Comparative ability to detoxify alder leaf litter in field larval mosquito collections

The larvicidal effects of polyphenols from dietary alder leaf litter were investigated in different field collections of three detritivorous Aedes taxa (Ae. detritus, Ae. cataphylla, Ae. rusticus) and compared to the cytochrome P450 monooxygenase, glutathione S-transferase, and esterase activities. Larvae from polyphenol-rich habitats had a higher tolerance for polyphenols and higher midgut cytochrome P450 and esterase activities than larvae from polyphenol-poor habitats. Furthermore, the role of P450 enzymes in the mechanism of resistance to alder polyphenols was suggested by the synergistic effect in vivo of piperonyl butoxide in the resistant Ae. rusticus. This confirms the importance of polyphenols to larval mosquito performance, and provides evidence for the importance of specific detoxification mechanisms for tolerance to dietary polyphenols. Arch.     

The role of environment in shaping the genetic diversity of the subalpine mosquito,Aedes rusticus (Diptera,Culicidae)

The relative involvement of larval dietary tolerance to the leaf-litter toxic polyphenols in shaping population genetic structure of the subalpine mosquito Aedes rusticus was examined. This was compared with other parameters such as geographical range, type of vegetation surrounding the breeding site, and occurrence of annual larvicidal treatments. Population genetic structure was analysed at 10 presumed neutral polymorphic isoenzyme loci. Toxicological comparisons involved standard bioassays performed on larvae fed on toxic decomposed leaf litter. Significant overall genetic differentiation was observed among the 22 studied populations and within the five defined geographical groups. Analysis of molecular variance revealed an absence of relation between genetic and environmental parameters, genetic variance being essentially found within populations. This suggested that the larval dietary tolerance to the toxic leaf litter and the other studied parameters poorly influence population genetic structure. The local adaptation of subalpine mosquito populations to the surrounding vegetation thus appears as a labile trait. Such a dynamic adaptation is also suggested by the correlation between geographical and toxicological distances and the correlation between dietary tolerance to the leaf-litter toxic polyphenols and annual larvicidal treatments.     

Comparative sensitivity of larval mosquitoes to vegetable polyphenols versus conventional insecticides

The larvicidal effects of polyphenols of natural crude decomposed alder leaf litter and commercially available tannic acid were experimentally compared with those of two common conventional insecticides (Bacillus thuringiensis ssp. israelensis: microbial insecticide; temephos: organophosphate insecticide). Comparative standard bioassays using third instar larval Aedes aegypti, A. albopictus, Culex pipiens and Coquillettidia richiardii as references indicated that Aedes and Culex taxa are far more sensitive to alder leaf litter than to tannic acid and conventional insecticides. C. richiardii is far more resistant to conventional insecticides than Aedes and Culex taxa, but its sensitivity to tannic acid is close to that of those taxa. Dietary vegetable polyphenols are thus proposed as new, practical, alternative chemicals for mosquito control when conventional insecticides are difficult and costly to be used (e.g., in the management of Aedes and Culex populations in man-made breeding sites and Coquillettidia control strategy).     

Observations on subgenus Aedes (genus Aedes, Diptera, Culicidae) in northeast Italy and first italian report of Aedes geminus Peus]

The paper reports some observations on the subgenus Aedes (genus Aedes, Diptera, Culicidae) in northeast Italy. Two species were collected: Ae. cinereus and Ae. geminus, the latter recorded for the first time in Italy. Morphological, ecological and biological data of the two species are presented. The identification is possible only on the male hypopigium; larvae, pupae and adult females show no differential characters. For both species, the larval breeding sites were fresh water marshes mainly within woods; preimaginal development took place twice a year, in Spring and Autumn. The females were strongly anthropophilic. No biological differences between the two species were noticed, but more data are needed to ascertain their relationships and the presence of subtle biological divergences.     

