The gas chromatographic-mass spectrometric analysis of A. scoparia EOs revealed that the major compounds were capillene (9.6-31.8%), methyleugenol (0.2-26.6%), ß-myrcene (1.9-21.4%), γ-terpinene (1.5-19.4%), trans-ß-caryophyllene (0.8-12.4%), and eugenol (0.1-9.1%). The EO of A. scoparia collected from the city of Attock at low elevation was the most active against Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Pseudomonas aeruginosa bacteria (minimum inhibitory concentration of 156-1250 µg/mL) and showed the best mosquito larvicidal activity (LC50, 55.3 mg/L). The EOs of A. scoparia collected from the high-altitude areas of Abbottabad and Swat were the most repellent for females of Ae. aegypti and exhibited repellency for 120 min and 165 min, respectively. The results of the study reveal that different climatic conditions and altitudes have significant effects on the chemical compositions and the biological activity of essential oils extracted from the same species.

Chemical Composition, Repellent, and Phytotoxic Potentials of the Fractionated Resin Essential Oil from Araucaria heterophylla Growing in Tunisia.

The resin essential oil (REO) of the Tunisian Araucaria heterophylla trunk bark was investigated for its chemical composition. Then, it was evaluated for its insecticidal and allelopathic activities. The REO was obtained by hydrodistillation for 9 h (yield of 4.2 % w/w). Moreover, fractional hydrodistillation was carried out at 3-hour intervals, resulting in 3 fractions (R1-R3), to facilitate chemical identification and localization of the aforementioned biological activities. GC/MS analysis of the obtained samples allowed the identification of 25 compounds, representing between 91.2 and 96.3 % of their total constituents, which consisted predominantly of sesquiterpene hydrocarbons, oxygenated sesquiterpenes and diterpene hydrocarbons. α-Copaene (10.8 %), γ-muurolene (5.8 %), α-copaen-11-ol (7.8 %), spathulenol (10.5 %), 15-copaenol (8.2 %), ylangenal (10.3 %), dehydrosaussurea lactone (7.7 %), and sandaracopimaradiene (11.4 %) were identified as major compounds. The second part aimed to assess the impact of the A. heterophylla EO and its three fractions for their insecticidal and repellent activity against Tribolium castaneum (Herbst), a stored grain pest, of which a strong repellent activity was noted. In addition, the studied samples showed high phytotoxic effects against Lactuca sativa. The third fraction (R3) performed a total inhibitory potential on seed germination and seedling growth of the target plant. Furthermore, alongside this discovery, an estimation was conducted through molecular docking analysis. Wherein the main compounds of the studied samples were docked into the active pocket of protoporphyrinogen IX oxidase (PDB: 1SEZ), a key enzyme in chlorophyll biosynthesis. Thus, it is recommended to use the REO of A. heterophylla as a natural herbicide.

Chemical Composition, Toxicity, and Repellency of Essential Oils from Three Hedychium Species Against Stored-Product Insects.

Stored products are constantly infested by insects, so finding eco-friendly bioinsecticides for insect management is important. The work aimed to assess the insecticidal and repellent activity of essential oil (EO) from Hedychium glabrum S. Q. Tong, Hedychium coronarium Koen., and Hedychium yunnanense Gagnep. against Tribolium castaneum, Lasioderma serricorne, and Liposcelis bostrychophila. Results showed that 88 chemical components were identified in the extracted Hedychium EOs, indicating that they exhibited diversity in components. According to principal component analysis (PCA), the composition of the EO from the H. yunnanense stem and leaf (EOHYSL) was significantly different from other EOs due to the different organs and species. The biological activity also varied continuously with plant species and organs. Only the EO of H. yunnanense (EOHY) showed strong fumigant toxicity. While in the contact tests, EOHGR showed the strongest toxicity effect on L. bostrychophila, with a LC50 value of 71.76 µg/cm2, which was closest to the positive control (Pyrethrin). All EOs had remarkable repellent activities against the three target insects, and repellency increased with concentration. According to the results of the comprehensive score, EOHY had the highest potential, which ranged from 0.7999 to 0.8689. Thus, Hedychium EOs possess potential biorational traits to be biological insecticides.

Development of terpenoid repellents against Aedes albopictus: a combined study of biological activity evaluation and computational modelling.

