Repellent efficacy of 20 essential oils on Aedes aegypti mosquitoes and Ixodes scapularis ticks in contact-repellency assays

Repellency of Essential Oils and Plant-Derived Compounds Against Aedes aegypti Mosquitoes

Plant-based oils have a long history of use as insect repellents. In an earlier study, we showed that in a 10% concentration, geraniol, 2-phenylethl propionate, and the plant-based essential oils clove and cinnamon effectively protected from mosquito bites for over 60 min. To expand on this study, we reanalyzed our GC-MS data to identify the short organic constituents of these oils. We then used an arm-in-cage assay to test the repellency of different concentrations and combinations of these oils and pure compounds. We found a sigmoidal relationship between the complete protection time from mosquito bites and the concentration of these oils. The complete protection times we recorded for combinations of these oils suggest an absence of additive effects. The results of this study can inform the development of novel, effective, and plant-based insect repellents.

Lemongrass essential oil and DEET inhibit attractant detection in infected and non-infected Ixodes scapularis ticks

Blacklegged tick, Ixodes scapularis Say (Arachnida: Ixodidae), is a growing health concern for humans as vectors the causative agent of Lyme disease, Borrelia burgdorferi, and many other pathogens. Given the potential health threat I. scapularis entails, and the need to find effective strategies to prevent tick bites, it is pivotal to understand the chemosensory system of ticks and their host-seeking behaviour when exposed to repellents. In this study, we investigated whether the exposure to synthetic and plant-derived repellents impairs the ability of I. scapularis to detect attractants and host volatiles (butyric acid), and ultimately how these repellents interfere with host-seeking behaviour in both wild and lab-reared ticks. Furthermore, we screened wild ticks used in electrophysiology and Y-tube behavioural assays for presence of pathogens (Borrelia, Anaplasma, and Babesia) to evaluate if the bacterial infection status would affect the detection of butyric acid under the exposure to repellents. We determined that the exposure to DEET, lemongrass essential oil, citral, and geraniol significantly inhibited the ability of both lab-reared and wild adult female I. scapularis to detect and respond to butyric acid. We found that tick infection status does not significantly impact host-seeking behaviour in adult female I. scapularis. The knowledge gained from our study contributes to advance our understanding of host-seeking behaviour in ticks and the impact that the exposure to repellent has on the tick chemosensory system. These findings will be important for elucidating the mechanism of repellence in ticks and for the development of effective tick repellent management tools.

Effective Mosquito Repellents: Myrcene- and Cymene-Loaded Nanohydrogels against Aedes aegypti

Aedes mosquito-borne diseases remain a significant global health threat, necessitating effective control strategies. This study introduces monoterpenes-based nanohydrogels for potential use as repellents against Aedes aegypti, the primary dengue vector worldwide. We formulated hydrogels using cymene- and myrcene-based nanoemulsions with different polymers: chitosan, carboxymethylcellulose (CMC), and carbopol®. Our evaluations of rheological, texture, and bioadhesive properties identified CMC hydrogel as the most promising gelling agent for topical application, exhibiting sustained monoterpene release over 12 h with low skin permeation and high retention in the stratum corneum. Myrcene-loaded CMC hydrogel achieved a 57% feeding deterrence compared to 47% with cymene hydrogel in the mosquito membrane-feeding model. Molecular docking studies revealed interactions between myrcene and an essential amino acid (Ile116) in the Ae. aegypti odorant-binding protein 22 (AeOBP22), corroborating its higher repellent efficacy. These findings suggest that myrcene-loaded CMC hydrogels offer a promising, minimally invasive strategy for personal protection against Ae. aegypti and warrant further investigation to optimize monoterpene concentrations for vector control.

Rancid rumors or Native wisdom: Evaluating the efficacy of animal fats as insect repellents attributed to historic-period Native Americans

Little is known about Native American adaptations to blood-sucking arthropods prior to and following European contact. Multiple accounts starting in the 16th century suggest that rancid animal grease was employed by Gulf Coast indigenes as a mosquito repellent. Although many Native American ethnobotanical remedies for biting insects have been recorded, the use of animal products for this purpose is not well documented. Moreover, few traditional Native American mosquito repellents have been examined using controlled laboratory methods for repellency testing. In this study, we tested the repellent efficacy of fats derived from alligator, bear, cod, and shark that were aged to various stages of rancidity. Using yellow fever mosquitoes, (Aedes aegypti), we performed an arm-in-cage assay to measure the complete protection times resulted from these fats, when applied to human skin. We used a Y-tube olfactometer assay to evaluate long-distance repellency and tested tick-repellency in a crawling assay. Our results suggest that rancid animal fats from cod, bear, and alligator are potent albeit short-lived mosquito repellents. We found that both rancid and fresh fats do not repel ticks. Our findings show the validity of traditional ethnozoological knowledge of Native American people and support aspects of the ethnohistorical record.