Involvement of cytochrome P450 monooxygenases in the response of mosquito larvae to dietary plant xenobiotics

The response of mosquito larvae to plant toxins found in their breeding sites was investigated by using Aedes aegypti larvae and toxic arborescent leaf litter as experimental models. The relation between larval tolerance to toxic leaf litter and cytochrome P450 monooxygenases (P450s) was examined at the toxicological, biochemical and molecular levels. Larvae pre-exposed to toxic leaf litter show a higher tolerance to those xenobiotics together with a strong increase in P450 activity levels. This enzymatic response is both time- and dose-dependent. The use of degenerate primers from various P450 genes (CYPs) allowed us to isolate 16 new CYP genes belonging to CYP4, CYP6 and CYP9 families. Expression studies revealed a 2.3-fold over-expression of 1 CYP gene (CYP6AL1) after larval pre-exposure to toxic leaf litter, this gene being expressed at a high level in late larval and pupal stages and in fat bodies and midgut. The CYP6AL1 protein has a high level of identity with other insect's CYPs involved in xenobiotic detoxification. The role of CYP genes in tolerance to natural xenobiotics and the importance of such adaptive responses in the capacity of mosquitoes to colonize new habitats and to develop insecticide resistance mechanisms are discussed.     

The first report of Aedes (Stegomyia) albopictus in Haiti

Aedes albopictus was found in six of the 10 departments of Haiti and in 14 of the 35 communes surveyed. The survey found the larvae of Ae. albopictus in 13 different types of containers. Used tires and tins were by far the most common breeding sites used by this mosquito species. At the breeding sites, Ae. albopictus was associated with other mosquito species, such as Aedes aegypti, Culex nigripalpus and Aedes mediovittatus. The highest proportion of association was with Ae. aegypti. This study represents the first report of Ae. albopictus in Haiti.     

The importance of an invasive tree fruit as a resource for mosquito larvae

Invasive plants are common and may provide resources through litter for container mosquito larvae. Invasive plant reproductive parts can make up a substantial part of litter but have mostly been ignored as a resource for mosquito larvae. We hypothesized that the reproductive fruits of the invasive eastern red cedar, Juniperus virginiana, provide high quality resources for the invasive, container mosquito Aedes albopictus at the western margin of its invasive range in North America. To test this hypothesis, we performed two laboratory experiments. The first examined the response of individual larvae of Ae. albopictus to different amounts of J. virginiana leaf (fresh and senesced) and J. virginiana fruit (ripe and unripe), as well as to a control leaf (Quercus virginiana, live oak). The second experiment examined the response of different densities of Ae. albopictus larvae to each litter type. We found significant differences in response by individual larvae to different amounts of litter and litter types. We also found J. virginiana litter components could support positive population growth rates as a function of initial larval density where the control leaf could not. We conclude that invasive plants may provide high quality resources, and that the reproductive parts (fruits, flowers, cones) may be an important and overlooked component in provisioning larval habitats. Therefore, the expansion of J. virginiana into grassland areas may contribute to the expansion of Ae. albopictus westward in North America.     

Differential toxicity of leaf litter to dipteran larvae of mosquito developmental sites

The relative toxicity of leaf litter to nematocerous dipteran larvae characteristic of mosquito developmental sites was investigated. Culicidae, Chironomidae, and Simuliidae taxa originating from alpine hydrosystems were tested together with two laboratory nonindigenous culicid taxa. Bioassays indicate that ingestion of 10-month-old decaying leaves from Alnus glutinosa, Populus nigra, and Quercus robur by larvae is more deleterious for Aedes aegypti, A. albopictus, Culex pipiens, Simulium variegatum, and Chironomus annularius than for A. rusticus. Histopathological observations reveal that the midgut epithelium is the main target organ of the toxic effect of dietary leaf litter, which appears to be stronger than that of previously reported tannic acid. There is a general response of the nematocerous larval midgut epithelium to dietary tannins-phenolic compounds: clear cells of the anterior midgut showing symptoms of intoxication before dark cells of the posterior midgut.     

