Fine structure of tarsal sensilla of Aedes aegypti (L.) (Diptera: Culicidae)

The tarsi of all three pairs of legs of both sexes of Aedes aegypti (L.) bear spine sensilla, five types of hair sensilla, which are designated A, B, C1, C2 and C3, and campaniform sensilla. Type A and B hairs, spines, and cam-paniform sensilla are innervated by one neuron with a tubular body, a characteristic of cuticular mechanoreceptors. In particular the hairs and spines are tactile receptors and the campaniform sensilla are proprioceptors. The C1, C2, and C3 hair sensilla have the morphological features of contact chemoreceptors. Type C1 and C3 hairs are innervated by five and four neurons, respectively, which extend to the tip of the hair. Type C2 is innervated by five neurons, one of which terminates at the base of the hair in a tubular body while the remaining four extend to the tip of the hair. The role of the type C hairs in oviposition behavior, nectar feeding, and recognition of conspecific females is discussed. Presumed efferent neurosecretory fibers occur near the spine and hair sensilla.     

Secretion in the male accessory glands of Aedes aegypti (L.) (Diptera : Culicidae)

The objective of this work is to determine the nature, the mode of synthesis, and release of the secretion of accessory glands of the male mosquito, Aedes aegypti (Diptera : Culicidae). Cytological studies revealed that each of the 2 glands has 2 morphological types of regionally specialized secretory cells, each type differing in the nature, and the release of its products. The anterior region (AR) cells secrete small (0.3–1.3 μm) electron-lucent granules, which during the peak period of synthesis seemed to transport the flocculent and fibrous granular substance to the apical cytoplasm of the cell by means of binary fusions. This cytoplasm, when filled with secretory granules, pinches off and falls freely into the lumen of the gland, thus, exhibiting a macroapocrine mode of secretion. The posterior region (PR) cells, however, elaborate large (mean diameter 2 μm) electron-dense granules, which are released into the lumen essentially by rupture of the cell membrane at the apical end. In males mated to depletion, the volume of the gland is reduced by 67%; there is no subsequent recovery of secretory capacity.     

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.     

Structural organization of the brain and subesophageal ganglion of male Aedes aegypti (L.) (Diptera : Culicidae)

The brain and subesophageal ganglion of male Aedes aegypti (L.) (Diptera : Culicidae) are described from cryofractures and silver-stained, semithin (0.5 μm) serial sections of whole heads observed in the scanning and light microscopes. The brain and subesophageal ganglion of male A. aegypti are fused. The major structures of the brain include the protocerebral lobes and bridge, the mushroom bodies, central complex of the protocerebrum, the mechanosensory regions and olfactory loves of the deutocerebrum, and the tritocerebrum. Major commissures of the brain are the anterior optic tract, central commissure, posterior dorsal commissure, and subesophageal commissure. The structural associations of brain components with each other and the subesophageal ganglion, as well as the paths of the major nerve tracts in male A. aegypti are described and compared with those in other Diptera.     

Key premises, a guide to Aedes aegypti (Diptera: Culicidae) surveillance and control

The population densities of Aedes aegypti Linneaus in four towns in Trinidad were studied using standard house-to-house inspections of all water-holding containers to determine whether persistently positive containers and premises existed over a three-month period in the wet season, from May to July 2002. From a total of 1503 houses inspected, 223 were positive with 41 persistently positive over the three month period and classified as 'key premises'. The definition of the term key premises is described and the rationale for its utilization discussed. A total of 24,439 containers was inspected from Santa Margarita (6407), Mt Lambert (5709), St Augustine (5384) and Curepe (6939) of which 1.3% or 334 containers were positive for A. aegypti larvae and pupae. A total of 16,507 immatures of A. aegypti were retrieved from these containers which comprised 17 container types but when these were ranked according to productivity levels, only water drums (average 53.5%), buckets (22.2%), tubs and basins (8.0%), water tanks (5.4%), brick holes (4.2%) and tyres (2.0%) were significant (P < 0.001) producers. The role that key premises play in the introduction and re-infestation of A. aegypti-free communities is described and illustrated. These results suggest that A. aegypti control programmes could be more cost effective and sustainable by concentrating efforts on key premises and key containers to control mosquito densities and Dengue transmission while reducing manpower needs and insecticide use.     

