Behavioral and toxicological responses of Rhodnius prolixus and Triatoma infestans (Hemiptera: Reduviidae) to 10 monoterpene alcohols

Effectiveness and chemical insights: Exploring interactions between nanomicelles and monoterpenoids for head lice treatment

This study evaluates the pediculicidal activity of nanoformulations containing different binary essential oil component mixtures (eugenol:linalool, 1,8 -cineole:linalool, and eugenol:thymol) using immersion bioassays. These have allowed us to evaluate the knockdown time affecting 50% of the individuals (KT50). In addition, the type of interaction between the components in each mixture was established in terms of the combination index (IC). The KT50 values were 6.07; 8.83; 7.17 and 27.23 h for linalool, 1,8 -cineole, eugenol, and thymol, respectively. For the eugenol:linalool mixtures, the efficacy was lower or equal to that obtained for the nanoformulations of the pure compounds, with values of KT50 about 13.33, 8.16 and 6.71 h for mixtures with ratios 3:1, 1:1 and 1:3, respectively. These mixtures present IC > 1, evidencing antagonistic interaction, which is enhanced with eugenol content. In the case of the binary mixtures of 1,8 -cineole: linalool, KT50 values were similar to those obtained for eugenol:linalool mixtures with similar ratios. In this case, IC assumes values close to unity, suggesting additive interactions independently of the mixture composition. On the other side, mixtures of eugenol:thymol with 1:1 and 1:3 ratios showed values of 9.40 and 32.93 h, while the mixture with a 3:1 ratio showed the greatest effectiveness (KT50 of 4.42 h). Eugenol:thymol mixtures show synergistic interaction (IC < 1) for combinations 3:1 and 1:1, while no interaction was observed for 1:3 combination. This indicates that eugenol enhances thymol activity. These results must be considered an important step forward to the development of effective pediculicidal nanoformulations based on botanical compounds.

Coconut fatty acids exhibit strong repellency and week-long efficacy against several urban pest arthropods of the southwestern United States

BACKGROUND

The southwestern United States is home to a variety of arthropods including Turkestan cockroaches, Blatta lateralis (Walker); hematophagous kissing bugs, Tritoma rubida (Uhler); and Arizona bark scorpions, Centruroides sculpturatus Ewing. These arthropods cause medical concern when they become established around homes and/or invade indoors. Traditionally, the management of these pests has relied primarily on the use of chemical insecticides; however, they offer poor prospects for control owing to their lack of efficacy as well as the effects of insecticides on humans and the environment. Botanical repellents are an option that has not been fully investigated for the management of these pests. Here, we investigated the behavioral responses of common urban pests of the southwestern USA to recently discovered coconut fatty acids (CFAs), to establish the potential use of these compounds as repellents.

RESULTS

Fresh residues of CFA mixture (CFAm) and their constituents caprylic acid, capric acid, capric acid methyl ester, lauric acid, and lauric acid methyl ester, tested at a concentration of 1 mg cm-2 , strongly repelled all arthropods. The repellent activity of CFAm lasted for at least 7 days, and the addition of lavender oil, used as an odor-masking agent, did not decrease this effect. Concentrations of CFAm ten times lower (0.1 mg cm-2 ) still repelled Turkestan cockroaches, and concentrations 100 times lower (0.01 mg cm-2 ) repelled T. rubida and scorpions.

CONCLUSIONS

CFAm and some of their constituents are efficacious, economical, and logistically feasible for inclusion in integrated pest management programs for these important urban pests of the southwestern USA. © 2023 Society of Chemical Industry.

Lethal and repellent effect of amitraz, eugenol and thymol against Triatoma infestans, the main vector of Trypanosoma cruzi in the southern of America

The lethal and repellent effect of the synthetic insecticide amitraz and the botanical insecticides eugenol and thymol separately and together in binary mixtures was tested against late-stage nymphs of a susceptible strain of Triatoma infestans, the main vector of Trypanosoma cruzi, the etiological agent of Chagas disease, in the Southern Cone of America. For the lethality study, the LD50 was determined for each insecticide alone and in binary mixture by topical application. The combination index (CI) was established to quantify interactions occurring between the insecticides. The repellent effect was tested using the area preference technique. The lethal effect of amitraz was 11 and 34 times more potent than that of thymol and eugenol, respectively. Only the combination of eugenol and amitraz at high concentrations showed a synergistic effect (CI: 0.3). The repellent activity of monoterpenes after 30 min of exposure was significant at 780 and 78 μg/cm2 for eugenol and thymol, respectively. The residual repellent effect of eugenol lasted for one week at the concentrations of 1170 and 1560 μg/cm2 , whereas thymol managed to retain its repellent effect for two weeks at concentrations of 1560 and 3900 μg/cm2 .

