Larvicidal effect of pepper plants on Aedes aegypti (L.) (Diptera: Culicidae).

Ethanolic extracts derived from three species of the Piperaceae (pepper) family, Piper longum L., P. ribesoides Wall., and P. sarmentosum Roxb. ex Hunt., were evaluated for efficacy against early 4th instar larvae of Aedes aegypti mosquitoes using larvicidal bioassays. The highest larvicidal efficacy was established from P. longum, followed by P. sarmentosum and P. ribesoides, with LC50 values of 2.23, 4.06, and 8.13 ppm, respectively. Observations of morphological alterations on treated 4th instar larvae revealed that most organs, except anal papillae, had a normal structural appearance that was similar to controls. Under light microscopy, the internal structures of anal papillae in the treated larvae showed shrinkage, while the external features were normal in appearance. Ultrastructural studies, however, clearly demonstrated external destruction, with extensive damage and shrunken cuticle of the anal papillae. The structural deformation of anal papillae probably led to their dysfunction, which may be intrinsically associated with the death of the larvae. This study affords some evidence regarding the action site of the pepper extracts and suggests their potential in developing new types of larvicides used for mosquito control.     

Influence of nutritional and physiological status on behavioral responses of Aedes aegypti (Diptera: Culicidae) to deltamethrin and cypermethrin.

Excito-repellency responses of Aedes aegypti (L.) exposed to deltamethrin and cypermethrin were assessed using an excito-repellency test system. Contact irritancy and non-contact repellency assays compared non-bloodfed (unfed) parous (post-gravid), nulliparous, early blood-fed, late blood-fed, sugar-fed, and unmated female mosquitoes for behavioral responses based on nutritional and physiological conditions at the time of testing. Rates of escape during contact exposure with either compound were most pronounced in parous mosquitoes, followed by unmated mosquitoes, when compared to other conditional states. Significantly higher numbers of parous females also escaped from control chambers compared to other cohorts (P <0.05). Irritability of blood- and sugar-fed mosquitoes was noticeably suppressed. We conclude that nutritional and physiological conditions (including age) of mosquitoes at the time of testing can significantly influence behavioral responses (excito-repellency) to insecticides. The findings indicate that whether due to chronological age, nutrition, physiological state, or innate (circadian) activity patterns, careful consideration must be given to the selection of appropriate conditioned mosquitoes for testing.     

Susceptibility and avoidance behavior by Culex quinquefasciatus Say to three classes of residual insecticides.

The behavioral responses ofthree colonized strains of Culex quinquefasciatus, two from recent field collections in Thailand (Nonthaburi and Mae Sot) and one from a long-established colony from the National Institute of Health (NIH), Ministry of Public Health, Thailand, were compared during and after exposure to deltamethrin (0.02 g/m2), propoxur (0.2 g/m2), and fenitrothion (0.2 g/m2) using an excito-repellency escape chamber system. We observed striking differences in behavioral response and excito-repellency between mosquito strains and test compounds. Greater escape responses were observed in the NIH strain during direct contact with deltamethrin and fenitrothion compared with the two field populations. Deltamethrin was the most irritant, followed by fenitrothion. Escape responses with propoxur were significantly delayed but increased slightly towards the end of the 30-min exposure period, more notably in the Nonthaburi strain (P < 0.05). Non-contact repellent responses were generally much weaker than irritancy, with the greatest escape response seen with NIH and Nonthaburi. Deltamethrin showed the weakest repellent response overall (< 10% escape), while propoxur again demonstrated a delayed effect (NIH and Mae Sot) before escape occurred. We conclude that irritant and repellent behavioral responses by Cx. quinquefasciatus are important components for assessing the impact of residual spraying in mosquito control programs. A better understanding of chemical properties that elicit behavioral responses in mosquitoes should be considered in formulating control strategies designed to control mosquitoes or mitigate disease transmission risk.     

Chemical composition and anti-mosquito potential of rhizome extract and volatile oil derived from Curcuma aromatica against Aedes aegypti (Diptera: Culicidae).

