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.     

Insecticide susceptible/resistance status in Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae) in Thailand during 2003-2005

Susceptibility baselines and diagnostic doses of the technical grade insecticides deltamethrin, permethrin, fenitrothion, and propoxur were established based on Aedes aegypti (L.), Bora (French Polynesia), a reference susceptible strain. Field-collected Aedes mosquitoes from each part of Thailand were subjected to bioassay for their susceptibility to the diagnostic doses of each insecticide. Almost all Ae. aegypti collected were incipient resistant or resistant to deltamethrin and permethrin, except those from some areas of Songkhla (southern) and Phan district of Chiang Rai (northern) province. Susceptibility to fenitrothion was found in mosquitoes from Bangkok (central), Chonburi (eastern), Chiang Rai, Kanchanaburi (western), and Songkhla, whereas they were resistant in almost all areas of Nakhon Sawan (north central) and Nakhon Ratchasima (northeastern) provinces. Most of Ae. aegypti were susceptible to propoxur except those from Mae Wong, Nakhon Sawan province. Various levels of insecticide resistance and susceptibility in adjacent areas revealed a focal susceptible/resistance profile in the country. It could be noted that almost all of Ae. albopictus were susceptible to the insecticides tested at the same diagnostic doses. In conclusion, resistance to pyrethroids (permethrin and deltamethrin) has developed in Ae. aegypti in most of the collected areas, suggesting that an alternative choice of insecticide or other control measures should be applied.     

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 of three Colombian Aedes aegypti Linnaeus (Diptera: Culicidae) populations to a Lysinibacillus sphaericus formulation

The increased incidence of vector-borne diseases and insecticide resistance in mosquitos constitute public health concerns in the tropics. Biological control is an effective alternative in the management of Aedes aegypti Linnaeus populations. Lysinibacillus sphaericus a bacterium proved to be harmless for non-target organisms has shown promising entomopathogenic activity. In Colombia, the control of A. aegypti using L. sphaericus has not been contemplated as part of vector control programmes. To assess the susceptibility of three A. aegypti populations to a L. sphaericus formulation consisting of vegetative cells of 2362 and III(3)7 strains, simulated-field bioassays were conducted in the municipalities of Ricaurte in Cundinamarca and Tauramena in Casanare, as well as in the municipal inspection of San Joaquin, Cundinamarca. Sixty larvae were deposited in a test device, which contained nine litres of chlorine-free tap water and 100 ml of the bacterial formulation (10⁹ CFU/ml). Six replicates were set-up for treatment with bacteria and for the control. Larvae mortality was recorded at 24 and 48 hr. The three A. aegypti populations were susceptible to L. sphaericus formulation with a mortality rate higher than 90% at 48 hr. The formulation of L. sphaericus comprised of vegetative cells of 2362 and III(3)7 strains showed promising results that could guarantee the implementation of this formulation to control of A. aegypti populations.     

Multiple insecticide resistance in Aedes aegypti (Diptera: Culicidae) populations compromises the effectiveness of dengue vector control in French Guiana

In French Guiana, pyrethroids and organophosphates have been used for many years against Aedes aegypti. We aimed to establish both the resistance level of Ae. aegypti and the ultra low volume spray efficacy to provide mosquito control services with practical information to implement vector control and resistance management. Resistance to deltamethrin and fenitrothion was observed. In addition, the profound loss of efficacy of AquaK'othrine? and the moderate loss of efficacy of Paluthion? 500 were recorded. Fenitrothion remained the most effective candidate for spatial application in French Guiana until its removal in December 2010. Further investigation of the mechanism of resistance to deltamethrin demonstrated the involvement of mixed-function oxidases and, to a lesser extent, of carboxylesterases. However, these observations alone cannot explain the level of insecticide resistance we observed during tube and cage tests.     

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.     

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.     

Cloning and daily expression of the timeless gene in Aedes aegypti (Diptera:Culicidae)

Molecular approaches for studying biological rhythms in insects have been well investigated in the model Drosophila melanogaster, in which a number of genes have been characterized in terms of sequence, expression, protein interactions and involvement in the control of locomotor activity and eclosion rhythms. However, only scattered information is available for insect vectors of diseases. In this paper, we report the cloning and expression analysis of the clock gene timeless in the mosquito Aedes aegypti, vector of Dengue and Yellow Fever viruses. In Drosophila, timeless has a crucial role in the control of the central pacemaker and the resetting mechanism that allows the clock to synchronize with the environment light-dark cycles. Comparison of the predicted protein sequence encoded by timeless in Ae. aegypti and D. melanogaster demonstrated high similarity in some important domains, suggesting functional conservation. Analysis of the daily expression of timeless in Ae. aegypti shows a peak in mRNA abundance around the light-dark transition.     

Adaptation of Aedes aegypti (Diptera: Culicidae) oviposition behavior in response to humidity and diet

Effects of humidity and sugar concentration on the fecundity, temporal oviposition patterns and survival of a tropical strain of Aedes aegypti (L.) were investigated. Fecundity was significantly reduced by low humidity, but was not affected by sugar concentration. Low humidity caused a significant decrease in percentage survival after 19 days as compared to high humidity. Oviposition was inhibited by host availability for eight successive days. When access to a host was no longer provided, oviposition continued for 10 days in three to four distinct cycles without additional bloodmeals. Humidity stress and high sugar concentration caused oviposition to be delayed for one to four days, which is the typical duration of extreme low humidity periods in nature. These responses are hypothesized to protect the eggs of ovipositing females against the environmental hardships of periodic humidity stress and lack of hosts, thus enabling the perpetuation of the vector and the diseases it transmits in hot and dry seasons.     

