Environmental effects on bet hedging in <Emphasis Type="Italic">Aedes</Emphasis> mosquito egg hatch

We used a path analysis procedure to examine the influence of environmental effects on the egg hatching response of the container breeding mosquito, Aedes triseriatus. A. triseriatus eggs were collected from 22 different sites across the eastern US, and exposed to repeated hatch stimuli in the laboratory. The resulting data were used to construct hatch indexes for each site. Structural equation modeling was used to discriminate among hypotheses relating to the functional relationships between population hatch trait and local climatic conditions. The results suggest that the delayed hatch pattern is an adaptive bet-hedging strategy that allows the species to manage desiccation risks. The selected model indicates that environmental variables differentially affect the immediate and the delayed, long term hatch patterns. High temperatures directly reduce the number of egg hatching on the first stimulus, but only indirectly affect the delayed hatch pattern. Low precipitation and high variability in precipitation directly increase the delaying pattern. The hatch trait appears to be a critical adaptation that allows the species to occupy a broad range in east North America.     

Successional mosquito dynamics in surrogate treehole and ground‐container habitats in the northeastern United States: Where does Aedes albopictus fit in?

This study assessed the risk of larval displacement of the eastern treehole mosquito, Aedes triseriatus, and the northern house mosquito, Culex pipiens, by Aedes albopictus, the Asian tiger mosquito, during the establishment and successional stages of novel larval mosquito treehole and ground‐container habitats in the state of New Jersey, U.S.A. Culex pipiens and Culex restuans were the first mosquito species to colonize ground‐container habitats and were the dominant larval species throughout the study period, whereas Ae. albopictus was late to colonize ground habitats and accounted for less than 15% of weekly larval collections once established. Ae. albopictus had a much stronger community presence within treehole ovitraps; however, Ae. albopictus never reached the average larval densities of the expected primary colonizer, Ae. triseriatus. Throughout the study period, the weekly abundances of Ae. triseriatus and Ae. albopictus were positively correlated and there were no significant differences between the abundances of each species. The larval dominance of Ae. triseriatus appears to be enhanced by the presence of Toxorhynchites rutilus septentrionalis, a large predatory mosquito species. When Tx. rut. septentrionalis was present, mature larvae (3^rd–4^th instar) of Ae. albopictus were also present in only 16.7% of collections, whereas mature larvae of Ae. triseriatus were collected concurrently with Tx. rut. septentrionalis in 53.8% of collections. These data suggest that Ae. triseriatus is at a greater risk of displacement by Ae. albopictus than are Cx. pipiens and Cx. restuans.     

To hatch or not to hatch? Egg hatch response to larval density and to larval contact in a treehole mosquito

Abstract.

• 1 We investigated the effect on egg hatch of exposure to: (1) varying larval density, and (2) larval contact in Aedes triseriatus Say (Diptera: Culicidae). For 2 days in the laboratory we submerged eggs into a treehole water medium containing 0 (control), 4, 12 or 24 larvae that could either contact the eggs directly or were separated from them by a screen. Following treatment, abundance of microorganisms on the egg surfaces, a food source for newly hatched larvae and a proposed hatching stimulus, was assessed by counts made from serial dilutions of samples.
• 2 We discovered a complex hatching response to larval contact and to larval density, and an interaction between these two factors in their effect on microbial growth. Hatching was inhibited in the 0-larva control, even though microorganisms grew abundantly on the eggs. Hatch rate, as well as microbial counts, were high for eggs in direct contact with 4 larvae. As density increased in the larval contact treatment, microorganisms disappeared from the egg surfaces and hatch rate declined.
• 3 When protected from larval grazing, eggs supported numerous microbial colonies irrespective of larval density. In contrast to the contact treatment group, egg hatch increased with increasing larval density. These observations suggest that the combination of microbial growth and a larval factor stimulates hatch. This hatching response may have evolved because both abundant microorganisms and numerous larvae reflect a habitat of good quality.

