Plagiorchis noblei (Plagiorchiidae) in Aedes aegypti: parasite acquisition and host mortality in trickle infections

The prevalence and intensity of experimental infections of Aedes aegypti with the digenean Plagiorchis noblei increased significantly with the level of trickle exposure to cercariae. Daily exposure to doses of 16 cercariae/day yielded a mean infection intensity of 13.0 metacercariae; doses of 1 cercaria/day resulted in only 2.4 metacercariae per infected mosquito larva. The prevalence of infection rose from 46% at an exposure of 1 cercaria/day to 99% at 16 cercariae/day. Host mortality rose concomitantly from 25% to 88%. Host mortality and parasite acquisition were independent of environmental temperatures (21-29 C), despite the fact that developmental times, and consequently the number of daily exposures, were more than 50% greater at the low end of the temperature scale. This may be attributable to low activity of mosquito larvae and the resulting decrease in the number of encounters with cercariae.     

The effects of single exposures of Aedes aegypti larvae and pupae to Plagiorchis noblei cercariae in the laboratory

The mortality of Aedes aegypti pre-imagos harboring metacercariae of Plagiorchis noblei Park, 1936, is governed by the stage of development of the host at the time of infection and the location of the parasite in the insect body. First and second instar larvae generally succumbed to infection, regardless of site. Infections of the head and thorax of third and fourth instar larvae were generally lethal or gave rise to imparied adults. However, older instars frequently survived abdominal infections. Pupae showed greater tolerance to cephalic, thoracic and abdominal infections and generally emerged as adults. Again, many such infected adults were impaired.     

Aerial dispersal and host plant selection by neonate Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae)

Abstract.  1. Neonate evergreen bagworms, Thyridopteryx ephemeraeformis (Haworth) (Lepidoptera: Psychidae), disperse by dropping on a strand of silk, termed silking , and ballooning on the wind. Larvae construct silken bags with fragments of plant foliage. This species is highly polyphagous, feeding on more than 125 species of woody plants of 45 families. The larvae commonly infests juniper ( Juniperus spp.) and arborvitae ( Thuja spp.), but rarely feed on deciduous hosts such as maples. The hypothesis is proposed that polyphagy in T. ephemeraeformis is maintained by variation among larvae in dispersal behaviour, and time constraints on the opportunity to disperse, but patterns of host species preference result from a predisposition for larvae to settle on arborvitae and juniper but disperse from other hosts.
2. Consistent with that hypothesis, laboratory experiments revealed: (a) starved larvae varied in their tendency to disperse from paper leaf models; (b) starved larvae readily silked only during their first day; (c) larvae became increasingly sedentary the longer they were exposed to plant foliage; (d) when provided with several opportunities to silk, larvae became sedentary after exposure to arborvitae foliage, but repeatedly silked after exposure to maple ( Acer species) foliage or paper; and (e) larvae were less inclined to silk from foliage of arborvitae than from maple.
3. Field experiments supported the hypothesis by demonstrating that: (a) neonates tended to disperse from maple leaves while larvae older than 1 day tended to settle and remain; and (b) neonates were less likely to disperse from arborvitae and juniper trees than from maples.     

Parasitoid-induced behavioral alterations of Aedes aegypti mosquito larvae infected with mermithid nematodes (Nematoda: Mermithidae).

