Sylvia Fischer

Urban rain pools: seasonal dynamics and entomofauna in a park of Buenos Aires

We describe the seasonal variations of the insect community of the rain pools in a park of Buenos Aires during a 1-year period, based on a weekly sampling programme. We also analyse the relationships between the observed biotic patterns and some physical and meteorological variables. Four periods, fairly coincident with the seasons of the year, were graphically identified as functions of temperature, rainfall, flooded area, number of rain pools, depth and taxonomic richness. A total of 45 insect taxa was identified: 18 Coleoptera, 15 Diptera, 9 Heteroptera, 1 Ephemeroptera and 2 Odonata. Culicidae represented 76% of the pooled abundance of insects, probably related to their rapid development before the pools dry up. Taxonomic richness was maximum at the end of the summer (32), in coincidence with maximum rainfalls and temperatures. Inversely, the minimum richness (2) was recorded during the spring drought. Richness was positively correlated to the flooded area and to the number of pools on each sampling date. The species recorded throughout the study period were: Aedes albifasciatus, Culex eduardoi, Culex pipiens, Liodessus sp., Rhantus signatus signatus, Tropisternus setiger, Tropisternus lateralis limbatus and Sigara platensis. Some of the species recorded are considered pioneer in temporary habitats. Both coleopterans and heteropterans are likely to persist in the park through repeated colonisation from more permanent habitats. Habitat duration and flooded area were positively correlated to richness in each one of the rain pools.

TILLER POPULATION DYNAMICS AND PRODUCTION ON SPARTINA DENSIFLORA (BRONG) ON THE FLOODPLAIN OF THE PARANÁ RIVER, ARGENTINA

Tiller dynamics and net aerial primary productivity (NAPP) were studied in permanent sample plots of Spartina densiflora at Otamendi Natural Reserve (Buenos Aires Province, Argentina) from June 1996 to December 1997. Emergence, growth, senescence, survival, biomass dynamics, and tiller productivity were analyzed. These population attributes varied seasonally during the first study year, whereas variations were not detected in the subsequent year. Tiller density increased slightly during the first twelve months, attaining 2445±869 tillers·m−2, then remaining constant until the end of the study period. Senescence rate was lower than emergence rate on most sampling occasions. Cohorts originated in different seasons showed distinct survival curves. The life span of tillers was at least 18 months, Green biomass reached maximum values (771±410 g·m−2) during the first year but decreased during the last six months of observations. Annual NAPP was 1450±566 g·m−2·y−1. Average productivity rate was higher in the first year than in the second year. The decrease in the population density of tillers of S. densiflora and the increase in its senescence rate were more likely related to grazing and flooding than to population dynamics.

Effects of Thermal Heterogeneity and Egg Mortality on Differences in the Population Dynamics of Aedes aegypti (Diptera: Culicidae) Over Short Distances in Temperate Argentina

ABSTRACT In temperate regions, the seasonal dynamics of Aedes aegypti (L.) (Diptera: Culicidae) is mainly influenced by temperature, whereas the probability of local extinction depends on the egg mortality during the cold season. The aim of the current study was to assess the importance of temperature and egg mortality in determining the differences in the oviposition dynamics of Ae. aegypti between favorable and less favorable areas in Buenos Aires City (Argentina). Year-round temperature dynamics were monitored, and oviposition dynamics were experimentally studied with ovitraps at two sites. Daily egg mortality values were calculated from a previous study performed at the same sites. The relative contribution of the differences in temperature and egg mortality between sites to the oviposition dynamics was assessed by means of a mathematical stochastic population dynamics model for Ae. aegypti. The results showed higher temperature and lower daily egg mortality at the site where higher oviposition activity was recorded. A larger influence of temperature than of egg mortality on population abundance during most of the activity season was detected in the results of the simulations. Our results showed a temperature gradient that relates to the distance to the Río de la Plata river and contributes to explaining the spatial heterogeneity in Ae. aegypti population abundances previously reported. The hypothesis of local extinctions because of egg mortality during the winter was not supported by the present analysis. The differences between field oviposition dynamics and simulation results suggest that rainfall might also be an important variable under extremely dry conditions.

