Managing incursions of *Vespa velutina nigrithorax* in the UK: an emerging threat to apiculture

The yellow-legged Asian hornet, Vespa velutina nigrithorax, is an invasive predator of invertebrates with a strong predilection for honey bees, threatening apiculture and pollinator health in Europe. After accidental introduction into France around 2004, the species spread rapidly (approximately 75–82 km/year in France; 12.4 km/year in Korea) and has colonised large areas of Western Europe, including Spain, Portugal, Belgium, Italy, and Germany, and three-quarters of France, as well as the Channel Islands. A single mated queen can produce thousands of individuals in a season (average ~6000 with sufficient protein resources), with late-season females (gynes) founding the next generation; French colonies can produce up to ~350 gynes in autumn. European honey bees (Apis mellifera) exhibit insufficient defensive responses compared to Asian honey bees (Apis cerana), leading to elevated predation, reduced foraging, and reported colony losses of 20–30% in France. In the UK, the first V. v. nigrithorax nest was found in Tetbury in 2016, and additional nests were subsequently discovered in geographically dispersed locations. This study aims to analyse UK-found nests to assess reproductive status, relatedness, and origin, informing incursion management and the risk of establishment.

Background literature documents rapid expansion of V. v. nigrithorax in Europe post-2004 via both natural spread and human-mediated dispersal, with colonisation of Spain (2010), Portugal and Belgium (2011), Italy (2012), and Germany (2014). Queens are highly fecund and mobile (>40 km/day reported), producing seasonal broods culminating in gynes; French colonies may produce up to ~350 gynes (September–November). Honey bee defensive behaviours differ between A. cerana and A. mellifera, with the latter exhibiting inadequate defence against hornet predation, contributing to substantial colony losses in invaded regions. Genetic studies indicate European populations show reduced diversity relative to Asian sources. Prior reports in Europe also note diploid male production (DMP) due to inbreeding and complementary sex determination, which can impact colony productivity without preventing invasion success. Predictive niche models suggest the UK climate is suitable for invasion, though colder conditions may reduce reproductive output.

Materials and sampling: All Asian hornet nests found in England up to the end of 2019 were included. Lone adults were also collected across several locations (Somerset 2016; Hull, Oxfordshire, Liskeard, Dungeness 2018; New Milton, Tenterden 2019). Nest detection, destruction, and collection: APHA officers conducted dusk-time destruction to maximize in-nest adult presence, using pesticide followed by overnight action, double-bagging, and freezing. Nests and any nearby hornets were frozen for a minimum of 72 h. Potential loss of adults during destruction was minimized and deemed unlikely to bias genetic analyses. Morphological assessments: Recorded numbers of adults, sex ratio, wet mass (Sartorius R2000; ±0.1 mg), nest and comb diameters, life stages present; teneral adults were identified as fully formed adults yet in sealed cells pre-hardening/coloration. Worker vs founder queen classification used weight thresholds from Rome et al. (wet weight limit 593 mg). DNA sampling and extraction: For each nest, at least 10 individuals per life stage (adults, teneral adults, pupae, larvae, eggs) were sampled. Hind legs were taken from adults/teneral/pupae; larvae were subsampled avoiding gut; eggs were whole. DNA extraction used QIAGEN DNeasy Blood & Tissue Kit. Microsatellite genotyping: Fifteen loci (R1-36, LIST2020, R1-80, R4-33, R4-114, R1-169, D2-185, LIST2018, D3-15, VMA6, LIST2015, R4-26, R1-75, R4-100, VMA8) were amplified in six multiplex PCRs (QIAGEN Multiplex kits). Cycling: 95°C 15 min; 35 cycles of 94°C 30 s, 59°C 90 s, 72°C 60 s; final elongation 60°C 30 min. Products were run on ABI 3031 with ROX 500 ladder. Alleles were scored in Geneious 9.1.4; error rates from repeat discrepancies. Kinship and ploidy inference: Parentage and sibship were inferred using COLONY (and manual checks). Ploidy inferred from marker zygosity (all-locus homozygosity taken as haploid). Allele size calibration was cross-checked with prior datasets. Allele frequencies per location were computed with CONVERT; genetic diversity indices (allelic diversity, observed and unbiased expected heterozygosity) were calculated in Genetix (excluding haploids). Lone individuals were assessed for compatibility with nearby nest parental genotypes; Factorial Correspondence Analysis (Genetix) was used to visualise clustering. Data analysis: For each nest, estimates included number of paternal genotypes, timing of haploid egg laying (based on life-stage ploidy), and presence/number of diploid males. Geographic and temporal associations among nests and lone individuals were tested by comparing inferred parental genotypes across years and sites to assess relatedness and potential descent.

