Size variation and reproductive success in the mosquito Aedes cantans

Abstract. A link between density-dependent larval competition and adult size of the mosquito Aedes cantans was demonstrated in northern England. Ponds containing high larval densities produced smaller larvae which, in turn, resulted in smaller adults at emergence.
In both 1989 and 1990, parous mosquitoes caught at human bait were larger than nulliparous ones, suggesting that larger mosquitoes are more successful at host location and egg-laying and also that they may be longer lived.
Larger mosquitoes produced larger egg clutches than smaller females: however, there was no difference in the size of eggs laid by large and small females.     

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.     

Sequential organization of grooming behaviors of the mosquito,Aedes triseriatus

Grooming behavior was studied in adult females of the mosquito Aedes triseriatus(Say). The grooming repertoire consisted of 12 different behaviors (accounting for bilateral symmetry) organized into five sequences. The proboscis and antennae were scraped by the forelegs, whereas the dorsal and ventral surfaces of the wings, and the forelegs, midlegs, hindlegs, and tip of the abdomen were scraped by the hindlegs. Each sequence ended with hindleg groomng. Tibial grooming combs were found on the ventral apices of the fore- and hind-tibiae but not on the mid-tibiae. A multidimensional scaling procedure grouped the grooming behaviors in two ways: (1) by the relative position of the groomed structure along the anteroposterior axis of the insect's body, and (2) by whether the groomed structure has a locomotory or sensory function. This suggests that mosquitoes groom both to clear sensilla of obstructive matter and to clean and smooth scales on legs and wings, possibly to decrease drag during flight.     

Natural infection of the tree hole breeding mosquito Aedes triseriatus (Diptera: Culicidae) with the fungus Funicularius triseriatus

In the course of a study of the population of Aedes triseriatus, a female adults was found to be infected with a fungus. The fungus, Funicularius triseriatus, is described as a new genus and species of the class Hyphomycetes (form-class Deuteromycetes). F. triseriatus is characterized by thallic arthrospores produced on conidiophores arising from hyphae which are funiculose in habit. This is the first report of the natural occurrence of a fungus on A. triseriatus adults.     

Effect of leaf litter and density on fitness and population performance of the hole mosquito Aedes triseriatus

Abstract.

• 1 We tested the effects of food, in the form of leaf litter, and density on population growth and fitness correlates of the tree hole mosquito Aedes triseriatus. Our field experiment used a 2 x 2 randomized block design, with three holes as blocks. In cages within three holes, we manipulated densities of Aedes triseriatus larvae, and presence of leaf litter. Our laboratory experiment used a 2 x 3 factorial design in which we also manipulated densities of larvae and availability of leaf litter within similar cages (inside, outside, or absent). For both experiments we determined effects on survival, days to and mass at eclosion of both sexes, and a composite index of population performance λ′, that estimates finite rate of increase.
• 2 In the field experiment, we found significant and large effects of leaf litter and of density on the performance of A. triseriatus. The effect of density did not depend on leaf litter availability. We documented significant variation among tree holes for most correlates of fitness. Often, the effects of treatments varied significantly depending on the tree hole tested.
• 3 In the laboratory experiment we found significant effects of density and litter, and that direct browsing on leaf litter is necessary for the effect of litter on performance of A. triseriatus. In the laboratory, the negative effect of increased density was dependent on leaf litter availability.
• 4 Our results demonstrate that leaf litter has important effects on population performance of tree hole mosquitoes in natural tree holes. Direct browsing on the surface of leaf litter is the most likely mechanism by which litter enhances population growth. The impact of litter on populations varies among tree holes.

