The stakes behind this seemingly modest insect are considerable. Culicoides biting midges, some of which measure barely a millimeter in length, are the principal transmitters of bluetongue virus, African horse sickness virus, and a suite of Simbu serogroup viruses including Akabane, Aino, and Peaton viruses. Bluetongue has wrought hundreds of millions of dollars in losses across European ruminant industries, while African horse sickness carries mortality rates in susceptible equine populations that can approach ninety percent. Simbu serogroup viruses, for their part, are notorious for causing congenital deformities in cattle, sheep, and goats when pregnant animals are infected. Because there are no curative treatments for these diseases and vaccination strategies vary in coverage and effectiveness, surveillance of the vectors themselves remains a cornerstone of veterinary public health. Knowing which midge species dominate at which sites, in which months, and in which years is the foundation upon which risk models, vaccination timing, and movement restrictions are built.

The research team, led by Jong-Uk Jeong and Jihun Ryu of the Foreign Animal Disease Division at the Animal and Plant Quarantine Agency in Gimcheon, with corresponding authors In-Soon Roh and SeEun Choe, structured their surveillance with unusual temporal density. Culicoides were collected biweekly from May through October—the window of peak midge activity on the Korean peninsula—during five consecutive field seasons. Trapping took place at seven cattle sheds and two horse racetracks, a deliberate mix of facility types designed to capture midge communities associated with both ruminants and equines, the two host groups most threatened by the viruses under investigation. Ultraviolet blacklight traps, the standard tool of Culicoides surveillance, were deployed at each site, yielding a total of 476 collection events over the study period. Of these, 277 produced Culicoides specimens, while 199 traps returned nothing, a pattern that itself carries ecological information about the patchiness of midge activity.

In total, 41,259 individual midges were collected, and each was identified morphologically using the taxonomic characters of wing patterns, palpal morphology, and antennal ratios that distinguish Culicoides species. Where morphology alone was insufficient to resolve an identification, the team turned to molecular barcoding, sequencing the mitochondrial cytochrome c oxidase subunit I gene—the COI marker that has become the workhorse of insect DNA taxonomy—to confirm species assignments. This combination of classical taxonomy and molecular confirmation is critical because misidentified vectors can silently corrupt surveillance data, leading risk assessments astray. The rigor paid off in a strikingly clear result: the Korean livestock-associated midge fauna proved overwhelmingly dominated by just three species, Culicoides arakawae, Culicoides punctatus, and Culicoides oxystoma, which together accounted for 95.1 percent of all individuals collected. The remaining species, though present, were numerically marginal.

One of the study’s most important contributions lies in its demonstration of how dramatically midge populations fluctuate from year to year. Mean abundance per collection event peaked during 2021 and 2022 and then fell substantially during the 2023, 2024, and 2025 seasons. Such swings are not merely academic; they have direct implications for how surveillance programs are evaluated. A single-season or single-site study, the authors note, can easily be misled by the idiosyncrasies of one year’s weather, one site’s microhabitat, or one farm’s management practices. Only by integrating data across multiple years and multiple locations can the underlying structure of a vector community be distinguished from its noisy year-to-year expression. This is precisely why multi-year datasets of this kind remain rare and why the Korean program’s persistence is scientifically valuable.

The temporal dimension extended to the composition of the community as well as its size. The relative proportions of the dominant species changed among years, meaning that even the identity of the primary vector candidates at a given site cannot be assumed to remain constant over time. When the researchers analyzed dominance at the level of individual sites, a consistent pattern emerged: most locations were dominated by either C. arakawae or C. punctatus, with C. oxystoma and other species playing secondary roles. This site-level dominance structure suggests that local environmental conditions—perhaps host availability, breeding substrate, moisture, or farm-level management—exert a stronger organizing force on midge communities than the regional forces that drive annual variation.

To test this formally, the team employed two complementary multivariate techniques that have become standard in community ecology. Permutational multivariate analysis of variance, or PERMANOVA, applied to Bray–Curtis dissimilarity matrices—a measure of how different two communities are in species composition and abundance—revealed that site effects on community composition exceeded year effects. In other words, where you trap matters more than when you trap, at least at the scale of this dataset. Non-metric multidimensional scaling, or NMDS, was used as a supplementary ordination tool to visualize these patterns, with the event-level and site-year-level ordinations presented as diagnostic figures rather than independent inferential tests. The NMDS analysis, based on the eight species whose total abundance exceeded one hundred individuals, showed clear clustering of collection events by dominant taxon, reinforcing the picture of a community structured primarily by local conditions.

