Interspecific Hierarchies, Aggressiveness & Body Size in Invasive Vespa velutina vs Native Korean Hornets

Background: Invasive Alien Species (IAS) & Vespa velutina

• IAS are expanding rapidly due to climate change + global trade, driving biodiversity loss and $\approx 70\ \text{billion US\$ yr}^{-1}$ in insect/arthropod damages.
  • Korean examples: black widow (Latrodectus hesperus), spotted lanternfly (Lycorma delicatula), frosted moth-bug (Metcalfa pruinosa), leaf-footed bug (Leptoglossus gonagra), black planthopper (Ricania speculum).
• Social insects are especially impactful because of: many workers, venomous stings, large nests.
• Yellow-legged hornet Vespa velutina nigrithorax (hereafter Vv)
  • Origin: southern China → Korea (2003 via shipping) → Japan (2012 Tsushima, 2015 Kyushu) → Europe (France 2004, then ES, PT, IT, DE, BE, CH, UK).
  • Impacts: removes $\sim 30\%$ of honey-bee colonies, highest nest-removal demand (>100,000 Korean events yr⁻¹), ≈15,000 sting injuries + ≈10 deaths yr⁻¹; 2 deaths in both France & Korea since invasion.
  • Designated “Ecological Disturbance Organism” (MOE Notice 2019-185).

Study Objectives

• Quantify interspecific hierarchies based on aggressiveness + body size between invasive Vv and five co-occurring native Korean hornets:
  1. Vespa simillima (Vs)
  2. Vespa mandarinia (Vm)
  3. Vespa analis (Va)
  4. Vespa crabro (Vc)
  5. Vespa dybowskii (Vd)
• Test hypotheses:
  • H₁ – Vv’s lower fighting ability/body size relative to natives slows its forest spread in Korea.
  • H₂ – Body size (head + thorax length) positively correlates with resource-holding potential (RHP).

Methods

• Study area: Piagol Valley, Jirisan National Park (Site B: $N35^{\circ}15'17.53''\ E127^{\circ}35'58.24''$, $417\ \text{m}$ elevation) – only locality with all six species.
• Sampling period: 12–13 Aug 2017 & 17–18 Aug 2017 (pre-gynes/males, workers only); 8 dawn/dusk sessions (8-10 a.m., 5-7 p.m.).
• Bait station: 1-m table, toilet paper pads soaked with 1:1:1 mixture of brown-sugar water :v vinegar :v ethanol (oak-sap mimic); attractant also nebulised 10-20 min before.
• Behavioural recording: digital camcorder at 50 cm.
• Behaviour classification (11 discrete acts ➔ 3 categories):
  • Threatening (no body contact) → intensity = 2.
  • Grappling (physical contact) → intensity = 3–4.
  • Killing (fatal) → intensity = 5.
• Scoring rule per interaction:
  • Win = act intensity score;
  • Loss = 0;
  • Tie = 1.
• Morphometrics: 30 workers/species; head width + thorax length (abdomen excluded to avoid elasticity bias); preserved in $95\%$ EtOH, dried 1 wk.
• Statistics:
  • Independent t-tests on aggressiveness scores (Vv vs each native).
  • One-way ANOVA $F(5,174)$ + Tukey HSD for body size.
  • Analyses run in IBM SPSS v23.

Results

Aggressive Behaviour — General

• Three overarching categories observed; examples:
  • Threatening: rushing, lifting antennae/front legs, wing shaking, mandible gaping, over-flying, chasing.
  • Grappling: aerial tussling, head-butting, grabbing/throwing, biting, stinging.
  • Killing: Vm occasionally killed Vv outright.

Pairwise Outcomes (wins calculated for Vv perspective)

• Vs (304 bouts):
  • t(304)=9.89,\ P<0.001; Vv wins $71\%$; dominant acts – rushing & wing-shaking.
• Vm (206 bouts):
  • t(206)=-22.75,\ P<0.001; Vm wins $91\%$; Vm specialises in grappling (head-butt) + occasional predation.
• Va (132 bouts):
  • t(132)=-8.38,\ P<0.001; Va wins $76\%$; Va rushes/over-flies; Vv weak counter-threat.
• Vc (184 bouts):
  • t(184)=-9.62,\ P<0.001; Vc wins $73\%$; Vc dominates via wing-shaking + head-pushes.
• Vd (262 bouts):
  • t(262)=-24.22,\ P<0.001; Vd wins $91\%$; Vd major acts – chasing & grabbing.