Occurrence of Toxorhynchites guadeloupensis (Dyar Knab) in oviposition trap of Aedes aegypti (L.) (Diptera: Culicidae)

Toxorhynchites guadeloupensis (Dyar Knab), a poorly known mosquito species, was observed preying upon Aedes aegypti (L.) larvae, in an oviposition trap placed for routine dengue entomological surveillance, during 2003-2004 in the urban area of Boa Vista, Roraima, Brazil. This is the first report for Tx. guadeloupensis using Ae. aegypti oviposition traps as breeding places. This finding may have important consequences in the epidemiology and local dengue control since Ae. aegypti density is a basic variable in dengue prediction. Whether predation of Ae aegypti by Tx. guadeloupensis in the Amazon is of significance, is a question to be examined. Also, larval predation may be a cause for underestimation of the actual Ae aegypti numbers. Together these hypotheses need to be better investigated as they are directly related to dengue epidemiology, to the success of any outbreak prediction and surveillance program.     

Bionomics of Aedes aegypti subpopulations (Diptera: Culicidae) from Argentina

Differences in biological features of immature and adult Aedes aegypti, as well as variability in vector competence, seem consistent with the existence of genetic variation among subpopulations and adaptation to local conditions. This work aims to compare the bionomics of four Ae. aegypti subpopulations derived from different geographical regions reared under temperate conditions. Life statistics of three Ae. aegypti subpopulations from the provinces of Córdoba, Salta, and Misiones were studied based on horizontal life tables. The Rockefeller strain was used as a control. The development time required to complete the larva and pupa stages varied from 6.91 to 7.95 and 1.87 to 2.41 days, respectively. Significant differences were found in mean larval development time between the Córdoba and Orán subpopulations. The larva‐pupa development time was similar in all the subpopulations. However, survival values varied significantly between the Orán and San Javier subpopulations. The proportion of emergent males did not differ from females within each subpopulation nor among them. Adult longevity was similar among the subpopulations. The average number of eggs laid by each female was significantly different. The Rockefeller strain laid a significantly greater number of eggs (463.99 eggs/female) than the rest of the subpopulations. Moreover, differences in the demographic growth parameter R_o were detected among the four subpopulations. The differences obtained in larval development time, larva‐pupa survival values, and net reproductive rates among the subpopulations might reflect underlying genetic differences as a result of colonization from different regions that probably involve adaptations to local conditions.     

Characterization of a leucokinin binding protein in Aedes aegypti (Diptera: Culicidae) Malpighian tubule

The insect myokinin (leucokinin-like) neuropeptide family includes peptides that have different physiological effects such as the induction of hindgut myotropic activity and stimulation of urine production. The C-terminal pentamer of myokinins Phe-X-(Ser/Pro/Ala)-Trp-Gly-amide [X=Phe, His, Asn, Ser or Tyr], had been previously determined as the minimum fragment able to elicit a functional response. The receptor(s) for these insect neuropeptides has not yet been identified. In order to characterize the Malpighian tubule leucokinin-like peptide receptor(s) from the yellow fever mosquito (Aedes aegypti), a leucokinin photoaffinity analogue (LPA) of sequence dAla-dTyr-Bpa-dLys-Phe-Phe-Ser-Trp-Gly-amide was designed based on structure/activity relationships for leucokinins. LPA caused depolarization of the transepithelial voltage (TEV) in female Malpighian tubule, confirming the activity of the peptide. The effective concentration to give half the maximum depolarization (EC₅₀) was 17 nM. The ¹²⁵I-LPA was then used to characterize leucokinin binding proteins in female Malpighian tubule membranes. It specifically labeled and saturated a protein(s) of about 54 kDa as shown by SDS-PAGE/autoradiography and by competition experiments with excess unlabeled leucokinin analogues. ¹²⁵I-LPA bound to the 54 kDa protein(s) with a K_d value of 13±3 nM in agreement with the EC₅₀ for the TEV bioassay. Altogether these data suggest that the 54 kDa protein is an Aedes-leucokinin receptor. This is the first characterization of an insect leucokinin receptor and reveals that LPA is a powerful tool to label insect myokinin receptors.     