Laboratory assessment of the acaricidal, repellent and anti-cholinesterase effects of Melaleuca alternifolia and Chamaemelum nobile essential oils against Hyalomma scupense ticks

In cattle, Hyalomma scupense serves as an important vector of several pathogens resulting in diseases, subsequently affecting the agricultural field as well as the economy. Resistance to chemical acaricides has become widespread affirming the need for new drugs to tick control. The goal of this study was to investigate the acaricidal, repellent activities as well as the putative mode of action of two essential oils (EOs) from Melaleuca alternifolia (Tea tree) and Chamaemelum nobile (Roman chamomile) on Hyalomma scupense. The chemical composition of EOs was also evaluated. Different concentrations of EOs were tested in vitro for their acaricidal property on adults and larvae of H. scupense using adult immersion test (AIT) and larval packet test (LPT). Additionally, using Ellman's spectrophotometric method, the anticholinesterase (AChE) inhibition activity of M. alternifolia and C. nobile EOs was assessed in order to understand their putative mode of action. The main compounds of C. nobile were α-Bisabolene (22.20%) and (E)-β-Famesene (20.41%). The major components in the analyzed M. alternifolia were Terpinen-4-ol (36.32%) and γ-Terpinene (13.69%). Adulticidal and larvicidal assays demonstrated a promising efficacy of the essential oils against tick H. scupense. The lethal concentration (LC50) values obtained for M. alternifolia and C. nobile oils were 0.84 and 0.96 mg/mL in the AIT and 0.37 and 0.48 mg/mL in the LPT, respectively. Regarding repellent activity, M. alternifolia achieved 100% repellency at the concentration of 1 mg/mL while C. nobile showed 95.98% repellency activity at concentration of 4 mg/mL. Also, M. alternifolia and C. nobile EOs displayed potent AChE inhibition with IC50 value of 91.27 and 100.12 μg/mL, respectively. In the present study, M. alternifolia and, to a lesser degree, C. nobile EOs were found to be effective in vitro acaricides, repellents and acetylcholinesterase inhibitor against H. scupense ticks. These plants may represent an economical and sustainable alternative to toxic synthetic acaricides in the management of ectoparasites of veterinary importance.

How to repel a killer; chemical identification and effective repellent activity of commercial essential oils against kissing bugs

Triatomines are haematophagous insects, some species are vectors of Trypanosoma cruzi, the aetiological agent of Chagas disease. The main strategy for interrupting T. cruzi transmission is to avoid contact of the vector populations with humans. Volatiles from commercial essential oils are excellent candidates to serve as repellents of kissing bugs. We used an exposure device to assess the repellence effect of eight commercial essential oils on Triatoma pallidipennis. The most effective oils were blended and evaluated against T. infestans, T. pallidipennis and Rhodnius prolixus. The blend was also evaluated on parasitised T. pallidipennis. Data were compared with the commercial repellent NN-diethyl-3-methylbenzamide. We recorded the time the insects spent in the proximity of the host and determined if any of the evaluated oils served as kissing bug repellent. We found commercial essential oils and a blend that significantly reduced the time spent in the proximity of the host. The blend was effective for use by human males and females, repelling infected and non-infected insects. The study of essential oils as repellents of blood-sucking disease-vector insects could shed light on the development of new control strategies.

Repellent activity of essential oils to the Lone Star tick, Amblyomma americanum

BACKGROUND

The Lone Star tick, Amblyomma americanum is important to human health because of a variety of pathogenic organisms transmitted to humans during feeding events, which underscores the need to identify novel approaches to prevent tick bites. Thus, the goal of this study was to test natural and synthetic molecules for repellent activity against ticks in spatial, contact and human fingertip bioassays.