     

Insights to host discrimination and host acceptance behaviour in a parasitoid (Diptera: Asilidae): Implications for fitness

The robber fly Mallophora ruficauda is one of the principal pests of apiculture in the Pampas region of Argentina. As adults they prey on honey bees and other insects, while as larvae they are solitary ectoparasitoids of third instar scarab beetle larvae. Females of M. ruficauda lay eggs away from the host in tall grasses. After being dispersed by the wind, larvae drop to the ground, where they dig in search of their hosts. It is known that second instar larvae of M. ruficauda exhibit active host searching behaviour towards its preferred host, third instar larva of Cyclocephala signaticollis. Although the means by which host location occurs has been studied and since superparasitism is a frequent scenario in the field, no information about host discrimination and host acceptance is available. We carried out studies in the field and behavioural experiments in the laboratory to determine if M. ruficauda is capable of quality host discrimination. We also studied if this parasitoid is capable of conspecific detection in order to avoid superparasitism. Finally, we analyzed the conditions under which superparasitism occurs in the field. We report here that the second instar larva of M. ruficauda is able to discriminate the parasitism status of the host by means of chemical cues, but is not capable of detecting conspecifics prior to attacking a host. We also found that the host cannot detect the presence of the parasitoid by means of chemical cues, so that no counter-defense against parasitism occurs. Furthermore, we determined that superparasitism occurs on the heavier hosts, i.e. those with more abundant resources which could harbor several parasitoid individuals. Finally, we discuss the possible implications of larval host location and host discrimination decisions on the fitness of this parasitoid.     

Effects of habitat drying on size at and time to metamorphosis in the tree hole mosquito Aedes triseriatus

Models of complex life cycles predict that greater mortality of immature stages should induce earlier metamorphosis at smaller sizes. We tested for effects of one source of mortality, habitat drying, on size at and time to metamorphosis of the tree hole mosquito Aedes triseriatus. In a laboratory experiment, we manipulated two variables associated with drying, volume of water and solute concentration, and recorded mean mass at and median days to eclosion for males and females in replicate cohorts. We also tested for treatment effects on the correlation of mass at and time to eclosion. For females, decreasing volume consistently induced metamorphosis at smaller sizes than did constant volume. Decreasing volume also led to earlier metamorphosis of females than did constant volume, but only in one of two experimental runs. For both sexes, increasing concentration led to greater size at metamorphosis and, for males, earlier metamorphosis than did constant volume, but again only in one of two experimental runs. Correlations of size at and time to metamorphosis tended to be positive for females and negative for males, and this difference was significant. For both sexes, decreasing volume led to larger (more positive) correlations than did constant volume, but only in one of two experimental runs. The effects of decreasing volume on females are consistent with the predictions of models of complex life cycles, and suggest that A. triseriatus can perceive volume changes and modify metamorphosis to escape a deteriorating habitat. The effects of increasing concentration are opposite to those predicted, and are consistent with enhanced growth rates, possibly due to enhanced microbial growth, as solutes become concentrated due to drying. The responses of these mosquitoes to habitat drying are complex, and we suggest that habitat drying increases both mortality and growth rates, yielding no simple predictions of how habitat drying will affect these mosquitoes in natural tree holes.     

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.     

Body size variation of the floodwater mosquito Aedes albifasciatus in Central Argentina

An inverse relationship between larval density and adult body size has been reported for several mosquito species, affecting their survival and vector competence, response to repellents and other factors. Larvae of the floodwater mosquito Aedes (Ochlerotatus) albifasciatus (Macquart) (Diptera: Culicidae) develop quickly in temporary pools, so intraspecific competition (for food or space) might regulate population abundance and affect the size of adult mosquitoes. We investigated the temporal variation of adult female wing-length (an index of body-size) in natural populations of Ae. albifasciatus, using adults collected during each phase of the rainy season. The relationships between adult mosquito abundance, female wing-length, rainfall and temperature were analysed through simple regressions. Skewness of the frequency distribution of wing-lengths showed a strong negative relationship with mean wing-length. The distribution of wing-lengths varied seasonally and was correlated with rainfall 7-15 days previously as the major consequence of breeding site volume. Thus temporal variation of body size in natural populations of Ae. albifasciatus reflected density-dependent changes in the aquatic habitat where immature stages develop, influenced more by rainfall than by temperature or other environmental variables.     

Fine structure and development of Pilosporella chapmani (Microspora: Thelohaniidae) in the mosquito, Aedes triseriatus (Say)

New information on the life cycle and fine structure of Pilosporella chapmani, a microsporidium of the mosquito Aedes triseriatus, is presented. Pilosporella chapmani is shown to have two sporulation sequences, one of them being involved in transovarial transmission. One sequence, involving meiosis and production of a moniliform sporogonial plasmodium, occurs in the larval fat body, resulting in eight uninucleate, spherical, and fully developed spores. The other occurs in oenocytes of adult mosquitoes and results in isolated, binucleate, elongate, and thin-walled spores. Also, for the first time, metabolic products are shown to be expelled into the surrounding host tissues through the wall of the sporocyst.     