Forest canopy,water level,and biopesticide interact to determine oviposition habitat selection in Aedes albopictus

Understanding how interacting abiotic and biotic factors influence colonization rates into different habitat types is critical for both conserving and controlling species. For example, the rapid global spread of Asian tiger mosquitoes, Aedes albopictus, has reduced native species abundances and produced disease outbreaks. Fortunately, bacterial endospores of two Bacillus species (biospesticide) are highly lethal to Ae. albopictus larvae and have been commercially developed to reduce populations. Oviposition habitat selection is the first defense Ae. albopictus females possess against any control substance added to breeding sites, and considerable variation exists in their response to biopesticides. In a field experiment, I crossed the presence/absence of biopesticides, with two canopy (open, closed) and water (high, low) levels at 64 breeding sites, to examine if these interacted to influence oviposition site choice. Avoidance of biopesticide was most pronounced in closed canopy sites and those with low water levels, as all main effects and two‐way interactions influenced oviposition. Oviposition habitat selection represents a possible mechanism of resistance to biopesticides and other methods used to kill mosquito larvae. Future experiments examining how larval density and mortality modify these results should allow for more effective control of this highly invasive species.     

Distributions of Competing Container Mosquitoes Depend on Detritus Types, Nutrient Ratios, and Food Availability

Coexistence of competitors may result if resources are sufficiently abundant to render competition unimportant, or if species differ in resource requirements. Detritus type has been shown to affect interspecific competitive outcomes between Aedes albopictus (Skuse) and Aedes aegypti (L.) larvae under controlled conditions. We assessed the relationships among spatial distributions of detritus types, nutrients, and aquatic larvae of these species in nature. We collected mosquitoes, water, and detritus from artificial containers across 24 Florida cemeteries that varied in relative abundances of Ae. aegypti and Ae. albopictus.We measured nutrient content of fine particulate organic matter in water samples as total N, P, and C and ratios of these nutrients. We quantified food availability via a bioassay, raising individual Aedes larvae in the laboratory in standard volumes of field-collected, particulate-containing water from each cemetery. Quantities of detritus types collected in standard containers were significant predictors of nutrients and nutrient ratios. Nutrient abundances were significant predictors of relative abundance of Ae. aegypti, and of larval survival and development by both species in the bioassay. Survival and development of larvae reared in particulate-containing water from sites decreased with decreasing relative abundance of Ae. aegypti. These data suggest that N, P, and C availabilities are determined by detritus inputs to containers and that these nutrients in turn determine the feeding environment encountered by larvae, the intensity of interspecific competition among larvae, and subsequent relative abundances of species at sites. Detritus inputs, nutrients, and food availability thus seem to contribute to distributions of Ae. aegypti and Ae. albopictus in cemetery containers throughout Florida.     

Competition and resistance to starvation in larvae of container-inhabiting Aedes mosquitoes

Abstract. 1. Hypotheses about declining populations of container-inhabiting Aedes mosquitoes following the invasion by additional species were tested.
2. The larval competition hypothesis was studied experimentally in pure and mixed cultures of Aedes aegypti (L.), A.albopictus (Skuse) and A.triseriatus (Say). The experiments used decomposing leaf litter in the laboratory, as opposed to most previous research which used non-natural food.
3. Resistance to starvation is introduced as a new measure of larval performance and competitiveness. The hypothesis is that more successful larvae store larger energy reserves and resist the lack of food longer.
4. Contrary to previous research showing better performance of A.aegypti in mixed cultures, A.albopictus developed faster and had greater survival when natural food was used.
5. Resistance to starvation was greater in the better performing species (i.e. A.aegypti with non-natural food and A.albopictus with leaf litter). Oxygen consumption by starved larvae was similar in the three container species, and in the ground-water mosquito, A.taeniorhynchus (Wied.), whose resistance to starvation was comparatively very low.     