A wide range of parasites are known to cause behavioral changes in their hosts and parasitized insects are especially amenable to the study of such changes. The majority of studies addressing parasite-induced behavioral alterations have focused on parasites with complex life cycles and the adaptive nature of such changes. Behavioral changes caused by parasitoids, single-host parasites that kill their host upon emergence, have been studied less and the adaptive nature of these changes is likely to be different than those in complex life cycles. I investigated behavioral alterations in Aedes aegypti mosquito larvae infected with parasitoid nematodes (family Mermithidae). I conducted several experiments in which I tested the following hypotheses: 1) Mermithid nematodes induce behavioral changes in mosquito larvae and the changes are density dependent. 2) Different species of mermithid nematodes induce similar changes in mosquito larvae behavior. 3) Behavioral alterations vary with mermithid developmental stage. 4) Mosquito larvae infected with mermithid nematodes behave similarly to uninfected food-deprived mosquito larvae. I found that 4th instar Ae. aegypti infected with Romanomermis culicivorax or Strelkovimermis spiculatus exhibited resting behaviors significantly more often than uninfected controls but that intensity of infection did not affect activity levels. In earlier instars, infected mosquito larvae were more active than uninfected control larvae in some behaviors associated with feeding. There was no significant difference between infected and uninfected food-deprived mosquitoes in nine of the ten behaviors observed. The decrease in activity of late instar Ae. aegypti larvae infected with mermithids may be a parasitoid adaptation that reduces the risk of predation and thus increases host and parasitoid survival. The increase in feeding activity in earlier instars as well as the similarity between uninfected food-deprived and infected Ae. aegypti behavior may indicate that these behaviors are adaptive for the parasitoid, increasing nutritional acquisition for successful parasitoid development.     

Ecology and epizootology of Microsporidia in malarial mosquitoes (diptera: culicidae) from the south of Western Siberia

The ecology and epizootology of Microsporidia were studied in the natural population of malarial mosquitoes from Western Siberia over a 30-year period. Symptoms of the disease, host specificity and character of parasite localization in host tissues were investigated. Microsporidia of 9 species from 4 genera, namely Amblyospora, Crepidulospora, Senoma, and Parathelohania, were found in the malarial mosquito larvae from the territory examined. The mosquito species Anopheles messeae was infested by larger number of the microsporidian species, than A. beklemishevi. Spores and active stages of the microsporidian Senoma sp. from mosquito larvae are localized in epithelial cells of the host's intestine. There are no external signs of the infestation. The infested larvae do not die, and the infection proceeds to the pupal stage. Microsporidians of the genus Parathelohania infest larvae of both male and female mosquitoes. The parasites localized in the fat body cells. Body of the infested host gets an opaque white color in the period of spore maturation. The infested larvae perish at 4th stage. Microsporidian spores are formed in the mosquito larvae of both sexes. However, infection rate is much higher in the male larvae (77.8%), than that in the female larvae (22.2%). Symptoms of the disease under the infestation by the genera Crepidulospora and Amblyospora are similar to those under the infestation by Parathelohania sp. There are no external signs of the infestation in the adult mosquito females. In these hosts microsporidians form large two-nuclear spores with a thin capsule accounted for transovarial transmission. An about two times decrease of fertility in the Anopheles messeae females infested by the Parathelohania microsporidians is revealed. Mean number of eggs laid by the infested females was 121, while that of the noninfested ones was 232. Thus, the cause of the decrease in the mosquito abundance is both the elimination of male larvae and the decrease of fertility in females. In Siberia malarial mosquitoes yield 2 or 3 generations per year. Infested larvae are rarely occurred in the beginning of summer. Maximal extensiveness of the invasion may be observed in the end of summer, because of the accumulation of the infection in water bodies. Number of the peaks of infestation coincides with the peaks of abundance of mosquito larvae of 4th stage. Peaks of infestation are delayed as compared with the peaks of mosquito abundance. It is an evidence of the development delay in the infested larvae. Long-term dynamics of the infestation by microsporidians is studied. The epizooty caused by Parathelohania was observed in the malarial mosquitoes in the eighties (with the infestation rate up to 62 %). In the last ten years the infestation rate was low (from 0.1 to 2.6%).     

Dispersal of first-instar gypsy moth larvae in relation to population quality

Summary Field studies of dispersal by first instar gypsy moth larvae indicate that almost all larvae undergo an initial dispersal episode. However, in laboratory studies large larvae (from large eggs) disperse more frequently than small larvae (from small eggs) in the presence of favored food. Large larvae may be better adapted for dispersal. When larvae encounter unacceptable food or are denied food, larvae disperse more frequently and dispersal by small larvae is nearly as frequent as dispersal by large larvae. Factors affecting egg size may contribute to shifts in dispersal patterns of gypsy moth larvae and distribution of populations.Paper No. 2041, Massachusetts Agricultural Experiment Station, University of Massachusetts at Amherst. This research supported (in part) from Experiment Station Project No. 355     

Plagiorchis elegans (Digenea: Plagiorchiidae) infections in Stagnicola elodes (Pulmonata: Lymnaeidae): host susceptibility, growth, reproduction, mortality, and cercarial production.