     

The mosquitoes of eastern Tennessee: studies on abundance,habitat preferences,and host‐seeking behaviors

A significant increase in the number of reported cases of La Crosse virus (LACV) infections in eastern Tennessee has occurred in the last ten years. The objective of this study was to determine the abundance and habitat preferences of the potential vectors of LACV in this region. Adult host‐seeking mosquitoes were collected using CO₂‐baited CDC light traps and a series of human‐landing catches in eastern Tennessee from 2004 to 2006. A total of 4,200 female mosquitoes of 23 species was collected by CO₂‐baited CDC trapping at ten sites during the study period. Aedes albopictus (Skuse) was the most abundant mosquito collected at all sites and vegetation types, with the ratios of total Ae. albopictus to Ae. triseriatus (Say) females collected being 2.1:1 in 2004, 3.8:1 in 2005, and 4.9:1 in 2006. Ten species were collected during a series of human‐landing catches made at four different sites; one probable and three confirmed case sites of LACV infections, totaling 528 female mosquitoes. Aedes albopictus was the most abundant species collected, with a 4:1 ratio of Ae. albopictus to Ae. triseriatus females. Aedes albopictus exhibited two clear peaks of “landing” activity, one in the early morning and one in the late afternoon or early evening. Simple and multiple regression analyses of the predictors of the number of mosquitoes collected showed that populations of Ae. albopictus were three times more likely to be collected overall than Ae. triseriatus. Species (Ae. albopictus), vegetation (residential), and the previous cumulative precipitation for the four weeks prior to collection were significantly (P < 0.05) associated with the number of mosquitoes collected by CO₂‐baited CDC trapping. Aedes albopictus was also more likely to be collected than Ae. triseriatus at confirmed cases of LACV infections.     

Temperature‐mediated survival,development and hatching variation of Pacific cod Gadus macrocephalus eggs

Laboratory‐validated data on the survival, development and hatching responses of fertilized Pacific cod Gadus macrocephalus eggs from the northern Japan stock were determined through an incubation experiment. The optimum temperature for survival until hatching ranged from 4 to 8° C. No significant difference in development rates was found between the populations from Mutsu Bay, Japan, and western Canadian coastal waters even though the samples may belong to different G. macrocephalus stocks. Gadus macrocephalus larvae hatched asynchronously from egg batches despite incubation under the same environment during their development. Both incubation temperature and temperature‐mediated hatch rank affect size and yolk reserve. These data suggest that variations in water temperatures within an ecological range markedly influence the development rates, survival and hatching of the eggs, as well as the stage at hatch larvae of G. macrocephalus. Asynchronous hatching and the production of offspring with variable sizes and yolk reserves are considered evolutionary bet‐hedging strategies that enable the species to maximize their likelihood of survival in an environment with variable temperatures.     

Egg hatching of mosquitoes Aedes caspius and Ae. vittatus stimulated by water vibrations

Simple laboratory experiments with eggs of wild-caught Aedes caspius (Pallas) and Aedes vittatus (Bigot) mosquitoes (Diptera: Culicidae) demonstrated that egg hatch rates (i.e. percentage hatching daily) increased significantly in response to water vibrations caused by finger-tip drumming on containers of submerged eggs. When disturbed by daily vibrations for only 30 s, eggs hatched sooner than when they remained flooded or were periodically drained and reflooded without agitation. For autogenous Ae. caspius from saltmarsh, egg hatch rates were three- to four-fold greater for groups of 50 eggs than for solitary eggs, possibly due to chemicals (functional kairomones) emanating from other eggs or larvae. For anautogenous Ae. vittatus from freshwater rock-pools, tapping the container daily (for 30 s each 2 h in 8 h) hatched 93% of eggs, compared with only 42% hatch of eggs agitated only when first flooded or 55% hatch of eggs dried and reflooded three times during 17 days without other disturbance. It is concluded that, apart from flooding and other factors, vibrations - simulating the patter of rainfall provide a very significant hatching stimulus for Aedes eggs.     

