The Asian hornet arrived in Galicia (NW Spain) in 2012, and since then it has expanded and spread, becoming very common and highly visible. It is particularly attracted to the sap of recently cut pine and eucalyptus logs, the latter being, ironically, another non-native species. I witnessed very large numbers of hornets around mainly eucalyptus log piles. Since these logs continue to exude sap for some days, it is possible that the hornets could be exported (and spread abroad) along with the eucalyptus logs. As well as predating domestic honey bees, it is undoubtedly having a negative effect on native pollinators and other beneficial species, although the scale of this impact is largely unknown, I would suggest, in recently invaded areas.

I spend some time, ever summer, in Galicia, Spain. Over recent years I have witnessed the arrival and spread of an invasive alien insect: the Asian or yellow‐legged hornet, Vespa velutina nigrithorax (Hymenoptera: Vespidae). This insect first arrived in Spain in 2010 (Lopez et al., 2011), having been accidentally introduced into France some six years previously. It took a couple of years to reach Galicia, but – as I wrote in a previous blog – it really seems to like this part of the world, and is certainly thriving in NW Spain (and Portugal). This year (2019), there seemed to me to be many more hornets around, noticeably more even than last year, and large numbers were being attracted to cut pine and Eucalyptus logs.

Eucalyptus trees were first introduced and planted in Spain in 1873. Initially, they were used for fuel and mine supports, but today, their value lies as wood pulp for paper; a billion euro industry. Contrary to popular opinion, reforestation with exotic eucalyptus mostly occurred on ‘spoiled soils’ without forests, and they mostly did not replace native woodland (according to Ruiz, 2010). Today however, exotic eucalyptus species occupy an area of about half a million hectares in Spain, of which the majority (c. 61%) is in the province of Galicia, in the NW of the country. The eucalyptus trees are usually felled (as a cash crop) every 15 years (12-20) or so.

In the past, the eucalyptus oil was extracted: E. globulous oil essence is used in many ways, including perfumes and medicines. The production of eucalyptus oil peaked in the 1950’s in Spain – when 300 tonnes were produced – and is now just a craft industry. The main chemical components of eucalyptus oil are  eucalyptol and alpha-terpineol. I was surprised to discover this year, just how attractive the sap of eucalyptus trees – presumably rich in these ingredients – is to the invasive Asian hornet, Vespa velutina. As well as feeding on tree sap, V. velutina also obtains sugars from flower nectar and ripening fruits.

One of my favourite walks is along the edge of the Ria Ortigueira – an inlet or fjord-like feature – which is nicely documented here. This year, in September, trees were being cut and felled alongside the road beside the ria (see below). As I walked past the cut tree stumps and piles of freshly cut logs, I was surprised by how many Asian hornet were flying around. In this location, they seemed to be the only wasp which was attracted to the sweetly smelling logs, although in another location, a few miles up the coast, I noticed large numbers of native yellow wasps (probably Vespula germanica) buzzing around a pile of logs. There were literally hundreds, perhaps even thousands, just around one particular pile of logs.

I took most of these images whilst standing very close to the hornets, which were in fact buzzing about all around me. It does not take long to realise that they are not at all aggressive, at least in this situation, and one can approach them very closely without eliciting any response. This situation is, I understand, very different, when you get close to their nests. Nests should be treated with the utmost respect and left to professional pest controllers to remove. People have died as a result of hornet stings.

The fact that so many hornets were attracted to the cut logs, suggested to me that there must have been some nests around. However, finding the nests in the tall eucalyptus trees – which cover thousands of hectares – must be next to impossible (see above). That said, over 26,000 nests were removed or inactivated in this region of Spain alone, in 2018. Unfortunately, due to the ability of this species to spread and reproduce, it is not considered feasible to eradicate it, and efforts are being concentrated on protecting honey bee colonies (Balmori, 2015). Unfortunately, this leaves the native bees and bumblebees – as well as a host of other invertebrates, including flies – to fend for themselves. Bees may represent only about a third of the diet of this hornet, in some situations (two-thirds in others), according to Monceau et al. (2018). So a lot of flies and other insects are going to get eaten as well. What overall impact this is having on pollinators, and on invertebrates in general, is hard to say, but urgently needs monitoring. My impression, for what it is worth, is that there were fewer bees around this year.

Asian hornets are monitored throughout the region (see Link below). The traps that are employed are often very simple devices, containing mixtures of sweet smelling substances, such as beer, fruit juices and sugar, for example (see below). Commercial traps and baits are also available (see Rodríguez-Flores et al., 2019), but unfortunately they all attract and catch a wide variety of other insects. Non-target insects caught in these non-selective traps include, native hornets, honey bees, flies and lepidopterans, so there is an urgent need to develop more effective traps. A sex pheromone, produced by the female hornets (gynes) has recently been discovered (Wen et al., 2017) which should at least result in more effective ways of trapping the males.

Some of the trees which were cut down beside pathways (described above) were quite large – and also included pines – and continued to exude sap for some time. I monitored one cut stump over a number of days, and wasps were still feeding on it four of five days after it was felled. It does make me wonder whether cut eucalyptus logs – which are exported in bulk – are a potential pathway for spreading the pest further?

There is no doubt that the Asian hornet will continue to spread throughout Galicia and into Spain more widely. It’s impact on domestic honey bees is severe but has been well documented, and can, to some extent be mitigated by certain protective measures. However, the I remain deeply worried about the effect of this invasive species on the native bee and insect fauna. Such native populations are not routinely monitored in most places, so it is very difficult to quantify an impact such as this. Nevertheless, given the severity of the threat, it needs to be urgently researched.

This pest has not managed to get a foothold in the UK, and given the magnitude of the potential impact, all efforts should be made to keep it out. Guidance, and updates, are provided here.

Links

https://mediorural.xunta.gal/es/areas/ganaderia/apicultura/vespa_velutina/

References

Balmori, A. (2015). On the real risk of a generalized expansion of the asestic avispa vespa Velutina Lepeletier, 1836. Boletín de la Sociedad Entomológica Aragonesa (S.E.A.), nº 56 (30/06/2015): 283–289.

López, S., González, M., & Goldarazena, A. (2011). Vespa velutina Lepeletier, 1836 (Hymenoptera: Vespidae): first records in Iberian Peninsula. EPPO Bulletin, 41(3), 439-441.

Monceau, K., Arca, M., Leprêtre, L., Bonnard, O., Arnold, G., & Thiéry, D. (2018). How Apis mellifera behaves with its invasive hornet predator Vespa velutina?. Journal of insect behavior, 31(1), 1-11.

Rodríguez-Flores, M. S., Seijo-Rodríguez, A., Escuredo, O., & del Carmen Seijo-Coello, M. (2019). Spreading of Vespa velutina in northwestern Spain: influence of elevation and meteorological factors and effect of bait trapping on target and non-target living organisms. Journal of Pest Science, 92(2), 557-565.

Wen, P., Cheng, Y. N., Dong, S. H., Wang, Z. W., Tan, K., & Nieh, J. C. (2017). The sex pheromone of a globally invasive honey bee predator, the Asian eusocial hornet, Vespa velutina. Scientific reports, 7(1), 12956.