Floodwater Mosquitoes Research Articles

Laboratory evaluation of citronella, picaridin, and deet repellents against Psorophora ciliata and Psorophora howardii.

Commercial formulations of 3 repellents: OFF Active (active ingredient [AI] 15% N,N-diethyl-meta-toluamide [deet]), OFF Family Care (AI 5% picaridin), and CVS Pharmacy Fresh Insect Repellent (AI 10% citronella, 2% lauryl sulfate, and 0.2% potassium sorbate) were evaluated to determine the mean protection time provided against the large floodwater mosquitoes, Psorophora ciliata and Ps. howardii. Each of these products contained different active ingredients and displayed different protection times, but repellency was in accordance with each product's label reapplication times/repellency durations. The CVS Pharmacy Fresh Insect Repellent provided the least protection (2 h and 26 min), followed by OFF Family Care (3 h and 46 min). OFF Active afforded the longest protection (5 h and 41 min), which was significantly higher than the other treatments (P < 0.001).

Worthy of Their Name: How Floods Drive Outbreaks of Two Major Floodwater Mosquitoes (Diptera: Culicidae)

An understanding of how climate variables drive seasonal dynamics of mosquito populations is critical to mitigating negative impacts of potential outbreaks, including both nuisance effects and risk of mosquito-borne infectious disease. Here, we identify climate variables most affecting seasonal dynamics of two major floodwater mosquitoes, Aedes vexans (Meigen, 1830) and Aedes sticticus (Meigen, 1838) (Diptera: Culicidae), along the lower courses of the Dyje River, at the border between the Czech Republic and Austria. Monthly trap counts of both floodwater mosquitoes varied both across sites and years. Despite this variability, both models used to fit the observed data at all sites (and especially that for Ae. sticticus) and site-specific models fitted the observed data quite well. The most important climate variables we identified-temperature and especially flooding-were driving seasonal dynamics of both Aedes species. We suggest that flooding determines seasonal peaks in the monthly mosquito trap counts while temperature modulates seasonality in these counts. Hence, floodwater mosquitoes indeed appear worthy of their name. Moreover, the climate variables we considered for modeling were able reasonably to predict mosquito trap counts in the month ahead. Our study can help in planning flood management; timely notification of people, given that these mosquitoes are a real nuisance in this region; public health policy management to mitigate risk from such mosquito-borne diseases as that caused in humans by the Tahyna virus; and anticipating negative consequences of climate change, which are expected only to worsen unless floods, or the mosquitoes themselves, are satisfactorily managed.

Immature Mosquitoes Associated with Urban Parklands: Implications for Water and Mosquito Management

The aim of the present study was to compare 2 urban habitat types: pools artificially filled with water from damaged or leaking water pipes (AF) and pools naturally filled by rainwater (NF), with regard to their favorability as breeding sites for mosquitoes. Two study areas were analyzed, 1 for 5 months and the other for 9 months, covering the whole period when AF pools contained water. The AF pools held water during the entire study, and showed lower fluctuations in surface area than NF pools. The AF pools showed higher levels of total mosquitoes and of stagnant-water mosquitoes. The floodwater mosquitoes were numerically (but not significantly) more abundant in NF pools. Nine mosquito species were identified. Habitat type, temperature, and season were significant in explaining the variability in species composition according to the canonical correspondence analysis. The most abundant species were Ochlerotatus albifasciatus (= Aedes albifasciatus, predominantly in NF pools), Culex dolosus, and Cx. pipiens (mainly in AF pools). The latter 2 species differed in their temporal dynamics, with Cx. dolosus associated with lower temperatures and Cx. pipiens with higher temperatures. Overall, the results indicate that although both habitat types harbored immature mosquitoes, the AF pools were more favorable than co-occurring rain pools. Easy-to-implement management actions such as the design of adequate drainage systems and the fast repair of broken pipes will be helpful to reduce the risk of human illness associated with mosquitoes in urban green areas.