Body Size (head + thorax length, $\bar x \pm SD$ mm)

• Vm $13.21\pm0.83$
• Va $10.14\pm0.51$
• Vc $9.82\pm0.40$
• Vd $8.88\pm0.49$
• Vv $8.04\pm0.41$
• Vs $7.80\pm0.29$
• ANOVA: F(5,174)=434.9,\ P<0.001; nonsignificant pairwise differences only between Vs–Vv and Va–Vc.

Discussion & Interpretation

Ecological Niche & Spread

• Korea hosts 9 native Vespa; Vv therefore interacts with a dense competitor guild unlike Europe (only Vc + V. orientalis).
• Ecological separation:
  • Vm – ground nests; hunts honeybees & large beetles.
  • Vc – ground/tree cavities.
  • Va – grasses/leaves nests; hunts small/medium insects.
  • Vd – tree-trunk nests; facultative social parasite.
  • Vs & Vv – tree-top aerial nests; similar phenology/resources ➔ highest niche overlap; Vs numbers decline post-invasion.
• Korean forest matrix (≈$63\%$ land cover) + multiple competitors slowed Vv: spread rate $10{-}20\ \text{km yr}^{-1}$ vs European $60{-}80\ \text{km yr}^{-1}$.
• During incubation phase, Vv colonised urban/suburban zones with fewer hornets, avoiding intense competition.

Colony Traits & Competitive Compensation

• Despite low individual RHP, Vv compensates through:
  • Colonies $2{-}4\times$ larger than native Vespa, boosting collective foraging.
  • Superior flight performance (max load‐lifting & range) and efficient hawking of bees.
  • Alarm-pheromone plasticity, deterring predators.
• Result: Gradual density increase even within forests, overcoming initial disadvantages.

Derived Korean Vespa Hierarchy (this study)

1. Vm (largest, most aggressive)
2. Vd
3. Va
4. Vc
5. Vv (invasive)
6. Vs
7. (Projected) V. ducalis would rank below Vs.

Implications & Connections

• Confirms body size → RHP → hierarchy → spread velocity paradigm.
• Demonstrates how colony-level traits can offset inferior individual competitiveness.
• Provides baseline for biosecurity & biocontrol modelling (e.g., CLIMEX predictions, $\text{risk}_{\text{spread}}$ formulas).
• Highlights necessity of urban management because cities act as invasion hubs.

Key Numerical & Statistical References

• Economic insect IAS damages $70\ \text{billion US\$ yr}^{-1}$.
• Vv removal calls in Korea >$100{,}000$/yr; sting injuries $\approx 15{,}000$/yr; deaths $\approx 10$/yr.
• Spread rates: Europe $60{-}80\ \text{km yr}^{-1}$; Korea $10{-}20\ \text{km yr}^{-1}$.
• Representative test stats:
  • $t(304)=9.89$ (Vv vs Vs);
  • $t(206)=-22.75$ (Vv vs Vm);
  • $F(5,174)=434.9$ (body size ANOVA).

Ethical, Management & Real-World Considerations

• Public-health protocols: improved sting-response training, safer nest removal (peak Aug–Oct).
• Apiculture: reinforcing apiary defenses, selective breeding for bee defensive traits.
• Conservation: monitoring of Vs decline; balancing eradication with ecosystem function.
• Legal: Vv listed under Korean biodiversity law Article 23; informs quarantine & trade inspections.

Study Limitations & Future Directions

• Four Korean Vespa not directly tested (V. ducalis, V. binghami, Vc subsp., Vs subsp.) – need broader behavioural matrix.
• Only worker caste examined; gynes & males may alter hierarchy late season.
• Laboratory flight & pheromone assays could quantify compensatory advantages.
• Long-term population genetics to trace human-mediated dispersal routes.

Definitions & Terminology

• RHP (Resource-Holding Potential): an individual’s ability to win contests; often scales with size/weaponry.
• Threatening: non-contact displays (antennae raising, mandible gaping).
• Grappling: physical combat (biting, stinging, head-butting).
• Hawking: mid-air capture of prey (e.g., bees returning to hive).
• Incubation phase: lag between invasion arrival & exponential range expansion.