Pond dyes are Culex mosquito oviposition attractants

Effects of soil on the development, survival, and oviposition of Culex quinquefasciatus (Diptera: Culicidae) mosquitoes

BACKGROUND

Water quality is known to influence the development and survival of larval mosquitoes, which affects mosquito-borne pathogen transmission as a function of the number of mosquitoes that reach adulthood and blood feed. Although water properties are known to affect mosquito development, few studies have investigated the link among soil properties, water quality, and mosquito development. Given the large number of ground-breeding mosquito species, this linkage is a potentially important factor to consider in mosquito ecology. In this study, we explored the effects of different soils on multiple life history parameters of the ground-breeding mosquito species Culex quinquefasciatus (Diptera: Culicidae).

METHODS

Cx. quinquefasciatus larvae were reared in water combined with different soil substrates (sandy, silt, or clay loam textures) at increasing soil to water volume ratios, with and without the addition of organic matter (fish food). Gravid mosquitoes were offered different soil-water extracts to investigate soil effects on oviposition preference.

RESULTS

Without the addition of organic matter, larval survival and development differed significantly among waters with different soil textures and volumes of substrate. Mosquitoes in water with clay loam soil survived longer and developed further than mosquitoes in other soil waters. Larvae survived for longer periods of time with increased volumes of soil substrate. Adding organic matter reduced the differences in larval survival time, development, and pupation among soil-water extracts. Adult female mosquitoes oviposited more frequently in water with clay loam soil, but the addition of organic matter reduced the soil effects on oviposition preference.

CONCLUSIONS

This study suggests soil composition affects larval mosquito survival and development, as well as the oviposition preference of gravid females. Future studies could differentiate abiotic and biotic soil features that affect mosquitoes and incorporate soil variation at the landscape scale into models to predict mosquito population dynamics and mosquito-borne pathogen transmission.

Biting the hand that feeds: Anthropogenic drivers interactively make mosquitoes thrive

Anthropogenic stressors on the environment are increasing at unprecedented rates and include urbanization, nutrient pollution, water management, altered land use and climate change. Their effects on disease vectors are poorly understood. A series of full factorial experiments investigated how key human induced abiotic pressures, and interactions between these, affect population parameters of the cosmopolitan disease vector, Culex pipiens s.l. Selected pressures include eutrophication, salinity, mean temperature, and temperature fluctuation. Data were collected for each individual pressure and for potential interactions between eutrophication, salinization and temperature. All experiments assessed survival, time to pupation, time to emergence, sex-ratio and ovipositioning behavior. The results show that stressors affect vector survival, may speed up development and alter female to male ratio, although large differences between stressors exist to quite different extents. While positive effects of increasing levels of eutrophication on survival were consistent, negative effects of salinity on survival were only apparent at higher temperatures, thus indicating a strong interaction effect between salinization and temperature. Temperature had no independent effect on larval survival. Overall, increasing eutrophication and temperatures, and the fluctuations thereof, lowered development rate, time to pupation and time to emergence while increasing levels of salinity increased development time. Higher levels of eutrophication positively impacted egg-laying behavior; the reverse was found for salinity while no effects of temperature on egg-laying behavior were observed. Results suggest large and positive impacts of anthropogenically induced habitat alterations on mosquito population dynamics. Many of these effects are exacerbated by increasing temperatures and fluctuations therein. In a world where eutrophication and salinization are increasingly abundant, mosquitoes are likely important benefactors. Ultimately, this study illustrates the importance of including multiple and combined stressors in predictive models as well as in prevention and mitigation strategies, particularly because they resonate with possible, but yet underdeveloped action plans.

Effects of Color and Light Intensity on the Foraging and Oviposition Behavior of Culex pipiens biotype molestus Mosquitoes

Mosquitoes are the most important vector of arboviruses; thus, controlling mosquito population is a key point for controlling these diseases. Two major factors that influence mosquito population size are the availability of blood hosts and suitable oviposition sites. Behavioral mechanisms by which Culex pipiens biotype molestus mosquitoes locate their hosts or oviposition sites are influenced by physical and chemical factors. The present study evaluated the impact of the colors (for human eyes) red, green, blue and yellow in combination with different light intensities on preferences for oviposition and foraging sites under laboratory conditions. We identified the color red as the overall favored color for both target behaviors, which was only surpassed by black as the foraging stimulus. Altogether, we described two new inexpensive and simple bioassays, which can be used as a mosquito-tracking method for behavioral tests and as an oviposition trap to monitor Culex pipiens biotype molestus populations.

