Excito-repellency of Myristica fragrans Houtt. and Curcuma longa L. extracts from Southern Thailand against Aedes aegypti (L.).

The development of local plant extracts as a mosquito repellent is environmentally safe, inexpensive, and easily accessible for residents. In this study, three extracts from two local plants, Myristica fragrans Houtt. (flesh and mace) and Curcuma longa L. (rhizome) from southern Thailand, were investigated for their inherent repellent activity using the excito-repellency (ER) assay system against insectary-colonized Aedes aegypti (L.) (Diptera: Culicidae). The escape responses of mosquitoes exposed to concentrations of 0.5% to 5.0% (w/v) were measured to determine the contact irritant and non-contact repellent properties of each extract. Both the flesh and mace extracts of M. fragrans had relatively limited contact irritants (28.1% and 34.6% escape) and non-contact repellent (16.7% and 18.3% escape) activities against Ae. aegypti, respectively. The C. longa rhizome extract produced higher escape responses in the non-contact (42.6% escape) and contact (41.4% escape) trials at concentrations of 5.0% and 1.0%, respectively. GC-MS analysis found diethyl malate (56.5%) and elemicin (11.7%) to be the main components of the flesh and mace extracts, respectively, while ar-turmerone (24.6%), ß-turmerone (15.2%), α-turmerone (10.5%) were the primary constituents of the rhizome extract. Overall, our results indicate that both M. fragrans extracts primarily caused Ae. aegypti escape through contact irritation. For C. longa, lower concentrations (0.5% and 1.0%) exhibited contact irritancy, but higher concentrations (2.5% and 5.0%) exhibited non-contact repellency against Ae. aegypti. Although they had limited efficacy, further experiments (e.g., mixing with other plant-based compounds) could enhance the ER of both local plant extracts. Additional evaluation of these extracts against other mosquito species and the ER of their chemical components, either alone or in combination, would also be beneficial for the development of green repellents. Our findings emphasize the possibility of utilizing plant-based mosquito repellent as an alternative personal protection method for future mosquito control programs.

Enhanced Excito-Repellency of Binary Mixtures of Plant-Based Mosquito Repellents Against Culex quinquefasciatus Say (Diptera: Culicidae), a Night Biting Mosquito Species.

Efficacies of essential oils (EOs) of Vetiveria zizanioides (L.) Nash. (Poales: Poaceae) (VZ EO), Cananga odorata (Lam) Hook. F. & Thomson (Magnoliales: Annonaceae) (CO EO), and crude extract (CE) of Andrographis paniculata (Burm.F.) Wall ex. Nees (Lamiales: Acanthaceae) (AP CE), against laboratory (lab) and field strains of Culex quinquefasciatus Say were investigated. Irritant and repellent activities of individual and binary mixtures of plant extracts were compared with N,N-diethyl-m-toluamide (DEET) using an excito-repellency system. The irritant activity (direct tarsal contact), the mean percent escape response of VZ EO (91.67%, 83.33%), and CO EO (80%, 88.33%) were not significantly different compared with DEET (88.33%, 95%) against lab and field strains, respectively. Similarly, irritant responses in combinations (1:1 and 1:2, v:v) of either VZ EO or CO EO with AP CE were not significantly different from DEET against both strains (P > 0.001). The repellent activity (no tarsal contact), the mean percent escape response of VZ EO (68.33%), CO EO (61.67%), and VZ EO+AP CE (1:1, v:v) (81.67%) against lab strain and CO EO (85%) against field strain were not significantly different from that of DEET (P > 0.001). Interestingly, the greatest contact irritancy of VZ EO+AP CE (1:1, v:v) (96.67%) (P = 0.0026) and a stronger repellency response of CO EO (85%) (P = 0.0055) produced significantly different patterns of escape response compared with DEET against both lab and field strains, respectively. The EOs of VZ EO and CO EO or their mixture with AP CE showed potential as plant-based active ingredients for mosquito repellents. In addition, the major chemical constituents of VZ EO were ß-vetivone (6.4%), khusimol (2.96%), and α-vetivone (2.94%) by gas chromatograpy-mass spectrometry.

Excito-repellency Activity of Andrographis paniculata (Lamiales: Acanthaceae) Against Colonized Mosquitoes.