Novel Essential Oils Blend as a Repellent and Toxic Agent against Disease-Transmitting Mosquitoes

Bio-insecticidal research has focused on long-term vector control using essential oils (EOs). This study examined the larvicidal, oviposition-deterrent, and repellent properties of five medicinal herb-based EO formulations (EOFs) on mosquitoes that are vectors of dengue, filariasis, and malaria. EOFs were significantly more toxic to the larvae and pupae of Culex quinquefasciatus, Anopheles stephensi, and Aedes aegypti with LC₅₀ = 9.23, 12.85, and 14.46 ppm, as well with 10.22, 11.39, and 12.81 ppm, with oviposition active indexes of -0.84, -0.95, and -0.92, respectively. Oviposition-deterrent repellence was found in 91.39%, 94.83%, and 96.09%. EOs and N, N-Diethyl-3-methylbenzamide (DEET) were prepared at various concentrations for time duration repellent bioassays (6.25-100 ppm). Ae. aegypti, An. stephensi, and Cx. quinquefasciatus were monitored for 300, 270, and 180 min, respectively. At 100 ppm, EOs and DEET had comparable repellence in terms of test durations. EOF's primary components d-limonene (12.9%), 2,6-octadienal, 3,7-dimethyl, (Z) (12.2%), acetic acid, phenylmethyl ester (19.6%), verbenol (7.6%), and benzyl benzoate (17.4%) may be combined to make a mosquito larvicidal and repellant equivalent to synthetic repellent lotions. In the molecular dynamics simulations, limonene (-6.1 kcal/mol) and benzyl benzoate (-7.5 kcal/mol) had a positive chemical association with DEET (-6.3 kcal/mol) and interacted with the OBP binding pocket with high affinity and stability. This research will help local herbal product manufacturers and the cosmetics industry in developing 100% herbal insect repellent products to combat mosquito-borne diseases, including dengue, malaria, and filariasis.

Novel Approach for a Controlled Delivery of Essential Oils during Long-Term Maize Storage: Clove Bud and Pennyroyal Oils Efficacy to Control Sitophilus zeamais, Reducing Grain Damage and Post-Harvest Losses

Maize grains represent a significant contribution for assuring food safety all over the globe. Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae), also known as the maize weevil, is one of the most destructive pests in stored maize, causing qualitative and quantitative losses. To control S. zeamais populations in maize storage sites, synthetical chemical insecticides are applied. However, these are often used wastefully, have environmental implications, and can induce the development of resistant populations. In this work, the insecticidal and grain protecting efficacy of an innovative macro-capsule delivery device, loaded with essential oils from Clove bud and Pennyroyal, as well as their combined solutions, was tested against naturally S. zeamais-infested maize grains. The blend of both compounds incorporated in a controlled release device reduced losses by more than 45% over a long storage period of twenty weeks, diminishing the survivability of maize weevils by over 90%. The usage of the blend at a concentration of 370 μL⋅Lair-1 with an antioxidant showed the best results, however, by halving the concentration (185 μL⋅Lair-1), a significant control of S. zeamais populations was still achieved.

Eugenol and menthol synergize the toxicity of permethrin in the blood-sucking bug, Triatoma infestans

The blood-sucking bug, Triatoma infestans, is one of the main vectors of Chagas disease in America. It is usually controlled with pyrethroids, but the emergence of resistance to these insecticides creates the need to look for alternative products. Eugenol, menthol and menthyl acetate are botanical monoterpenes, which produce lethal and sublethal effects on insects. The purpose of this work was to determine what type of toxicological interactions occur when binary mixtures, formed by the pyrethroid permethrin and sublehtal doses of eugenol, menthol or menthyl acetate, are applied to T. infestans. First instar nymphs were exposed to filter papers impregnated with the insecticides. The number of knocked down insects was registered at different times and Knock Down Time 50% (KT₅₀) values were calculated. The following KT₅₀ values with their corresponding 95% Confidence Intervals were obtained: permethrin, 47.29 (39.92 - 56.32) min; permethrin + eugenol, 34.08 (29.60 - 39.01) min; permethrin + menthol, 27.54 (23.28 - 32.55) min; permethrin + menthyl acetate, 43.62 (39.99 - 47.59) min. Eugenol and menthol increased the speed of action of permethrin (synergism), but menthyl acetate had no effect on it (additivity). These results provide the basis to further explore interactions between conventional insecticides and plant monoterpenes as potential tools for controlling T. infestans.

A rapid method for screening mosquito repellents on Anopheles pseudopunctipennis and Aedes aegypti

One of the main vectors for malaria in Latin America is Anopheles pseudopunctipennis (Theobald), whereas Aedes aegypti (L.) is the primary vector of dengue, yellow fever, Zika and chikungunya viruses. The use of repellents is recommended as a personal protection method against these mosquitoes. However there are very few studies evaluating the effect of repellents on An. pseudopunctipennis. The use of a Petri dish to study repellence has been applied by several authors on flies, cockroaches, kissing bugs and mosquitoes, being a valuable technique for species that are difficult to breed under laboratory conditions, such as An. pseudopunctipennis. In the present study, we evaluated the repellence of the essential oil of the Eucalyptus nitens (Shining gum), its main component (1,8-cineole) and the commercial repellent DEET on An. pseudopunctipennis and Ae. aegypti adult females using the plaque repellency method coupled to EthoVision XT10.1 video-tracking software. Repellent effect and locomotor activity were studied through a repellence index (RI) together with an axis constructed from the behavioural variables obtained using the tracking software. DEET repellent effect was observed at 0.01 mg/mL for Ae. aegypti and 0.01 and 0.1 mg/mL for An. pseudopunctipennis. In addition, the essential oil showed significant repellence at 1 and 10 mg/mL for Ae. aegypti, and 1, 5, 10 and 25 mg/mL for An. pseudopunctipennis. Neither of these species were repelled at any concentration of 1,8-cineole. This is the first study that evaluates these compounds on An. pseudopunctipennis females and quantifies their effects on the activity of both species.

