Some boundary layer characteristics of microhabitats occupied by larval black flies (Diptera: Simuliidae)

Microhabitats occupied by larvae of 12 species or complexes of black flies at 30 sites in western Canada were investigated for fluid flow parameters. An attached boundary layer was a feature of almost all microhabitats occupied by larvae of black flies. Although not measured directly, acceleration of water resulting from flow around an object appeared to alter flow characteristics and make microhabitats more favourable. This predictability in the flow characteristics of microhabitats occupied by larvae of black flies implies that the contagious dispersion of these larvae is a response to the heterogeneity of flow over a stream bed. It is suggested that microhabitats characterized by suitable flow characteristics may constitute a resource that limits population size. Larvae occupied microhabitats with both laminar and turbulent boundary layers, and were found in microhabitats with a wide range of water velocities. Mainstream flow parameters investigated were; water velocity, water depth, and whether flow was subcritical, critical or supercritical. Boundary layer parameters investigated were; whether flow was laminar or turbulent, and the point at which separation from the substratum occurred.     

Consequences of the introduction of the planarian Girardia anceps (Tricladida: Dugesiidae) in artificial containers with larvae of the mosquitoes Aedes aegypti and Culex pipiens (Diptera: Culicidae) from Argentina

Aedes aegypti and Culex pipiens are container-dwelling mosquito species that are vectors of important diseases to man, such as dengue and lymphatic filariasis, respectively. Predators of these pests are an interesting alternative to be incorporated to biological control measures. We tested the consequences of introducing individuals of Girardia anceps, a native freshwater flatworm species, within artificial water containers where larvae of these mosquitoes thrive. Our goals were to ascertain if mosquito species, density of larvae (high or low), type of water container (tires or ovitraps), and presence or absence of planarians affected mosquito survivorship (measured as number of individuals reaching the pupa stage) in manipulated artificial containers. Furthermore, we monitored ovitraps in the field along several months in order to explore the long-term effect of the presence of planarian on the colonization of these containers by feral mosquitoes under natural conditions. We found that the presence of planarians reduced the number of mosquitoes reaching pupation and that such reduction depends on the initial density of larvae. Reduction of populations of A. aegypti was high along the breeding season of this mosquito, being the effect less evident in C. pipiens. G. anceps could be an agent of control against container-breeding mosquitoes if its release in small water containers is complemented with other suitable management strategies.     

Global diversity of black flies (Diptera: Simuliidae) in freshwater

Black flies are a worldwide family of nematocerous Diptera whose immature stages are confined to running waters. They are key organisms in both aquatic and terrestrial ecosystems, but are perhaps best known for the bloodsucking habits of adult females. Attacks by black flies are responsible for reduced tourism, deaths in wild and domestic birds and mammals, and transmission of parasitic diseases to hosts, including humans. About 2,000 nominal species are currently recognized; however, certain geographical regions remain inadequately surveyed. Furthermore, studies of the giant polytene chromosomes of larvae reveal that many morphologically recognized species actually consist of two or more structurally indistinguishable (yet reproductively isolated) sibling species. Calculations derived from the best-known regional fauna—the Nearctic Region—reveal that the actual number of World black fly species exceeds 3,000. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: E.V. Balian, C. Lévêque, H. Segers & K. Martens Freshwater Animal Diversity Assessment     

Variation in larval black fly populations at three sites in a stream system over five years (Diptera:Simuliidae)

Five years of monthly samples taken from three sites in the Broad Cove River, two at lake outlets and one a kilometer downstream from an outlet were analyzed. Considerable variations in the population of each species between sites and years were observed. Fluctuations in the populations were not always consistent from site to site or between species at the same site from one year to the next. As all sites were on the same stream and within 3 km of each other with only slight altitude differences climatic factors were not a major factor producing variations observed between sites. Also the previous years population level did not appear to be directly related to the following seasons population level. Apparently localized conditions at a site were exerting a marked influence on the larval simuliid population at each site.     

Growth of Anopheles mosquito larvae on dietary microbiota in aquatic surface microlayers

Abstract. Hydrophobic organic matter accumulates under the surface film of water bodies to form the surface microlayers. Heterotrophic microorganisms use this organic matter for growth, and they, in turn, are fed upon by Anopheles mosquito larvae and other animals. From laboratory experiments we show that two species of mosquito larvae, Anopheles gambiae and An.quadrimaculatus , grew most rapidly where surface micro-layers were present and, especially, where labile dissolved organic matter was added to promote growth of microorganisms. The importance of microorganisms was confirmed by the addition of gentamicin antibiotic, which suppressed the microbiota and reduced the growth of larvae feeding on surface microlayers. Anopheles larvae grew well on a suspension of finely ground fish food to which the antibiotic had been added, showing that reduced growth was not due to gentamicin itself. Because sub-surface microorganisms are the components of the larval diet that most affect growth, we discuss their relevance to strategies for larval control of Anopheles mosquitoes.     

