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.     

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.     

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.     

Morphological effects of insect growth regulating compounds on Aedes aegypti (Diptera: Culicidae) larvae.

Cuticular development of $\text{Aedes aegypti}$ larvae was examined by electron microscopy and comparisons were made between larvae exposed to methoprene, isopropyl (E, E)-11-methoxy-3, 7, 11-trimethyl-2, 4-dodecadienoate, those treated with the fluorescent insect growth regulator, 5-[[[5-(dimethylamino)-1-naphthalenyl]-sulfonyl]amino]-1, 3-benzodioxole (DNSAB), and untreated larvae. Larvae of all three groups were routinely fixed at 24, 48, and 72 hr posttreatment. Thin sections of the sixth-abdominal segment, anal papillae, midgut tissue, and Malpighian tubules were examined for morphological variations from controls.     

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.     

Evaluation of methoprene effect on Aedes aegypti (Diptera: Culicidae) development in laboratory conditions

Several Brazilian Aedes aegypti populations are resistant to the larvicidae temephos. Methoprene, that inhibits adult emergence, is one of the alternatives envisaged by the Brazilian Dengue Control Program (PNCD). However, at Brazil vector infestation rates are measured through larvae indexes and it has been claimed that methoprene use in the field could face operational problems. In order to define a standardized protocol, methoprene effect was evaluated in laboratory conditions after continuous exposure of larvae (Rockefeller strain) to a methoprene formulation available to the PNCD. Methoprene-derived mortality occurs mainly at the pupa stage and pupa development is inversely proportional to methoprene concentration. Number and viability of eggs laid by treated and control females are equivalent. A methoprene dose-dependent delay in the development was noted; however, strong correlations were found for total mortality or adult emergence inhibition if data obtained when all control mosquitoes have emerged are compared to data obtained when methoprene-treated groups finish development. The cumulative record of total methoprene-induced mortality at the time control adults emerge is proposed for routine evaluation of field populations. Mortality of all specimens, but not of larva, could account for adult emergence inhibition, confirming the inadequacy of larvae indexes to evaluate methoprene effect.     

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.     

Effect of body size and sugar meals on oviposition of the yellow fever mosquito,Aedes aegypti (Diptera: Culicidae)

The effects of dietary sugar and body size on the oviposition of Ae. aegypti were studied under laboratory conditions. In female mosquitoes provided with sugar, the start of maximum fecundity was significantly delayed and the oviposition period was longer than in females provided with water. The peak of oviposition was also delayed in sugar‐fed females. Large females oviposited more eggs per day than small females at maximum fecundity and during eight days of observations. Large females also visited significantly more water‐containing cups in their cages per day than small females at maximum fecundity. During the eight days of observations, large females and sugar‐fed females visited more water‐containing cups in their cages than water‐fed small females. Both large females and sugar‐fed females oviposited their eggs at sites higher above the water line than water‐fed small females. These results suggested that large and sugar‐fed female Ae. aegypti mosquitoes had more energy reserves and oviposited their eggs at higher sites, which would lead to a time lag in hatching.     

The chemical characterization of the epicuticular hydrocarbons of Aedes aegypti (Diptera: Culicidae)

The chemical characterization of the hydrocarbon fraction of the epicuticular lipids of the vector mosquito Aedes aegypti (Linnaeus) was performed using gas chromatography (GC) and gas chromatography-electron impact mass spectrometry (GC-MS). Seventy eight compounds were detected in purified hexane extracts and of these, 42 hydrocarbons were identified and several of the remaining compounds were partially characterized. The hydrocarbon classes present were n-alkanes, monomethylalkanes, dimethylalkanes and alkenes and the results were similar to those published for other Aedes species. Quantitative comparisons of cuticular hydrocarbon profiles were made between males and females, different age groups and between a standard laboratory strain and a recently colonized strain of A. aegypti. These results provide baseline data for further studies on the possible role of mosquito cuticular hydrocarbons in the modification of mating behaviour.     

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.     

Field optimisation of MosquiTRAP sampling for monitoring Aedes aegypti Linnaeus (Diptera: Culicidae)

A sticky trap designed to capture gravid Aedes (Stegomyia) aegypti mosquitoes, MosquiTRAP, has been evaluated for monitoring this species in Brazil. However, the effects of trap densities on the capture rate of Ae. aegypti females and the sensitivity of vector detection are still unknown. After a preliminary study has identified areas of high and low female mosquito abundance, a set of experiments was conducted in four neighbourhoods of Belo Horizonte (state of Minas Gerais, Brazil) using densities of 1, 2, 4, 8, 16, 32 and 64 traps per block. Trap sensitivity (positive MosquiTRAP index) increased significantly when 1-8 MosquiTRAPs were installed per block in both high and low abundance areas. A strong fit was obtained for the total number of mosquitoes captured with increasing trap densities through a non-linear function (Box-Lucas) (r2 = 0,994), which likely exhibits saturation towards an equilibrium level. The capacity of the Mean Female Aedes Index to distinguish between areas of high and low Ae. aegypti abundance was also investigated; the achieved differentiation was shown to be dependent on the MosquiTRAP density.     