Eggs of Plagiorchis elegans were readily ingested by Stagnicola elodes of all ages, but were more infective to immature than mature snails. Infection enhanced the growth of the host in a dose-dependent manner. The number of cercariae released by immature snails increased with the age of the snail host; mature snails yielded fewer cercariae. Heavily infected snails tended to die prematurely, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails that acquired the infection as juveniles never produced eggs. Actively reproducing snails ceased egg laying within days of infection and never recovered. All parasite effects on the growth and reproduction of the snail host first manifested themselves during the early stages of infection, long before the development of daughter sporocysts and cercariae, and are therefore attributable to the effects of mother sporocysts. The study provides insight into how this natural enemy of mosquito larvae may be established in natural snail populations by means of strategically timed introductions of parasite eggs, with a goal of maximizing cercarial production for the biological control of sympatric mosquito larvae.     

INFECTION OF AEDES AEGYPTI LARVAE BY AXENIC CULTURES OF THE FUNGAL GENUS SMITTIUM (TRICHOMYCETES)

Spores from axenic cultures of Smittium spp., isolated from various geographical locations and from different dipteran hosts (mosquito, black fly, and chironomid larvae), were fed to mosquito larvae (Aedes aegypti). We were able to demonstrate some host specificity at the insect family level, but no significant differences in infectivity by isolates from different geographical areas. Spore germination and thallus attachment were observed in the host hindgut within 1.5 hr post spore ingestion. Preliminary studies indicate that S. culisetae has little effect on A. aegypti larvae when they are reared under conditions that promote pupation within 5–6 days. However, ingestion of large numbers of spores by 1st instar larvae growing under suboptimal nutritional conditions may produce fungal growth detrimental to larvae.     

Distributional, morphological and genetic consequences of dispersal for temporary pond water mites

1. To determine the consequences of dispersal and gene flow for temporary pond water mites (Hydrachnida), we compared distributional, genetic and morphological characters in the closely related species Arrenurus angustilimbatus and A. rufopyriformis. The former has larvae that parasitise and disperse on adult mosquitoes, whereas larvae of the latter forego any association with hosts. 2. Allometrically adjusted egg size and gonopore size were found to be useful characters for distinguishing between females of the two species. 3. Arrenurus angustilimbatus possesses a broader and more continuous geographic distribution than its ‘direct developing’ counterpart. Allozyme heterozygosity was higher and population differentiation lower in A. angustilimbatus. In addition, populations of A. rufopyriformis were morphologically divergent, whereas populations of A. angustilimbatus were not. Isolation by distance analyses on both genetic and morphological characters indicated that the results were not biased by different sampling regimes for the two species. 4. These results demonstrate the importance of mosquito parasitism for maintaining ecological and genetic linkages between A. angustilimbatus populations. More broadly, we hypothesise that insect‐mediated dispersal has contributed to the ecological and evolutionary success of water mites, because the Hydrachnida lack other obvious adaptations for dispersing in space or time.     

Mosquitocidal toxins, genes and bacteria: the hit squad

Certain entomopathogenic species of bacilli and Clostridium produce one or more toxins that kill mosquito larvae even at concentrations in the picomolar range. Altogether, 19 distinct genes are known that encode mosquitocidal toxins, which vary in their potency, species specificity and mode of action. Unlike chemical insecticides, mosquitocidal bacilli used as larvicides are safe for animals and the environment, and do not affect non-pest insects. Mosquitocidal bacteria are effective to varying degrees against Culex, Anopheles and Aedes mosquito larvae, but their rapid sedimentation from the larval feeding zone, UV-light sensitivity and narrow host range have hampered their development. New genetic engineering approaches are being investigated that could overcome these limitations and allow stable expression of broad host range combinations of toxins in UV-resistant, buoyant recombinant bacteria, as discussed here by Alan Porter.     