Behavioral Responses of the Common Bed Bug to Essential Oil Constituents

Simple Summary

Bed bugs (Cimex lectularius L.) are blood-sucking insects that have emerged worldwide in the last two decades causing serious public health and economic impact. Today, control of bed bug infestations relies on the use of synthetic insecticides, but their frequent use has led to the development of resistance in bed bug populations. Therefore, there is a growing demand for the development of safer, green, and more effective tools for bed bug control. Plant-derived pesticides are part of the proposed “green” methods for bed bug control. We evaluated behavioral responses of bed bugs to essential oil constituents (EOCs) and detected that bed bugs did not rest on areas treated with geraniol, eugenol, citronellic acid, and carvacrol. Barriers of these constituents did not deter bed bugs from reaching warmed blood meal and feeding. Our results show that novel formulations of natural product insecticides that include geraniol, eugenol, carvacrol, or citronellic acid have potential to repel bed bugs. However, little benefit of protection against bed bug bites can be expected when EOC-based products are applied to items present in close proximity to a sleeping host such as mattress covers, liners, or around the bed.

Abstract

Botanical-derived pesticides have arisen as an attractive alternative to synthetic insecticides to effectively manage infestations of bed bugs (Cimex lectularius L.). While information on contact, residual, and fumigant toxicity of plant-essential oils against bed bugs have been recently published, there is a gap of information regarding the repellent activity of these products and their constituents. Identification of essential oil constituents (EOCs) with repellent activity will help develop potentially efficacious essential oil-based formulations for use in bed bug management programs. In this study, we first screened fresh and 24 h-aged residues of geraniol, eugenol, carvacrol, thymol, citronellic acid, linalool, menthone, trans-cinnamaldehyde, α-pinene, β-pinene, and limonene for avoidance behavior from individual bed bugs with a video-tracking system. Six EOCs, geraniol, eugenol, citronellic acid, thymol, carvacrol, and linalool were further evaluated overnight in choice tests to determine whether 24-h aged residues were still avoided by groups of bed bugs. While bed bugs avoided resting on filter papers treated with 24-h aged residues of geraniol, eugenol, citronellic acid, and carvacrol, bed bugs aggregated in areas treated with linalool-aged residues. Barriers of EOCs did not prevent bed bugs from reaching a warmed blood source and acquiring blood meals. Our results show that novel formulations of natural product insecticides that include geraniol, eugenol, carvacrol, or citronellic acid have potential to repel bed bugs. The presence of host-associated cues might interfere with these responses.

Bed bugs, Cimex lectularius L., exhibiting metabolic and target site deltamethrin resistance are susceptible to plant essential oils

Pyrethroid resistance has been a major hurdle limiting the effective control of bed bugs (Cimex lectularius L.). Alternative approaches that include the use of plant essential oils (EOs) have been proposed for effective management of bed bugs. However, EO resistance level comparisons between pyrethroid susceptible and resistant bed bug populations have not been previously conducted. The goal of this study was twofold: (i) determine deltamethrin resistance levels and associated resistance mechanisms in the field-collected Knoxville strain and (ii) quantify resistance levels of the Knoxville strain to five EOs (thyme, oregano, clove, geranium and coriander), their major insecticidal constituents (thymol, carvacrol, eugenol, geraniol and linalool) and an EO-based product (EcoRaider®). First, we found that the Knoxville strain was 72,893 and 291,626 fold more resistant to topically applied deltamethrin in comparison to the susceptible Harlan strain at the LD₂₅ and LD₅₀ lethal dose levels, respectively. Synergist bioassays and detoxification enzyme assays revealed significantly higher activity of cytochrome P450 and esterase enzymes in the resistant Knoxville strain. Further, Sanger sequencing revealed the presence of the L925I mutation in the voltage-sensitive sodium channel α subunit gene. The Knoxville strain that possesses both enzymatic and target site deltamethrin resistance, however, did not show any resistance to EOs, their major insecticidal constituents and EcoRaider® in topical bioassays (resistance ratio of ~1). In conclusion, this study demonstrated that a deltamethrin-resistant strain of bed bugs is susceptible to EOs and their insecticidal constituents.

Botanical monoterpenes synergise the toxicity of azamethiphos in the vector of Chagas disease, Triatoma infestans (Hemiptera: Reduviidae)

OBJECTIVE

To investigate what toxicological interactions occur when binary combinations of azamethiphos and botanical monoterpenes (eugenol, menthol or menthyl acetate) are applied to Triatoma infestans.

METHODS

The toxicity of binary mixtures of azamethiphos and sublethal doses of a monoterpene (eugenol, menthol or menthyl acetate) was evaluated in nymphs of the first stage of T. infestans. Experiments using exposure to filter papers and topical application were carried out. Values of Lethal Concentration 50% (LC50) were calculated in the first case, and values of Lethal Dose 50% (LD50) in the second.

RESULTS

The LC50 of azamethiphos applied on filter paper was 50.3 µg/cm² . However, when it was simultaneously applied with a sublethal concentration of monoterpene, its toxicity increased (LC50 with eugenol = 11.20 µg/cm² , LC50 with menthyl acetate = 5.30 µg/cm² , LC50 with menthol = 7.26 µg/cm² ). When applied topically, the LD50 of azamethiphos was 7.85 µg/insect, but its toxicity drastically increased when it was applied together with sublethal doses of menthol (LD50 = 0.00016 µg/insect) or menthyl acetate (LD50 = 0.00051 µg/insect). The simultaneous application with eugenol did not significantly change azamethiphos toxicity (LD50 = 12.79 µg/insect).

