Differential lipid reserves influence host-seeking behaviour in the mosquitoes Aedes cantans and Aedes punctor

ipid reserves of bait-caught female Ae.cantans and Ae.punctor mosquitoes (Diptera: Culicidae) were significantly higher than in teneral females. Female Ae.cantans given access to 10% w/v sucrose solution post-emergence showed an ability to synthesize lipid and, after 192 h, they were willing to take a bloodmeal from a human volunteer. At this point, mean lipid reserves were not significantly different from mean lipid reserves of bait-caught females. Prior to 192 h, females would not take a bloodmeal and lipid reserves were significantly lower than in bait-caught females. Female Ae.punctor given access to 10% w/v sucrose solution post-emergence also showed an ability to synthesize lipid. Females of this species were willing to feed from a human host after only 48 h, at which point lipid content was not significantly different from that in bait-caught females. The level of lipid reserves in females coming to bait differs significantly between species: Ae.cantans has lipid reserves approximately double those of Ae.punctor . In addition, Ae.punctor is able to synthesize lipid to a level comparable with that found in bait-caught females after only 24 h, whilst it takes 192 h for Ae.cantans females to synthesize the amount of lipid found in host-seeking females, when allowed free access to sugar. Physiological differences in lipid synthesis and the level of lipid reserves required may therefore explain the differences observed between the species in the time taken to initiate host seeking.     

Abdominal distention terminates subsequent host-seeking behaviour of Aedes aegypti following a blood meal

The failure of Aedes aegypti females to engage in host-seeking behaviour following a replete blood meal was investigated. Abdominal distention appears to be responsible for this immediate inhibition after feeding. Large enemas of saline had the same effect as blood in terminating host-seeking; this was not due merely to the presence of large amounts of fluid, but rather to the distention produced by these liquids. Since transection of the ventral nerve cord anterior to the 2nd abdominal ganglion did not release the inhibition in blood-fed females, we restricted the degree of distention of abdominal segments with wax. Distention of the abdomen anteriorly by a blood meal more effectively inhibited host-seeking than did distention posteriorly, suggesting that stretch receptors in the anterior portion of the abdomen regulate the response towards a host.     

Evaluation of boric acid as toxic sugar bait against resistant Aedes aegypti mosquitoes

Current methods of broad area application of contact insecticides used in mosquito control are becoming less effective, primarily due to resistance within mosquito populations. New methods that can deliver ingestible insecticides are being investigated as a means to mitigate resistance. This study evaluated insecticide delivery through toxic sugar baits (TSB) and resulting mortality of susceptible and resistant strains of Aedes aegypti. Two Ae. aegypti strains were evaluated using a 1% boric acid TSB: the susceptible Orlando 1952 (ORL) strain and the resistant Puerto Rican (PR) strain. The TSB resulted in high mortality for both ORL and PR strain of Ae. aegypti. Average mortality of female mosquitoes given TSB was 90.8% for PR and 99.3% for ORL. Our study suggests that targeting resistant mosquitoes with ingestible insecticides through TSBs could be a viable alternative to current mosquito control strategies and should be considered when developing an integrated vector management program.     

Role of the fat body in the regulation of host-seeking behaviour in the mosquito,Aedes aegypti

Following a blood meal that initiates oöcyte development, the host-seeking behaviour of Aedes aegypti mosquitoes is inhibited by a haemolymph-borne factor that is released in response to a humoral signal from a vitellogenic ovary. This inhibition is accompanied by a decrease in the sensitivity of the peripheral lactic acid receptors. Implantation of corpora allata, medial neurosecretory cells, or terminal abdominal ganglia from blood-fed donors could not induce the inhibition in sugar-fed recipients. However, fat body transplanted from blood-fed into sugar-fed females suppressed host-seeking behaviour as well as the sensitivity of lactic acid receptors, suggesting that the source of the behavioural inhibitor is the fat body. Resting-stage ovaries from other mosquito species inhibited host-seeking after the A. aegypti host was fed on blood only if the fat body was activated by the donor ovary.     

Effects of intraspecific larval competition on adult longevity in the mosquitoes Aedes aegypti and Aedes albopictus

Abstract Larval competition is common in container‐breeding mosquitoes. The impact of competition on larval growth has been thoroughly examined and findings that larval competition can lead to density‐dependent effects on adult body size have been documented. The effects of larval competition on adult longevity have been less well explored. The effects of intraspecific larval densities on the longevity of adults maintained under relatively harsh environmental conditions were tested in the laboratory by measuring the longevity of adult Aedes aegypti (L.) and Aedes albopictus (Skuse) (Diptera: Culicidae) that had been reared under a range of larval densities and subsequently maintained in high‐ or low‐humidity regimes (85% or 35% relative humidity [RH], respectively) as adults. We found significant negative effects of competition on adult longevity in Ae. aegypti, but not in Ae. albopictus. Multivariate analysis of variance suggested that the negative effect of the larval environment on the longevity of Ae. aegypti adults was most strongly associated with increased development time and decreased wing length as adults. Understanding how larval competition affects adult longevity under a range of environmental conditions is important in establishing the relationship between models of mosquito population regulation and epidemiological models of vector‐borne disease transmission.     