Early and late developmental arrest as complementary embryonic bet‐hedging strategies in African killifish

The production of dormant eggs is a crucial adaptation for African killifish of the genus Nothobranchius to survive in temporary waters. These habitats are often characterized by unpredictable variation in the suitability of growing seasons as a result of variable lengths of inundations and temporary colonization by piscivorous fish. Incomplete hatching could enable killifish to buffer against reproductive failure during unsuitable inundations. Although this phenomenon has been tentatively linked to variation in dormancy states, it has never been investigated under controlled conditions and its viability as a bet hedging strategy to distribute offspring over several inundations remains unclear. In the present study, we used common garden experiments to assess the contribution of environmental modulation and bet hedging to delayed hatching in Nothobranchius killifish by testing the feasibility of arrested development in the presence and absence of environmental cues. Overall, the results confirmed that the presence of cues signalling a threat (predator kairomones) inhibited hatching. However, delayed development also occurred independent of cues and was regulated at two stages. Developmental arrest in energy‐efficient dormancy stages could present a means for long‐term bet hedging over years, whereas arrest in the energy‐consuming final stage may serve a similar purpose over shorter time scales. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 114 , 941–948.     

Evidence of vertical transmission of West Nile virus in field‐collected mosquitoes

Male and nulliparous female mosquitoes were surveyed for evidence of vertical WNV infection in East Baton Rouge Parish, Louisiana. Adult male mosquitoes collected by trapping and aspiration, and adult male and nulliparous female mosquitoes reared from field‐collected larvae were tested. Adult male Culex spp., female Aedes albopictus (Skuse), and female Culex quinquifasciatus Say mosquitoes that were collected as larvae were test‐positive for WNV RNA. Infectious WNV was detected using virus isolation in field‐collected male Aedes triseriatus Say and Culex salinarius Coquillett; these data represent the first field evidence of vertical transmission of WNV in Ae. triseriatus and Cx. salinarius.     

The role of priority effects in limiting the success of the invasive tiger mosquito,Aedes albopictus

Priority effects (PE), wherein species colonizing a habitat early have a negative impact on later colonizers, can have profound and legacy effects on community organization. In temperate zones, larval mosquito habitats are emptied each year in the winter and recolonized in the spring. There are phenological differences among common species but the role of PE in these communities is largely unexplored. Aedes albopictus, the invasive tiger mosquito, is considered a superior competitor to resident species during the larval phase when conditions are initiated with same-staged heterospecific larvae. However in nature, Ae. albopictus hatches, and resumes activity, later in the spring than other species, suggesting it encounters larger later developed individuals, and denser populations, of species such as Aedes triseriatus. Additionally, despite their competitive inferiority, these species often coexist with Ae. albopictus in larval habitats, with Ae. albopictus often occurring at relatively low abundances in sylvan habitats. Using lab and near field experiments, we tested the hypothesis that PE with early hatching species reduces survivorship and population growth for the invasive Ae. albopictus. When Ae. albopictus larvae encountered larger, later developed heterospecific larvae at greater densities, under controlled lab conditions and in artificial and natural mesocosms, they experienced significant reductions in survival and estimated finite rate of population increase. Additionally, we found that intraguild predation of Ae. triseriatus on Ae. albopictus may be an important mechanism through which PE works. We conclude that PE is a potential mechanism for coexistence between invasive and resident mosquitoes and should be further explored.

     

Identification of super-infected Aedes triseriatus mosquitoes collected as eggs from the field and partial characterization of the infecting La Crosse viruses

Background

La Crosse virus (LACV) is a pathogenic arbovirus that is transovarially transmitted by Aedes triseriatus mosquitoes and overwinters in diapausing eggs. However, previous models predicted transovarial transmission (TOT) to be insufficient to maintain LACV in nature.