METHODS

The efficacy of essential oils and naturally derived compounds as repellents to Am. americanum nymphs was compared in three different bioassays: contact, spatial and fingertip repellent bioassays.

RESULTS

Concentration response curves after contact exposure to 1R-trans-chrysanthemic acid (TCA) indicated a 5.6 μg/cm2 concentration required to repel 50% of ticks (RC50), which was five- and sevenfold more active than DEET and nootkatone, respectively. For contact repellency, the rank order of repellency at 50 μg/cm2 for natural oils was clove > geranium > oregano > cedarwood > thyme > amyris > patchouli > citronella > juniper berry > peppermint > cassia. For spatial bioassays, TCA was approximately twofold more active than DEET and nootkatone at 50 μg/cm2 but was not significantly different at 10 μg/cm2. In spatial assays, thyme and cassia were the most active compounds tested with 100% and 80% ticks repelled within 15 min of exposure respectively and was approximately twofold more effective than DEET at the same concentration. To translate these non-host assays to efficacy when used on the human host, we quantified repellency using a finger-climbing assay. TCA, nootkatone and DEET were equally effective in the fingertip assay, and patchouli oil was the only natural oil that significantly repelled ticks.

CONCLUSIONS

The differences in repellent potency based on the assay type suggests that the ability to discover active tick repellents suitable for development may be more complicated than with other arthropod species; furthermore, the field delivery mechanism must be considered early in development to ensure translation to field efficacy. TCA, which is naturally derived, is a promising candidate for a tick repellent that has comparable repellency to commercialized tick repellents.

Comparative analysis of essential oil efficacy against the Asian longhorned tick Haemaphysalis longicornis (Acari: Ixodidae)

This study evaluated the potential repellent and acaricidal effects of 4 essential oils (clove, eucalyptus, lavender, and mint) against the Asian longhorned tick Haemaphysalis longicornis, a vector of various tick-borne diseases in medical and veterinary contexts. Selected for their potential repellent and acaricidal properties, the 4 essential oils were tested on adult and nymph H. longicornis ticks at different concentrations. The experiment assessed mortality rates and repellency, particularly during tick attachment to host skin. There was a significant increase (p<0.05) in tick mortality and repellency scores across all groups. At a 1% concentration, adult tick mortality ranged from 36% to 86%, while nymph mortality ranged from 6% to 97%. Clove oil exhibited notable efficacy, demonstrating high mortality rates of nymphs and adults. Clove oil also displayed strong repellency properties, with a repellency index of 0.05, surpassing those of mint, eucalyptus, and lavender oils. Clove oil showed the highest effectiveness in deterring nonattached adult ticks (90%) and nymphs (95%) when applied to skin. Clove oil was the most effective against adult and nymph ticks, achieving mortality rates of 86% and 97%, respectively, and led to the highest nonattachment rates when applied to skin. In conclusion, essential oils such as clove, eucalyptus, lavender, and mint oils present promising results for tick population control.

Differential analysis of volatiles in five types of mosquito-repellent products by chemometrics combined with headspace GC-Orbitrap HRMS nontargeted detection

This paper reports a method for the differential analysis of volatile chemical components in five novel types of mosquito-repellent products based on chemometrics combined with headspace gas chromatography-Orbitrap high-resolution mass spectrometry (HS-GC-Orbitrap HRMS) nontargeted screening. A total of 358 unknown substances were detected in 30 samples under specific headspace conditions. Through principal component analysis and orthogonal partial least-squares discriminant analysis, 36 significantly different substances with variable importance in the projection values greater than 1 were further screened, and these substances were accurately identified by GC-Orbitrap HRMS. Most substances were found for the first time in mosquito-repellent products. The clustered heat map, Venn diagram and peak area histogram showed that the mosquito-repellent products had similar volatile composition, and the volatile species and content of different types of mosquito-repellent products significantly varied. Substances, such as eucalyptol, d-limonene, α-pinene, β-pinene, dl-menthol and methyl salicylate, may be the main sources of odour in mosquito-repellent products. This work explored the characteristic volatile components in mosquito-repellent products and comparatively analysed the chemical composition of different types of products. It can be generalised to consumer products as a case study and has positive implications for promoting product quality and safety and improving production processes.