Effects of manipulating apoptosis on Sindbis virus infection of Aedes aegypti mosquitoes

Improved control of vector-borne diseases requires an understanding of the molecular factors that determine vector competence. Apoptosis has been shown to play a role in defense against viruses in insects and mammals. Although some observations suggest a correlation between apoptosis and resistance to arboviruses in mosquitoes, there is no direct evidence tying apoptosis to arbovirus vector competence. To determine whether apoptosis can influence arbovirus replication in mosquitoes, we manipulated apoptosis in Aedes aegypti mosquitoes by silencing the expression of genes that either positively or negatively regulate apoptosis. Silencing of the A. aegypti anti-apoptotic gene iap1 (Aeiap1) caused apoptosis in midgut epithelium, alterations in midgut morphology, and 60 to 70% mosquito mortality. Mortality induced by Aeiap1 silencing was rescued by cosilencing the initiator caspase gene Aedronc, indicating that the mortality was due to apoptosis. When mosquitoes which had been injected with Aeiap1 double-stranded RNA (dsRNA) were orally infected with Sindbis virus (SINV), increased midgut infection and virus dissemination to other organs were observed. This increase in virus infection may have been due to the effects of widespread apoptosis on infection barriers or innate immunity. In contrast, silencing the expression of Aedronc, which would be expected to inhibit apoptosis, reduced SINV midgut infection and virus dissemination. Thus, our data suggest that some level of caspase activity and/or apoptosis may be necessary for efficient virus replication and dissemination in mosquitoes. This is the first study to directly test the roles of apoptosis and caspases in determining mosquito vector competence for arboviruses.     

Fluid excretion by adult Aedes aegypti mosquitoes

Only clear cloacal fluid was eliminated during the first 30–60 min after emergence of feeding on human blood. Neither meconium nor blood was voided. More liquid was excreted after withdrawing blood than after ecdysis, but the pattern of emission was the same, in that fluid was ejected rapidly only or mostly over the first 10–15 min after these events. Diuresis was not exhibited after adults fed on either sucrose or fructose. Injury prevents the release of the diuretic hormone.     

Genetic structure among Thai populations of Aedes aegypti mosquitoes

Thirty‐one field populations of Aedes aegypti (L.) were compared using isozyme starch gel electrophoresis to characterize genetic variation between populations. Ae. aegypti were collected from seven provinces in Thailand. Thirty‐one isozyme encoding loci, including 19 polymorphic loci, were characterized. Only small levels of genetic differentiation were observed among the 31 district populations in the seven provinces. Isolation by distance among populations from the seven provinces showed no correlation between genetic variation and geographical distance.     

Pathogenicity of Spiroplasma taiwanense for larval Aedes aegypti mosquitoes

Helical replicative forms, but not the persistent non-replicative forms, of Spiroplasma taiwanense Abalain-Colloc et al. (isolated from the mosquito Anopheles sinensis Wiedemann in Taiwan) were shown to reduce significantly the survival of Aedes aegypti (L.) mosquito larvae reared in 10 ml of water with 0.3 ml of S.taiwanense suspensions added on days 0 and 3. The suspensions contained, respectively, helical forms at a concentration of 10(9) Colour Change Units (CCU)/ml and persistent forms at 10(6) CCU/ml. It is suggested that S.taiwanense, or toxins produced from it, are potentially useful for use in integrated mosquito control programmes.     

Characterization of hemocytes from the mosquitoes Anopheles gambiae and Aedes aegypti

Hemocytes are an essential component of the mosquito immune system but current knowledge of the types of hemocytes mosquitoes produce, their relative abundance, and their functions is limited. Addressing these issues requires improved methods for collecting and maintaining mosquito hemocytes in vitro, and comparative data that address whether important vector species produce similar or different hemocyte types. Toward this end, we conducted a comparative study with Anopheles gambiae and Aedes aegypti. Collection method greatly affected the number of hemocytes and contaminants obtained from adult females of each species. Using a collection method called high injection/recovery, we concluded that hemolymph from An. gambiae and Ae. aegypti adult females contains three hemocyte types (granulocytes, oenocytoids and prohemocytes) that were distinguished from one another by a combination of morphological and functional markers. Significantly more hemocytes were recovered from An. gambiae females than Ae. aegypti. However, granulocytes were the most abundant cell type in both species while oenocytoids and prohemocytes comprised less than 10% of the total hemocyte population. The same hemocyte types were collected from larvae, pupae and adult males albeit the absolute number and proportion of each hemocyte type differed from adult females. The number of hemocytes recovered from sugar fed females declined with age but blood feeding transiently increased hemocyte abundance. Two antibodies tested as potential hemocyte markers (anti-PP06 and anti-Dox-A2) also exhibited alterations in staining patterns following immune challenge with the bacterium Escherichia coli.     

