The yellow-legged hornet, also known as the Asian hornet (Vespa velutina), is an invasive exotic species native to Southeast Asia and has been present in Europe since 2005. Since then, it has spread rapidly to different territories, reaching Spain in 2010. This rapid dispersion is explained both by its high flying capacity and by the possibility of being passively transported through the trade of goods.
In addition, there are other factors that have facilitated the expansion of this species, such as its generalist diet, which allows it access to abundant food sources, favorable climatic conditions, and the limited number of direct competitors in the invaded areas.
The 'Vespa velutina' feeds on a wide range of insects
Another feature that favors their spread is polyandry, an unusual trait that allows the founding queens to be fertilized by multiple males. This increases the genetic diversity of the offspring, which reduces the risk of potential issues stemming from inbreeding.
Health and ecosystem effects
The invasion of V. velutina is causing significant impacts. Among the most well-known issues are the socio-sanitary effects resulting from the risk of their stings, as their venom can trigger allergic reactions and, in some cases, has led to death.
At an ecological level, adult individuals capture insects to feed the larvae that require proteins. Although their prey includes honey bees, which causes economic losses in beekeeping, V. velutina feeds on a wide range of insects, which can also have impacts on various ecosystem services such as pollination.
Asian wasp on the lookout for bees perched on the borage leaves
But these are not the only issues associated with this species. Adult individuals need sugars that they obtain, in many cases, from fruit, causing losses in various crops, such as vineyards.
In fact, wine producers from the Protected Geographical Indication (PGI) of Betanzos (Galicia) have reported an increase in damage caused by V. velutina in recent years. They also suspect that bites on the grapes may be favoring the entry of fungal diseases into the vines, resulting in an increase in crop losses and, therefore, economic losses as well.
How to reduce damage in the vineyards
Although there are no magical formulas to avoid the presence of wasps in vineyards, if the damage is very high, the use of physical protection methods can be tested. For example, anti-hail nets have been used to protect different types of crops not only against weather phenomena, but also against attacks by different types of organisms.
In a study we conducted at the University of A Coruña, we designed an experiment to evaluate the effectiveness of using this type of netting to mitigate the damage caused by V. velutina in two grape varieties grown in this region, Blanco Lexítimo and Godello.
During the grape ripening period, which is when the damage occurs, some groups of vines were covered and sealed with nets to prevent wasps from entering, while others were left uncovered. During the harvest, damage from wasps and birds, which also reduce yields, was recorded in the uncovered vines. Additionally, to assess whether there were changes in the sugar content of the grapes from the covered vines, the sugar content was measured both at the time of installing the nets and during the harvest.
Vines covered with mesh
The results show the suitability of the chosen mesh to completely prevent damage caused by both V. velutina and birds, thereby increasing the amount of harvested grapes. However, they also suggest that the mesh could affect the final sugar content of Blanco Lexítimo grapes, and even the susceptibility to fungal infections.
Uncertainty about the future
The economic and infrastructure investment required for the implementation of this protection system may not be feasible for producers in areas like the one studied, where the size of the farms is small and using mechanical means is challenging.
Furthermore, although physical protection systems such as hail nets are effective in preventing damage from wasps, it is essential to understand the role they may play as modifiers of the final characteristics of the grape, as they could affect the organoleptic properties of the end product, wine.
All of this currently generates a lot of uncertainty among producers, who are torn between investing in a system they don't know if they will be able to amortize and that may change the characteristics of the wine produced, or leaving the vineyards at the mercy of wasp attacks, which can be highly variable due to the natural fluctuations suffered by V. velutina populations.
Thus, while it is unlikely that Asian wasps will leave us without wine, it is possible that we may see a reduced supply of different varieties in the future, especially those coming from small productions.