Field Evaluation of a Large-Scale Barrier Application of Bifenthrin on a Golf Course to Control Floodwater Mosquitoes

In order to better understand the role a barrier application plays in an integrated mosquito management program, we applied a bifenthrin barrier application to a large golf course community and evaluated the mosquito population, service requests, and money spent for control from our barrier site as well as a site that was only treated via ground ultra-low-volume (ULV) application or that was left as a control site. Both the barrier application (t = 2.63, df = 126, P = 0.0147) and the ground ULV application (t = 2.49, df = 126, P = 0.0054) significantly reduced mosquito populations compared to the control, but means within treatments did not differ from each other. Mosquito populations in Centers for Disease Control and Prevention traps were reduced by 84% by barrier application and by 52% by ground ULV application at 6 wk posttreatment compared with untreated control. The 2 major species collected during the evaluation period were Anopheles crucians (81%) and Aedes atlanticus (13%). Anopheles crucians was significantly reduced compared with controls for the barrier application (t = 1.55, df = 126, P = 0.003) and ground ULV application (t = 1.85, df = 126, P = 0.052). Aedes atlanticus was significantly reduced by the barrier application (t = 1.54, df = 128, P = 0.002) but not by ground ULV application (P = 0.083). Overall the barrier application resulted in a significant decrease (chi2 = 35.4, df = 1, P < 0.0001) in the number of service requests and ground ULV application (chi2 = 5.3, df = 1, P = 0.02) compared to the site that was only treated with ground ULV application. The barrier application saved an estimated $2,700. Thus, large-scale barrier applications are an appropriate tool for control of floodwater mosquito populations.

Oviposition strategies of temporary pool mosquitoes in relation to weather, tidal regime and land use in a temperate wetland

Wetlands have traditionally been associated with harbouring mosquitoes, a well-known nuisance and vectors of diseases. Within mosquito life cycle, oviposition is a determinant event by shaping their individual fitness and vectorial capacity. The study was conducted in one of the main temperate wetlands in South America. We used Generalized Linear Models to study the relation between temperature, precipitation, tidal regime, land use, microenvironment, and the occurrence of floodwater (Ochlerotatus and Psorophora spp.) and raft-laying (Culex and Uranotaenia spp.) mosquitoes using temporary pools as larval habitats. Pool occurrence was negatively associated with temperature, and positively related to precipitation and high tides. As regards the land use, it was lowest in domestic areas and plantations, intermediate in secondary forests, and highest in marshes. Each oviposition strategy was best modelled as a function of different environmental factors. The occurrence of floodwater mosquitoes was positively associated with high cumulative precipitation and low tide records. Raft-laying mosquitoes were related to low temperature records, while the effect of flooding varied with the land use. In view of these results, physical (water inputs and movement), chemical, and biological (egg and larval flushing, and predatory interactions) considerations are given to provide insight in the oviposition patterns of mosquitoes occurring in this complex wetland. We finally propose the generation of a tidal flow as a control measure against floodwater mosquitoes, which are the most anthropophilic in the study area.

Detection of temporary flooded areas with potential floodwater mosquito production using imaging radar

Temporarily flooded areas can produce enormous numbers of floodwater mosquitoes, causing tremendous nuisance to people living in the vicinity. The aim of this study is to develop a remote-sensing method for detecting temporary flooded areas that can produce floodwater mosquitoes. For this objective, synthetic aperture radar (SAR) imagery from the European Remote Sensing satellite (ERS-2) and the Environmental Satellite (Envisat) are chosen. The images cover both flooded and dry periods around Lake Färnebofjärden, located in the lowlands of the River Dalälven, central Sweden, during the vegetation season between 2000 and 2006. Unsupervised classification and principal component analysis (PCA) are tested as methods for detecting floodwater mosquito production sites. In the unsupervised classification experiment, four types of images are tested. The classification of a synthetic colour image gives the best result with an overall accuracy of 85.7% and a kappa value of 0.7, as well as a 46% detection rate of field-mapped flooded areas. PCA is performed on a data set of 16 time series radar images. The resulting principal component (PC) bands provide information about flooding probability as well as vegetation structures. Regular flooding increases the probability for an area to provide breeding sites for floodwater mosquitoes. Thus, this approach will be very useful in estimating the risk of floodwater mosquito establishment.

The flight distances of floodwater mosquitoes (Aedes vexans, Ochlerotatus sticticus and Ochlerotatus caspius) in Osijek, Eastern Croatia

In spring 2004, the mark-release-recapture study was conducted in the Osijek area, covering the total of 171 km2, to describe dispersal pattern of three floodwater mosquito species (Aedes vexans, Ochlerotatus sticticus and Oc. caspius). Forty CO2 baited Center for Disease Control and Prevention (CDC) traps were set at distances varying from 0.95 to 16.7 km from release site. Fifty thousand mosquitoes were released, and a total of 22 marked mosquitoes were recaptured in 12 traps, 82% of which were collected in the first six days after release. The maximum distance of recaptured mosquito (Oc. sticticus) was found at 11.68 km from the release site. During the study, the average dispersal rate per day for mosquitoes ranged from 0.96 km to 3.6 km in regard to different species.