Results

To investigate this issue, we reared mosquitoes from field-collected eggs and assayed adults individually for LACV antigen, viral RNA by RT-PCR, and infectious virus. The mosquitoes had three distinct infection phenotypes: 1) super infected (SI+) mosquitoes contained infectious virus, large accumulations of viral antigen and RNA and comprised 17 of 17,825 (0.09%) of assayed mosquitoes, 2) infected mosquitoes (I+) contained no detectable infectious virus, lesser amounts of viral antigen and RNA, and comprised 3.7% of mosquitoes, and 3) non-infected mosquitoes (I-) contained no detectable viral antigen, RNA, or infectious virus and comprised 96.21% of mosquitoes. SI+ mosquitoes were recovered in consecutive years at one field site, suggesting that lineages of TOT stably-infected and geographically isolated Ae. triseriatus exist in nature. Analyses of LACV genomes showed that SI+ isolates are not monophyletic nor phylogenetically distinct and that synonymous substitution rates exceed replacement rates in all genes and isolates. Analysis of singleton versus shared mutations (Fu and Li's F*) revealed that the SI+ LACV M segment, with a large and significant excess of intermediate-frequency alleles, evolves through disruptive selection that maintains SI+ alleles at higher frequencies than the average mutation rate. A QTN in the LACV NSm gene was detected in SI+ mosquitoes, but not in I+ mosquitoes. Four amino acid changes were detected in the LACV NSm gene from SI+ but not I+ mosquitoes from one site, and may condition vector super infection. In contrast to NSm, the NSs sequences of LACV from SI+ and I+ mosquitoes were identical.

Conclusions

SI+ mosquitoes may represent stabilized infections of Ae. triseriatus mosquitoes, which could maintain LACV in nature. A gene-for-gene interaction involving the viral NSm gene and a vector innate immune response gene may condition stabilized infection.     

Maternal source of variability in the embryo development of an annual killifish

Organisms inhabiting unpredictable environments often evolve diversified reproductive bet‐hedging strategies, expressed as production of multiple offspring phenotypes, thereby avoiding complete reproductive failure. To cope with unpredictable rainfall, African annual killifish from temporary savannah pools lay drought‐resistant eggs that vary widely in the duration of embryo development. We examined the sources of variability in the duration of individual embryo development, egg production and fertilization rate in Nothobranchius furzeri. Using a quantitative genetics approach (North Carolina type II design), we found support for maternal effects rather than polyandrous mating as the primary source of the variability in the duration of embryo development. The number of previously laid eggs appeared to serve as an internal physiological cue initiating a shift from rapid‐to‐slow embryo developmental mode. In annual killifish, extensive phenotypic variability in progeny traits is adaptive, as the conditions experienced by parents have limited relevance to the offspring generation. In contrast to genetic control, with high phenotypic expression and heritability, maternal control of traits under natural selection prevents standing genetic diversity from potentially detrimental effects of selection in fluctuating environments.     