Excito-repellency activity of plant extracts have been increasingly studied as mosquito repellents. In this study, the crude extract of Andrographis paniculata was evaluated for its noncontact repellency, contact excitation (irritancy + repellency), and knockdown/toxicity response against five colonized mosquitoes; Aedes aegypti (L.), Aedes albopictus (Skuse), Anopheles dirus Peyton & Harrison, Anopheles epiroticus Linton & Harbach, and Culex quinquefasciatus Say (Diptera: Culicidae) using an excito-repellency assay system under laboratory-controlled conditions. The escape responses were observed at four different concentrations (0.5-5.0% w/v) with A. paniculata showing strong spatial repellency against Ae. albopictus (96.7% escape) and Ae. aegypti (71.7% escape) at the 2.5% and 0.5% concentrations, respectively. At 0.5% and 5.0% concentrations, the greatest repellency was seen for An. dirus (48.2% escape) and Cx. quinquefasciatus (59.7% escape), respectively. Comparatively, low repellency action was observed against An. epiroticus (1.6-15.0% escape). Escape in contact assays (before adjustment) was generally less pronounced compared to noncontact spatial repellency, with Ae. albopictus showing highest percent escape (71.4% escape) in the contact assay at 1.0% concentration. After adjusting for spatial repellency, escape due to contact irritancy alone was either not present or an insignificant contribution to the overall avoidance response for all species. No knockdown or mortality at 24-h postexposure was observed in any trials. These findings indicate that the A. paniculata crude extract is more active against day-biting mosquitoes; however, this may be a reflection of the time of testing. This study demonstrates compelling evidence that A. paniculata extract performs primarily as a spatial repellent. Further investigations exploring the use A. paniculata as a potential active ingredient in repellent products are needed.

Comparative Excito-Repellency of Three Cambodian Plant-Derived Extracts Against Two Mosquito Vector Species, Aedes aegypti and Anopheles minimus.

A study of the behavioral responses of Aedes aegypti and Anopheles minimus to 3 Cambodian plant extracts at 3 different concentrations (1%, 2.5%, and 5%) was performed using an excito-repellency test system. These 3 plants were Strophanthus scandens, Capparis micracantha, and Dioscorea hispida, selected according to traditional healer's knowledge, bibliographic studies and market surveys. Results showed that S. scandens leaves' hexane extract was the only one to exert repellency against Ae. aegypti with 23.3% of escaped mosquitoes at a concentration of 5%. Capparis micracantha was responsible for an irritant activity against An. minimus with 20.2% of escaped mosquitoes at a concentration of 2.5% and 22.8% escaping at a concentration of 5%. Dioscorea hispida showed an irritant activity on both mosquito species with 23.2% of escaped Ae. aegypti at a concentration of 5% and about 20% of escaped An. minimus at 2.5% and 5%. This is the first report on the irritant and repellent activities of S. scandens , D. hispida , and C. micracantha against mosquito species.

Avoidance Behavior to Essential Oils by Anopheles minimus, a Malaria Vector in Thailand.

Essential oils extracted from 4 different plant species--citronella (Cymbopogon nardus), hairy basil (Ocimum americanum), sweet basil (Ocimum basilicum), and vetiver (Vetiveria zizanioides)-were investigated for their irritant and repellent activities against Anopheles minimus, using an excito-repellency test system. Pure essential oils were used in absolute ethanol at the concentrations of 0.5%, 1%, 2.5%, and 5% (v/v) compared with deet. At the lowest concentration of 0.5%, hairy basil displayed the best irritant and repellent effects against An. minimus. Citronella and vetiver at 1-5% showed strong irritant effects with>80% escape, while repellent effects of both oils were observed at 1% and 2.5% citronella (73-89% escape) and at 5% vetiver (83.9% escape). Sweet basil had only moderate irritant action at 5% concentration (69.6% escape) and slightly repellent on test mosquitoes (<50% escape). The results found that hairy basil, citronella, and vetiver are promising potential mosquito repellent products for protection against An. minimus.

Plants traditionally used as mosquito repellents and the implication for their use in vector control.

Numerous plants with insect repelling properties are native to the tropics where they are produced for a wide range of medicinal purposes. In Thailand, these native plant species have a history of use for personal protection against biting insects. From our investigation we identified 37 plant species within 14 plant families that showed some mosquito repellent properties. Of these, 9 plant species were characterized using an excito-repellency test system against several Thai mosquito species. Results from these studies revealed that five essential oils extracted from plants demonstrated promising insect repellent activity. These active ingredients show promise for further development into formulations that may serve as alternatives to DEET or possibly be used as natural bio-pesticides to kill mosquitoes.

The effects of plant essential oils on escape response and mortality rate of Aedes aegypti and Anopheles minimus.

The High Throughput Screening System (HITSS) has been applied in insecticide behavioral response studies with various mosquito species. In general, chemical or natural compounds can produce a range of insect responses: contact irritancy, spatial repellency, knock-down, and toxicity. This study characterized these actions in essential oils derived from citronella, hairy basil, catnip, and vetiver in comparison to DEET and picaridin against Aedes aegypti and Anopheles minimus mosquito populations. Results indicated the two mosquito species exhibited significantly different (P<0.05) contact irritant escape responses between treatment and control for all tested compound concentrations, except with the minimum dose of picaridin (P>0.05) against Ae. aegypti. Spatial repellency responses were elicited in both mosquito species when exposed to all compounds, but the strength of the repellent response was dependent on compound and concentration. Data show that higher test concentrations had greatest toxic effects on both mosquito populations, but vetiver had no toxic effect on Ae. aegypti and picaridin did not elicit toxicity in either Ae. aegypti or An. minimus at any test concentration. Ultimately, this study demonstrates the ability of the HITSS assay to guide selection of effective plant essential oils for repelling, irritating, and killing mosquitoes.