Field Efficacy of Granular Formulations of Bacillus thuringiensis israelensis - Strain Am65-52 Against Floodwater Mosquitoes in Poland and Germany

This paper presents results of studies evaluating the efficacy of various granular formulations of VectoBac (Bacillus thuringiensis israelensis de Barjac [Bti] H-14 strain AM65-52) against immature mosquitoes in 2 distinct habitats in Poland and Germany. Meadows intermittently flooded with sewage water in the city of Wroclaw, Poland, provide ideal habitats for immature stages of Aedes caspius, Ae. vexans, and Culex pipiens pipiens. Helicopter applications of VectoBac G and VectoBac TP sand granules (VectoBac TP-SG) at rates of 5 and 10 kg/ha to the meadows resulted in between 70.9% and 97.4% larval mortality. In Germany, some swampy woodlands in the upper Rhine Valley provide ideal habitats for snowmelt mosquitoes, Aedes cantans, Ae. punctor, Ae. rusticus, and Ae. communis. Ground applications of VectoBac G, VectoBac TP-SG, and VectoBac WG IcyPearls (VectoBac WG-IP) were made to these habitats when the water temperature was approximately 7 degrees C. Larval mortality at 7-day post-application ranged from 90.9% (VectoBac TP-SG: 10 kg/ha) to 98.0% (VectoBac WG-IP: 15 kg/ha). In a separate microcosm trial within the same habitat, all 3 granule formulations controlled larvae of Ae. cantans for 3 wk.

Evaluation of Lurex 3™, Octenol, and CO2Sachet as Baits in Mosquito Magnet® Pro Traps Against Floodwater Mosquitoes

Field evaluation of Lurex 3, 1-octen-3-ol (octenol), and CO2 sachet as baits in Mosquito Magnet Pro traps (MMP) for collecting adult mosquitoes was conducted at 2 different locations in northeast Florida. A total of 18 species of mosquitoes were collected by the MMP baited with the 3 attractant baits in St. Augustine, FL. The MMP baited with octenol collected significantly more mosquitoes than the traps baited with Lurex 3 and the CO2 sachet. A total of 6 floodwater mosquito species were collected by the MMP baited with the 3 attractant baits in Elkton, FL. The MMP baited with Lurex 3 or octenol collected more mosquitoes than the MMP baited with CO2 sachet or MMP alone.

Mosquito density forecast from flooding: population dynamics model forAedes caspius(Pallas)

Insect population dynamics depend strongly on environmental factors. For floodwater mosquitoes, meteorological conditions are crucial in the rhythm of mosquito abundances. Indeed, rainfall triggers the egg hatching after flooding breeding sites, and temperature controls the duration of the aquatic immature development up to adult emergence. According to this, we have developed a simple mechanistic and tractable model that describes the population dynamics of floodwater mosquitoes as a function only of the most accessible meteorological variables, rainfall and temperature. The model involves three parameters: development duration tdev of the immature aquatic stages, the adult emergence rate function f(t) (characterized by the emergence time scale tau and shaping the profile of adult population abundance), and the depletion rate, alpha, of adult disappearance. The developed model was subsequently applied to fit experimental field data of the dynamics of Aedes caspius (Pallas), the main pest mosquito in southern France. First, it was found that the emergence rate function of adult mosquitoes very well reproduce experimental data of the dynamics of immature development for all sampled temperatures. The estimated values of tdev and tau both exhibit Arrhenius behaviour as a function of temperature. Second, using the meteorological records of rainfall and temperature as inputs, the model correctly fit data from a two-site CO2 trapping survey conducted in 2004 and 2005. The estimated depletion rates (summation of the mortality and the emigration rates) were found to be a concave quadratic function of temperature with a maximum of 0.5 per days at about 22 degrees C.