CONCLUSIONS

The toxicity of azamethiphos in T. infestans was synergised when it was applied together with eugenol, menthol or menthyl acetate on a filter paper. However, only menthol and menthyl acetate synergysed azamethiphos when mixtures were topically applied. The drastic effects of menthol and menthyl acetate in topical application experiments should be further studied as they could be the basis for developing more efficient triatomicidal products with a lower content of conventional insecticides than those currently used for controlling T. infestans.

Surfactantless Emulsions Containing Eugenol for Imidacloprid Solubilization: Physicochemical Characterization and Toxicity against Insecticide-Resistant Cimex lectularius

Synthetic insecticides have been used for a long time as one of the most effective tools for insect pest control. However, the re-emergence of insect pests and their fast development of resistance, as has occurred for pyrethroid-resistant bed bugs Cimex lectularius L., make it necessary to develop new and safe strategies for effective pest control. This has fostered the research on new eco-sustainable formulations based on essential oils, which allows reducing the impact associated with the intensive use of synthetic insecticides on the environment and their effects on human health. This research explores the stability of water/eugenol/ethanol surfactantless emulsions loaded with imidacloprid (0.003 wt%), and their toxicity against a resistant bed bug strain. The results have shown that these emulsions enable the solubilization of a poorly water-soluble drug, such as the imidacloprid, without any significant modification of their stability. Furthermore, the application of the obtained formulations against the pyrethroid-resistant bed bug results in mortality in the 50-85% range upon topical and spray applications, with the increase of the eugenol content enhancing the effectiveness of the formulations. It may be expected that the ternary water/eugenol/ethanol mixtures could be further developed in the preparation of ready to use formulations, enabling the dispersion of insecticides for pest control.

Deorphanization of an odorant receptor revealed new bioactive components for green mirid bug Apolygus lucorum (Hemiptera: Miridae)

BACKGROUND

The Apolygus lucorum is one of the most destructive insect pests in China with a wide range of host plants. Interaction of A. lucorum with surrounding environment heavily relies on chemical communication. Deorphanization of receptors involved in odors detection elevates our understanding of the olfactory system of this pest and may help to develop a chemical ecology-based control strategy.

RESULTS

AlucOR80, an odorant receptor (OR) in A. lucorum was newly cloned. Gene expression analysis showed that this receptor was mainly expressed in the antennae and head of both sexes but with a male bias. The Xenopus oocytes heterologous expression system coupled with the two-electrode voltage-clamp (TEVC) recording revealed that AlucOR80 was tuned to 21 selected compounds. Furthermore, electroantennogram (EAG) tests confirmed that all 21 ligands of AlucOR80 were electrophysiologically active in antennae of both sexes. Behavioral trials in a three-cage olfactometer indicated that 16 compounds were behaviorally active, amongst which, 12 components were attractants and four components were repellents for adults of both sexes. Butyl butyrate and Dimethyl disulfide (DMDS) were the strongest attractive and repellant compounds, respectively. Importantly, we found the repellency of 1, 8-Cineole, S-(-)-cis-Verbenol and (1S)-(1)-beta-Pinene against adults of A. lucorum.

CONCLUSION

Although AlucOR80 is a general OR, may play important role in the olfactory perception of A. lucorum. Screening of AlucOR80 ligands by behavioral assay provided valuable insights by which olfactory-based management approaches could be developed by utilizing the behaviorally active components as attractants or repellents. © 2019 Society of Chemical Industry.

Novel repellents for the blood-sucking insects Rhodnius prolixus and Triatoma infestans, vectors of Chagas disease

BACKGROUND

Studying the behavioral response of blood-sucking disease-vector insects to potentially repellent volatile compounds could shed light on the development of new control strategies. Volatiles released by human facial skin microbiota play different roles in the host-seeking behavior of triatomines. We assessed the repellency effect of such compounds of bacterial origin on Triatoma infestans and Rhodnius prolixus, two important vectors of Chagas disease in Latin America.

METHODS

Using an exposure device, insects were presented to human odor alone (control) and in the presence of three individual test compounds (2-mercaptoethanol, dimethyl sulfide and 2-phenylethanol, the latter only tested in R. prolixus) and the gold-standard repellent NN-diethyl-3-methylbenzamide (DEET). We quantified the time the insects spent in the proximity of the host and determined if any of the compounds evaluated affected the behavior of the insects.

RESULTS

We found volatiles that significantly reduced the time spent in the proximity of the host. These were 2-phenylethanol and 2-mercaptoethanol for R. prolixus, and dimethyl sulfide and 2-mercaptoethanol for T. infestans. Such an effect was also observed in both species when DEET was presented, although only at the higher doses tested.

CONCLUSIONS

The new repellents modulated the behavior of two Chagas disease vectors belonging to two different triatomine tribes, and this was achieved using a dose up to three orders of magnitude lower than that needed to evoke the same effect with DEET. Future efforts in understanding the mechanism of action of repellent compounds such as 2-mercaptoethanol, as well as an assessment of their temporal and spatial repellent properties, could lead to the development of novel control strategies for these insect vectors, refractory to DEET.