To explore novel terpenoid repellents, 22 candidate terpenoid derivatives were synthesized and tested for their electroantennogram (EAG) responses and repellent activities against Aedes albopictus. The results from the EAG experiments revealed that 5-(2-hydroxypropan-2-yl)-2-methylcyclohex-2-en-1-yl formate (compound 1) induced distinct EAG responses in female Aedes albopictus. At concentrations of 0.1, 1, 10, 100, and 1000 mg/L, the EAG response values for compound 1 were 179.59, 183.99, 190.38, 193.80, and 196.66 mV, demonstrating comparable or superior effectiveness to DEET. Repellent activity analysis indicated significant repellent activity for compound 1, closest to the positive control DEET. The in silico assessment of the ADMET profile of compound 1 indicates that it successfully passed the ADMET evaluation. Molecular docking studies exhibited favourable binding of compound 1 to the active site of the odorant binding protein (OBP) of Aedes albopictus, involving hydrophobic forces and hydrogen bond interactions with residues in the OBP pocket. The QSAR model highlighted the influential role of hydrogen-bonding receptors, positively charged surface area of weighted atoms, polarity parameters of molecules, and maximum nuclear-nuclear repulsion force of carbon-carbon bonds on the relative EAG response values of the tested compounds. This study holds substantial significance for the advancement of new terpenoid repellents.

Discovery and properties of novel analogues of the aphid pheromones nepetalactone and nepetalactol.

BACKGROUND: Pheromones have unique advantages for pest control. Current aphid pheromone research focuses on alarm and sex pheromones. However, practical applications are limited so far, as (E)-ß-farnesene has only been investigated to a small extent as an alarm pheromone and only male aphids are targeted by sex pheromones. Previous literature reports electrophysiological responses and repellent behavior of asexual aphids to nepetalactone (1B), therefore our objective was to modify nepetalactone's structure to identify key fragments responsible for repellent effects, as guidance for subsequent modifications and further investigation. RESULTS: In this study, seven derivatives were designed and synthesized based on nepetalactol (1A) and nepetalactone (1B) as lead compounds. Free-choice tests, conducted using cowpea aphids (Aphis craccivora), revealed that the lactone moiety was crucial for the repellent activity, and the removal of the carbonyl group eliminated the repelling effect. Compound (±)1I, an analogue of nepetalactone (1B), demonstrated a significantly higher repellent value than nepetalactone (1B) at three different concentrations, and even at 0.1 mg/mL it maintained a considerable repellent effect (26.5%). Electrostatic potential and density functional theory calculations supported the importance of the carbonyl group for the repellent effects. CONCLUSION: The newly discovered para-pheromone (±)1I shows improved repellent effects and potential for development as a novel biological control agent. Based on our innovative findings, analogues with improved efficacy and properties can be designed and prepared. Our research contributes to understanding the effects of structural modifications on pheromone activity and properties, which is crucial for exploring novel pheromone-based products for crop protection. © 2024 Society of Chemical Industry.

The Odorant Receptor Recognizing Camphor in a Camphor Tree Specialist Orthaga achatina (Lepidoptera: Pyralidae).

Camphor has been used as an effective repellent and pesticide to stored products for a long history, but Orthaga achatina (Lepidoptera: Pyralidae) has evolved to specifically feed on the camphor tree Cinnamomum camphora. However, the behavioral response of O. achatina to camphor and the molecular basis of camphor perception are totally unknown. Here, we demonstrated that both male and female adults were behaviorally attracted to camphor, suggesting the adaptation of O. achatina to and utilization of camphor as a signal of C. camphora. Second, in 40 O. achatina OR genes obtained by analyzing antenna transcriptomes, only OachOR16/Orco significantly responded to camphor in the Xenopus oocyte system. Finally, by molecular docking analysis and site-directed mutagenesis, the Ser209 residue is confirmed to be essential for binding of the oachOR16 with camphor. This study not only reveals the camphor-based host plant choice and olfactory mechanisms of O. achatina but also provides a molecular target for screening more potential insect repellents.

Rational design, synthesis and binding mechanisms of novel benzyl geranate derivatives as potential eco-friendly aphid repellents.