Development and ageing of the salivary glands of adult female Aedes aegypti (L.) and Aedes togoi (Theobald) mosquitoes (Diptera : Culicidae)

Histological sections of the salivary glands of adult female Aedes aegypti and Aedes togoi (Diptera : Culicidae), varying in age from less than 1 day to 48 days after emergence were examined by light microscopy. Following emergence, the glands underwent morphological differentiation and also enlarged as secretory product accumulated, especially in the posterior regions. Development was complete within about 3 days (A. aegypti) or 7 days (A. togoi). Ageing, which began between 10 and 15 days after emergence in both species, predominantly affected the posterior regions of the glands. Regression, involving loss of stored secretion and cytological evidence of reduced protein synthesis, was accompanied by degenerative changes including substantial cytoplasmic vacuolation and increased prominence of large cyst-like structures. Although, in general, both regression and degeneration increased with time, there was marked variation between one gland and another in the speed with which ageing changes progressed.     

Phylogeography of Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) (Diptera: Culicidae) based on mitochondrial DNA variations

Aedes (Stegomyia) aegypti (l.) and Aedes (Stegomyia) albopictus (Skuse) are the most important vectors of the dengue and yellow-fever viruses. Both took advantage of trade developments to spread throughout the tropics from their native area: A. aegypti originated from Africa and a. albopictus from South-East Asia. We investigated the relationships between A. aegypti and A. albopictus mosquitoes based on three mitochondrial-DNA genes (cytochrome b, cytochrome oxidase I and NADH dehydrogenase subunit 5). Little genetic variation was observed for a. albopictus, probably owing to the recent spreading of the species via human activities. For A. aegypti, most populations from South America were found to be genetically similar to populations from South-East Asia (Thailand and Vietnam), except for one sample from Boa Vista (northern Amazonia), which was more closely related to samples from Africa (Guinea and Ivory Coast). This suggests that African populations of A. aegypti introduced during the slave trade have persisted in Boa Vista, resisting eradication campaigns.     

Fine structure and role in behavior of sensilla on the terminalia of Aedes aegypti (L.) (Diptera: Culicidae)

The terminalia of male and female Aedes aegypti (L.) bear numerous hairs of various shapes and lengths, all of which are mechanoreceptors. Each hair is innervated by one bipolar neuron which contains ciliary rootlets, two basal bodies, and a region assuming the structure of a non-motile cilium. At the distal tip of the dendrite is a tubular body, a characteristic of cuticular mechanoreceptors. Covering the outer dendritic segment is a cuticular sheath which ends proximally in a net-like felt-work and distally attaches to the hair base. Each hair sensillum has two sheath cells. Presumed efferent fibers are associated with the sheath cells. On the insula of the female terminalia are a few campaniform sensilla, the domes of which are raised into small pegs. The sensilla on the terminalia function in copulation and oviposition and probably in warning. A sequence of neurological events is traced for copulation and oviposition. Other cuticular structures, viz., scales, microtrichia, acanthae, and aedeagal spines, which occur on the terminalia are not innervated.     

Calibration and evaluation of field cage for oviposition study with Aedes (Stegomyia) aegypti female (L.) (Diptera: Culicidae)

Differences among results gathered from insect behavior studies conducted in laboratory and field situations are due to ambient variables that differ greatly between both environments. In laboratory studies the environmental conditions can be controlled whereas in field temperature, humidity and air velocity vary uncontrollably. The objective of this study was to calibrate and evaluate an experimental area (field cage) (14 x 7 x 3.5 m) subdivided into eight test cages (2.5 x 2.5 x 2 m) for use in behavioral oviposition tests of Aedes aegypti (L.) mosquitoes for developing a new methodology to assess attractants and oviposition traps. Test cage calibration involved: (1) minimal experiment duration tests; (2) optimal female release number per traps test and (3) trap placement tests. All tests used gravid A. aegypti females; 3-4 days post blood meal and the sticky trap MosquiTRAP to catch adults. Ninety percent of the females released were recaptured 2h after the beginning of the experiment, and this allowed up to 32 test repetitions/day to be conducted in the field cage. The minimum number of females necessary to conduct statistical analyses was 20 females/trap/test per cage. No significant difference was found in the behavioral response of gravid females to four different trap positions within test cages. Field trapping results with attractant were similar to those in the field cage. Therefore, the field cage could replace field trapping for evaluating at least mosquito traps and oviposition attractants for A. aegypti.     

