Proportions of eggs laid by Aedes aegypti on different substrates within an ovitrap in Trinidad, West Indies

Abstract. Ovitraps were exposed outdoors during wet and dry seasons in Trinidad to determine the proportions of eggs laid on three substrates: the widely-used hardboard 'paddle' (P), the water surface (W) and the inside surface of the ovitrap wall (S) in three types of blackened ovitrap: round, glass (RG), round, plastic (RP) and rectangular, plastic (SP). Of the 3556 eggs obtained, most (86.4%) were laid during the wet season when statistically significant differences were detected between substrates but not between ovitrap types. Most eggs (>80%) were laid on P, significantly fewer ( P < 0.005) on W, on S and on W + S, thus confirming the superiority of the paddle as a device for monitoring oviposition activity. Implications of these findings for the interpretation of Aedes studies using ovitraps are discussed.     

Host feeding responses of Aedes aegypti (L.) exposed to deltamethrin

Escape responses of mated and unmated nulliparous Aedes aegypti mosquitoes were compared using three different concentrations of deltamethrin in the presence or absence of a live animal host using an excito-repellency (ER) test system. Both insecticide contact (excitation) and non-contact (repellency) test configurations were compared. For contact trials, mated mosquitoes showed similar escape movements among the three concentrations when host stimuli were absent. Significant differences in responses were seen between the lower concentrations of (LC(50) and LC(75) ) deltamethrin with and without hosts present (P<0.05). Presence or absence of host stimuli produced no significant differences in escape response for unmated females when exposed to the highest concentration (LC(90) ) of deltamethrin. Our findings indicate that as deltamethrin concentrations decrease to sublethal levels, mating status and host cues play a more significant role in escape behavior. Therefore, insemination can influence the outcome of feeding success and flight movement of nulliparous female Ae. aegypti in contact with deltamethrin and in the presence of live host stimuli. The ER assay system serves as a useful tool for observing excitation and repellency responses of Ae. aegypti to insecticides in the presence or absence of other environmental and biological cues that can affect mosquito behavior.     

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.     

Efficacy of sticky and standard ovitraps for Aedes aegypti in Trinidad,West Indies

The double sticky trap (DST) is described for the first time and is evaluated along with standard ovitraps and sticky traps (STs) to determine population densities of Ae. aegypti in the urban township of St. Augustine and the rural community of Tamana, Trinidad, West Indies. Ten houses were selected at each study site. At each of the ten houses, one ovitrap, one ST, and one DST were placed using the criteria established for placement of ovitraps. The results showed the three trapping methods successfully collected Ae. aegypti mosquitoes. All three traps collected significantly more adults or eggs in St. Augustine than in Tamana. DSTs collected 2,286 adults from St. Augustine vs 316 adults from Tamana (p<0.002), STs collected 1,480 and 220 adults, respectively (p<0.01), and the ovitraps collected 2,735 and 517 eggs, respectively from St. Augustine and Tamana (p<0.002). Based on these results, the DSTs collected significantly (P<0.02) more adults than the STs. The DSTs and STs collected both adult and immature stages which can be used for toxicology, virology, and PCR studies and are suitable alternative Ae. aegypti surveillance tools for the Caribbean and Latin American region.     

Intraspecific variation in desiccation survival time of Aedes aegypti (L.) mosquito eggs of Australian origin

Aedes aegypti (L.) mosquitoes preferentially oviposit in natural and artificial receptacles where their eggs are able to withstand drying as water levels fluctuate. Desiccation‐resistant eggs also increase the potential for establishment in non‐native habitats while providing logistical impediments to control programs. Viability and mean survival times of eggs stored under three dryness conditions for up to 367 days were investigated among three field‐derived colonies of Australian Ae. aegypti to understand variation in desiccation survival. Further investigations compared egg survival between an established colony and its wild counterpart. Our results confirmed that Ae. aegypti eggs can withstand desiccation for extended periods of time with approximately 2–15% egg viability recorded after one year and viability remaining above 88% under all conditions through 56 days. Intraspecific variations in egg survival times were recorded, suggesting local adaptation while each of the colonies demonstrated a consistent preference for higher humidity. Egg volume varied between the populations, suggesting a relationship between egg volume and survival time, with the marginally larger eggs (Charters Towers and Innisfail) having greater desiccation resistance over the range of conditions. The strong survivorship of Charters Towers eggs in dry, warm conditions demonstrates the adaptive significance of a desiccation‐resistant egg.     