Scale, time, space, and predictability: species distributions of preimaginal black flies (Diptera: Simuliidae)

This study examines species-specific distributions of preimaginal black flies (Diptera: Simuliidae) over two large spatial scales (within and across ecoregions) and two seasons (spring and summer). The study area in the western two-thirds of South Carolina, United States, was divided into three ecoregions (Mountains, Piedmont, Sandhills). The mean correct classification of species distributions among stream sites was 81.3%. Predictability of species distributions varied with spatial scale, location, and time. Species occurrence was not independent of ecoregion; distributions of 12 of the 13 species examined at this scale were predictable on the basis of changes in water column (temperature, percentage dissolved oxygen) and channel characters (velocity, streambed-particle size). However, with the exception of the Mountains during the summer, predicting species distributions within ecoregions was far less successful than predicting distributions across these regions; predictability was particularly poor in the Sandhills. Seasonal effects on predictability were most pronounced in the Mountains. As stream sites became more homogeneous, predictive capability declined. Species-level identifications, aided by cytogenetic techniques, were of paramount importance in recognizing the association between species distributions and stream conditions, emphasizing that considerable ecological information is lost when species are not considered. This study emphasizes the critical role of taxonomic resolution, linked with spatial and temporal scale of observation, in elucidating patterns of species assemblages. It also demonstrates that predictability of species distributions at a fixed scale can vary with geographic location and time. Received: 2 May 1997 / Accepted: 22 September 1997     

Long-term emergence patterns of black flies (Diptera: Simuliidae) in northwestern Pennsylvania

Spatial and temporal patterns of black fly emergence at three sites over a 7-year to 11-year period are presented. This study is the longest, continuous collection of emerging black flies ever undertaken. Because ovipositing females were able to enter emergence traps, analysis of emergence patterns for the Simulium vittatum Zetterstedt and S. tuberosum Lundström complexes, was restricted to males. Year, season and stream temperature had significant effects on mean emergence for these complexes. Sampling stations on the same system and within close proximity exhibited synchronous emergence patterns, whereas streams in close proximity but in separate basins showed asynchronous patterns. Dissimilarities might reflect divergent temperature regimes. We suggest that emergence-trap data might provide a less variable picture of community structure than larval data.     

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.     

The ecology of mosquito larvae (Diptera: Culicidae) in the subalpine zone of the eastern Sierra Nevada Mountains,California, U.S.A.

Mosquito larvae were collected from the subalpine region of the eastern Sierra Nevada Mountains from 2011 to 2014. Two watersheds were sampled and sites selected were mainly vernal snow‐melt pools and wet meadows. Seven Aedes species, Culiseta incidens (Thomson), and Culex tarsalis Coquillett were collected. The most abundant and widely distributed species were Ae. hexodontus Dyar and Ae. tahoensis Dyar. Aedes tahoensis was the predominate species in woodland snow‐melt habitats. Some species were found at most elevations while others were found more often at specific elevations. The most restrictive species was Ae. ventrovittis Dyar which occurred almost exclusively between 3,219 m a.s.l. and 3,390 m a.s.l. Shannon and Simpson species diversity indices demonstrated that species diversity was greater in meadow habitats compared to woodland habitats. Mixed woodland/meadows, rock pools, and shallow grass pools were intermediate in species diversity. Abiotic factors such as snowpack and water temperature impacted species development times and when habitats dried. It was concluded that spatial and temporal patterns of habitats, along with elevation, influenced species presence and larval development. The results of the present study and previous work in the eastern Sierras will help guide future research that focuses on the potential change in the distribution and seasonality of subalpine mosquitoes and disease potential in the eastern Sierras as climatic conditions change.     

Evaluation of a peridomestic mosquito trap for integration into an Aedes aegypti (Diptera: Culicidae) push-pull control strategy

We determined the feasibility of using the BG‐Sentinel? mosquito trap (BGS) as the pull component in a push‐pull strategy to reduce indoor biting by Aedes aegypti. This included evaluating varying numbers of traps (1–4) and mosquito release numbers (10, 25, 50, 100, 150, 200, and 250) on recapture rates under screen house conditions. Based on these variations in trap and mosquito numbers, release intervals were rotated through a completely randomized design with environmental factors (temperature, relative humidity, and light intensity) and monitored throughout each experiment. Data from four sampling time points (05:30, 09:30, 13:30, and 17:30) indicate a recapture range among treatments of 66–98%. Furthermore, 2–3 traps were as effective in recapturing mosquitoes as 4 traps for all mosquito release numbers. Time trends indicate Day 1 (the day the mosquitoes were released) as the “impact period” for recapture with peak numbers of marked mosquitoes collected at 09:30 or 4 h post‐release. Information from this study will be used to guide the configuration of the BGS trap component of a push‐pull vector control strategy currently in the proof‐of‐concept stage of development in Thailand and Peru.     