Hatching Response of Aedes aegypti (Diptera: Culicidae) Eggs at Low Temperatures: Effects of Hatching Media and Storage Conditions

ABSTRACT In temperate regions, Aedes aegypti (L.) (Diptera: Culicidae) populations remain in the egg stage during the cold season. To ensure the start of a new breeding season, eggs should hatch at the beginning of a favorable period. The aim of the current study was to investigate the hatching response of two Ae. aegypti egg batches collected and stored for 3 mo under different conditions, to different low immersion temperatures. Two different hatching media (water and yeast solution) were used for the first batch and only one (water) for the second egg batch. Eggs were immersed for 8 d, during which the number of hatched eggs was recorded daily. The proportion of hatched eggs, delay of the hatching response, proportion of dead larvae, and proportion of remaining eggs within the first egg batch were compared between the two hatching media at each temperature. These parameters also were compared between the two batches immersed in water. Hatching rates were higher and faster in the yeast solution. The hatching response was lower at lower immersion temperatures and among eggs stored under field conditions at colder temperatures (second batch). Among the eggs stored in the laboratory (first batch), older eggs exhibited lower hatching response. The proportion of dead larvae was higher in the yeast solution and in the eggs stored in the laboratory. The conditions that triggered a lower hatching response led to higher proportions of remaining eggs, allowing the population to maintain an egg bank for future favorable opportunities.

Cold Season Mortality Under Natural Conditions and Subsequent Hatching Response of Aedes (Stegomyia) aegypti (Diptera: Culicidae) Eggs in a Subtropical City of Argentina

ABSTRACT In temperate and subtropical regions, populations of Aedes (Stegomyia) aegypti (L.) survive unfavorable winter conditions in the egg stage. Knowing their survival rates can be of great interest for the health authorities in charge of control activities. In this study, we analyzed the mortality of Ae. aegypti eggs exposed to the cold season as well as their hatching patterns under laboratory conditions in the city of Resistencia, Chaco, Argentina. The mortality rate was 48.6%. No statistically significant differences were observed in the mortality of eggs exposed at different sites. Hatching response differed significantly among the successive postexposure immersions, with the highest proportion of hatched eggs during the first immersion. These results show that the mortality rate of Ae. aegypti eggs exposed to the cold season in a subtropical city of Argentina was higher than those from temperate climate region. The additional mortality of eggs in our study might be related to fungal development (an unexpected event), which was not observed in research in temperate climate. The hatching pattern observed in this study ensures a rapid increase of the population at the beginning of the favorable breeding season, but it also maintains a batch with delayed hatching eggs, posing a risk for the community.

Effect of habitat complexity on the predation of Buenoa fuscipennis (Heteroptera: Notonectidae) on mosquito immature stages and alternative prey

ABSTRACT: Notonectids are well-known predators in aquatic habitats, where mosquito larvae, chironomids, and cladocerans constitute their main diet. Our purpose was to assess the effect of structural complexity on the predatory ability of Buenoa fuscipennis, a common predator in aquatic habitats of Buenos Aires city (Argentina). Buenoa fuscipennis showed type 2 functional responses in both the presence and absence of prey refuge and no differences in attack rate or handling time between refuge treatments. Regarding mosquito size classes, B. fuscipennis exhibited a significantly higher preference for 2nd instar larvae and no predation on pupae. In the presence of mosquito larvae and alternative prey, B. fuscipennis preferred mosquitoes over chironomid larvae and adult cladocerans over mosquito larvae. No switching behavior was detected in our experiments. Habitat structure only slightly affected the predators consumption rates on mosquito larvae. Overall, preference for prey did not vary with the presence of refuge, except for the preference for mosquitoes over chironomid larvae, which was significantly decreased in the presence of refuge as a consequence of reduced predation on mosquito larvae. The results suggest that B. fuscipennis could efficiently control mosquitoes in structurally simple habitats where chironomids are the most abundant alternative prey but not in temporary pools where cladocerans are abundant.