Oviposition by Aedes aegypti and Aedes albopictus: Influence of congeners and of oviposition site characteristics

We investigated the oviposition behavior of Ae. aegypti and Ae. albopictus. In particular we examined whether small‐scale site characteristics and the presence of conspecifics or congeners altered oviposition by these mosquitoes. Various combinations of females of the two species were allowed to oviposit inside cages among either vegetation (potted plants) or structural components (wood and concrete blocks). Numbers of eggs deposited per female were compared between species, sides, and treatments. Most significant differences between treatments and species involved differences between single species and mixed species treatments. Ae. aegypti deposited more eggs/female in the vegetation side than in the structure side whereas the opposite pattern was evident for Ae. albopictus. Ae. aegypti females had higher frequency of skip oviposition than Ae. albopictus. An average of 63% of the containers in the two‐species treatments contained eggs of both species, with more frequent joint occurrences observed in the treatment with three females of each species than in the treatments with one of each. Our results point to the existence of various interactions between gravid Ae. aegypti and Ae. albopictus females at or near the oviposition sites but further experimental work is necessary to fully characterize the interactions and their specific mechanisms.     

Synergistic repellent and irritant effect of combined essential oils on Aedes aegypti (L.) mosquitoes

This study was designed to compare the behavioral responses of Aedes aegypti to a single essential oil and to a mixture of two or three essential oils using an excito‐repellency test chamber. Mixtures were prepared from essential oils extracted from Litsea cubeba (LC), Litsea salicifolia (LS), and Melaleuca leucadendron (ML). In general, the mixture of essential oils produced a much stronger escape response by Ae. aegypti, regardless of the test conditions. No significant difference in escape responses was seen when the mixture of oils was compared with a standard commercial product containing DEET. Greater contact irritancy was seen from mixed oils of LC and LS than with other mixed oils. Mixtures of LC and LS at 0.075% showed the highest synergistic action (65.5% escaped) compared to that with unmixed oil alone at the same concentration (LC/20% and LS=32.2%). In addition, mixtures of LC and LS at 0.075% demonstrated the highest non‐contact repellency (62.7%) and showed a greater effect than the use of LC (20%) or LS (20.3%) alone. We conclude that mixtures of two essential oils show potential as active ingredients for mosquito repellents.     

Some mechanical properties of the flagellar-pedicellar suspension in Aedes aegypti (L.)

Abstract The rotation of the flagellum of male and female Aedes aegypti (L.) was measured in a wind tunnel at wind speeds between 0 and 116cms^-1. Although the surface area of the female flagellum is only 10% of the male, it rotates through double the angle of the male flagellum for the same increase in wind speed. The larger rotation of the female flagellum could result from: a reduced resistive torque in the flagellar-pedicellar suspension; a larger antennal-positioning reflex; a smaller initial angle of attack. The resistive torque in the flagellar-pedicellar suspension was measured in both male and female Aedes aegypti. In ten adult females the average torque at 8^o rotation was 2.70 ± 1.33 times 10^-10Nm and in ten males it was 6.46 ± 3.46 times 10^-10Nm. The significance of these results is discussed with reference to the sexual dimorphism in the sensory tasks required of Johnston's organ in the two sexes.     

Electronic mosquito repellers induce increased biting rates in Aedes aegypti mosquitoes (Diptera: Culicidae)

Studies have demonstrated that electronic mosquito repellers are useless and that some of them could even increase the attraction of mosquitoes. While testing some electronic repellers, we noted that they also promoted an increase in biting rates. The present work has evaluated three commercial devices and a computer program working on five different sound frequencies. In a test chamber, the number of Aedes aegypti L. bite attempts was computed during four cycles of 3 min each by alternately turning the devices off and on. The mosquito biting rates for five sound frequencies (ranging from 9.6 kHz to 18.2 kHz) initially demonstrated a significant increase (ranging from around 20% to 50%), which decreased from 8.3% to 25.1% when the repellers were turned off. The biting rate significantly increased at 11.8 kHz (33.7%) when the device was turned on again. The danger of using electronic repellers and the role of sound frequencies stimulating mosquito biting are discussed.     