The Octopamine Receptor Is a Possible Target for Eugenol-Induced Hyperactivity in the Blood-Sucking Bug Triatoma infestans (Hemiptera: Reduviidae)

Eugenol is a major component of the essential oils in cloves and other aromatic plants. In insects, it produces toxic effects and repellency, and there is evidence that its site of action is the octopamine receptor. The objective of the present study was to explore whether the octopamine receptor is involved in the hyperactivity produced by eugenol in the blood-sucking bug Triatoma infestans (Klug). This insect is the main vector of Chagas disease in Latin America. Four treatments were topically applied on third instar nymphs: 1) octopamine, 2) eugenol, 3) phentolamine hydrochloride (an antagonist of the octopamine receptor) followed by octopamine, and 4) phentolamine hydrochloride followed by eugenol. Both octopamine and eugenol hyperactivated the nymphs. However, pretreatment with phentolamine hydrochloride inhibited the hyperactivating effect of both compounds. These results are in agreement with previous works on Drosophila melanogaster (Meigen) (Diptera: Drosophilidae) and the American cockroach. They suggest that the octopamine receptor is a possible site of action for eugenol.

Behavioral and Electrophysiological Effects of Ajowan (Trachyspermum ammi Sprague) (Apiales: Apiaceae) Essential Oil and Its Constituents on Nymphal and Adult Bean Bugs, Riptortus clavatus (Thunberg) (Hemiptera: Alydidae)

We investigated the repellent effect of 12 Apiaceae plant essential oils on nymphal and adult (male and female) forms of the bean bug, Riptortus clavatus (Thunberg) (Hemiptera: Alydidae), using a four-arm olfactometer. Among the essential oils tested, ajowan (Trachyspermum ammi Sprague) essential oil showed the strongest repellent activity against the nymphal and adult bean bugs. For female adults, the repellent activity was significantly different between an ajowan oil-treated chamber and an untreated chamber down to a concentration of 14.15 μg/cm². We also investigated the repellent activity of individual ajowan essential oil constituents. Of the compounds examined, carvacrol and thymol showed the most potent repellent activity against the nymphal and adult bean bugs. Carvacrol and thymol exhibited 73.08% and 70.0% repellent activity for the bean bug nymph at 0.71 and 2.83 μg/cm², respectively, and 82.6% and 80.7% at 5.66 and 11.32 μg/cm², respectively, for male adults. Carvacrol and thymol exhibited strong repellent activity against female adult bean bugs down to a concentration of 2.83 μg/cm². Ajowan essential oil, thymol and carvacrol elicited a negative electroantennogram (EAG) response from adult bean bugs. This could explain the repellent activity of ajowan essential oil and its constituents. Our results indicate that ajowan essential oil and its constituents carvacrol and thymol can be potential candidates as the 'push' component in a 'push-pull' strategy for bean bug control.

Insecticidal Activity of Compounds of Plant Origin on Mahanarva spectabilis (Hemiptera: Cercopidae)

The damage caused by spittlebugs varies according to the species of grass, and the losses can reach alarming levels. Measures for population control are currently restricted to the use of resistant grasses and the diversification of pastures. Therefore, alternative control measures are necessary, such as the use of botanical insecticides. The aim of this study was to evaluate the insecticidal activities of thymol, carvacrol, eugenol, cinnamaldehyde, and trans-anethole on Mahanarva spectabilis eggs, nymphs, and adults under laboratory conditions. In the egg tests, treatments with eugenol, carvacrol, and thymol showed the highest mortalities, presenting efficiencies higher than 85% after 48 h of application. In the nymph tests, the treatments with thymol and carvacrol at 2.5% and eugenol at 2.0% and 2.5% showed intermediate efficiencies, with values above 61%. The highest mortality was observed in the treatment with trans-anethole at 2.5%, with an efficiency of 95%. In the tests with adults, only treatment with trans-anethole at 2.5% obtained an efficiency reaching 90%; in the other treatments, the efficiency did not exceed 51%. These results showed that, at these concentrations, trans-anethole presents a high rate of insecticidal activity on M. spectabilis nymphs and adults and, therefore, is recommended as a potential natural insecticide for the control of this pest.

Insecticide resistance modifies mosquito response to DEET and natural repellents

Background

Pyrethroid and organophosphate resistance in the malaria vector Anopheles gambiae has led to the search for not only alternative insecticides, but also repellent chemical compounds. However, little is known about the potential actions of repellents and the cross-resistance risk between insecticide and repellent compounds.

Methods

Here we show the action of permethrin, DEET, geraniol, carvacrol, culminaldehyde and cinnamaldehyde against three A. gambiae strains: ‘Kis’ (Kisumu susceptible strain), ‘KdrKis’ (pyrethroid resistant strain) and ‘AcerKis’ (organophosphate resistant strain), the last two differing from the first by a mutation on the kdr and ace1 genes, respectively.

Conclusions

Results from the DEET assays show it induced repellency for the resistant KdrKis and AcerKis strains but maintained irritancy for the susceptible strain. More generally, we show resistance genes modify the behavior of An. gambiae, increasing or decreasing the effectiveness of DEET and natural compounds, depending on the mutation. These findings offer a new avenue for research on the target and mechanism of repellent compounds. We discuss these findings in the context of vector control strategies.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3343-9) contains supplementary material, which is available to authorized users.

Toxicity and neurophysiological impacts of plant essential oil components on bed bugs (Cimicidae: Hemiptera)

Bed bugs (Cimex lectularius L.) are globally important human parasites. Integrated pest management (IPM) approaches, which include the use of essential oil-based insecticidal compounds, have been proposed for their control. This study aimed to define insecticidal activity and neurophysiological impacts of plant essential oil constituents. The topical and fumigant toxicity of 15 compounds was evaluated against adult male bed bugs. Neurological effects of the 6 most toxicologically active compounds were also determined. In both topical and fumigant bioassays, carvacrol and thymol were the most active compounds. The potency of bifenthrin (a pyrethroid insecticide) in topical bioassays was 72,000 times higher than carvacrol, while vapors of dichlorvos (an organophosphate insecticide) were 445 times more potent than thymol. Spontaneous electrical activity measurements of the bed bug nervous system demonstrated neuroinhibitory effects of carvacrol, thymol and eugenol, whereas linalool produced an excitatory effect. Although citronellic acid and (±)-camphor increased baseline activity of the nervous system their effects were not statistically significant. Bifenthrin also caused neuroexcitation, which is consistent with its known mode of action. These comparative toxicity and neurological impact findings provide new information for formulating effective essential oil-based insecticides for bed bug IPM and conducting mode-of-action studies on individual essential oil components.