Dynamics and Characterization of Aedes aegypti (L.) (Diptera: Culicidae) Key Breeding Sites

The present study aimed to analyze the dynamics of containers used as breeding sites by Aedes aegypti (L.) in the city of Aracaju, SE, one of the Northeast Brazilian states. A total of three entomological surveys were performed during different precipitation levels. Breeding sites were categorized according to their function into storage, disposable containers, and reusable containers. “Mean number of pupae” and “frequency of each type of breeding site” were the criteria considered to identify key breeding sites. House index and Breteau index were calculated in each survey. A total of 3,647 water reservoirs were found, of which 220 were breeding sites, where 22,880 immature forms were identified. There were no differences in the mean number of larvae of several types of breeding sites and in the number of larvae among surveys. Larval indices showed a reduction in the second visit, but with no effect on adult occurrence when the number of pupae was considered. Key breeding sites resulted from containers used for water storage. The area studied showed conditions favorable to a new epidemic of dengue fever.     

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.     

Susceptibility of two different strains of Aedes aegypti (Diptera: Culicidae) to plant oils

The toxicity of 11 oils extracted from plants commonly grown in the Himalayan region was studied using larvae of two Aedes aegypti (L.) strains. A strain from Liverpool, England, was highly susceptible to these oils. The LC50 values were much higher in a local laboratory strain. Daucus carota L. oil was highly toxic in both strains. Differences in the susceptibility of these strains to the action of the test oils and their potential use in integrated pest management are discussed.     

Diurnal test periods influence behavioral responses of Aedes aegypti (Diptera: Culicidae) to repellents

There is a broad understanding of the influence of environmental factors on various aspects of normal mosquito behavior. How these external factors influence responses to repellent compounds is far less clear. The objective of this study was to investigate the effect of different daytime periods combining the normal circadian activity of a laboratory colony of Aedes aegypti (L.) with behavioral responses of mosquitoes exposed to three different compounds possessing repellent properties. Using an excito-repellency test chamber with different test designs (contact irritancy + repellency and noncontact repellency), female mosquitoes were exposed to each chemical or matching blank control during four different 3-h time intervals beginning 0600 to 1800?h. Mosquitoes showed more significant avoidance responses (escape movement away from the chemical) when exposed to either DEET or hairy basil during the afternoon periods. With deltamethrin, there was no significant difference in repellent escape movement during any period of testing. Escape activity with deltamethrin was significantly greater during all diurnal periods in contact tests compared to DEET and hairy basil. From this study, it was shown that time of diurnal testing can significantly influence behavioral responses of Ae. aegypti exposed to chemical-based repellents. Therefore, the assessment of chemicals (toxins, repellents, attractants) and must carefully consider time-of-test as a potential confounding factor during evaluation and comparisons.     

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.     

Long-lasting effects of a Bacillus thuringiensis serovar israelensis experimental tablet formulation for Aedes aegypti (Diptera: Culicidae) control

Dengue is a growing public health problem in many tropical and subtropical countries worldwide. At present, the only method of controlling or preventing the disease is to eliminate its vector, Aedes aegypti (L.) (Diptera: Culicidae). In the current study, an experimental larvicide tablet formulation XL-47 based on Bacillus thuringiensis serovar israelensis (Bti) and containing 4.8% of technical powder was developed. This formulation was evaluated against Ae. aegypti in three different sets of experiments, under field-simulated conditions: two experiments were indoors and under partial sunlight exposure and one experiment was outdoors with sunlight exposure. Larvae were added throughout the experiment two times per week, and the residual larvicidal activity was recorded daily. Pupal formation was reduced in the containers with Bti by > 80% in relation to the containers without treatment for 12 wk; to our knowledge, this is the longest period of control reported for a Bti tablet formulation outdoors under sunlight exposure. Moreover, samples from the top, middle, and bottom of the water column were collected to perform bacterial plate counts and toxicity assays. The Bti population and the active ingredient of the tablet formulation remained mainly at the bottom of the containers and mosquito larvae reached the formulation by diving and shredding the tablet's material. In conclusion, the experimental tablet formulation XL-47 showed an inhibition of pupal formation that lasted for long periods under sunlight exposure.