Experience‐ and age‐mediated oviposition behaviour in the yellow fever mosquito Stegomyia aegypti (=Aedes aegypti)

In repeated behaviours such as those of feeding and reproduction, past experiences can inform future behaviour. By altering their behaviour in response to environmental stimuli, insects in highly variable landscapes can tailor their behaviour to their particular environment. In particular, female mosquitoes may benefit from plasticity in their choice of egg‐laying site as these sites are often temporally variable and clustered. The opportunity to adapt egg‐laying behaviour to past experience also exists for mosquito populations as females typically lay eggs multiple times throughout their lives. Whether experience and age affect egg‐laying (or oviposition) behaviour in the mosquito Stegomyia aegypti (=Aedes aegypti) (Diptera: Culicidae) was assessed using a wind tunnel. Initially, gravid mosquitoes were provided with a cup containing either repellent or well water. After ovipositing in these cups, the mosquitoes were blood‐fed and introduced into a wind tunnel. In this wind tunnel, an oviposition cup containing repellent was placed in the immediate vicinity of the gravid mosquitoes. A cup containing well water was placed at the opposite end of the tunnel so that if the females flew across the chamber, they encountered the well water cup, in which they readily laid eggs. Mosquitoes previously exposed to repellent cups became significantly more likely to later lay eggs in repellent cups, suggesting that previous experience with suboptimal oviposition sites informs mosquitoes of the characteristics of nearby oviposition sites. These results provide further evidence that mosquitoes modify behaviour in response to environmental information and are demonstrated in a vector species in which behavioural plasticity may be ecologically and epidemiologically meaningful.     

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.     

Renewal of sexual receptivity in mated female mosquitoes, Aedes aegypti

ABSTRACT. Females of Aedes aegypti (L.) (Diptera, Culicidae) were not receptive to mating 1 h after the initial insemination or at later times throughout the first gonotrophic cycle. A significant proportion of these females (75–90%) were inseminated again prior to the second gonotrophic cycle. Insemination was determined by the detection in females of sperm labelled with ³ H(G)-adenine or ³²PO₄ after matings with radiolabeled males. Females deprived of males after the first insemination displayed an abnormal reduction in fertility throughout successive gonotrophic cycles.     

Temperature induces trade-offs between development and starvation resistance in Aedes aegypti (L.) larvae

Heightened temperature increases the development rate of mosquitoes. However, in Aedes aegypti (Diptera: Culicidae), the larvae of which commonly experience limited access to food in urban habitats, temperature effects on adult production may also be influenced by changes in the capacity of larvae to survive without food. We carried out experiments to investigate the effects of temperatures increasing at intervals of 2 °C from 20 °C to 30 °C on the growth, maturation rate and longevity of optimally fed larvae placed in starvation. Overall, both growth rate and starvation resistance were lower in the first three larval instars (L1-L3) compared with L4, in which growth of >75% occurred. Although increasing the temperature reduced the duration of each instar, it had a U-shaped impact in terms of the effect of initial growth on starvation resistance, which increased from L1 to L2 at 20 °C and 30 °C, remained constant at 22 °C and 28 °C, and decreased at 24 °C and 26 °C. Growth from L2 to L3 significantly increased starvation resistance only from 26 °C to 30 °C. Increased temperature (>22 °C) consistently reduced starvation resistance in L1. In L2-L4, increments of 2 °C decreased starvation resistance between 20 °C and 24 °C, but had weaker and instar-specific effects at >24 °C. These data show that starvation resistance in Ae. aegypti depends on both instar and temperature, indicating a trade-off between increased development rate and reduced starvation survival of early-instar larvae, particularly in the lower and middle temperatures of the dengue-endemic range of 20-30 °C. We suggest that anabolic and catabolic processes in larvae have distinct temperature dependencies, which may ultimately cause temperature to modify the density regulation of Ae. aegypti populations.     

Major chorion proteins and their crosslinking during chorion hardening in Aedes aegypti mosquitoes

The chorion of Aedes aegypti eggs undergoes a hardening process following oviposition and individual chorion proteins become insoluble thereafter. Our previous studies determined that peroxidase-catalyzed chorion protein crosslinking and phenoloxidase-mediated chorion melanization are primarily responsible for the formation of a hardened, desiccation resistant chorion in A. aegypti eggs. To gain further understanding of peroxidase- and phenoloxidase-catalyzed biochemical processes during chorion hardening, we analyzed chorion proteins, identified three low molecular weight major endochorion proteins that together constituted more than 70% of the total amount of endochorion proteins, and assessed their insolubilization in relation to phenoloxidase- and peroxidase-catalyzed reactions under different conditions. Our data suggest that the three low molecular weight endochorion proteins undergo disulfide bond crosslinking prior to oviposition in A. aegypti eggs, and that they undergo further crosslinking through dityrosine or trityrosine formation by peroxidase-catalyzed reactions. Our data suggest that chorion peroxidase is primarily responsible for the irreversible insolubilization of the three major endochorion proteins after oviposition. The molecular mechanisms of chorion hardening are also discussed.     