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.     

Laboratory evaluation of Toxorhynchites splendens (Diptera: Culicidae) for predation of Aedes albopictus mosquito larvae

Biology of the mosquito Toxorhynchites splendens (Wiedemann) was studied in the laboratory to provide baseline data for using the predatory larvae of this species against those of Aedes albopictus (Skuse) in a biological control programme. The mean incubation time of Tx.splendens eggs was 43.8 h and the time required for newly-hatched larvae to initiate predation was 2.5 h. Mean numbers of prey larvae consumed and killed by each Tx.splendens larva totalled 389 and 345 respectively. The larval period of Tx.splendens was not significantly different for rearing individually or in groups of nine, with equal prey density, and duration of larval development was proportional to prey density. In mass rearing, larval cannibalism was usually observed during days 1-3 post-eclosion. The incidence of cannibalism decreased sharply on the fourth day after hatching when some larvae became fourth-instar. Adult female Tx.splendens usually commenced oviposition on day 4 after emergence. The number of eggs laid daily increased on day 7 and the peak oviposition of 6.3 eggs/female/day occurred on day 11. When oviposition containers were provided only intermittently, gravid females of Tx.splendens scattered most of their eggs on the dry floor of the cage. Viability of eggs laid by females aged 4-14 days was high (60-90%) but decreased to less than 40% as the females aged.     

Climate and geographic trends in hatch delay of the treehole mosquito,Aedes triseriatus Say (Diptera: Culicidae)

Eggs of Aedes triseriatus mosquitoes are stimulated to hatch when inundated with water, but only a small fraction of eggs from the same batch will hatch for any given stimulus. Similar hatching or germination patterns are observed in desert plants, copepods, rotifers, insects, and many other species. Bet hedging theory suggests that parents stagger offspring emergence into vulnerable life history stages in order to avoid catastrophic reproductive failures. For Ae. triseriatus, a treehole breeding mosquito, immediate hatching of an entire clutch leaves all of the parent's progeny vulnerable to extinction in the event of a severe drought. Natural selection has likely favored parents that pursued a bet hedging strategy where the risk of reproductive failure is distributed over time. Considering treehole mosquitoes, bet hedging theory could be used to predict that hatch delay would be positively correlated with the likelihood of drought. To test this prediction, we collected Ae. triseriatus from habitats that varied widely in mean annual precipitation and exposed them to several hatch stimuli in the laboratory. Here we report that, as predicted, Ae. triseriatus eggs from high precipitation regions showed less hatch delay than areas of low precipitation. This strategy probably allows Ae. triseriatus to cope with the wide variety of climatic conditions that it faces in its extensive geographical range.     

Prevalence and seasonality of Ascogregarina culicis (Apicomplexa: Lecudinidae) in natural populations of Aedes aegypti (Diptera: Culicidae) from temperate Argentina

Ascogregarina infections from South America were recently documented in Brazil and Argentina. The aim of this study was to report our recent findings on the prevalence and seasonality of Ascogregarina culicis in Aedes aegypti adults from temperate Argentina. Between December 2003 and May 2005, 391 females of Ae. aegypti were captured in two areas of Greater Buenos Aires. Overall prevalence of A. culicis was 21.2% (83/391), and a significant difference was observed between both areas (28.4% vs. 8%). Infected Ae. aegypti were found from November to May, with highest values in March (24-37%). Parasite prevalence and host abundance showed similar seasonal patterns. Our observations suggest a widespread infestation of A. culicis among Ae. aegypti populations from temperate Argentina.     

Multiple symbiotes in larval black flies (Diptera: Simuliidae): associations of trichomycete fungi with ichthyosporeans, mermithid nematodes, and microsporidia

Studies of the interactions between two or more species of symbiotic microorganisms in a single host animal have lagged behind those involving interactions between hymenopteran or dipteran parasitoids. We screened the larvae of four species of black flies (Cnephia ornithophilia, Simulium furculatum, Simulium innoxium, and Simulium vandalicum) to determine if the colonization and prevalence of trichomycete fungi were influenced by ichthyosporeans, mermithid nematodes, microsporidia, or other species of trichomycetes. Differences in the layering of the peritrophic matrix of microsporidian-infected larvae, compared with that of uninfected larvae, were associated with greater levels of colonization by the midgut-inhabiting trichomycete Harpella melusinae. Colonization levels of different species of trichomycetes were significantly correlated with one another in only 2 of 11 cases. Aside from the consistent attachment of the trichomycete Simuliomyces microsporus to the ichthyosporean Paramoebidium chattoni, we found little evidence that prevalence and colonization levels of trichomycetes were influenced by other symbiotes, including other trichomycetes. This trend might reflect the physical separation of trichomycetes, which are gut specific, from other symbiotes. In addition, any combinations of symbiotes that were wholly incompatible would not have been observed in our field-collected material.     