Reappearance of Aedes aegypti (Diptera: Culicidae) in Lima, Peru

We report here the reappearance of Aedes aegypti in the Rimac district, and summarize the history of this mosquito species in Peru since its first detection in 1852. On March 17 2000 were found Ae. aegypti and Culex quinquefasciatus in Mariscal Castilla town, Flor de Amancaes, San Juan de Amancaes, El Altillo and Santa Rosa in the Rimac district, Lima Province.     

Insecticide susceptibility status of Aedes aegypti (Diptera: Culicidae) from Townsville

Insecticide susceptibility of a 1995 Townsville Aedes aegypti (L.) population was investigated in the laboratory according to World Health Organization guidelines. Using baseline data from two Townsville populations (1955 and 1989), larval bioassays detected significant increases in susceptibility to synthetic pyrethroids and malathion, and significant reductions in susceptibility to most organophosphates and propoxur. Adult bioassays detected significant resistance to bendiocarb and DDT. Comparison of larval data with the international reference ROCK strain showed substantial resistance to have developed to malathion and fenthion. Further analysis revealed the presence of distinct substrains in the baseline 1989 population, which displayed varying levels of temephos susceptibility. We concluded that susceptibility investigations should assess mosquito populations collected from many sites within an area rather than taking a single population from one site, and that the 1989 Ae. aegypti colony would be unsuitable for use as a susceptible reference population.     

Oviposition activity and seasonal pattern of a population of Aedes (Stegomyia) aegypti (L.) (Diptera: Culicidae) in subtropical Argentina

Monthly oviposition activity and the seasonal density pattern of Aedes aegypti were studied using larvitraps and ovitraps during a research carried out by the Public Health Ministry of Salta Province, in Tartagal, Aguaray and Salvador Mazza cities, in subtropical Argentina. The A. aegypti population was active in both dry and wet seasons with a peak in March, accordant with the heaviest rainfall. From May to November, the immature population level remained low, but increased in December. Ae. aegypti oviposition activity increased during the fall and summer, when the relative humidity was 60% or higher. Eggs were found in large numbers of ovitraps during all seasons but few eggs were observed in each one during winter. The occurrence and the number of eggs laid were variable when both seasons and cities were compared. The reduction of the population during the winter months was related to the low in the relative humidity of the atmosphere. Significant differences were detected between oviposition occurrences in Tartagal and Aguaray and Salvador Mazza cities, but no differences in the number of eggs were observed. Two factors characterize the seasonal distribution pattern of Ae. aegypti in subtropical Argentina, the absence of a break during winter and an oviposition activity concomitant of the high relative humidity of the atmosphere.     

Frequency of pyrethroid resistance in Aedes aegypti and Aedes albopictus (Diptera: Culicidae) in Thailand

Thirty‐two Aedes aegypti populations collected throughout Thailand and five populations of Aedes albopictus from southern Thailand were subjected to standard WHO contact bioassays to assess susceptibility to three commonly used synthetic pyrethroids: permethrin, deltamethrin, and lambda‐cyhalothrin. A wide degree of physiological response to permethrin was detected in Ae. aegypti, ranging from 56.5% survival (Lampang, northern Thailand) to only 4% (Kalasin in northeastern and Phuket in southern Thailand). All 32 populations of Ae. aegypti were found to have evidence of incipient resistance (62.5%) or levels of survival deemed resistant (37.5%) to permethrin. Four populations of Ae. albopictus were found with incipient resistance (97 – 80% mortality) and one with resistance (< 80%) to permethrin. The majority of Ae. aegypti populations (68.7%) was susceptible (> 98% mortality) to deltamethrin, with incipient resistance (observed 97–82% mortality) in other localities. In contrast, all populations of Ae. aegypti were completely susceptible (100% mortality) to the recommended operational dosage of lambda‐cyhalothrin. All five populations of Ae. albopictus were found completely susceptible to both deltamethrin and lambda‐cyhalothrin. Evidence of defined incipient or resistance to synthetic pyrethroids mandates appropriate response and countermeasures to mitigate further development and spread of resistance. In light of these findings, we conclude that routine and comprehensive susceptibility monitoring of dengue mosquito vectors to synthetic pyrethroids should be a required component of resistance management policies and disease control activities.