Host plant availability potentially limits butterfly distributions under cold environmental conditions

Species ranges are shaped by both climatic factors and interactions with other species. The stress gradient hypothesis predicts that under physiologically stressful environmental conditions abiotic factors shape range edges while in less stressful environments negative biotic interactions are more important. Butterflies provide a suitable system to test this hypothesis since larvae of most species depend on biotic interactions with a specific set of host plants, which in turn can shape patterns of occurrence and distribution. Here we modelled the distribution of 92 butterfly and 136 host plant species with three different modelling algorithms, using distribution data from the Swiss biodiversity monitoring scheme at a 1 × 1 km spatial resolution. By comparing the ensemble prediction for each butterfly species and the corresponding host plant(s), we assessed potential constraints imposed by host plant availability on distribution of butterflies at their distributional limits along the main environmental gradient, which closely parallels an elevational gradient. Our results indicate that host limitation does not play a role at the lower limit. At the upper limit 50% of butterfly species have a higher elevational limit than their primary host plant, and 33% have upper elevational limits that exceed the limits of both primary and secondary hosts. We conclude that host plant limitation was not relevant to butterfly distributional limits in less stressful environments and that distributions are more likely limited by climate, land use or antagonistic biotic interactions. Obligatory dependency of butterflies on their host plants, however, seems to represent an important limiting factor for the distribution of some species towards the cold, upper end of the environmental gradient, suggesting that biotic factors can shape ranges in stressful environments. Thus, predictions by the stress gradient hypothesis were not always applicable.     

Nestedness patterns of container-dwelling mosquitoes: effects of larval habitat within variable terrestrial matrices

Distributions of mosquito larvae likely are a consequence of multiple factors, although two commonly studied factors (quality of the larval environment and the terrestrial matrix in which these habitats reside) have rarely and simultaneously been varied in the field to understand distributions of larvae. We monitored aquatic containers of two leaf detritus levels within a forest, prairie, and industrial habitat across five months to understand the temporal and spatial colonization of aquatic invertebrates in Northcentral Illinois, USA. Data were collected monthly on mosquito populations and the composition of other invertebrates colonizing containers. Overall, six species of mosquitoes colonized containers, with Culex restuans and Aedes triseriatus having the highest relative abundances. There were strong seasonal abundance patterns for these two mosquito species, with the dominant species changing over time in the forest habitat. The responses of other mosquito taxa were more variable, with abundances reflective of either the terrestrial matrix or larval habitat quality. High detritus containers supported the highest abundances of most species encountered, regardless of habitat. Non-mosquito taxa were less common numerically, but analyses suggested that some taxa, such as syrphid larvae, often co-occurred with mosquitoes. Nested subset analysis indicated communities were strongly nested, and that both habitat type and detritus level were important in explaining nested patterns of aquatic invertebrates. Our data show that both the larval habitat and the surrounding terrestrial matrix shape patterns of container mosquitoes, and that other container invertebrates vary in similar ways as mosquitoes. Handling editor: K. Martens     

The effects of Plagiorchis noblei metacercariae on the development and survival of Aedes aegypti larvae in the laboratory

Plagiorchis noblei infections impair the survival and development of fourth instar Aedes aegypti larvae. Mortality during the larval and pupal stages reached 92%, and 60% of the emerging adults were malformed. The metacercariae interfere with pupation and the emergence of adults. Larvae and pupae that fail to transform to the next developmental stage within the normal time characteristically persist for extended periods, but invariably die without transforming. Whereas 82% of the control larvae gave rise to functional adults, only 4% of infected larvae managed to do so. Such effects may facilitate the transmission of the parasite.     