The seasonal timing of each dominant species adds another layer of practical value. Mean seasonal abundance peaked in June for C. punctatus, in August for C. arakawae, and in September for C. oxystoma—a staggered phenology that effectively extends the window of vector risk across the entire summer and early autumn. For veterinarians and animal health officials, this staggered peak has concrete consequences. Bluetongue vaccination campaigns, movement restrictions, and vector control measures such as insecticide-treated shielding of livestock housing would ideally be timed to the local dominant species’ activity curve. A farm dominated by C. punctatus faces its highest-risk period in early summer, while one dominated by C. oxystoma remains at risk well into September. A one-size-fits-all national calendar, the data suggest, may obscure important regional and site-level differences in transmission risk.

The virological screening, meanwhile, delivered the study’s most consequential negative result. All 766 Culicoides pools—groups of midges pooled by site, date, and species for testing—were analyzed by reverse transcription polymerase chain reaction, or RT-PCR, for the RNA of bluetongue virus, African horse sickness virus, and Simbu serogroup viruses. Not a single pool tested positive. Over five years and more than 41,000 midges, none of the screened arboviruses was detected in the vector population at these livestock-associated sites. The authors are appropriately measured in their interpretation: a negative screening result does not prove the absence of viral circulation in the country or in wild vector populations beyond the sampled sites, and the study’s design—a surveillance dataset rather than a spatially balanced nationwide survey—means the finding cannot be generalized nationally. Still, the absence of viral RNA over such a sustained effort provides a meaningful baseline against which future detections can be judged, and it demonstrates that the surveillance pipeline, from trap to molecular assay, functions reliably.

The study is candid about its limitations, which strengthens rather than weakens its value. The authors explicitly state that the dataset does not constitute a spatially balanced nationwide survey; the nine sites were operational surveillance locations, not a statistically representative sample of Korean livestock environments. Nevertheless, they argue that integrating multi-year livestock-associated collection data provides a descriptive baseline for the spatio-temporal occurrence of dominant Culicoides species that can directly support future vector surveillance and risk assessment. Funding for the work came from the Animal and Plant Quarantine Agency of the Republic of Korea, and the authors acknowledge the regional Animal Disease Control centers, the Korea Racing Authority, and the many livestock producers who granted access to their facilities.

For the international community of vector biologists and veterinary epidemiologists, the Korean dataset speaks to a broader question: how should countries with limited surveillance resources allocate them? The finding that site effects dominate over year effects suggests that establishing a network of well-characterized sentinel sites, each monitored consistently over many years, may be more informative than rotating trapping efforts across many locations for short bursts. The staggering of seasonal peaks among the three dominant species suggests that sentinel trapping should extend across the full May-to-October window rather than concentrating on a single assumed peak month. And the clean virological results underscore the importance of screening not just for one virus but for the full panel of pathogens a vector community is capable of transmitting, since the same midges that carry bluetongue are candidates for Simbu serogroup circulation.

As climate change alters temperature and precipitation patterns across East Asia, the phenology, abundance, and geographic distribution of Culicoides species are all expected to shift, potentially lengthening transmission seasons and allowing vector species to colonize new territory. In that context, the five-year baseline established by the Korean team is not merely a record of what was, but a reference point against which future change will be measured. Whether the low abundances of 2023 through 2025 represent a temporary dip or the beginning of a longer trend, and whether the current virological silence in Korean midge populations persists, will be answerable only through continued surveillance of exactly the kind this study exemplifies—patient, methodical, and sustained across the years.

Researchers from the Leibniz-Centre for Agricultural Landscape Research and the Friedrich-Loeffler-Institut, Germany’s federal research institute for animal health, deployed UV-light traps weekly inside cattle, sheep, goat and horse stables at between 12 and 55 locations across Germany over four consecutive winters, from November 1 to March 31 in the years 2021 through 2025. The monitoring window deliberately straddled the dramatic emergence of bluetongue virus serotype 3 (BTV-3), which swept through German ruminant flocks starting in late 2023, giving the team a rare opportunity to compare midge ecology and virus circulation under both endemic and outbreak conditions.

The scale of the trapping effort allowed the researchers to assemble an unusually detailed picture of winter biting midge activity. Collected female midges of the genus Culicoides — the principal biological vectors of bluetongue virus, Schmallenberg virus and epizootic hemorrhagic disease virus — were morphologically assigned to two major taxonomic groupings, the Obsoletus Group and the Pulicaris Complex, both of which contain proven or suspected vector species. In total, 1,581 pools comprising 46,358 female midges were screened by real-time reverse-transcription polymerase chain reaction (real-time RT-PCR) for all three viruses.