State‐wide survey of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) in Florida

Aedes aegypti and Aedes albopictus are invasive mosquito species with geographic ranges that have oscillated within Florida since their presence was first documented. Local transmission of dengue, chikungunya, and Zika viruses serves as evidence of the public health importance of these two species. It is important to have detailed knowledge of their distribution to aid in mosquito control efforts and understand the risk of arbovirus transmission to humans. Through a partnership involving the University of Florida Institute of Food and Agricultural Sciences Cooperative Extension Service and the Florida Medical Entomology Laboratory; the Florida Department of Health; and mosquito control agencies throughout Florida, a container mosquito surveillance program involving all life stages was launched in the summer of 2016 to detect the presence of Ae. aegypti and Ae. albopictus. Results from this survey were mapped to provide a picture of the current known distribution of Ae. aegypti and Ae. albopictus in Florida. Aedes aegypti and/or Ae. albopictus were detected in the 56 counties that were part of the survey. Only Aedes albopictus was detected in 26 counties, primarily in the panhandle region of Florida. The results of this work underscore the importance of maintaining container mosquito surveillance in a state where chikungunya, dengue, and Zika viruses are present and where there is continued risk for exotic arbovirus introductions.     

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.     

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.     

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.     

Characterisation of DDT and pyrethroid resistance in Trinidad and Tobago populations of Aedes aegypti

Insecticide resistance is an important factor in the effectiveness of Aedes aegypti control and the related spread of dengue. The objectives of this study were to investigate the status of the organochlorine dichlorodiphenyltrichloroethane (DDT) and pyrethroid (permethrin and deltamethrin) resistance in Trinidad and Tobago populations of Ae. aegypti and the underlying biochemical mechanisms. Nine populations of Ae. aegypti larvae from Trinidad and Tobago were assayed to DDT and PYs using the Centers for Disease Control and Prevention (CDC) time-mortality-based bioassay method. A diagnostic dosage (DD) was established for each insecticide using the CAREC reference susceptible Ae. aegypti strain and a resistance threshold (RT), time in which 98-100% mortality was observed in the CAREC strain, was calculated for each insecticide. Mosquitoes which survived the DD and RT were considered as resistant, and the resistance status of each population was categorised based on the WHO criteria with mortality <80% indicative of resistance. Biochemical assays were conducted to determine the activities of α and β esterases, mixed function oxidases (MFO) and glutathione-S-transferases (GST) enzymes which are involved in resistance of mosquitoes to DDT and PYs. Enzymatic activity levels in each population were compared with those obtained for the CAREC susceptible strain, and significant differences were determined by Kruskal-Wallis and Tukey's non-parametric tests (P<0.05). The established DDs were 0.01 mg l(-1), 0.2 mg l(-1) and 1.0 mg l(-1) for deltamethrin, permethrin and DDT, respectively; and the RTs for deltamethrin, permethrin and DDT were 30, 75 and 120 min, respectively. All Ae. aegypti populations were resistant to DDT (<80% mortality); two strains were incipiently resistant to deltamethrin and three to permethrin (80-98% mortality). Biochemical assays revealed elevated levels of α-esterase and MFO enzymes in all Ae. aegypti populations. All, except three populations, showed increased levels of β-esterases; and all populations, except Curepe, demonstrated elevated GST levels.Metabolic detoxification of enzymes is correlated with the manifestation of DDT and PY resistance in Trinidad and Tobago populations of Ae. aegypti. The presence of this resistance also suggests that knock down (kdr)-type resistance may be involved, hence the need for further investigations. This information can contribute to the development of an insecticide resistance surveillance programme and improvement of resistance management strategies aimed at combatting the spread of dengue in Trinidad and Tobago.     

Synthesis and larvicidal activity of indole derivatives against Aedes aegypti (Diptera: Culicidae)

In light of the challenges to control Aedes aegypti and the critical role that it plays as arbovirus vector, it is imperative to adopt strategies that provide fast, efficient and environmentally safe control of the insect population. In the present study, we synthesized six indole derivatives (C1‐C6) and examined their larvicidal activity and persistence against Ae. aegypti larvae, as well as their toxicity towards Raw 264.7 macrophages, Vero cells, Chlorella vulgaris BR017, Scenedesmus obliquus BR003, Caenorhabditis elegans N2 and Galleria mellonella. Among the bioactive compounds (C1, C2, C4 and C5), C2 exerted the strongest larvicidal activity against Ae. aegypti, with LC50 = 1.5 μg/ml (5.88 µM) and LC90 = 2.4 μg/ml (9.50 µM), indicating that the presence of chlorine or bromine groups in the aromatic ring improved the larvicidal activity of the indole derivatives. C1, C2, C4 and C5 did not reduce viability of RAW 264.7 macrophages, Vero cells, C. elegans N2 and G. mellonella. Compounds C1, C2 and C5 did not affect the growth of C. vulgaris BR017 and S. obliquus BR003. Analysis of larvicidal persistence under laboratory conditions revealed that the effect of compounds C1, C2, C4 and C5 lasted for 30 days and caused 100% of larvae mortality within few hours. Altogether, our findings demonstrate that the indole derivatives C1, C2, C4 and C5 effectively control Ae. aegypti larvae population, without clear signs of toxicity to mammalian cells, algae, C. elegans and G. mellonella.