Bioassays for comparative infectivity of mermithid nematodes (Romanomermis iyengari,Romanomermis culicivorax and Strelkovimermis spiculatus) for culicine mosquito larvae

Two improved bioassays were developed to establish infectivity baselines for selection experiments using mermithid nematode variants. Comparative infectivity of Romanomermis iyengari, Romanomermis culicivorax and Strelkovimermis spiculatus using larvae of three mosquito spp. Aedes sierrensis, Aedes aegypti and Culex pipiens were evaluated with “plate” and “tray” bioassays at selected intensity of infections. Using the “plate” bioassay, single mosquito larvae were immersed in 2 ml of water within individual depressions of 12-well, polystyrene tissue culture plates. One, three, or five preparasitic juveniles (J2) were added to each well. In the “tray” bioassay, polyethylene trays containing 500 ml water and 100 mosquito larvae were exposed to 500 (5:1, nematode:insect host) or 1000 (10:1) J2s. Percentage infection (PINF, infectivity) and intensity of infection (IINF, #nematodes per infected larvae) number were determined only after emergence of post-parasitic J3 juveniles. Under the bioassay conditions, all three species of nematodes resulted in infections in all mosquito hosts, but R. iyengari exhibited better effectiveness in the parasitism of mosquito larvae. The three species of mosquitoes presented high levels of susceptibility to each of the three species of nematodes, but in general Cx. pipiens and Ae. sierrensis were slightly more susceptible than Ae. aegypti. The “plate” bioassay was more efficient in measurement of infectivity of the mermithid species and in establishing baseline characteristics for these mosquito-parasitic nematodes. The “tray” bioassay was an effective bioassay for large cohorts of both infective juveniles and host larvae and, potential for field interactions.     

A parasite's modification of host behavior reduces predation on its host

Parasite modification of host behavior is common, and the literature is dominated by demonstrations of enhanced predation on parasitized prey resulting in transmission of parasites to their next host. We present a case in which predation on parasitized prey is reduced. Despite theoretical modeling suggesting that this phenomenon should be common, it has been reported in only a few host–parasite–predator systems. Using a system of gregarine endosymbionts in host mosquitoes, we designed experiments to compare the vulnerability of parasitized and unparasitized mosquito larvae to predation by obligate predatory mosquito larvae and then compared behavioral features known to change in the presence of predatory cues. We exposed Aedes triseriatus larvae to the parasite Ascogregarina barretti and the predator Toxohrynchites rutilus and assessed larval mortality rate under each treatment condition. Further, we assessed behavioral differences in larvae due to infection and predation stimuli by recording larvae and scoring behaviors and positions within microcosms. Infection with gregarines reduced cohort mortality in the presence of the predator, but the parasite did not affect mortality alone. Further, infection by parasites altered behavior such that infected hosts thrashed less frequently than uninfected hosts and were found more frequently on or in a refuge within the microcosm. By reducing predation on their host, gregarines may be acting as mutualists in the presence of predation on their hosts. These results illustrate a higher‐order interaction, in which a relationship between a species pair (host–endosymbiont or predator–prey) is altered by the presence of a third species.     

Prey preferences of aquatic insects: potential implications for the regulation of wetland mosquitoes

Wetlands are potential sites for mosquito breeding and are thus important in the context of public health. The use of chemical and microbial controls is constrained in wetlands in view of their potential impact on the diverse biota. Biological control using generalist aquatic insects can be effective, provided a preference for mosquito larvae is exhibited. The mosquito prey preferences of water bugs and larvae of odonate species were evaluated using chironomid larvae, fish fingerlings and tadpoles as alternative prey. Manly's selectivity (α_i) values with 95% confidence intervals (CIs) were estimated to judge prey preference patterns. Multivariate analysis of variance (manova) and standardized canonical coefficients were used to test the effects of density on prey selectivity. The α_i values indicated a significant preference (P < 0.05) in all of the insect predators tested for mosquito larvae over the alternative prey as a density‐dependent function. On a comparative scale, chironomid larvae had the highest impact as alternative prey. In a multiple‐prey experiment, predators showed a similar pattern of preference for mosquito larvae over alternative prey, reflecting a significant (P < 0.05) niche overlap. The results suggest that, in a laboratory setting, these insect predators can effectively reduce mosquito density in the presence of multiple alternative prey.     