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.     

Attractive properties and physicochemical modifications of water following colonization by Aedes aegypti larvae (Diptera: Culicidae)

The authors investigated in this paper the attractiveness of a water colonized ('positive' breeding site) or not ('negative' breeding site) by larvae of Aedes aegypti on gravid females and then characterized their physicochemical properties to identify and quantify the principal components. The results showed that the water that sheltered several generations of larvae was more attractive to gravid females than water that had never been colonized. The water in a 'positive' breeding site contains 2.5 times more ammonium ions (NH(+)(4)), four times more nitrite ions (NO(-)(2)) and 20 times more nitrate ions (NO(-)(3)) than 'negative' breeding water. The authors suggest that in the 'positive' breeding sites, where the larvae absorb the organic matter, the turbidity of water decreases, which makes it possible for organic nitrogen to be converted into nitrites and nitrates via the nitrifying activity of the bacteria (NH(+)(4)-->NO(-)(2)-->NO(-)(3)). It is likely that chitin, while accumulating and decomposing at the bottom of the breeding site, may increase the percentage of organic carbon and ammonium ions, then reinforcing the nitrification process. Conversely, in the 'negative' breeding site where the conditions of evolution of the nitrifying bacteria are more restrictive, the nitrogen remains primarily in its organic form.     

Infection and pathogenicity of the mosquito densoviruses AeDNV, HeDNV, and APeDNV in Aedes aegypti mosquitoes (Diptera: Culicidae)

These studies compared three genetically distinct mosquito densoviruses Aedes aegypti (AeDNV), Hemagogus equinus (HeDNV), and Aedes Peruvian (APeDNV) densoviruses in a laboratory investigation to begin to evaluate their potential as mosquito control agents. A real-time polymerase chain reaction (PCR) assay for quantification of viral genomes and a standardized mosquito infection protocol were developed. Mortality associated with exposure to AeDNV increased in a dose-dependent manner, with the maximum mortality of 75.1% occurring in those organisms exposed to the highest dose of virus. The majority of death occurred as larvae. Similar results were observed with AeDNV produced from ground larvae and AeDNV produced from cell culture. Exposure of mosquitoes to HeDNV and APeDNV resulted in lower mortality, with values peaking at 33.5% for HeDNV and 27.8% for APeDNV. AeDNV-exposed larvae develop at a slower rate than nonexposed and HeDNV- and APeDNV-exposed larvae. Decreased virulence does not reflect a decrease in virus replication. PCR analysis of infectivity rates and titers in adults revealed reproduction of all three viruses, with an average viral titer of approximately 10 logs/mosquito after exposure to the highest dose of each virus. Accumulation of virus in the larval-rearing water was also observed with values approaching 10-11 logs/ml for each virus. These data indicate that there are dramatic differences in the pathogenicity among mosquito densoviruses.     

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

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

Pathogenicity of entomopathogenic fungus,Metarhizium anisopliae MET-GRA4 isolate on dengue vectors,Aedes albopictus and Aedes aegypti mosquito larvae (Diptera: Culicidae)

Considering the rapid transmission of the dengue virus, substantial efforts need to be conducted to ward-off the epidemics of dengue viruses. The control effort is depending on chemical insecticides and had aroused undesirable conflicts of insecticide resistance. Here, we study the entomopathogenic fungus, Metarhizium anisopliae as a promising new biological control agent for vector control. The pathogenicity effects of Metarhizium anisopliae against field and laboratory strains of Aedes albopictus and Aedes aegypti larvae were tested using the larvicidal bioassay technique. The results demonstrate that the treatments using M. anisopliae isolate MET-GRA4 were highly effective and able to kill 100% of both Ae. albopictus and Ae. aegypti mosquito larvae at a conidia concentration of 1 × 10?/ml within 7 days of the treatment period. The fungus displayed high larvicidal activity against laboratory and field strain of Ae. aegypti larvae with LC₅₀ values (9.6 × 10³/ml, 1.3 × 10³/ml) and LC₉₅ values (1.2 × 10?/ml, 5.5 × 10⁵/ml) respectively. For Ae. albopictus, LC₅₀ values for laboratory and field strains were (1.7 × 10⁴/ml, 2.7 × 10⁴/ml) and the LC₉₅ values were (2.1 × 10?/ml, 7.0 × 10⁵/ml) respectively. Interestingly, the susceptibility of field strain towards M. anisopliae was higher as compared to the laboratory strain Aedes larvae. In which, the causative agents of all the dead larvae were verified by the virulence of M. anisopliae and caused morphological deformities on larval body. The findings from this study identify this isolate could be an effective potential biocontrol agent for vector mosquitoes in Malaysia.