Herpetomonas,with Bacterium-like Inclusions,in Malaysian Aedes aegypti and Aedes albopictus*

SYNOPSIS Herpetomonas sp. was found repeatedly in the Malpighian tubules of laboratory-reared male and female Aedes aegypti and Aedes albopictus in Malaysia. The flagellates occurred irregularly, in batches, and were absent during long periods. The data suggest an exogenous source of infection for the mosquitoes, presumably from another insect, probably of another genus. Thirty to 40% of the flagellates of Aedes contained intracytoplasmic rod-shaped structures strongly resembling bacteria. These were found often in groups suggesting intracellular multiplication. They were passed to the Herpetomonas daughter cells during division. Parasitism of Aedes by Herpetomonas is extremely unusual, only one previous record, an inconclusive one, having been found. Parasitism by Herpetomonas containing bacterium-like rods has apparently never been reported.     

Effects of natal habitat odour, reinforced by adult experience, on choice of oviposition site in the mosquito Aedes aegypti

The effects of natal experience on the oviposition behaviour of adult female mosquitoes were investigated in the laboratory using Aedes aegypti (L.) (Diptera: Culicidae). 'Treatment' mosquitoes were exposed to a dilute repellent (inducing stimulus) in their breeding water (aquatic stages) and/or in the air (adults) during various combinations of life stages [larval only (L regime); larval and pupal (LP regime); larval, pupal and emergent adult (LPE regime); larval, pupal, emergent adult and adult (LPEA regime); pupal, emergent adult and adult (PEA regime); adult only (A regime)]. 'Control' mosquitoes were raised in an identical manner, but were not exposed to the inducing stimulus. The oviposition behaviour of treatment and control females was assessed in an oviposition assay that presented a choice of water with or without the inducing stimulus. Of the 435 mosquitoes tested in the experiment, 176 were non-distributors (i.e. laid all of their eggs in only one of the choices). Treatment females (distributors plus non-distributors) reared in the presence of the inducing stimulus throughout their lives (LPEA regime) showed a significant preference for the oviposition option containing the inducing stimulus (24/36 females) compared with corresponding controls (5/39 females). Distributors reared under the LPEA and PEA regimes also showed this preference (6/6 treatment vs. 2/29 control females, and 13/18 treatment vs. 7/23 control females, respectively). Females that had been exposed to the inducing stimulus as either immatures or adults only showed no preference for, and some showed an aversion to, the treatment oviposition option. This is interpreted as evidence for a natal habitat preference induction (NHPI) in this species, albeit one that requires extensive reinforcement in the adult stage. This adult experience-reinforced NHPI (AER-NHPI) is discussed in terms of its adaptive significance for container breeders, the possible timing mechanism and sensory basis of induction and potential practical consequences.     

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.     

A novel multiple membrane blood‐feeding system for investigating and maintaining Aedes aegypti and Aedes albopictus mosquitoes

A novel multiple membrane blood‐feeding system for mosquitoes has been developed for the study and routine maintenance of Aedes aegypti L. and Aedes albopictus Skuse that require a meal of vertebrate blood to produce eggs. This blood‐feeding system uses cattle collagen sausage‐casing membrane to facilitate feeding. The efficiency of this blood‐feeding system was compared to a live mice blood source. We observed that Ae. aegypti that fed on pig whole blood had 89.7% (w/o ATP) and 90.7% (w/ ATP) blood‐feeding rates, which were not significantly different from the mice‐fed ones (98.0%). Ae. albopictus fed on pig whole blood (w/ ATP) had a success rate of 84.4%, which was significantly different from the mice‐fed mosquitoes (51.1%). The feeding rates did not differ between sausage‐casing membrane and Parafilm‐M®. The survival rate, fecundity, pupation, and pupal emergence rates of Aedes females fed on pig whole blood were not significantly different from the mice‐fed ones. The artificial blood feeder can be applied to replace live animals as blood sources. Considering that this simple, inexpensive, convenient, and efficient feeding device can be built with common laboratory materials for research on Aedes mosquitoes.     

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.     

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.