Crude rhizome extracts and volatile oils of Curcuma aromatica were evaluated for chemical composition and anti-mosquito potential, including larvicidal, adulticidal, and repellent activities against the Aedes aegypti mosquito. Chemical identification achieved by GC/MS analysis revealed that xanthorrhizol, 1H-3a, 7-methanoazulene and curcumene at 35.08 and 13.65%, 21.81 and 30.02%, and 13.75 and 25.71%, were the main constituents in hexane extracts and volatile oils, respectively. Volatile oil of Cu. aromatica possessed a significantly higher larvicidal activity against the 4th instar larvae of Ae. aegypti than that of hexane extracts, with LC50 values of 36.30 and 57.15 ppm, respectively. In testing for adulticidal activity, on the other hand, hexane-extracted Cu. aromatica (LC50: 1.60 microg/mg female) was found to be slightly more effective against female Ae. aegypti than volatile oil (LC50: 2.86 microg/mg female). However, the repellency of these two products against Ae. aegypti adult females differed significantly. The hexane-extracted Cu. aromatica, with a median complete protection time of 1 h (range = 1-1.5 h) when applied at a concentration of 25%, appeared to have significantly higher repellency than that of distillate oil (0.5 h, range = 0-0.5 h). The different results obtained from both products of Cu. aromatica were probably due to variety in quantity and type of active ingredients as well as the biological and physiological characteristics that differed between both developmental stages of mosquitoes, larvae, and adults.     

The in vitro effect of albendazole, ivermectin, diethylcarbamazine, and their combinations against infective third-stage larvae of nocturnally subperiodic Brugia malayi (Narathiwat strain): scanning electron microscopy.

Scanning electron microscopy (SEM) was employed to observe the effects of ivermectin (IVM), diethylcarbamazine (DEC), and albendazole (ALB) alone, and the drugs in combination (ALB+IVM and ALB+DEC) against infective third stage larvae (L3) of nocturnally subperiodic (NSP) Brugia malayi (Narathiwat strain) in vitro. IVM, at a concentration of 10(-4) M, killed L3 within 1-2 h. The SEM data showed damage to the L3 surface and loss of regular cuticular annulations. The cuticles were grooved in the middle region of the body. In comparison with normal L3 before treatment with IVM, the cuticular surface showed transversed striations with periodic annulations. The result demonstrated that IVM showed a larvicidal activity against L3 of NSP B. malayi cultivated in vitro. Compared with those larvae in the control group, the treated larvae had no morphological changes in the cuticular surface at the head, body, and tail regions after cultivation with all drugs alone, and in their combinations at a concentration of 10(-5) M for 7 d. In this system, and at that concentration, only the larvae cultured with ALB alone remained highly motile. Although no morphological changes had been observed by SEM, those drugs used alone (IVM and DEC) and in combinations (ALB+IVM and ALB+DEC), reduced larval motility throughout the experiments at a concentration of 10(-5) M. The minimum lethal concentration (MIC) of IVM against NSP B. malayi was 10(-4) M.     

Chemically induced behavioral responses in Anopheles minimus and Anopheles harrisoni in Thailand

Behavioral responses of female mosquitoes representing two species in the Minimus Complex exposed to an operational field dose of bifenthrin or DEET (N,N-diethyl-m-toluamide) were described using an excito-repellency test system. Two test populations of An. minimus, one from the field (Tak Province, western Thailand), the other from a long-established laboratory colony, and Anopheles harrisoni collected from Kanchanaburi Province, western Thailand, were used. Results showed that all test populations rapidly escaped after direct contact with surfaces treated with either bifenthrin or DEET compared to match-paired untreated controls. Greater escape response by exposed females to bifenthrin and DEET were observed in the An. minimus colony compared to the two field populations. Field-collected An. minimus demonstrated a more rapid escape response to DEET than to bifenthrin, whereas An. harrisoni showed a converse response. Although fewer females escaped from test chambers without direct contact with treated surfaces compared to contact tests, the spatial repellency response was significantly pronounced in all test populations compared to match-paired controls (P < 0.05). DEET was found to perform as both a contact stimulant and moderate spatial repellent.     