Cyst bank life-history model for a fairy shrimp from ephemeral ponds

1. Ephemeral wetland habitats provide a useful model system for studying how life‐history patterns enable populations to persist despite high environmental variation. One important life‐history trait of both plants and crustaceans in such habitats involves hatching/germination of only some of the eggs/seeds at any time. This bet‐hedging leads to the development of a bank composed of dormant propagules of many ages. 2. The San Diego fairy shrimp, Branchinecta sandiegonensis (Crustacea: Anostraca), a dominant faunal element of ephemeral ponds in San Diego, California, is a suitable organism for studying the consequences of highly fluctuating environmental conditions. As a result of large‐scale habitat loss, the species is also endangered, and this motivated our specific study towards understanding the hatching dynamics of its egg bank for planning conservation efforts such as pool restoration and re‐creation. 3. We formulated a matrix population model using egg age within the bank to study the relationship between adult survival and reproduction, and survival in and hatching from the egg bank. As vital rates for fairy shrimp are only poorly known, we generated 48 matrices with parameters encompassing ranges of likely values for the vital rates of B. sandiegonensis. We calculated population growth rates and eigenvalue elasticities both for a static model and a model with periodic reproductive failure. 4. The model shows that in good filling events, population growth rate is very high and the egg bank is increased dramatically. While population growth rate is insensitive to long‐term survival in the egg bank in our static deterministic model, it becomes sensitive to survival in the egg bank when a regime of periodically failed reproductive events is imposed. 5. Under favourable conditions, it was best for shrimp to hatch from eggs as soon as possible. However, under a regime where failed reproductive events were common, it was best to hatch after several pool fillings. Because conditions change from favourable to unfavourable unpredictably, variation in age within the egg bank appears to be critical for the persistence of the population. This attribute needs to be carefully considered when restoring or creating new pools for conservation purposes.     

Drought and the organization of tree-hole mosquito communities

Summary In southeastern North America (North Florida, USA), the duration, frequency, and timing of drought differentially affect the survivorship of pre-adult tree-hole mosquitoes. Drought affects survivorship both by the direct action of dehydration on developing larvae and pupae and by the indirect modulation of predation. The drought-susceptible species, Toxorhynchites rutilus, Orthopodomyia signifera, and Anopheles barberi co-occur in more permanent holes that are larger, with larger, more vertical openings, lower down in larger trees, and contain darker water with higher conductivity, pH, and tannin-lignin content than the holes occupied by Aedes triseriatus that has drought-resistant eggs and rapid larval development. Ovipositing mosquitoes cue on physical and chemical attributes of tree holes independently of host tree species. These same attributes differ among drought-prone and drought-resistant holes but mosquitoes track these attributes more faithfully than the attributes predict tree-hole stability.     

Reproductive interference between Aedes albopictus and Aedes flavopictus at a place of their origin

Aedes (Stegomyia) albopictus and Aedes (Stegomyia) flavopictus are related species that have overlapping distributions from southern to central Japan. To understand how they interact, we studied reproductive interference between them, particularly focusing on the body size difference between the mating pair. Here, we examined the effects of conspecific, heterospecific and double mating (i.e. heterospecific mating followed by conspecific mating) on copulation duration, egg production and hatchability of eggs using mosquitoes that varied in body size. Females mated only with heterospecific males produced few viable eggs, indicating that post‐mating isolation is almost complete. When mated with heterospecific males before conspecific mating, the production of viable eggs was lower than when mated only with conspecific males, revealing the occurrence of reproductive interference. The degree of reproductive interference was larger in Ae. flavopictus than in Ae. albopictus when the female size was small but did not differ between them when the female size was large. Aedes albopictus females appear to be able to distinguish Ae. flavopictus males from conspecific males and larger females are more successful in the rejection of heterospecific males. On the other hand, Ae. flavopictus were not able to discriminate between conspecific and heterospecific males.     

The response of eggs of the white potato cyst nematode Globodera pallida to diffusate from potato and mustard roots

Brief exposure of eggs of Globodera pallida to potato root diffusate not only initiated hatching but also caused the majority of unhatched juveniles to respond more rapidly to subsequent treatment with diffusate. Eggs previously exposed to diffusate had a peak hatch after 1 or 2 days compared with 4 days for untreated eggs. Mustard root diffusate prevented hatch, and further stimulation with potato root diffusate was necessary to re-initiate it. Eggs previously treated with potato root diffusate for 24 h were much more sensitive to drought than untreated eggs. These results are discussed in relation to the theory that potato root diffusate alters the permeability of the eggshell as an initial step in the hatching process.     