The seasonal pattern that emerged was strikingly lopsided. November and March together accounted for a remarkable 95 percent of all midges captured during the winter monitoring periods, while the true mid-winter months of December through February yielded just 5 percent of the total. January and February were the quietest of all, each contributing a mere 1 percent of captures. The researchers also noted pronounced variation between sites and months, reflecting the patchy microclimates of livestock housing — stables, with their residual warmth, Organic matter and resident animal hosts, can sustain small populations of adult midges long after outdoor conditions have become lethal.

Activity peaks in March 2024 and March 2025 coincided with mild temperatures and, in part, with the inclusion of new monitoring sites, and the authors suggest that one or both factors helped drive those surges. Temperature data recorded alongside the trapping allowed the team to relate midge activity directly to thermal conditions, an important analytical step because Culicoides development, activity and — critically — the capacity of arthropod-borne viruses to replicate within their insect hosts are all strongly temperature-dependent. Below a certain thermal threshold, even if a midge ingests virus-laden blood, the virus cannot complete its extrinsic incubation period, meaning the insect never becomes infectious.

Perhaps the most consequential finding concerns blood-feeding. Although blood-fed females were very rare during the depths of mid-winter, their abundance at other points in the cold season often exceeded the European Union threshold used to define a so-called “vector-free period” — a threshold of fewer than five parous (egg-laying-experienced) female Culicoides per trap per night. Parous females are the epidemiologically dangerous ones, because only females that have already taken a blood meal can have acquired and potentially transmitted virus. The regular presence of blood-fed midges above the regulatory threshold in livestock stables, the researchers conclude, directly challenges the concept of a true vector-free period in winter as it is currently applied.

Yet the virological results tell a very different story from the entomological ones. Across the entire four-year dataset — spanning 1,581 pools and nearly 47,000 midges, including two winters during an active BTV-3 outbreak — only a single pool of Culicoides, collected in November 2024, tested positive for BTV-3 RNA. Every other pool was negative for bluetongue virus, Schmallenberg virus and epizootic hemorrhagic disease virus alike. A single RNA-positive pool among tens of thousands of midges is, epidemiologically, a whisper rather than a signal, and the team interprets it as evidence that genuine virus circulation during the central European winter is, in their words, hardly existent, even under endemic or outbreak conditions.

Reconciling these two strands of evidence — abundant, feeding midges on one hand and an almost complete absence of detectable virus on the other — points to temperature as the decisive factor. Even when midges are active enough to bite livestock in the sheltered environment of a stable, winter temperatures in Germany generally remain too low to permit viral replication within the insect, breaking the transmission cycle regardless of vector presence. Combined with the overall scarcity of midges in mid-winter, the researchers conclude that the risk of BTV infection of ruminants via biting midges between December and March is negligible.

The findings nonetheless leave a significant biological puzzle unresolved: how does bluetongue virus survive the winter in central Europe at all? Overwintering is essential for the virus to re-emerge each season, and the new data effectively rule out sustained winter transmission by vectors. The authors propose two candidate mechanisms. One is that rare infection events in ruminants or vectors during winter — such as the single positive pool they detected in November — occasionally sustain low-level viremia in animals until spring, providing a seed for renewed transmission when temperatures rise. The other is that the virus persists by means currently unknown to science, which could include mechanisms within vectors or reservoir animals that have not yet been characterized. Distinguishing between these possibilities will be essential for improving models of BTV-3 recurrence and for designing surveillance strategies that catch the virus as it reawakens.

For veterinary authorities and farmers, the practical implications are double-edged. On one hand, the study supports treating the deep winter months as a period of negligible transmission risk, which is welcome news for the timing of animal movements, vaccination campaigns and other control measures. On the other hand, the documented presence of blood-fed female midges above the EU’s vector-free threshold means that blanket assumptions about winter safety inside livestock housing should be treated with caution, particularly in the shoulder months of November and March, when more than nine in ten winter midges were captured. Stable-dwelling midge populations, the study suggests, are a real if seasonally constrained feature of the European livestock landscape.

The research also fills a long-standing gap. Despite the central importance of winter vector ecology to seasonal risk assessment for Culicoides-borne diseases, data on winter activity of these midges have remained sparse, and information on virus circulation in winter-active vectors — under either endemic or outbreak conditions — has been almost non-existent. By combining four consecutive years of standardized weekly trapping, temperature recording and molecular virus screening, the German team has produced one of the first datasets robust enough to quantify both the insect and the virus sides of the winter equation simultaneously.

The work was supported by the German Federal Ministry of Agriculture, Food and Regional Identity through the Federal Office for Agriculture and Food, and the authors emphasize that the results are best understood as an integrated picture: vectors are present and feeding in winter, but thermal constraints keep virus replication — and therefore transmission — at levels that are functionally close to zero. As BTV-3 continues to reshape livestock disease management in northwestern Europe, understanding what the virus does between outbreaks may prove as important as understanding what it does during them.