Prevalence of Parasitism and Adult Survival Time of Aedes albifasciatus (Diptera: Culicidae) Parasitized by Strelkovimermis spiculatus (Nematoda: Mermithidae)

ABSTRACT: We described the carryover of Strelkovimermis spiculatus (Poinar and Camino) (Nematoda: Mermithidae) from mosquito larvae, the primary site of maturation, to adults. We analyzed the survival time of male and female Aedes albifasciatus (Macquart) (Diptera: Culicidae) parasitized by S. spiculatus, the time of emergence of nematodes from adult mosquitoes, and the state of parasitism in the same mosquito cohorts during the immature stages. Mosquito larvae with single and multiple parasitism (up to 11 parasites) were observed. The mortality of mosquito larvae and adults was produced in all cases where at least one mermithid emerged. The mortality of S. spiculatus showed an increasing trend in mosquito larvae with larger numbers of nematodes and was higher in larvae parasitized by eight or more nematodes. Maximum survival of parasitized adult females of Ae. albifasciatus was 38 days, while non-parasitized adult males and females survived 39 and 41 days, respectively. Strelkovimermis spiculatus mortality was observed in Ae. albifasciatus larvae with single or multiple parasitisms. The spread of mermithid parasitism in adult mosquito populations is discussed.

Effects of scarcity and excess of larval food on life history traits of Aedes aegypti (Diptera: Culicidae)

ABSTRACT: Few studies have assessed the effects of food scarcity or excess on the life history traits of Aedes aegypti (L.) (Diptera: Culicidae) independently from larval density. We assessed immature survival, development time, and adult size in relation to food availability. We reared cohorts of 30 Ae. aegypti larvae from newly hatched to adult emergence with different food availability. Food conditions were kept constant by transferring larvae each day to a new food solution. Immature development was completed by some individuals in all treatments. The shortest development time, the largest adults, and the highest survival were observed at intermediate food levels. The most important effects of food scarcity were an extension in development time, a decrease in the size of adults, and a slight decrease in survival, while the most important effects of food excess were an important decrease in survival and a slight decrease in the size of adults. The variability in development time and adult size within sex and treatment increased at decreasing food availability. The results suggest that although the studied population has adapted to a wide range of food availabilities, both scarcity and excess of food have important negative impacts on fitness.

Effects of Larval Density and Habitat Drying on Developmental Success of Ochlerotatus albifasciatus (Diptera: Culicidae) in Urban Rain Pools: Evidence From Field and Experimental Studies

ABSTRACT Ochlerotatus albifasciatus (Macquart) (Diptera: Culicidae) larvae develop synchronously after rainfall events in ephemeral or temporary pools, where they occasionally attain very high abundance. The aims of the current study were to analyze the response of life history parameters such as daily larval mortality, time to pupation, and adult size of Oc. albifasciatus to increasing larval density under controlled conditions, and to analyze the relationships of daily larval mortality with density and environmental variables (drying rate, temperature, and season) in urban rain pools in Buenos Aires, Argentina. An exponential increase in mortality was observed at high larval densities under controlled conditions. Development times and adult size (wing length) differed between males and females, and were also affected by density. Development times extended for 0.36 d for each order of magnitude of increase in larval density, and wing length decreased 0.0021 mm per additional larva in 600 cm2. Larval density in the field varied from <1 larva per square meter to nearly 1100 larvae per square meter. Daily larval mortality values in the field were variable (0.02–0.91), positively related to the drying rate, and exhibited seasonal differences. No significant relation with larval density or temperature was found in the field. It remains to be established whether the density-independent mortality observed in this study is a generalized pattern of Oc. albifasciatus populations in Buenos Aires Province or a pattern restricted only to urban habitats.