BACKGROUND: The push-pull strategy is considered as a promising eco-friendly method for pest management. Plant volatile organic compounds (PVOCs) act as semiochemicals constitute the key factor in implementing this strategy. Benzyl alcohol and geraniol, as functional PVOCs, were reported to regulate insect behavior, showing the potential application in pest control. Using geraniol as lead, a geraniol derivative 5i with fine repellent activity was discovered in our previous work. In order to explore novel, eco-friendly aphid control agents, a series of benzyl geranate derivatives was designed and synthesized using 5i as the lead and benzyl alcohol as the active fragment. RESULTS: Benzyl alcohol was firstly evaluated to have repellent activity to Acyrthosiphon pisum. Based on this repellent fragment, a series of novel benzyl geranate derivatives was rationally designed and synthesized using a scaffold-hopping strategy. Among them, compound T9, with a binding affinity (Kd = 0.43 µm) and a substantial repellency of 64.7% against A. pisum, is the most promising compound. Molecule docking showed that hydrophobic and hydrogen-bonding interactions substantially influenced the binding affinity of compounds with ApisOBP9. Additionally, T9 exhibited low-toxicity to honeybees and ladybugs. CONCLUSION: Using a simple scaffold-hopping strategy combined with active fragment benzyl alcohol, a new derivative T9, with high aphid-repellency and low-toxicity to nontarget organisms, can be considered as a novel potential eco-friendly aphid control agent for sustainable agriculture. © 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.

The essential oil of Kochia scoparia (L.) Schrad. as a potential repellent against stored-product insects.

Chemical composition of the essential oil from Kochia scoparia (L.) Schrad. (syn. Bassia scoparia (L.) A. J. Scott) was analyzed in quality and quantity by GC-MS and GC-FID. Repellent activities of the essential oil from K. scoparia (KSEO) were evaluated against two common species of stored-product insects Tribolium castaneum Herbst and Liposcelis bostrychophila Badonnel. Results indicated that KSEO mainly consisted of eugenol, ß-caryophyllene, and α-humulene, accounting for 75.6%, 8.2%, and 1.4% of the total oil, respectively. KSEO and the three major components were repellent to T. castaneum and L. bostrychophila adults. Notably, KSEO exerted significant effects, comparable to the positive control DEET at 2 and 4 h post-exposure. Eugenol at 63.17-2.53 nL/cm2 exhibited high percentage repellency ranging from 96 to 70% against L. bostrychophila during 4-h exposure. To gain further insights into the repellent activity, molecular docking simulation was performed with eugenol as the ligand and an odorant binding protein TcOBPC12 (gene: TcOBP10B) from the model insect T. castaneum as the receptor. Docking calculation results revealed that TcOBPC12 had binding affinity to eugenol (△G = - 4.52 kcal/mol) along with a hydrogen bond of 0.18 nm (1.8 Å) long forming between them, which could be an important target protein associated with identifying volatile repellent molecules. This work highlights the promising potential of KSEO as a botanical repellent for controlling stored-product insects.

Assessment of Contact Toxicity and Repellent Effects of Essential Oils from Piper Plants Piper yunnanense and Piper boehmeriifolium against Three Stored-Product Insects.

Storage is a crucial part during grain production for the massive spoilage caused by stored product insects. Essential oils (EOs) of plant origin have been highly recommended to combating insects which are biodegradable and safe mode of action. Hence, to make the fullest use of natural resources, essential oils of different parts from Piper yunnanense (the whole part, PYW; fruits, PYF; leaves, PYL) and Piper boehmeriifolium (leaves, PBL) were extracted by steam distillation method in the present study. Gas chromatography-mass spectrometry (GC-MS) characterization revealed bicyclogermacrene (PYW), γ-muurolene (PYF), δ-cadinene (PYL) and methyl 4,7,10,13,16,19-docosahexaenoate (PBL) as the principal compound of each essential oil. Sesquiterpene hydrocarbons were also recognized as the richest class accounting for 56.3 %-94.9 % of the total oil. Three storage pests, Tribolium castaneum, Lasioderma serricorne and Liposceis bostrychophila, were exposed to different concentrations of EOs to determine their insecticidal effects. All tested samples performed modest contact toxicity in contrast to a bioactive ingredient pyrethrin, among which the most substantial effects were observed in PYF EOs against T. castaneum (35.84 µg/adult), PBL EOs against L. serricorne (15.76 µg/adult) and PYW EOs against L. bostrychophila (57.70 µg/cm2 ). In terms of repellency tests, essential oils of PYF at 78.63 nL/cm2 demonstrated to have a remarkable repellence against T. castaneum at 2h and 4h post-exposure. The investigations indicate diverse variations in the chemical profiles and insecticidal efficacies of P. yunnanense and P. boehmeriifolium EOs, providing more experimental evidence for the use of the Piper plants.