Diving beetle assemblages of flooded wetlands in relation to time, wetland type and Bti-based mosquito control

We investigated the abundance and taxonomic composition of the aquatic predatory insect fauna, with focus on adult diving beetles (Coleoptera: Dytiscidae), in eight temporary flooded wet meadows and two alder swamps in the River Dalälven floodplains, central Sweden from 2002 to 2006. Diving beetles are generalist predators and often abundant in various waters, including temporary wetlands. In the River Dalälven floodplains, recurrent floods induce massive hatching of flood-water mosquitoes (Diptera: Culicidae), which constitute a superabundant patchy and irregular food resource for aquatic predatory insects. Our aims were (1) to characterize the assemblage of adult diving beetles occurring in the wetlands during floods in relation to time and wetland type and (2) to evaluate the effect on the aquatic predator assemblage of strongly reducing the abundance of a potential prey, flood-water mosquito larvae with Bacillus thuringiensis var. israelensis (Bti) during floods. We found diving beetles to be the dominating aquatic predatory insect taxa in all 10 wetlands. There was a difference in Dytiscidae species richness but not in diversity between wet meadows and alder swamps after rarefaction. The cluster analysis based on dytiscid species and abundances showed very high similarities between the wetlands. The variance component analysis was unable to distinguish any factor that could explain more than 7.4% of the variation in the dytiscid species assemblages. The only effect of Bti-treatment against flood-water mosquito larvae, potential food for the predatory dytiscids, was a slight increase in abundance of the medium-sized dytiscid species. Our results are in accordance with previous studies, suggesting that irregular and recurrent flood dynamic structure the dytiscid fauna more than food limitations and environmental factors.

The present distribution and predicted geographic expansion of the floodwater mosquito Aedes sticticus in Sweden

The mass emergence of floodwater mosquitoes, in particular Aedes sticticus and Aedes vexans, causes substantial nuisance and reduces life quality for inhabitants of infested areas and can have a negative impact on the socio-economic conditions of a region. We compared the previous, present, and predicted geographic distribution of Ae. sticticus in Sweden. Previous records from the literature until 1990 list the species in three out of 21 Swedish counties. Beginning in 1998, studies show that the present distribution of the species covers 11 counties, with highest abundances in an east-west belt in Central Sweden. Using climate data from the present and predicted climate scenarios, the expected distribution of Ae. sticticus in 2020, 2050, and 2080 could be modelled using GIS. As variables, mean temperatures and cumulative precipitation between May and August and degree slope were chosen. The predicted geographic distribution of Ae. sticticus will continue to increase and include 20 out of 21 Swedish counties. The expected temperature rise will increase the suitable area towards the northern part of Sweden by 2050. Some non-suitable areas can be found along the south-east coast due to insufficient amount of precipitation in 2050 and 2080. Modelling the expected distribution of a species using predicted climate change scenarios provides a valuable tool for risk assessments and early-warning systems that is easily applied to different species and scenarios.

Production of wetland Chironomidae (Diptera) and the effects of using Bacillus thuringiensis israelensis for mosquito control

Massive mosquito nuisance problems, caused by the flood-water mosquito Aedes sticticus, occur after floods in the flood plains of the River Dalälven, central Sweden. Since 2002, the biological mosquito larvicide Bacillus thuringiensis var. israelensis (Bti) has been used to control these mosquitoes. Here, we report results from the first six years of monitoring Chironomidae, the most susceptible non-target organisms, in three wetlands with Bti-treatment against mosquitoes and in three wetlands without treatment. Emergence traps were used for continuous insect sampling from May to September each year, 2002-2007, and were emptied once a week. A total of 21,394 chironomids of 135 species were collected, and the subfamily Orthocladiinae dominated the fauna followed by Chironominae and Tanypodinae. The annual chironomid production in the wetlands was low, with an average of 1917 individuals per m(2), and 42 g ash-free dry weight per m(2). We found no reduced production of chironomids at neither family, nor subfamily level, in Bti-treated as compared to untreated wetlands. This is the first long-term follow-up study of the possible effects of Bti-based mosquito larval control on chironomid species production. In the short-term view, one species had higher production in treated areas. In the long-term view, four species had higher and one species had lower production in treated areas. We conclude that the Bti-based control of flood-water mosquitoes does not cause any major direct negative effects on chironomid production, and therefore does not seem to induce any risk for indirect negative effects on birds, bats or any other predators feeding on chironomids.