Disease Vector Relative Spatio-Temporal Abundances to Water Bodies and Thermal Fitness Across Malaria Endemic Semi-Arid Areas

The biophysical environment plays an important role in the spatio-temporal abundance and distribution of mosquitoes. This has implications for the spread of vectors and diseases they cause across diverse landscapes. Here, we assessed vector mosquito abundances in relation to large water bodies, from three malaria districts in a semi-arid environment. Furthermore, we explored thermal limits to activity of the dominant and most medically important malaria vector across malaria-endemic areas. Mosquitoes were trapped near permanent water bodies across different districts. Critical thermal limits (critical thermal-maxima and -minima) to activity of wild adults and 4th instar larvae Anopheles arabiensis (Diptera: Culicidae) were assessed. Our results showed that Anopheles spp. dominate mosquito communities across all three districts, but that their numbers were far greater in Okavango than in other regions. At the Okavango sites, the numbers of Anopheles spp. decreased with distance from main water source. Anopheles spp. sampled in this region comprised Anopheles gambiae (Giles,1902) and Anopheles funestus (Giles, 1900) species complexes, with the former dominating in numbers. Thermal activity assays showed An. arabiensis females had wider thermal tolerance windows than males while larval thermal activity limits differed significantly across space. These results confirm that the Okavango district should be prioritized for vector control measures. Moreover, intervention strategies should consider recommendations for proximity effects to large water bodies, given the differential risk associated with distance from water. The wider thermal window on female vectors has implications for possible future malaria transmission and diverse habitat utilization under changing environments.

Sink trap: duckweed and dye attractant reduce mosquito populations

Duckweeds, such as Lemna minor Linnaeus (Alismatales: Lemnaceae), are common in aquatic habitats and have been suggested to reduce larval mosquito survivorship via mechanical and chemical effects. Furthermore, pond dyes are used increasingly in aquatic habitats to enhance their aesthetics, although they have been shown to attract mosquito oviposition. The present study examined the coupled effects of L. minor and black pond dye on the oviposition selectivity of Culex pipiens Linnaeus (Diptera: Culicidae) mosquitoes in a series of laboratory choice tests. Subsequently, using outdoor mesocosms, the combined influence of duckweed and pond dye on mosquito abundances in aquatic habitats was quantified. Mosquitoes were strongly attracted to duckweed, and oviposited significantly greater numbers of egg rafts in duckweed-treated water compared with untreated controls, even when the duckweed was ground. The presence of pond dye interacted with the duckweed and further enhanced positive selectivity towards duckweed-treated water. The presence of duckweed caused significant and sustained reductions in larval mosquito numbers, whereas the relative effects of dye were not evident. The use of floating aquatic plants such as duckweed, combined with dye, may help reduce mosquito populations via the establishment of population sinks, characterized by high rates of oviposition coupled with high levels of larval mortality.

The effect of pond dyes on oviposition and survival in wild UK Culex mosquitoes

British Culex pipiens complex [Culex pipiens sensu lato) mosquito distribution, abundance, and potential for disease transmission are intimately linked to their environment. Pond and lake dyes that block light to restrict algal photosynthesis are a relatively new product assumed to be an environmentally friendly since they are based on food dyes. Their use in urban garden ponds raises questions linked to mosquito oviposition, since coloured water can be an attractant. Culex (mostly pipiens) is commonly found in UK gardens and is a potential vector of viruses including the West Nile Virus (WNV). Any factors that significantly change the distribution and population of Cx pipiens could impact future risks of disease transmission. A gravid trap was used to catch female Cx pipiens mosquitoes for use in oviposition choice tests in laboratory and semi-field conditions. Two types of pond dye, blue and shadow (which looks slightly red), were tested for their impact on oviposition and survival of wild caught Cx pipiens. There were no significant differences in the number of egg batches laid when gravid mosquitoes were given a choice between either blue dye and clear water or shadow dye and clear water indicating that these dyes are not attractants. Larvae hatched from egg batches laid by wild-caught gravid females were used to measure survival to adulthood with or without dye, in a habitat controlled to prevent further colonisation. The experiment was run twice, once in the summer and again in the autumn, whereas the dyes had no impact on emergence in the summer, there were highly significant reductions in emergence of adults in both dye treated habitats in the autumn. Containers with or without shadow dye were placed outside to colonise naturally and were sampled weekly for larvae and pupae over a 6 month period through summer and autumn. There was a significant negative effect of shadow dye on pupal abundance in a three week period over the summer, but otherwise there was no effect. It is likely that population abundance and food was a more powerful factor for mosquito survival than the dye.