The required day number and timely induction of diapause in geographic strains of the mosquito Aedes atropalpus

Diapause is triggered in A. atropalpus from 14°N lat when 9 or more short-day photoperiod cycles occur during the photosensitive 4th instar and pupa. Temperatures of 22°C and higher cause part or all of this population to complete the photosensitive period in less than 9 days thereby avoiding diapause. In contrast, 4 short-day cycles during the photosensitive period are sufficient to trigger diapause in A. atropalpus from 45°N lat. This population experiences the 4 required days at temperatures as high as 28°C. A strain from an intermediate location of 34°N lat must “see” 7 short days during the photosensitive stages for induction of diapause. This requirement is met at temperatures of 24°C and less. These results suggest that southern strains of A. atropalpus have evolved a means of avoiding short-day induced diapause if temperatures are high and other requirements for growth are available.A correlation between critical daylength for induction of diapause and geographic origin has been shown for several insect species. The evolution of a requirement for different numbers of diel cycles during the photosensitive period is another means of modifying the diapause response in populations from different latitudes and geographic areas.     

Mitochondrial DNA variation within and among populations of the mosquito Aedes albopictus.

A survey of restriction fragment polymorphism in mitochondrial DNA (mtDNA) of 17 populations of the mosquito Aedes albopictus was undertaken. The mtDNA size was estimated to be about 17.5 kbp. The level of polymorphism was low, with over 99% of the fragments being shared in common among the 17 populations. Three populations, Mauritius, Singapore, and Hong Kong, contained individuals with both the ancestral and novel mtDNA haplotypes. We conclude that the low level of mtDNA polymorphism in A. albopictus is a result of recent range expansion and that the mixture of haplotypes is a likely result of human-aided gene flow among populations.     

Interactions with native mosquito larvae regulate the production of Aedes albopictus from bromeliads in Florida

Abstract. 1. Immatures of the invasive container mosquito Aedes albopictus occur in water-holding tanks and axils of ornamental bromeliads in Florida, where this species established and became abundant in the late 1980s and early 1990s.
2. The numbers of A. albopictus in bromeliads in gardens vary geographically in Florida, being significantly higher in northern sites where two species of Wyeomyia mosquitoes, native specialists of this microhabitat, are absent.
3. Causes of the negative relationship between abundances of A. albopictus and Wyeomyia spp. were investigated experimentally using Billbergia pyramidalis , an introduced bromeliad popular in Florida gardens.
4. Egg laying by A. albopictus in B. pyramidalis confined in a large outdoor cage was unaffected by the presence or absence of Wyeomyia spp. fourth instars, indicating that immatures of the native bromeliad mosquito species do not deter oviposition by the invasive species in this microhabitat.
5. Growth and survivorship of A. albopictus first instars in tanks of B. pyramidalis were negatively affected by the presence of fourth instar, but not first instar, Wyeomyia spp. larvae at natural densities. Stimulative effects of leaf litter on A. albopictus growth and survivorship in this microhabitat were independent of the presence or absence of Wyeomyia spp. immatures.
6. These results demonstrate that interspecific larval interactions regulate the production and explain the geographic patterns in abundance of A. albopictus from ornamental bromeliads in Florida.     

Identity and transfer of male reproductive gland proteins of the dengue vector mosquito, Aedes aegypti: potential tools for control of female feeding and reproduction

Male reproductive gland proteins (mRGPs) impact the physiology and/or behavior of mated females in a broad range of organisms. We sought to identify mRGPs of the yellow fever mosquito, Aedes aegypti, the primary vector of dengue and yellow fever viruses. Earlier studies with Ae. aegypti demonstrated that "matrone" (a partially purified male reproductive accessory gland substance) or male accessory gland fluid injected into virgin female Ae. aegypti affect female sexual refractoriness, blood feeding and digestion, flight, ovarian development, and oviposition. Using bioinformatic comparisons to Drosophila melanogaster accessory gland proteins and mass spectrometry of proteins from Ae. aegypti male accessory glands and ejaculatory ducts (AG/ED) and female reproductive tracts, we identified 63 new putative Ae. aegypti mRGPs. Twenty-one of these proteins were found in the reproductive tract of mated females but not of virgin females suggesting that they are transferred from males to females during mating. Most of the putative mRGPs fall into the same protein classes as mRGPs in other organisms, although some appear to be evolving rapidly and lack identifiable homologs in Culex pipiens, Anopheles gambiae, and D. melanogaster. Our results identify candidate male-derived molecules that may have an important influence on behavior, survival, and reproduction of female mosquitoes.     