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 studies on the biology and filarial susceptibility of selected blood-feeding and autogenous Aedes togoi sub-colonies

Blood-feeding and autogenous sub-colonies were selected from a laboratory, stock colony of Aedes togoi, which was originally collected from Koh Nom Sao, Chanthaburi province, Southeast Thailand. Comparative biology and filarial susceptibility between the two sub-colonies (blood-feeding: F11, F13; autogeny: F38, F40) were investigated to evaluate their viability and vectorial capacity. The results of comparison on biology revealed intraspecific differences, i.e., the average egg deposition/gravid female (F11/F38; F13/F40), embryonation rate (F13/F40), hatchability rate (F11/F38; F13/F40), egg width (F11/F38), wing length of females (F13/F40), and wing length and width of males (F11/F38) in the blood-feeding sub-colony were significantly greater than that in the autogenous sub-colony; and egg length (F11/F38) and width (F13/F40), and mean longevity of adult females (F11/F38) and males (F13/F40) in the blood-feeding sub-colony were significantly less than that in the autogenous sub-colony. The results of comparison on filarial susceptibility demonstrated that both sub-colonies yielded similar susceptibilities to Brugia malayi [blood-feeding/autogeny = 56.7% (F11)/53.3%(F38), 60%(F13)/83.3%(F40)] and Dirofilaria immitis [blood-feeding/autogeny = 85.7%(F11)/75%(F38), 45%(F13)/29.4%(F40)], suggesting autogenous Ae. togoi sub-colony was an efficient laboratory vector in study of filariasis.     

Intraspecific hybridization of Anopheles sinensis (Diptera: Culicidae) strains from Thailand and Korea

Anopheles (Anopheles) sinensis [Wiedemann (1828)] is a member of the hyrcanus species group, and it has been incriminated as the natural or experimental malaria vectors in the Republic of Korea, Japan, China, and Indonesia. In Thailand, however, An. sinensis seems to be of little medical importance. Hybridization tests among the three iso-female lines (isolines) of An. sinensis [i.e., Form A (X, Y1) and Form B (X, Y2) (Thailand strain), and Form B (X, Y2) (Korean strain)] were established based on two distinct types of metaphase chromosomes and geographical differences The chromosomal form of the Korean strain was first identified from this study. Results of reciprocal and back crosses indicated that both karyotypic forms of theAn. sinensis Thailand and Korean strains were genetically compatible, and provided viable progenies and completely synaptic polytene chromosomes. The sequences of the rDNA internal-transcribed spacer 2 (ITS2) and mitochondrial cytochrome c oxidase subunit II (COII) among the An. sinensis strains were nearly identical to each other, and the intraspecific sequence variability was very low (0.0-0.6%). Sequence comparisons among the cryptic inter-species (i.e., An. sinensis, An. lesteri, and An. yatsushiroensis), however, revealed extensive divergence, and the intraspecific variability ranged from 12.2 to 34.6%. Therefore, it is concluded from these results and previous vector ability studies that the An. sinensis Forms A and B exhibit cytological polymorphic races that have different vector abilities in their transmission of malaria, depending on their geographical locations.     

Excito-Repellent Responses between <Emphasis Type="Italic">Culex quinquefasciatus</Emphasis> Permethrin Susceptible and Resistant Mosquitoes

The behavioral responses exhibited between four permethrin-resistant field-derived populations and a long-established, permethrin susceptible colony population of Culex quinquefasciatus mosquitoes was assessed for behavioral responses to permethrin in an excito-repellency (ER) test system. After a 30-min exposure to two different insecticide concentrations (200 and 500 mg/m²) in paired contact and noncontact test designs, the initial knockdown and escape (exit) responses were recorded followed by mortality after 24-h. All five test populations rapidly escaped from the chambers treated with permethrin regardless of background insecticide susceptibility status or chemical concentration. The greatest contact escape response was seen from the colonized population with 88 % of specimens escaping the chamber within 30-min. Contact irritancy was the predominant response compared to relatively weak noncontact spatial repellency in all test populations at both low and high concentrations. At the lower concentration, the contact escape responses did not appear influenced by background susceptibility status, whereas at the higher concentration the permethrin-resistant field populations exhibited significantly reduced avoidance behavior compared to the laboratory susceptible strain.