- Surveillance 2016–2019 identified nine nests across England: Tetbury (2016; 1), Woolacombe (2017; 1), Fowey (2018; 2), New Alresford (2018; 1), Brockenhurst (2018; 1), Drayton Bassett (2019; 1), Christchurch (2019; 2). Follow-up monitoring detected no new nests in these regions in subsequent years. - None of the nests had produced new queens (gynes) by the time of destruction. - Nest size and brood structure: UK nests were small (diameters ~13–27 cm; typically ≤4 combs; Woolacombe had 7 combs, Tetbury 5). Compared to France (7–10 combs in September), UK nests were generally smaller. - Ploidy and diploid males: Diploid males were detected in multiple nests (Tetbury 9/9 genotyped males; Fowey 1: 3/3; Fowey 2: 10/10; New Alresford: 10/10). Brockenhurst adult males were haploid; many life stages there were haploid, suggesting worker-laid eggs or sperm depletion. - Reproductive timing: Estimated onset of haploid egg laying varied by nest (e.g., Fowey 2 ~30 Aug; New Alresford ~3 Sept; Brockenhurst ~29 Aug; Woolacombe ~2 Sept; Christchurch 1 ~1 Oct; Tetbury ~21 Sept). - Mating structure: Most nests were consistent with a single queen mated to few males: Tetbury (1 queen × 1 drone), Woolacombe (1 × 3), Fowey 1+2 (offspring of the same queen × 1 drone; primary/secondary pair), New Alresford (1 × 2), Brockenhurst (1 × 2), Drayton Bassett (1 × 1), Christchurch 1+2 (same queen × 1 drone; likely secondary/primary pair). - Genetic diversity: UK samples exhibited low diversity relative to France and Asia. Across UK nests, mean alleles per locus ~1.60–2.00; observed heterozygosity ~0.38–0.57; expected heterozygosity ~0.24–0.39. Allelic richness showed UK had 35 alleles (subset of France’s 60), and France’s alleles were a subset of Asia’s 178; no private UK alleles were detected. - Relatedness and origin: All UK nests’ allele profiles matched the European secondary colonisation pool, not Asia directly. No UK nest was the direct descendant of another UK nest (parental genotypes incompatible). Foundress queens/drones from nests within the same year were not full siblings (could be half-siblings). Individuals captured near nests (≤2 km) matched the local nest; most isolated captures (>15 km) were unrelated to recovered nests (exception: a Liskeard male compatible with Fowey, ~17 km away). Findings support multiple independent incursions from continental Europe rather than an established UK population.

Genetic evidence indicates UK incursions derive from continental Europe, with UK allele sets entirely nested within French variation, itself a subset of Asian diversity. The absence of direct descent among UK nests across years, and the lack of full-sibling relationships among foundresses within years, combined with isolated captures, support repeated independent introductions, likely via human-mediated transport rather than natural dispersal. UK nests were generally smaller than French nests in September, consistent with either environmental constraints (cooler UK climate) or later founding leading to limited seasonal growth. Queens in UK nests typically mated with one to two males (one case with three), fewer than averages reported in France, consistent with reduced mate diversity and leading to diploid male production (observed in most nests). While DMP can reduce colony efficiency, it has not prevented invasion success elsewhere. Intensive UK surveillance and rapid nest destruction, combined with genetic assignment of captured individuals to local nests, likely contributed to preventing establishment and spread. Altogether, the findings address the research questions by demonstrating low reproductive success (no gynes), low genetic diversity, and lack of continuity among nests, supporting management actions focused on early detection and eradication of repeated incursions.

Comprehensive dissection and microsatellite analysis of all UK-detected Vespa velutina nigrithorax nests (2016–2019) show that none had produced gynes, genetic diversity was low, many nests produced diploid males, and queens typically mated with few males. All nests traced to European secondary colonisation sources, and no nest was a direct descendant of another UK nest, indicating repeated independent introductions rather than an established population. Coupled with targeted surveillance and eradication, these incursions have not led to detectable local establishment. Continued rapid-response surveillance, genetic assignment of captured individuals, and public engagement are recommended to maintain early detection and prevent establishment. Future work could refine estimates of founding times, examine environmental constraints on nest growth and gyne production in UK climates, and integrate genomic tools to improve source attribution and incursion pathway analysis.

Adult hornet loss during nest destruction and removal likely reduced adult counts and could bias life-stage representation, though genetic analyses were considered robust. Some samples failed or were degraded (e.g., Woolacombe larvae), and one nest (Drayton Bassett) was too damaged for certain measurements. Diversity metrics were computed within closely related cohorts (siblings), potentially underestimating broader genetic variability. The precise location and context of queen mating could not be determined. While data strongly suggest multiple incursions without establishment, this inference is not conclusive; undetected nests may have occurred.