Eugenol-hyperactivated nymphs of Triatoma infestans become intoxicated faster than non-hyperactivated nymphs when exposed to a permethrin-treated surface

BACKGROUND

Eugenol is a botanical monoterpene that hyperactivates the blood-sucking bug Triatoma infestans, and permethrin is a pyrethroid with a strong triatomicide effect. In the present work, we tested the hypothesis that eugenol-hyperactivated nymphs of T. infestans pick up more insecticide, and then become intoxicated faster, than non-hyperactivated nymphs when exposed to a permethrin-treated surface.

RESULTS

Values of knockdown time 50% (KT50) for third-instar T. infestans exposed to a paper impregnated with permethrin were obtained under the following situations: (a.i.) immediately after topical application of eugenol (KT50: 66.75 min for acetone pre-treated controls, and 46.27 min for eugenol pre-treated nymphs); (a.ii.) 30 min after topical application of eugenol (KT50: 66.79 min for controls, and 66.79 min for eugenol pre-treated nymphs); (b) simultaneously with exposure to eugenol vapors (KT50: 51.90 min for controls, and 39.5 min for nymphs exposed to an eugenol-treated filter paper); and (c) immediately after an injection of eugenol (on average, controls were knocked down after 63.00 min, whereas nymphs injected with eugenol were knocked down after 65.30 min). In other experimental series, the distance traveled (DT) by nymphs exposed to eugenol was quantified in the same situations previously described, but without exposure to permethrin. In (a.i.), the DT in interval 0-30 min after topical application of eugenol was 487.00 (control) and 1127.50 (eugenol) cm; in (a.ii.), the DT in the interval 31-60 min after topical application was 336.75 (control) and 256.75 (eugenol) cm; in (b), DT was 939.08 (control) and 1048.53 (eugenol) cm; and in (c), it was 589.20 (control) and 700.00 (eugenol) cm. The KT50 values for permethrin decreased significantly in situations (a.i.) and (b), and eugenol only produced a significant hyperactivity in the same situations. Finally, the amount of permethrin picked up by non-hyperactivated and hyperactivated nymphs exposed to a film of permethrin was quantified by gas chromatography. Non-hyperactivated nymphs picked up 0.34 μg/insect of permethrin, while hyperactivated nymphs picked up 0.65 μg/insect.

CONCLUSION

Results support the hypothesis that eugenol-hyperactivated nymphs of T. infestans pick up more insecticide, and then become intoxicated faster, than non-hyperactivated nymphs when exposed to a permethrin-treated surface.

The Botanical Monoterpenes Linalool and Eugenol Flush-Out Nymphs of Triatoma infestans (Hemiptera: Reduviidae)

Monoterpenes are the main components of essential oils. Some members of this chemical family present insecticidal activity. Triatoma infestans (Klug) is the main vector of Chagas disease in Argentina, Bolivia, Paraguay, and Perú. The objective of this work was to evaluate the effect of six monoterpenes (1,8-cineole, eugenol, linalool, menthol, α-terpineol, and thymol) on the locomotor and flushing out activity of T. infestans. A video tracking technique was used to evaluate the locomotor activity of nymphs exposed to different concentrations of these chemicals applied as films on filter paper. Papers treated with acetone alone were used as negative controls, while solutions of tetramethrin were applied as positive controls. Only linalool and menthol produced hyperactivation. Flushing out was assessed under laboratory conditions using a standardized aerosolization method. All monoterpenes were applied at 1.5 g/m3. 1,8-Cineole, α-terpineol, and thymol flushed out 10% or less nymphs. The average flushing out produced by eugenol was 36.7%. Values of median flushing out time (FT50) could only be calculated for linalool and menthol (16.67 and 42.98 min, respectively). The FT50 value for the positive control tetramethrin (applied at 0.006 g/m3) was 8.29 min. Following these results, the flushing out activity of a mixture of linalool and eugenol was evaluated. The FT50 of this 2:1 linalool:eugenol mixture was 40.73 min. Finally, flushing out assays performed in semifield conditions showed similar results to those obtained at the laboratory.

Toxicity, repellency and flushing out in Triatoma infestans (Hemiptera: Reduviidae) exposed to the repellents DEET and IR3535

DEET and IR3535 are insect repellents present worldwide in commercial products; their efficacy has been mainly evaluated in mosquitoes. This study compares the toxicological effects and the behavioral responses induced by both repellents on the blood-sucking bug Triatoma infestans Klug (Hemiptera: Reduviidae), one of the main vectors of Chagas disease. When applied topically, the Median Lethal Dose (72 h) for DEET was 220.8 µg/insect. Using IR3535, topical application of 500 µg/insect killed no nymphs. The minimum concentration that produced repellency was the same for both compounds: 1,15 µg/cm². The effect of a mixture DEET:IR3535 1:1 was similar to that of their pure components. Flushing out was assessed in a chamber with a shelter containing groups of ten nymphs. The repellents were aerosolized on the shelter and the number of insects leaving it was recorded for 60 min. During that time, 0.006 g/m³ of the positive control tetramethrin flushed out 76.7% of the nymphs, while 1.76 g/m³ of DEET or IR3535 flushed out 30 and 0%, respectively. The concentrations required for both compounds to produce toxicity or flushing out are too high to have any practical applications. However, they showed a promising repellency. Additional research should be done to evaluate their possible use for personal protection against T. infestans bites.