Secretory proteins from the salivary glands of adult Aedes aegypti mosquitoes

We determined whether the salivary proteins of mosquitoes are constant throughout adult life and whether the nature of the feeding stimulus determined salivary composition. Acrylamide gel electrophoresis and subsequent silver-staining of harvested saliva yielded as many as 20 polypeptides; those precipitating silver most readily generally incorporated abundant isotope. Autoradiography revealed 12 prominant secretory polypeptide fractions, each synthesized beginning soon after ecdysis and throughout the first week of adult life. Synthesis was limited to the posterior regions of the salivary gland, mainly within the lateral lobes. Virtually all secretory polypeptides were antigenic and were secreted into hosts during blood-feeding. We concluded that the secretory salivary proteins of adult mosquitoes are continuously synthesized throughout the first week of adult life, and that most are secreted during blood-feeding.     

The effects of allatectomy and juvenile hormone replacement on the development of host-seeking behaviour and lactic acid receptor sensitivity in the mosquito Aedes aegypti

Host-seeking behaviour in newly emerged Aedes aegypti (L.) females is not expressed immediately after adult eclosion but develops gradually over a period of approximately 3-4 days. This development is accompanied by an apparent maturation of the antennal chemosensory afferent neurons involved in the detection of the airborne host attractant lactic acid. Since these events coincide in time with juvenile hormone-dependent previtellogenic ovarian growth and since the expression of other reproduction-associated behaviour has previously been shown to be dependent on juvenile hormone, the effects of juvenile hormone removal and replacement on the development of host-seeking behaviour and the response characteristics of the lactic acid-sensitive receptors were investigated. No effect of juvenile hormone removal by allatectomy or juvenile hormone replacement or augmentation by topical application of the juvenile hormone mimic methoprene was found. It was concluded that this hormone is not involved in the appearance of host-seeking behaviour or the apparent maturation of the lactic acid receptors that occurs during early imaginal life.     

Risk of dengue occurrence based on the capture of gravid Aedes aegypti females using MosquiTRAP

We assessed the risk classification of dengue fever based on the capture of Aedes aegypti adults using MosquiTRAP, a type of sticky trap, in comparison with traditional larval infestation indices. A total of 27 MosquiTRAPs were installed, with one trap per block, and were inspected weekly between November 2008-February 2009. Infestation baseline data were obtained from a survey conducted prior to trap installation. The index generated by MosquiTRAP and house index (HI) classified the area "in alert situation". The set for risk of dengue occurrence proposed by the use of MosquiTRAP classify areas in the same way of the traditional HI.     

Effects of sublethal pyrethroid exposure on the host-seeking behavior of female mosquitoes

A common method of adult mosquito control consists of residual application on surfaces and aerial spraying often using pyrethroids. However, not all insects that contact insecticides are killed. Sublethal exposure to neurotoxic compounds can negatively affect sensory organs and reduce efficiency of host location. Flight tracks of host-seeking female Culex quinquefasciatus, Anopheles albimanus, and Aedes aegypti in a wind tunnel were video-recorded to compare activation of host-seeking and patterns of flight orientation to host odors. During host-seeking flights, all three mosquito species differed significantly in flight duration, velocity, turn angle, and angular velocity. Mosquitoes were then exposed to sublethal levels (LD(25) ) of pyrethroid insecticides to evaluate the effects of the neurotoxicants 24 hours post-exposure. Significant reductions in time of activation to flight and flight direction were observed in mosquitoes exposed to deltamethrin and permethrin. Additionally, pesticide-treated Cx. quinquefasciatus mosquitoes flew significantly slower, spent more time in flight, and turned more frequently than untreated controls.     

A floral‐derived attractant for Aedes aegypti mosquitoes

The reproductive success of Aedes aegypti (L.) (Diptera: Culicidae) is strongly dependent on the availability of carbohydrates in the environment and the ability of the mosquitoes to locate them. The most significant source of carbohydrates for mosquitoes is nectar from flowering plants, which mosquitoes locate by their volatile compounds. The aim of our work was to identify plant volatile compounds that elicit a behavioral response in Ae. aegypti, which may be included in a mosquito trap for surveillance and/or control purposes. Landing‐preference bioassays were performed with plants of three species—Plectranthus neochilus Schltr. (Lamiaceae), Tagetes patula L. (Asteraceae), and Lobularia maritima (L.) Desv. (Brassicaceae)—as lures and toxic sugar baits as landing markers. Mosquitoes landed only on L. maritima. Freshly cut inflorescences of L. maritima elicited a positive flight response in both sexes of mosquitoes. The analysis of the compounds in the static head space of L. maritima was performed by solid phase microextraction (SPME). Of the single volatile compounds tested, acetophenone was attractive and 1‐octanol caused a flight aversive response. These findings are relevant as there are no reported plant‐derived compounds attractive to A. aegypti. As both the male and female mosquitoes sugar feed, traps baited with plant odors are able to lure the whole adult population, making it an interesting option for including in future mosquito surveillance traps.