Habitat segregation among African treehole mosquitoes

Abstract. 1. The division of macrohabitats, seasons and microhabitats among mosquitoes which inhabit water-filled treeholes was investigated in a wet/dry seasonal environment near the coast of Kenya. Twenty-two species from four genera (Aedes fourteen, Culex three, Eretmapodites three, Toxorhynchites two) were identified from treeholes and bamboo traps. These included five which were more abundant in other microhabitats: leaf axils, fruit husks, or abandoned snail shells. 2. Trapping in different vegetation zones or heights in forests revealed that all seventeen species examined showed opposition preferences for one or more macrohabitats. Segregation of related species by macrohabitat was particularly apparent among members of the subgenus Stegomyia of Aedes which accounted for 57% of individuals identified from one locality. 3. Seasonal overlap was low between Aedes, which tracked rainfall patterns, and Culex and Eretmapodites. Aedes bromeliae differed from other Aedes found in treeholes in that maximal larval numbers did not follow heavy rainfall. 4. Negative associations between Aedes fulgens and two congeners which co-occur in the same forest and season were attributed to preferences for tree-holes of different sizes. Positive associations between Aedes aegypti and several other Aedes were reduced when root buttresses, a subclass of larger treeholes containing more species, were removed. 5. Separation of species by growth stages may be accomplished by two schemes of egg hatching which desynchronize larval cohorts in the same tree-hole: the early pattern of Stegomyia whereby 50–95% of all eggs hatch at first immersion, and the delayed pattern of other subgenera (Diceromyia, Finlaya, Pseudarmigeres) in which less than one-third of all eggs hatch at first immersion. 6. The number of wet treeholes changed seasonally. During one year five or six periods occurred when 50% or more of the censused holes contained sufficient water for larval survival. The number of individuals and species of mosquitoes in treeholes were positively correlated with treehole volume. In a comparison of treeholes and their mosquito fauna from two forests, treehole volume and mean number of species per hole differed significantly, but mosquito density did not. 7. Ecotonal trapping sites of mixed grasses, trees, and scrub yielded the highest mean numbers of species per sample, and traps placed outside a village, the lowest. Specific dominance was significantly less in the ecotone than elsewhere. The predatory larvae of Toxorhynchites brevipalpis in ecotonal and forested traps were not correlated with species richness.     

Better eggs today than psocids tomorrow: Aedes aegypti (Diptera: Culicidae) eggs exposed to Liposcelis sp. (Psocodea: Liposcelididae) has reduced hatching rates

For different research purposes, there is a need to mass rear mosquitoes, such as Aedes aegypti, under laboratory conditions. The rearing process begins with egg production followed by egg storage in a dry environment, inside containers. Stored eggs are susceptible to environmental threats when storage conditions are suboptimal. Some terrestrial insects can invade this environment and attack stored eggs. In this brief report, we assessed whether Ae. aegypti eggs exposed to Liposcelis sp. individuals had reduced hatching and immature development rates. We exposed 100 eggs in different treatment conditions (fixed in porous paper and loosed) to 30 Liposcelis sp. individuals for ten days and then we induced hatching. We observed a hatching rate of 99% reduced for those eggs adhered to porous paper and loosed eggs showed a hatching rate of 45% decreased for those eggs exposed to Liposcelis sp. The remaining larvae took longer to develop into pupae as well, showing a four-day delay on average to the final metamorphosis of the aquatic stage. These results reinforce the need to frequently monitor egg storage conditions to maintain laboratory colonies stable and free from pests that can interfere with mosquito life-history traits.     

STRUCTURE AND DEVELOPMENT OF EMBRYOS OF EPHESTIA CAUTELLA (WALKER) DURING ANOXIA AND PHOSPHINE TREATMENT

Exposure of Ephestia cautella eggs to anoxic conditions retards the development of the embryo, causing a marked delay in hatching. Phosphine treatment results in little or no hatch delay in surviving embryos and produces characteristic ultrastructural lesions, mainly manifested as changes in mitochondrial organization.     

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.