Insecticidal Properties of Essential Oils and Some of Their Constituents on the Turkestan Cockroach (Blattodea: Blattidae)

The Turkestan cockroach, Blatta lateralis (Walker), has become the most important peridomestic species in urban areas of the Southwestern United States. The aim of this study was to evaluate the use of botanical compounds to control this urban pest. We tested the acute toxicity and repellency of six botanical constituents and three essential oils on Turkestan cockroach nymphs. Chemical composition of the essential oils was also determined. Topical and fumigant assays with nymphs showed that thymol was the most toxic essential oil constituent, with a LD50 of 0.34 mg/nymph and a LC50 of 27.6 mg/liter air, respectively. Contact toxicity was also observed in assays with trans-Cinnamaldehyde, eugenol, geraniol, methyl eugenol, and p-Cymene. Methyl eugenol and geraniol had limited fumigant toxicity. The essential oils from red thyme, clove bud, and Java citronella exhibited toxicity against nymphs. Cockroaches avoided fresh dry residues of thymol and essential oils. Chemical analysis of the essential oils confirmed high contents of effective essential oil constituents. Our results demonstrated that essential oils and some of their constituents have potential as eco-friendly insecticides for the management of Turkestan cockroaches.

Insecticidal action of synthetic girgensohnine analogues and essential oils on Rhodnius prolixus (Hemiptera: Reduviidae)

Introducción. El alcaloide natural girgensohnina se ha usado como modelo en la síntesis de nuevos análogos de alcaloidales alfa-aminonitrílicos con efecto insecticida en vectores de enfermedades.Objetivo. Evaluar la actividad biocida de análogos de girgensohnina y de aceites esenciales de las plantas Cymbopogon flexuosus, Citrus sinensis y Eucalyptus citriodora en ninfas de estadios I y V de Rhodnius prolixus.Materiales y métodos. Se empleó la aplicación tópica en terguitos, esternitos y superficies tratadas con diferentes dosis exploratorias de cada una de las moléculas y aceites esenciales para determinar las dosis letales (LD50 y LD95).Resultados. El análogo 3 tuvo la mayor actividad insecticida, con una mortalidad de 83,3±16,7% en los terguitos, de 38,9±4,8 % en los esternitos y de 16,7±0 % a las 72 horas en ninfas de estadio I expuestas a superficies tratadas y 500 mg.L-1. En las ninfas de estadio V solo se presentó mortalidad en los esternitos (11,1±9,6 % con el análogo 6 y 5,5±4,7 % con los análogos 3 y 7 a las 72 h y 1.500 mg.L-1). Las dosis letales para la molécula 3 en los terguitos de ninfas de estadio I fueron las siguientes: DL50, 225,60 mg.L-1y DL95, 955,90 mg.L-1. En cuanto a los aceites esenciales, el efecto insecticida solo se presentó con C. flexuosus (11,1±4,8%) en los esternitos de ninfas de estadio I expuestas a superficies tratadas; con C. sinensis (5,6±4,8%) en los terguitos y en los esternitos (8,3±0%) a las 72 horas y 1.000 mg.L-1.Conclusión. Los análogos sintéticos del alcaloide girgensohnina y los aceites esenciales de C. flexuosus y C. sinensis exhibieron actividad insecticida en R. prolixus. El análogo 3 exhibió la mayor actividad insecticida de todas las moléculas evaluadas bajo las condiciones de laboratorio.

Rhodnius prolixus intoxicated

Rhodnius prolixus (Hemiptera: Reduviidae) is a hematophagous insect native from South America. By the end of the 20th century, it was one of the main vectors of Chagas disease in Venezuela, Colombia, several Central American countries and southern Mexico. The aim of the present article is to review the literature regarding R. prolixus toxicology. British entomologist Vincent B. Wigglesworth carried out the first studies on this subject over seventy years ago. A wide bibliographical search allowed to locate one hundred and thirty scientific articles describing the effects of different insecticides on R. prolixus. About one-third of these articles report the acute toxicity and/or sublethal effects produced by the main synthetic neurotoxic families of insecticides (organochlorines, organophosphates, carbamates and pyrethroids). Only a couple of these studies have regarded the toxicokinetics or toxicodynamics of these insecticides. Insect growth or development disruptors, such as juvenoids, chitin synthesis inhibitors, precocenes, azadirachtin and lignoids, have been thoroughly studied in R. prolixus. Important aspects on the mode of action of ureases were also described in this species. By the end of the 1960's, resistance to insecticides was detected in R. prolixus from Venezuela. Some years later, the existence of pyrethroid-resistant individuals was also reported. Control programmes for R. prolixus in countries where Chagas is endemic have only used synthetic neurotoxic insecticides. In 2011, Central America and southern Mexico were declared free of this insect. The recent sequencing of the R. prolixus genome will provide valuable information to understand the molecular basis of insecticide resistance in this species.