Excito-repellency of essential oils against an Aedes aegypti (L.) field population in Thailand.

An investigation of the behavioral responses of Aedes aegypti (= Stegomyia aegypti) to various concentrations of essential oils (2.5, 5, and 10%) extracted from hairy basil (Ocimum americanum Linn), ginger (Zingiber officinale Roscoe), lemongrass (Cymbopogon citratus Stapf), citronella grass (Cymbopogon nardus Rendle), and plai (Zingiber cassumunar Roxb) were performed using an excito-repellency test chamber. Results showed that Ae. aegypti exhibited varying levels of escape response in both the contact and noncontact chambers in response to different essential oils. The magnitude of the behaviors changed in a dose-response fashion depending on the percent volume to volume concentration of oil used. A 2.5% concentration of hairy basil oil produced a significantly greater escape response compared to the other extracts at the same concentration (P< 0.05). Oils of ginger, lemongrass, and citronella produced stronger irritant and repellent responses at the median 5% concentration compared to the lowest and highest concentrations. There was marked suppression of escape for both contact and noncontact tests using 10% concentrations of hairy basil, lemongrass, and citronella, with high knockdown for all three oils after 30 min. Hairy basil and lemongrass had the highest insecticidal activity to Ae. aegypti, with LC50 values of 6.3 and 6.7 percent, respectively. We conclude that the essential oils from native plants tested, and likely many other extracts found in plants, have inherent repellent and irritant qualities that should to be screened and optimized for their behavior-modifying properties against Ae. aegypti and other biting arthropods of public health and pest importance.

Comparative Behavioral Responses of Pyrethroid-Susceptible and -Resistant Aedes aegypti (Diptera: Culicidae) Populations to Citronella and Eucalyptus Oils.

The objective of this study was to compare the behavioral responses (contact irritancy and noncontact spatial repellency) between susceptible and resistant populations of Aedes aegypti (L.) (=Stegomyia aegypti) to essential oils, citronella, and eucalyptus, Eucalyptus globulus, extracts, using an excito-repellency test system. N, N-diethyl-meta-toluamide (DEET) was used as the standard reference repellent. Mosquitoes included two long-standing insecticide susceptible colonies (U.S. Department of Agriculture and Bora Bora) and two pyrethroid-resistant populations recently obtained from Phetchabun and Kanchanaburi provinces in Thailand. Both DEET and citronella produced a much stronger excitation ("irritancy") and more rapid flight escape response in both pyrethroid-resistant populations compared with the laboratory populations. Noncontact repellency was also greater in the two resistant populations. Eucalyptus oil was found to be the least effective compound tested. Differences in responses between long-established pyrethroid-susceptible colonies and newly established and naturally resistant colonies were clearly demonstrated. These findings also demonstrate the need for further comparisons using natural pyrethroid-susceptible populations for elucidation of factors that might contribute to different patterns of escape behavior.

Excito-repellency properties of essential oils from Melaleuca leucadendron L., Litsea cubeba (Lour.) Persoon, and Litsea salicifolia (Nees) on Aedes aegypti (L.) mosquitoes.

We compared the behavioral responses of Aedes aegypti females to essential oils extracted from native plants Melaleuca leucadendron (ML), Litsea cubeba (LC), and Litsea salicifolia (LS) by using an excito-repellency test chamber. Mortality of Ae. aegypti females following 24 h holding period post-contact and non-contact trials were observed. No mortality was seen in escaped mosquitoes after direct contact with essential oils ML and LS, and low mortality (2.3-20.4%) with LC. Likewise, in all non-contact trials, no mortality was observed in escaped females from all three treatments, whereas low mortality was seen in non-escaped mosquitoes exposed to LC (0-14.3%) and LS (0-17.1%). Ae. aegypti showed significantly higher escape rates from contact chambers treated with ML and LC compared to LS, regardless of test concentrations (P<0.05). Non-contact repellent responses were significantly pronounced with LS, except at the highest (6.0%) concentration. We conclude that essential oils from all three botanicals exhibit significant irritant and repellent properties against Ae. aegypti and deserve further investigation for possible use as active ingredients in topical (skin) and indoor dispersed repellent systems.

Behavioral responses of catnip (Nepeta cataria) by two species of mosquitoes, Aedes aegypti and Anopheles harrisoni, in Thailand.

An investigation of the biological effect of catnip oil (Nepeta cataria L.) on the behavioral response of field collected Aedes aegypti and Anopheles harrisoni was conducted using an automated excitorepellency test system. Aedes aegypti showed significantly higher escape rates from the contact chamber at 5% catnip oil compared to other concentrations (P < 0.05). With Anopheles harrisoni, a high escape response was seen at 2.5% catnip oil from the contact chamber, while in the noncontact chamber a higher escape response was observed at a concentration of 5%. Results showed that this compound exhibits both irritant and repellent actions.