Seasonal emergence, host-seeking activity, age composition and reproductive biology of the mosquito Aedes punctor

Abstract. 1. Seasonal patterns of adult emergence, host-seeking and reproductive ageing in a population of the woodland mosquito Aedes punctor (Kirby) are described from field studies in northern Britain in 1984 and 1985.
2. The pattern of emergence showed little variation between years but duration did vary. Males started emerging before females. Sex ratios at emergence showed a marked female bias. A late, second period of emergence apparently represented a second generation in this normally univoltine species.
3. Host-seeking females were first caught at human bait within a few days of the start of emergence and peak numbers at bait occurred 3–4 weeks later. High numbers lasted for c. 2 weeks. The duration of host-seeking activity was related to weather. The second emergence in 1985 resulted in a second peak of activity in early September and probably extended the flight season.
4. As the season progressed the proportion of females that were parous increased. However, this proportion never reached 1.0, indicating variation in individual reproductive success, which differed between years.
5. The reproductive biology of individuals within the population was investigated. No autogeny was detected, suggesting that the population was entirely anautogenous.     

Geographic variation and the evolution of reproductive allocation in the pitcher-plant mosquito, Wyeomyia smithii

We measured the egg size of six geographic populations of the pitcher-plant mosquito, Wyeomyia smithii, from Florida (30 degrees N) to Ontario (49 degrees N). Populations from northern latitudes produced larger eggs than populations from southern latitudes. Egg size increased with increasing latitude more rapidly when larvae were reared under low rather than high density. One southern (30 degrees N) and one northern (49 degrees N) population of W. smithii that persisted through 10 generations of selection for increased persistence under conditions of chronic thermal- and nutrient-limiting stress (conditions similar to southern rather than northern habitats) produced smaller eggs more rapidly than unselected control lines. However, there were no differences in lifetime fecundity or fertility between control and selected lines. Thus, laboratory evolution in an environment representative of extreme southern latitudes caused evolutionary changes consistent with geographic patterns of egg size. These results implicate temperature as a selective factor influencing the geographic variation of egg size in W. smithii, and demonstrate a novel trade-off in reproductive allocation between egg size and egg maturation time.     

Costs of reproduction in Nyssa sylvatica: sexual dimorphism in reproductive frequency and nutrient flux

Summary We examined the influence of differential reproductive frequency between the sexes on tertiary (phenotypic) sex ratios in the the dioecious tree Nyssa sylvatica (Nyssaceae). Reproduction was evaluated in relation to sex, size and canopy exposure using flowering data collected from 1229 marked trees over a four year period. For subsets of each population we used data on flower number, fruit crop size, fruit/flower ratios, and individual flower and fruit mass to compare biomass invested in reproductive structures of males and females. We also examined seasonal changes in stem nitrogen and soluble carbohydrate content in relation to flower and fruit production for trees of each sex. Our results indicate that: 1) Male-biased tertiary sex ratios could be explained by more frequent reproduction by male trees; 2) Estimated secondary sex ratios based on sums of all known males and females were not significantly different from 1:1; 3) Flowering frequency of males and females was significantly related to plant size (DBH) and exposure of the canopy to light; 4) Estimtes of reproductive biomass allocation ranged from 1.36 to 10.8 times greater for females relative to males; 5) Flower production was related to stem nutrient status for both sexes, but nutrient depletion and its effect on subsequent flowering was much more pronounced for female trees. We conclude that less frequent flowering by female trees may result from depletion of stored reserves, and that differential flowering frequency in N. sylvatica may ultimately reduce apparent sexual differences in the costs of reproduction.