Lethal and sublethal effects of essential oil of Lippia sidoides (Verbenaceae) and monoterpenes on Chagas' disease vector Rhodnius prolixus

The aim of this study was to identify the composition of the essential oil from leaves of Lippia sidoides (EOLS), a typical shrub commonly found in the dry northeast of Brazil, popularly known as “alecrim-pimenta”. Additionally, we investigated the nymphicidal, ovicidal, phagoinhibitory and excretion effects of EOLS, its major constituent thymol and its isomer carvacrol, on fourth instar nymphs and eggs of Rhodnius prolixus, the Chagas’ disease vector. The nymphicidal and ovicidal activity of thymol, carvacrol, and EOLS was assessed by tests using impregnated Petri dishes. The lethal concentration values (LC50) for EOLS, carvacrol, and thymol were 54.48, 32.98, and 9.38 mg/cm², respectively. The ovicidal test showed that both carvacrol and thymol (50 mg/cm²) inhibited hatching (50% and 23.3%, respectively), while treatments with 10 mg/cm² or 50 mg/cm² EOLS did not affect the hatching rate at all (80% and 90%, respectively). We observed an anti-feeding effect in insects fed with blood containing natural products at the higher concentrations (100 µg/mL). Finally, excretion rate was affected by EOLS and carvacrol, but not by thymol. These findings offer novel insights into basic physiological processes that make the tested natural compounds interesting candidates for new types of insecticides.

Potential mode of action of a novel plumbagin as a mosquito repellent against the malarial vector Anopheles stephensi, (Culicidae: Diptera)

Plumbagin was isolated and characterized from the roots of Plumbago zeylanica using chromatography: TLC, Column chromatogram, HPLC, FTIR and ¹H NMR. The isolated pure compounds were assayed for potency as inhibitors of: acetylcholine esterase (AchE), glutathione S-transferases (GST), superoxide dismutase (SOD), cytochrome P450 and α, β-esterase, and for repellency with Anopheles stephensi at four different concentrations (25, 50, 75 and 100ppm). The enzyme assay against the pure compound reveals that the level of esterase and SOD was decreased significantly in contrast the level of GST and cytochrome P450 was increased significantly. Our results suggests that novel Plumbagin has significantly alters the level of enzyme comparable to the control. Evaluations resulted in Plumbagin producing maximum repellency scores against An. stephensi mosquitoes in dose dependent manner with highest repellence was observed in the 100ppm. Histological examination showed that the midgut, hindgut and muscles are the most affected tissues. These tissues affected with major changes including separation and collapse of epithelial layer and cellular vacuolization. The results support the utility of plant compound Plumbagin for vector control as an alternative to synthetic insecticides, however, more vigorous field trials are needed to determine viability under natural conditions.

Behavioral and Toxicological Responses of Rhodnius prolixus (Hemiptera: Reduviidae) to the Insect Repellents DEET and IR3535

,N-diethyl-3-methylbenzamide (DEET) is a broad-spectrum insect repellent used by millions of people since the 1950s. Ethyl 3-[acetyl(butyl)amino]propanoate (IR3535) is a repellent developed more recently that is still not used as extensively. This study compares the behavioral and toxicological effects of both substances in fifth-instar nymphs of the blood-sucking bug Rhodnius prolixus Stål (Hemiptera: Reduviidae), one of the main vectors of American trypanosomiasis (Chagas disease). Repellency was quantified using filter paper discs as experimental arenas. Half the discs were treated with solution of repellent in acetone, and the other half with acetone alone. The lowest observed effect level was identical for both substances, 74 µg/cm2. Nymph age (between 1-3 h and 99 d from last ecdysis) had no influence on repellency. Topical application of 750 µg of DEET per nymph produced a mortality rate between 0% (24 h after application) and 40% (7 d later). The same dose of IR3535 produced no mortality during the same period of time. Simultaneous treatment with piperonylbutoxide (a mixed function microsomal oxidase inhibitor) synergized the lethal effect of DEET. Only DEET increased locomotor activity in nymphs exposed to a treated surface. Nymph antennectomy abolished DEET repellency but not its effect on locomotor activity. The concentrations of both these compounds required to produce either behavioral or toxicological effects are too high to have any practical applications in the control of R. prolixus.

Repellent, irritant and toxic effects of 20 plant extracts on adults of the malaria vector Anopheles gambiae mosquito

Pyrethroid insecticides induce an excito-repellent effect that reduces contact between humans and mosquitoes. Insecticide use is expected to lower the risk of pathogen transmission, particularly when impregnated on long-lasting treated bednets. When applied at low doses, pyrethroids have a toxic effect, however the development of pyrethroid resistance in several mosquito species may jeopardize these beneficial effects. The need to find additional compounds, either to kill disease-carrying mosquitoes or to prevent mosquito contact with humans, therefore arises. In laboratory conditions, the effects (i.e., repellent, irritant and toxic) of 20 plant extracts, mainly essential oils, were assessed on adults of Anopheles gambiae, a primary vector of malaria. Their effects were compared to those of DEET and permethrin, used as positive controls. Most plant extracts had irritant, repellent and/or toxic effects on An. gambiae adults. The most promising extracts, i.e. those combining the three types of effects, were from Cymbopogon winterianus, Cinnamomum zeylanicum and Thymus vulgaris. The irritant, repellent and toxic effects occurred apparently independently of each other, and the behavioural response of adult An. gambiae was significantly influenced by the concentration of the plant extracts. Mechanisms underlying repellency might, therefore, differ from those underlying irritancy and toxicity. The utility of the efficient plant extracts for vector control as an alternative to pyrethroids may thus be envisaged.