The characteristics of the most troublesome mosquito species (Diptera: Culicidae) in Poland

Twelve of the 47 Polish mosquito species can pose serious health problems to man or at least heavy nuisance. They fly from breeding sites (where huge numbers can be produced) into houses and/or farm buildings transmitting severe human and animal diseases. Problematic ones are: Anopheles messae and An. atroparvus, Ochlerotatus communis, Oc. cataphylla, Oc. punctor, Oc. cantans, Oc. sticticus, Aedes cinereus, Ae. vexans, Coquillettidia richiardii, Culex pipiens and Culiseta annulata. The problems of distribution, phenology under different climatic conditions, habitat requirements and preferences both of larvae and adults, behaviour, fertility and vector competence of all these species are analyzed. Most of the species in the Temperate Zone produce more than one generation during the growing season or at least have several emergences. Only Oc. communis and Oc. cataphylla are early spring univoltine species, although they can breed twice or more during the growing season when there are favourable conditions. Univoltine Oc. cantans, which can develop several times during a season, occurring in the breeding sites often together with multivoltine Oc. punctor. The period of their occurrence is usually much longer than that of Oc. communis and Oc. cataphylla. Flood-water mosquitoes - Oc. sticticus and both Aedes species occur along river valleys and their larvae can breed in different pools situated in a variety of environments. Culex pipiens, especially its form molestus and Culiseta annulata can breed in natural and artificial ponds, even in heavily polluted water. Although mosquitoes are known to be competent vectors of serious human and animal diseases, in Poland they are considered to cause nuisance only. Despite the fact that malaria was eradicated in Poland in 1960-s, at least two Polish anopheline species can still transmit the human malarial parasite very effectively. The remaining species (except for only one) described in the paper can transmit several arboviruses, while several species are vectors of filariosis (Dirofilaria sp.).

Effect of diet composition on the development of the floodwater Mosquito Ochlerotatus (Ochletotatus) albifasciatus (Macquart) (Diptera: Culicidae)

One important step for the colonization of Ochlerotatus albifasciatus (Macquart) is to determine the optimal conditions for larval rearing, which makes possible the development of experiments comprising larval rearing in the laboratory. In this research the effect of diet composition on the development of O.albifasciatus was studied. For this purpose, cohorts of 20 first instars were reared using five diets: 1:1 mix of yeast and TetraMin(R), 1:1 mix of fine and coarse organic matter, grass cuts soaked in water for 1h or 24h before larvae incorporation, and 1:1:1 mix of grass cuts, fine organic matter and coarse organic matter. Survival was recorded for each cohort, while development time from first instar to pupa, and adult wing length were recorded for each individual. The effects of the diet on the observed traits were analyzed by one way ANOVA. Both survival and wing length were affected by diet, being significantly higher (79-100% and ~ 4.46 mm) on diets consisting of organic matter or organic matter plus grass than on those consisting of yeast plus TetraMin(R) or grass. Development time was not affected by the diet, although the shortest time (8.1 to 8.3 days) and lower variation between replicates were recorded when larvae were reared using organic matter or organic matter plus grass. Thus, it is concluded that the diets consisting of organic matter or a mix of it and grass are the most effective for O.albifasciatus rearing.

Effects of mosquito larvae removal with Bacillus thuringiensis israelensis (Bti) on natural protozoan communities

The protein crystals produced by Bacillus thuringiensis israelensis (Bti) are used against the larvae of pestiferous flood-water mosquitoes in ephemeral wetlands. Although mosquito larvae are considered important predators on protozoans and bacteria, it is not known how a distinct reduction of mosquito larvae density in natural wetlands caused by application of Bti may indirectly affect these microbial communities. Here we show, in a large scale experiment in six natural wetlands, that the densities of heterotrophic protozoans was on an average 4.5 times higher in wetland areas treated with Bti than in control areas. In addition, the taxonomic richness of heterotrophic protozoans increased on an average of 60% in areas with Bti application compared to control areas. The increase in protozoan density and richness was fairly consistent among sites of different wetland habitats. We discuss the potential implications of our results for other parts of the ecosystem.

SPATIOTEMPORAL MONITORING OF FLOODWATER MOSQUITO DISPERSAL IN OSIJEK, CROATIA

This paper demonstrates the possibility of using geostatistics to monitor the dispersal of mosquitoes for mosquito control programs at the municipal level. The case study objective was to quantify the dispersal of floodwater mosquitoes from the natural marshland Kopacki rit into the city of Osijek, Croatia, and to analyze the main factors controlling it. Fifty thousand adult Aedes vexans, Ochlerotatus sticticus, and Ochlerotatus caspius mosquitoes were marked with a powdered fluorescent pigment and released from the southern part of Kopacki rit on April 28, 2004. Forty CO2-baited Centers for Disease Control and Prevention traps were set in the area of the municipality of Osijek (171 km2) and were monitored for 10 days. A total of 582,471 mosquitoes were captured and examined in the laboratory. The mosquito counts from different sites were then interpolated using ordinary kriging and visualized dynamically to detect the dominant migrational directions. Mosquito dispersal and frequency were greatly influenced by wind speed (r = 0.82). The marked mosquitoes were found at 12 sites located from 1 km to 11.7 km away from the release point. The recapture rate was 0.044% (54% Oc. sticticus, 32% Ae. vexans, and 14% Oc. caspius). Based on the Lincoln index, the estimated total population size for floodwater mosquitoes in the study area ranged from 875 million to 2.0 billion mosquitoes. Limitations of the approach, recommendations for the improvement of the monitoring network, and spatial predictions are further discussed.

