Quantifying species traits related to oviposition behavior and offspring survival in two important disease vectors

Oviposition site selection and subsequent offspring performance of Aedes aegypti in short- and long-term detritus accumulation conditions

The "oviposition preference-offspring performance" hypothesis proposes that females lay their eggs in habitats that maximize the fitness of their offspring. The aim of this study was to assess the oviposition site selection by Aedes aegypti females and the success of their larvae in habitats with different detritus accumulation times, under conditions representative of the natural spatial variability of detritus quality and quantity in the Metropolitan Area of Buenos Aires, Argentina. Two experiments were performed, one assessing oviposition site selection and the other analyzing developmental success. In both experiments, two levels of detritus accumulation time were compared, one with short-time detritus accumulation (2 weeks), and the other with long-time detritus accumulation (8 weeks). Naturally fallen detritus was used in both experiments, collected in ten sites across the Metropolitan Area of Buenos Aires. In the oviposition experiment, two contiguous ovitraps corresponding to each accumulation time were placed at each of the ten sites and the number of eggs received for each accumulation time was compared. In the development experiment, always 19 larvae were raised in containers of both accumulation times and overall performance was compared using an integrated index that considers survival, development time and female size. A large variability in the amount of detritus collected at the different sites was observed. The number of eggs was significantly higher in the long-time than in the short-time detritus accumulation containers, and approximately proportional to the amount of detritus in each ovitrap. The performance was not affected by the detritus accumulation time, but a better performance was detected in containers that received a higher amount of organic detritus, regardless of the accumulation time. Leaves were on average the most abundant type of detritus, with an average of 53 % of the total detritus collected. The amount of leaves added 2 weeks before hatching showed a positive effect on larval performance. Our results do not support the "oviposition preference-offspring performance" hypothesis, since Ae. aegypti females laid eggs in containers where larvae did not show a better performance. Moreover, at larval densities related to the number of eggs actually laid in each of the accumulation times, it is expected that the performance would be even worse in the most selected containers, due to the density-dependent effects of crowding. Since the results obtained reflect the natural heterogeneity of the environmental conditions in the region studied, they might be a fairly good indicator of what occurs in natural larval habitats.

Taking it with a grain of salt: tolerance to increasing salinization in Culex pipiens (Diptera: Culicidae) across a low-lying delta

BACKGROUND

Salinity, exacerbated by rising sea levels, is a critical environmental cue affecting freshwater ecosystems. Predicting ecosystem structure in response to such changes and their implications for the geographical distribution of arthropod disease vectors requires further insights into the plasticity and adaptability of lower trophic level species in freshwater systems. Our study investigated whether populations of the mosquito Culex pipiens, typically considered sensitive to salt, have adapted due to gradual exposure.

METHODS

Mesocosm experiments were conducted to evaluate responses in life history traits to increasing levels of salinity in three populations along a gradient perpendicular to the North Sea coast. Salt concentrations up to the brackish-marine transition zone (8 g/l chloride) were used, upon which no survival was expected. To determine how this process affects oviposition, a colonization experiment was performed by exposing the coastal population to the same concentrations.

RESULTS

While concentrations up to the currently described median lethal dose (LD50) (4 g/l) were surprisingly favored during egg laying, even the treatment with the highest salt concentration was incidentally colonized. Differences in development rates among populations were observed, but the influence of salinity was evident only at 4 g/l and higher, resulting in only a 1-day delay. Mortality rates were lower than expected, reaching only 20% for coastal and inland populations and 41% for the intermediate population at the highest salinity. Sex ratios remained unaffected across the tested range.

CONCLUSIONS

The high tolerance to salinity for all key life history parameters across populations suggests that Cx. pipiens is unlikely to shift its distribution in the foreseeable future, with potential implications for the disease risk of associated pathogens.

Influence of natural and non-natural diets on the fitness and rearing of Pectinophora gossypiella Saunders

In order to develop integrated management approaches for Pectinophora gossypiella, basic studies are crucial. The two-sex life table is the most important tool for describing the fitness and population parameters of both sexes (male and female) of an insect, while the traditional life table only explains the female sex of an insect. However, no study has reported on the biology of P. gossypiella using two-sex life table tools. Therefore, this study explains the rearing dynamics of P. gossypiella on a cotton seed-based artificial diet and a natural diet (mature cotton bolls). According to the results, the oviposition period of P. gossypiella was recorded to be longer on the artificial diet (9.07 ± 0.24) compared to the natural diet (7.40 ± 0.11). The total fecundity of P. gossypiella was greater on the artificial diet (125.94 ± 3.06) in comparison to the natural diet (60.37 ± 1.10). The population parameters, including intrinsic rate of increase, finite rate of increase, gross reproductive rate, and net reproductive rate of P. gossypiella were highest on the artificial diet in comparison to the natural diet. This study concluded that the cotton seed-based artificial diet was most suitable for the rearing of P. gossypiella. In the future, P. gossypiella may be studied in depth in light of the findings in this study.

The Role of Tire Leachate in Condition-Specific Competition and the Persistence of a Resident Mosquito from a Competitively Superior Invader

(1) Background: Condition-specific competition, when the outcome of competition varies with abiotic conditions, can facilitate species coexistence in spatially or temporally variable environments. Discarded vehicle tires degrade to leach contaminants into collected rainwater that provide habitats for competing mosquito species. We tested the hypothesis that more highly degraded tires that contain greater tire leachate alters interspecific mosquito competition to produce a condition-specific advantage for the resident, Culex pipiens, by altering the outcome of competition with the competitively superior invasive Aedes albopictus. (2) Methods: In a competition trial, varying densities of newly hatched Ae. albopictus and Cx. pipiens larvae were added to tires that had been exposed to three different ultraviolet (UV)-B conditions that mimicked full-sun, shade, or no UV-B conditions in the field. We also measured Cx. pipiens and Ae. albopictus oviposition preference among four treatments with varying tire leachate (high and low) and resources (high and low) amounts to determine if adult gravid females avoided habitats with higher tire leachate. (3) Results: We found stronger competitive effects of Cx. pipiens on the population performance and survival of Ae. albopictus in tires exposed to shade and full-sun conditions that had higher concentrations of contaminants. Further, zinc concentration was higher in emergent adults of Ae. albopictus than Cx. pipiens. Oviposition by these species was similar between tire leachate treatments but not by resource amount. (4) Conclusions: These results suggest that degraded tires with higher tire leachate may promote condition-specific competition by reducing the competitive advantage of invasive Ae. albopictus over resident Cx. pipiens and, combined with Cx. pipiens' preferential oviposition in higher resource sites, contribute to the persistence of the resident species.