Organochlorines and organophosphates susceptibility of Aedes albopictus Skuse larvae from agricultural and non-agricultural localities in Peninsular Malaysia

Aedes albopictus larvae obtained from different types of agricultural and non-agricultural localities in Peninsular Malaysia were subjected to several larvicides at World Health Organization (WHO) recommended dosages. Upon 24 h of WHO larval bioassay using two organochlorines and six organophosphates, high resistance against dichlorodiphenyltrichloroethane (DDT), temephos, chlorpyrifos and bromophos were demonstrated among all larval populations. Aedes albopictus larvae from both paddy growing areas (92.33% mortality) and rubber estates (97.00% mortality) were moderately resistant to dieldrin while only Ae. albopictus larvae from dengue prone residential areas (89.00% mortality) showed high resistance against dieldrin. All Ae. albopictus larval populations also developed either incipient or high resistance to both malathion (33.67%–95.33% mortality) and fenitrothion (73.00%–92.67% mortality). Only Ae. albopictus larvae from fogging-free residential areas that were tolerant to fenthion (97.33% mortality), whereas Ae. albopictus larvae from dengue prone residential areas were highly resistant to the same organophosphate (88.33% mortality). Cross resistance between intraclass and interclass larvicides of organochlorines and organophosphates were also exhibited in this study. The present study provided baseline data on various susceptibility levels of Ae. albopictus larval populations from different types of agricultural and non-agricultural localities against organochlorines and organophosphates at WHO recommended dosages. Nevertheless, further susceptibility investigations are suggested using revised doses of larvicides established from the local reference strain of Ae. albopictus to prevent the underestimation or overestimation of insecticide resistance level among Ae. albopictus field strains of larvae.     

Occurrence of Toxorhynchites guadeloupensis (Dyar Knab) in oviposition trap of Aedes aegypti (L.) (Diptera: Culicidae)

Toxorhynchites guadeloupensis (Dyar Knab), a poorly known mosquito species, was observed preying upon Aedes aegypti (L.) larvae, in an oviposition trap placed for routine dengue entomological surveillance, during 2003-2004 in the urban area of Boa Vista, Roraima, Brazil. This is the first report for Tx. guadeloupensis using Ae. aegypti oviposition traps as breeding places. This finding may have important consequences in the epidemiology and local dengue control since Ae. aegypti density is a basic variable in dengue prediction. Whether predation of Ae aegypti by Tx. guadeloupensis in the Amazon is of significance, is a question to be examined. Also, larval predation may be a cause for underestimation of the actual Ae aegypti numbers. Together these hypotheses need to be better investigated as they are directly related to dengue epidemiology, to the success of any outbreak prediction and surveillance program.     

Biochemical and cytoimmunological evidence for the control of Aedes aegypti larval trypsin with Aea-TMOF

Trypsin and chymotrypsin-like enzymes were detected in the gut of Aedes aegypti in the four larval instar and pupal developmental stages. Although overall the amount of trypsin synthesized in the larval gut was 2-fold higher than chymotrypsin, both enzymes are important in food digestion. Feeding Aea-Trypsin Modulating Oostatic Factor (TMOF) to Ae. aegypti and Culex quinquefasciatus larvae inhibited trypsin biosynthesis in the larval gut, stunted larval growth and development, and caused mortality. Aea-TMOF induced mortality in Ae. aegypti, Cx. quinquefasciatus, Culex nigripalpus, Anopheles quadrimaculatus, and Aedes taeniorhynchus larvae, indicating that many mosquito species have a TMOF-like hormone. The differences in potency of TMOF on different mosquito species suggest that analogues in other species are similar but may differ in amino acid sequence or are transported differently through the gut. Feeding of 29 different Aea-TMOF analogues to mosquito larvae indicated that full biological activity of the hormone is achieved with the tetrapeptide YDPA. Using cytoimmunochemical analysis, intrinsic TMOF was localized to ganglia of the central nervous system in larvae and male and female Ae. aegypti adults. The subesophageal, thoracic, and abdominal ganglia of both larval and adult mosquitoes contained immunoreactive cells. Immunoreactive cells were absent in the corpus cardiacum of newly molted 4th instar larvae but were found in late 4th instar larvae. In both males and females, the intrinsic neurosecretory cells of the corpus cardiacum were filled with densely stained immunoreactive material. These results indicate that TMOF-immunoreactive material is synthesized in sugar-fed male and female adults and larvae by the central nervous system cells.     