In vitro acaricidal activity of essential oils and their binary mixtures against ixodes scapularis (Acari: Ixodidae)

Ixodes scapularis ticks are vectors of infectious agents that cause illness in humans, including Lyme disease. Recent years have seen a surge in tick-borne diseases (TBD) resulting in a high demand for tick management products. Plants offer a valuable source of active compounds for the development of novel, eco-friendly tick control products, reducing potential risks to human and animal health. Essential oils (EOs) have emerged as potential acaricides and repellents against ticks providing an alternative to synthetic chemicals and aiding in the prevention of TBD by lowering the risk of tick bites. We investigated the acaricidal activity of EOs from lemongrass (Cymbopogon citratus), geranium (Pelargonium x asperum), savory thyme (Thymus saturejoides), and white thyme (Thymus zygis) on I. scapularis. The interactions (i.e., synergistic, antagonistic, or additive) of their binary mixtures were also evaluated. EO samples were analyzed via gas chromatography-mass spectrometry to determine their chemical composition. The adult immersion test was used to determine the lethal concentration (LC50) of each EO alone and in mixtures. Quantitative assessment of synergistic, additive, or antagonistic effect of the binary mixtures was performed by calculating the combination index. Strong acaricidal activity was recorded for savory thyme and white thyme EOs, with LC50 values of 28.0 and 11.0 μg/μL, respectively. The LC50 of lemongrass and geranium EOs were 49.0 and 39.7 μg/μL, respectively. Among the tested EOs, savory thyme and white thyme had a strong acaricidal effect on I. scapularis, which might be linked to the presence of carvacrol (26.05 % ± 0.38) and thymol (53.6 % ± 2.31), main components present in savory thyme and white thyme EOs, respectively. The tick killing efficacy of lemongrass and geranium EOs was lower when mixed than when used separately (LC50 of 65.3 µg/µL). The same happened with savory thyme and white thyme EOs, except at 9.75 µg/µL where they had a synergistic effect (LC50 of 58.3 µg/µL). Lemongrass and savory thyme EOs had a synergistic effect at low concentrations, and an antagonistic effect at higher concentrations (LC50 of 95.4 µg/µL). Lemongrass and white thyme EOs had a synergistic effect against ticks from 15 to 120 µg/µL (LC50 of 18.5 µg/µL) similar to white thyme EO. Geranium and savory thyme EOs had an antagonistic effect at all concentrations, with an LC50 of 66.8 µg/µL. Geranium and white thyme EOs also had an antagonistic effect, except at 12.7 µg/µL where they had a synergistic effect (LC50 of 66.8 µg/µL). The interaction observed when combining selected essential oils suggests promising potential for developing acaricidal formulations aimed at controlling ticks and curbing the transmission of tick-borne disease agents.

Efficacy of unregulated minimum risk tick repellent products evaluated with Ixodes scapularis nymphs in a human skin bioassay

BACKGROUND

The majority of vector-borne disease cases in the USA are caused by pathogens spread by ticks, most commonly the blacklegged tick, Ixodes scapularis. Personal protection against tick bites, including use of repellents, is the primary defense against tick-borne diseases. Tick repellents registered by the Environmental Protection Agency (EPA) are well documented to be safe as well as effective against ticks. Another group of tick repellent products, 25(b) exempt or minimum risk products, use alternative, mostly botanically derived, active ingredients. These are considered to pose minimal risk to human health and therefore are exempt from EPA registration; efficacy testing is not mandated for these products.

METHODS

We used a finger bioassay to evaluate the repellency against I. scapularis nymphs for 11 formulated 25(b) exempt products together with two positive control DEET-based EPA registered products. Repellency was assessed hourly from 0.5 to 6.5 h after product application.

RESULTS

The DEET-based products showed ≥ 97% repellency for all examined timepoints. By contrast, an average of 63% of ticks were repelled in the first 1.5 h after application across the 11 25(b) exempt products, and the average fell to 3% repelled between 2.5 and 6.5 h. Ten of the 11 25(b) exempt products showed statistically similar efficacy to DEET-based products at 30 min after application (repellency of 79-97%). However, only four 25(b) exempt products maintained a level of repellency similar to DEET-based products (> 72%) at the 1.5-h mark, and none of these products were effective in repelling ticks at the timepoints from 2.5 to 6.5 h after application.