The glycoprotein toxin of Bacillus thuringiensis subsp. israelensis indicates a lectinlike receptor in the larval mosquito gut.

The mosquito-active protein crystals produced by Bacillus thuringiensis subsp. israelensis contain covalently attached aminosugars which are critical for their larvicidal activity. The 50% lethal concentrations toward Aedes aegypti larvae were increased up to 10-fold by mild periodate treatment, up to 40-fold by forming the protein crystals in the presence of tunicamycin, and up to 7-fold by the presence during the mosquito bioassays of N-acetylglucosamine or its trimer, triacetylchitotriose. Periodate-treated crystals and crystals formed in the presence of tunicamycin had greatly reduced binding capacities for wheat germ agglutinin, an N-acetylglucosamine-specific lectin. These results suggest that the B. thuringiensis subsp. israelensis glycoprotein toxin binds to a lectinlike receptor in the larval mosquito gut. Furthermore, the distinct lectin-binding patterns exhibited by diptera-active versus lepidoptera-active B. thuringiensis crystals suggest that host specificity for the microbial insecticides is determined, in part, by the carbohydrate portion of their glycoprotein crystals.     

The glycoprotein toxin of Bacillus thuringiensis subsp. israelensis indicates a lectinlike receptor in the larval mosquito gut

The mosquito-active protein crystals produced by Bacillus thuringiensis subsp. israelensis contain covalently attached aminosugars which are critical for their larvicidal activity. The 50% lethal concentrations toward Aedes aegypti larvae were increased up to 10-fold by mild periodate treatment, up to 40-fold by forming the protein crystals in the presence of tunicamycin, and up to 7-fold by the presence during the mosquito bioassays of N-acetylglucosamine or its trimer, triacetylchitotriose. Periodate-treated crystals and crystals formed in the presence of tunicamycin had greatly reduced binding capacities for wheat germ agglutinin, an N-acetylglucosamine-specific lectin. These results suggest that the B. thuringiensis subsp. israelensis glycoprotein toxin binds to a lectinlike receptor in the larval mosquito gut. Furthermore, the distinct lectin-binding patterns exhibited by diptera-active versus lepidoptera-active B. thuringiensis crystals suggest that host specificity for the microbial insecticides is determined, in part, by the carbohydrate portion of their glycoprotein crystals.     

Dragonfly endemism in the Brazilian Amazon: competing hypotheses for biogeographical patterns

Many hypotheses have been proposed to explain the origin and maintenance of the Amazonian diversity with special place for the theory of isolation by rivers and a set of hypothesis related to contemporary environmental dissimilarity. We explore those hypotheses here using the biogeographic distributional patterns of dragonflies in interfluve areas of the Amazonian biome and also evaluate how differences among in dispersal capabilities between the Anisoptera and Zygoptera suborders may contribute to those patterns. We used distributional information of 392 odonate species in the Amazonian forest in a cladistic analysis of distributions and endemism and the estimated faunistic similarity among interfluves with the Sorensen index. The environmental similarity among interfluves was analysed by discriminant analysis based on eight environmental metrics. Different metrics for geographic distance (connectivity) among interfluves were evaluated and their relation to the other variables tested by the Mantel test. The number of endemic species was linearly correlated to the area of the interfluves. General endemism patterns showed consistent resemblance to those reported for vertebrates, especially the similarity among the Rond?nia and Inambari interfluves. Geographical distance has no predictive value for dragonflies distribution, but the environmental similarity is a good predictor of proportion of shared species. The low dispersal group (Zygoptera) presented more clear patterns of distribution and a lower proportion of shared species among different interfluves. The environmental similarity can be considered the determinant factor of the distribution of dragonflies, possibly due to environmental specificity evolved during a long history of some clades in this system. The low dispersal group (Zygoptera) retained more biogeographical information about possible historical factors that determine current distribution. Also, the transport of larvae by macrophyte banks, the lateral change of river courses, the reversal of the drainage basin, together with the capacity to disperse across rivers for some species may be explanations for the lack of effect of isolation by rivers, especially for Anisoptera.