Role of vegetable tannins in habitat selection among mosquito communities from the Alpine hydrosystems

The role of vegetable tannins in habitat selection among mosquito communities in Alpine hydrosystems was investigated through ecotoxicological comparison of 19 arthropod species characteristic of 12 breeding sites known for their abiotic environmental factors and their different riparian vegetation. The toxicity of tannins was experimentally compared among species representative of both the dipteran fauna and the crustacean fauna associated with the mosquito breeding sites. Bioassays using tannic acid solutions at concentrations from 0.1 to 11 mM separated the dipteran taxa into five groups of differential sensitivity and the crustacean taxa into four groups. The different levels of sensitivity among taxa were correlated with the various amounts of total phenolics and tannins found in the most prominent plant types associated with the different breeding sites. This suggested that tannins and, more generally, phenolic compounds may be involved in plant-arthropod interactions in Alpine hydrosystems.     

Synergistic effects of the invasive Chinese tallow (Triadica sebifera) and climate change on aquatic amphibian survival

Changes in climate and the introduction of invasive species are two major stressors to amphibians, although little is known about the interaction between these two factors with regard to impacts on amphibians. We focused our study on an invasive tree species, the Chinese tallow (Triadica sebifera), that annually sheds its leaves and produces leaf litter that is known to negatively impact aquatic amphibian survival. The purpose of our research was to determine whether the timing of leaf fall from Chinese tallow and the timing of amphibian breeding (determined by weather) influence survival of amphibian larvae. We simulated a range of winter weather scenarios, ranging from cold to warm, by altering the relative timing of when leaf litter and amphibian larvae were introduced into aquatic mesocosms. Our results indicate that amphibian larvae survival was greatly affected by the length of time Chinese tallow leaf litter decomposes in water prior to the introduction of the larvae. Larvae in treatments simulating warm winters (early amphibian breeding) were introduced to the mesocosms early in the aquatic decomposition process of the leaf litter and had significantly lower survival compared with cold winters (late amphibian breeding), likely due to significantly lower dissolved oxygen levels. Shifts to earlier breeding phenology, linked to warming climate, have already been observed in many amphibian taxa, and with most climate models predicting a significant warming trend over the next century, the trend toward earlier breeding should continue if not increase. Our results strongly suggest that a warming climate can interact with the effects of invasive plant species, in ways we have not previously considered, to reduce the survival of an already declining group of organisms.     

Aedes albopictus (Diptera: Culicidae) in Rome: experimental study of relevant control strategy parameters

Since 1997, Aedes albopictus has colonised and then rapidly invaded the city of Rome (Italy) and its peripheral areas. Presently, the control of this mosquito in Italy relies mainly on larvicidal treatment of street storm sewer catch basins with the organophosphate temephos. We have therefore obtained baseline data on the susceptibility to temephos of the Roman Ae. albopictus population by laboratory bioassays on F1 fourth-instar larvae following standard WHO protocols. Estimated lethal concentrations were 0.014 mg/l (LC50) and 0.022 mg/l (LC90) indicating a lack of resistance to this compound. The persistence of temephos in sewer catch basins was evaluated by follow-up of catch basins treated with a dose of 1.5 mg of active ingredient. Mosquito larvae were recovered in 10% and 50% of the treated basins at 9 and 18 days posttreatment, respectively. In order to understand the relative contribution of this larval habitat to adult populations, we conducted a survey in the Zoo of Rome to estimate the larval density of mosquitoes breeding in sewer catch basins. A complete census of a 16.5 ha area mapped 243 catch basins, but only 25 (10.3%) contained water; of the latter 8 (32.0%) hosted mosquito larvae. All positive catch basins contained larvae of Culex pipiens, which were associated with Culiseta longiareolata and/or Ae. albopictus in 6 and 3 cases, respectively. A longitudinal survey of one catch basin over 4 months showed that the mean larval density of Ae. albopictus was markedly lower than that of Cx pipiens and Cs. Iongiareolata, ranging between 0 and 1.3 larvae/dip as compared to 0-33.2 and 0-22.7 larvae/dip, respectively. However, adult densities of Ae. albopictus in this area estimated during the same period with 20 ovitraps showed consistently high values (proportion of positive ovitraps around 100%). These preliminary observations suggest that whenever alternative larval biotopes other than sewer catch basins are widely available, they might be more productive and/or preferred substrates to catch basins for Ae. albopictus breeding.     