CONCLUSIONS

Neither the claims on the labels nor specific active ingredients and their concentrations appeared to predict the duration of efficacy we observed for the 25(b) exempt products. These products are not registered with the EPA, so the methods used to determine the application guidelines on their labels are unclear. Consumers should be aware that both the level of efficacy and the duration of repellency may differ among unregulated 25(b) exempt repellent products labeled for use against ticks. We encourage more research on these products and the 25(b) exempt active ingredients they contain to help determine and improve their efficacy as repellents under different conditions.

A critical review of current laboratory methods used to evaluate mosquito repellents

Pathogens transmitted by mosquitoes threaten human health around the globe. The use of effective mosquito repellents can protect individuals from contracting mosquito-borne diseases. Collecting evidence to confirm and quantify the effectiveness of a mosquito repellent is crucial and requires thorough standardized testing. There are multitudes of methods to test repellents that each have their own strengths and weaknesses. Determining which type of test to conduct can be challenging and the collection of currently used and standardized methods has changed over time. Some of these methods can be powerful to rapidly screen numerous putative repellent treatments. Other methods can test mosquito responses to specific treatments and measure either spatial or contact repellency. A subset of these methods uses live animals or human volunteers to test the repellency of treatments. Assays can greatly vary in their affordability and accessibility for researchers and/or may require additional methods to confirm results. Here I present a critical review that covers some of the most frequently used laboratory assays from the last two decades. I discuss the experimental designs and highlight some of the strengths and weaknesses of each type of method covered.

Efficacy of Unregulated Minimum Risk Products to Kill and Repel Ticks

Human-biting ticks threaten public health in the United States. Registration by the Environmental Protection Agency of products to kill host-seeking ticks or repel ticks contacting humans is indicative of their safety and effectiveness. Unregulated minimum risk products, exempt from Environmental Protection Agency registration and often based on botanical oils, are proliferating in the marketplace, but there is concern about their effectiveness to kill and repel ticks. Evaluations of such products are limited in the published literature. A review showed considerable variability among minimum risk products to kill host-seeking blacklegged ticks, with effectiveness similar to chemical pesticide products for some minimum risk products but minimal impact on the ticks for other products. Evaluations of minimum risk tick repellents have typically focused on individual active ingredients rather than formulated products, which often combine multiple active ingredients. Consumers should be aware that effectiveness to kill and repel ticks can differ among unregulated minimum risk products.

Comparison of in vitro and in vivo repellency bioassay methods for Ixodes scapularis nymphs

BACKGROUND

Numerous bioassay methods have been used to test the efficacy of repellents for ticks, but the comparability of results across different methods has only been evaluated in a single study. Of particular interest are comparisons between bioassays that use artificial containers (in vitro) with those conducted on a human subject (in vivo) for efficacy testing of new potential unregistered active ingredients, which most commonly use in vitro methods.

METHODS

We compared four different bioassay methods and evaluated three ingredients (DEET [N,N-Diethyl-meta-toluamide], peppermint oil and rosemary oil) and a negative control (ethanol) over a 6-h period. Two of the methods tested were in vivo bioassay methods in which the active ingredient was applied to human skin (finger and forearm bioassays), and the other two methods were in vitro methods using artificial containers (jar and petri dish bioassays). All four bioassays were conducted using Ixodes scapularis nymphs. We compared the results using nymphs from two different tick colonies that were derived from I. scapularis collected in the US states of Connecticut and Rhode Island (northern origin) and Oklahoma (southern origin), expecting that ticks of different origin would display differences in host-seeking behavior.

RESULTS

The results between bioassay methods did not differ significantly, even when comparing those that provide the stimulus of human skin with those that do not. We also found that tick colony source can impact the outcome of repellency bioassays due to differences in movement speed; behavioral differences were incorporated into the assay screening. DEET effectively repelled nymphs for the full 6-h duration of the study. Peppermint oil showed a similar repellent efficacy to DEET during the first hour, but it decreased sharply afterwards. Rosemary oil did not effectively repel nymphs across any of the time points.

CONCLUSIONS

The repellency results did not differ significantly between the four bioassay methods tested. The results also highlight the need to consider the geographic origin of ticks used in repellency bioassays in addition to species and life stage. Finally, our results indicate a limited repellent efficacy of the two essential oils tested, which highlights the need for further studies on the duration of repellency for similar botanically derived active ingredients and for evaluation of formulated products.