A Mondrian matrix of seasonal patterns of Culicoides nulliparous and parous females at different latitudes in Spain

Unraveling the relationships between midge abundance and incidence, microbial communities, and soil and water properties in a protected natural tallgrass prairie

BACKGROUND

Biting midges (Culicoides spp.) are small blood-feeding flies (Diptera: Ceratopogonidae) that transmit numerous pathogens that impact animal and human health. The larvae of several Culicoides spp., including vectors, are often found in organically enriched, moist soil habitats. However, the influence of biotic (e.g., cohabiting fauna, potential prey taxa) and abiotic factors (e.g., soil or water properties, time) on abundance and incidence of larval Culicoides in natural habitats is not well understood. This study evaluated the relationships between bacterial and protistan communities, soil and water physicochemical properties, and the abundance and incidence of Culicoides species in larval habitats at the Konza Prairie Biological Station in Kansas.

METHODS

Soil and water samples were collected monthly from March 2021 to February 2022 from four midge larval habitat sites, including three grazed (low-production cattle-grazed (LPCG), high-production cattle-grazed (HPCG), and bison-grazed sites) and one formally ungrazed (i.e., no managed large mammals) site. Midge incidence and abundance were evaluated using emergence assays, which assessed the number of adults emerging from collected soil samples, and bacterial and protistan communities in these samples were characterized through amplicon sequencing of the 16S and 18S rRNA genes. Physicochemical properties of water and soil were also analyzed.

RESULTS

Irrespective of site, the highest midge abundance was reported in warmer months between March and September, except June. Moreover, the greatest midge abundance, incidence, and prevalence occurred at the HPCG and bison-grazed sites, which had a persistent water source. Specific lineages of bacterial and protistan communities, soil texture, organic matter, and total dissolved solids in water samples were directly associated with the abundance of Culicoides spp. that emerged from soil samples.

CONCLUSIONS

Both biotic (bacterial and protistan communities, presence of host animals), and abiotic (soil and water properties, season) factors affected the abundance and incidence of Culicoides spp. in natural habitats. The results presented in this study expand our understanding of the ecological and environmental factors influencing larval ecology of biting midges in natural developmental substrates. These insights have important implications for identifying potential developmental sites, which can be used for targeted management of Culicoides.

The use of path analysis to determine effects of environmental factors on the adult seasonality of Culicoides (Diptera: Ceratopogonidae) vector species in Spain

Culicoides biting midges (Diptera: Ceratopogonidae) are the main vectors of livestock diseases such as bluetongue (BT) which mainly affect sheep and cattle. In Spain, bluetongue virus (BTV) is transmitted by several Culicoides taxa, including Culicoides imicola, Obsoletus complex, Culicoides newsteadi and Culicoides pulicaris that vary in seasonality and distribution, affecting the distribution and dynamics of BT outbreaks. Path analysis is useful for separating direct and indirect, biotic and abiotic determinants of species' population performance and is ideal for understanding the sensitivity of adult Culicoides dynamics to multiple environmental drivers. Start, end of season and length of overwintering of adult Culicoides were analysed across 329 sites in Spain sampled from 2005 to 2010 during the National Entomosurveillance Program for BTV with path analysis, to determine the direct and indirect effects of land use, climate and host factor variables. Culicoides taxa had species-specific responses to environmental variables. While the seasonality of adult C. imicola was strongly affected by topography, temperature, cover of agro-forestry and sclerophyllous vegetation, rainfall, livestock density, photoperiod in autumn and the abundance of Culicoides females, Obsoletus complex species seasonality was affected by land-use variables such as cover of natural grassland and broad-leaved forest. Culicoides female abundance was the most explanatory variable for the seasonality of C. newsteadi, while C. pulicaris showed that temperature during winter and the photoperiod in November had a strong effect on the start of the season and the length of overwinter period of this species. These results indicate that the seasonal vector-free period (SVFP) in Spain will vary between competent vector taxa and geographic locations, dependent on the different responses of each taxa to environmental conditions.

Development and lifespan of Culicoides obsoletus s.s. (Meigen) and other livestock-associated species reared at different temperatures under laboratory conditions

Culicoides Latreille (Diptera: Ceratopogonidae) transmit arboviruses affecting wild and domestic ruminants such as bluetongue (BTV) and Schmallenberg virus (SBV). The sub-adult development and lifespan of Culicoides obsoletus s.s. (Meigen), Culicoides circumscriptus Kieffer and Culicoides paolae Boorman were examined at three different temperatures under laboratory conditions. Insects were collected from field between spring and autumn 2015 in two livestock farms located in Majorca (Spain). Gravid females were held individually at 18, 25 or 30 °C. Low temperatures increased the adult lifespan, time to oviposit and rate of development, whereas high temperatures increased the number of eggs, successful pupation and adult emergence as well as the larvae growth rate. The results showed that C. obsoletus s.s. have optimum development at 18 °C, whereas the optimal rearing temperature for C. circumscriptus and C. paolae was under warmer conditions of 25-30 °C. Variations in temperature/humidity and assays with different materials and substrates for oviposition should be considered in future studies. Understanding the requirements of the different species of Culicoides optimizing the results should be of special interest for predicting environmental change effects on these species, in addition to determining the rearing conditions for candidate Culicoides vectors.

Environmental Drivers of Adult Seasonality and Abundance of Biting Midges Culicoides (Diptera: Ceratopogonidae), Bluetongue Vector Species in Spain

Bluetongue is a viral disease affecting wild and domestic ruminants transmitted by several species of biting midges Culicoides Latreille. The phenology of these insects were analyzed in relation to potential environmental drivers. Data from 329 sites in Spain were analyzed using Bayesian Generalized Linear Mixed Model (GLMM) approaches. The effects of environmental factors on adult female seasonality were contrasted. Obsoletus complex species (Diptera: Ceratopogonidae) were the most prevalent across sites, followed by Culicoides newsteadi Austen (Diptera: Ceratopogonidae). Activity of female Obsoletus complex species was longest in sites at low elevation, with warmer spring average temperatures and precipitation, as well as in sites with high abundance of cattle. The length of the Culicoides imicola Kieffer (Diptera: Ceratopogonidae) female adult season was also longest in sites at low elevation with higher coverage of broad-leaved vegetation. Long adult seasons of C. newsteadi were found in sites with warmer autumns and higher precipitation, high abundance of sheep. Culicoides pulicaris (Linnaeus) (Diptera: Ceratopogonidae) had longer adult periods in sites with a greater number of accumulated degree days over 10°C during winter. These results demonstrate the eco-climatic and seasonal differences among these four taxa in Spain, which may contribute to determining sites with suitable environmental circumstances for each particular species to inform assessments of the risk of Bluetongue virus outbreaks in this region.