An experimental field study of delayed density dependence in natural populations of Aedes albopictus

Aedes albopictus, a species known to transmit dengue and chikungunya viruses, is primarily a container-inhabiting mosquito. The potential for pathogen transmission by Ae. albopictus has increased our need to understand its ecology and population dynamics. Two parameters that we know little about are the impact of direct density-dependence and delayed density-dependence in the larval stage. The present study uses a manipulative experimental design, under field conditions, to understand the impact of delayed density dependence in a natural population of Ae. albopictus in Raleigh, North Carolina. Twenty liter buckets, divided in half prior to experimentation, placed in the field accumulated rainwater and detritus, providing oviposition and larval production sites for natural populations of Ae. albopictus. Two treatments, a larvae present and larvae absent treatment, were produced in each bucket. After five weeks all larvae were removed from both treatments and the buckets were covered with fine mesh cloth. Equal numbers of first instars were added to both treatments in every bucket. Pupae were collected daily and adults were frozen as they emerged. We found a significant impact of delayed density-dependence on larval survival, development time and adult body size in containers with high larval densities. Our results indicate that delayed density-dependence will have negative impacts on the mosquito population when larval densities are high enough to deplete accessible nutrients faster than the rate of natural food accumulation.     

Aedes albopictus in Rome: results and perspectives after 10 years of monitoring

In 1997, Aedes albopictus (Skuse 1894) was detected in Rome in two opposite areas of the city. In the following 2 years, the species quickly spread. In 2000, scattered foci of the species were reported in the whole urban area and in the outskirts of the capital city. In Rome, Ae. albopictus seems to have found optimal environmental conditions to proliferate and to overwinter through and without diapausing eggs. In ten years Ae. albopictus has colonized the whole urban area through three phases: first massive spread, following maintenance of infestation, and colonization of alternative winter breeding sites with favorable climatic conditions. Data collected during the 2007 show that rainfall is no longer the most important factor for the development of the species, with respect to the past. In fact Ae. albopictus probably has found new alternative larval breeding sites through the colonization of small water collections refilled periodically by human activities. During 2007-2008 winter season, in order to evaluate the species adaptability, a study of eggs hatching and length of larval cycle at low temperatures, was carried out in laboratory and in simulated field conditions. Data and results are showed and discussed also by the light of existing literature.     

A geospatial evaluation of Aedes vigilax larval control efforts across a coastal wetland,Northern Territory,Australia

Adjacent to the northern suburbs of Darwin is a coastal wetland that contains important larval habitats for Aedes vigilax (Skuse), the northern salt marsh mosquito. This species is a vector for Ross River virus and Barmah Forest virus, as well as an appreciable human pest. In order to improve aerial larval control efforts, we sought to identify the most important vegetation categories and climatic/seasonal aspects associated with control operations in these wetlands. By using a generalized linear model to compare aerial control for each vegetation category, we found that Schoenoplectus/mangrove areas require the greatest amount of control for tide‐only events (30.1%), and also extensive control for tide and rain events coinciding (18.2%). Our results further indicate that tide‐affected reticulate vegetation indicated by the marsh grasses Sporobolus virginicus and Xerochloa imberbis require extensive control for Ae. vigilax larvae after rain‐only events (44.7%), and tide and rain events coinciding (38.0%). The analyses of vector control efforts by month indicated that September to January, with a peak in November and December, required the most control. A companion paper identifies the vegetation categories most associated with Aedes vigilax larvae population densities in the coastal wetland. To maximize the efficiency of aerial salt marsh mosquito control operations in northern Australia, aerial control efforts should concentrate on the vegetation categories with high larval densities between September and January.