Rainfall triggered dynamics of Aedes mosquito aggressiveness

Inspired by Davidson method of estimating daily survivals of a structureless population of mosquitoes, we present a model which describes the behavior of floodwater mosquitoes in terms of emergence functions following a rainfall event, blood feeding frequency and parous stages, and survival at various stages. As a generalization of the Davidson formula, we have developed an approach for dealing with the dynamics of structured population of mosquitoes, and derived various formulas allowing assessment of demographic parameters like durations of gonotrophic cycles and (apparent) daily survivals. The method was subsequently applied to field data of floodwater mosquitoes Aedes vexans arabiensis, potential vectors of Rift Valley fever in West Africa, collected during the 2003 rainy season in Barkedji, Senegal. We found that mosquitoes emerged about 3 to 4 days following an efficient rainfall, and mosquito emergences, described by a bell shaped function, lasted for about 2 days. The mean duration of the gonotrophic cycle was 3 days and the apparent daily survival about 0.87 .

Study of the Insects Associated with the Floodwater Mosquito Ochlerotatus Albifasciatus (Diptera: Culicidae) and their Possible Predators in Buenos Aires Province, Argentina

Insects associated with the floodwater mosquito Ochlerotatus albifasciatus(Diptera: Culicidae) were studied from intermittent puddles in temperate Argentina in an attempt to detect the main predators. Forty-one taxa occurred in the puddles from spring to fall. Coleoptera and Diptera were dominant and diverse. Ephemeroptera and Odonata were scarce in numbers and species, and Heteroptera occurred in low numbers of species and high abundance of individuals. The main predators of immature O. albifasciatus were detected on the basis of relative abundance (ISA index), ecological dominant groups, and species association (“I” index). Liodessus sp. and Rhantus signatus signatus(Coleoptera: Dytiscidae) were the most abundant predators in the puddles and Liodessus sp., Lancetes marginatus(Dytiscidae) and Psorophora ciliata(Culicidae) were the most frequent. Liodessus sp. and O. albifasciatus were the best associated species in all seasons. Liodessus sp. was indistinctly associated with all larval instars and pupae, while R. signatus signatus, L. marginatus and Psorophora ciliata were more associated with the pupal stage of the mosquito. Desmopachria concolor(Coleoptera: Dytiscidae), Tropisternus lateralis limbatus and Tropisternus setiger(Coleoptera: Hydrophilidae) were other potential predators inhabiting the puddles, but their relevance was minor.

Different responses of two floodwater mosquito species, Aedes vexans and Ochlerotatus sticticus (Diptera: Culicidae), to larval habitat drying

Insect larvae that live in temporary ponds must cope with a rapidly diminishing resource. We tested the hypothesis that floodwater mosquitoes would react to diminishing water levels by accelerating larval development time and emerging as smaller adults. Since a reduction in habitat size leads to increased larval densities, we also included two larval densities. Newly-hatched floodwater mosquito larvae, Aedes vexans (87.9% of emerged adults) and Ochlerotatus sticticus (12.0% of emerged adults), were taken from the field and randomly assigned to one of three water level schedules. Survival to adult emergence was significantly affected by the water level schedule. Ae. vexans adults emerged later in the decreasing schedule than the constant water schedule, but time to emergence was not affected by larval density. In the drying water schedule, Ae. vexans adults emerged 6 to 14 days after complete water removal. Adult size was significantly affected by both water level schedule and larval density. Adults of Oc. sticticus emerged earlier in the decreasing than the constant water schedule which was in accordance with our hypothesis, but size was not affected. Our results indicate two different responses of two floodwater mosquito species to diminishing larval habitat. Oc. sticticus accelerated larval development while Ae. vexans larvae showed remarkable survival in humid soil. Both species are often numerous in inundation areas of large rivers, and climatic conditions after a flood might influence which species dominates the adult mosquito fauna.