The chestnut gall wasp is a small invasive species from China that is causing a big headache for chestnut growers in Spain and threatening a major harvest.
The BBC reports that the gall wasp has impacted this year’s crop on many Spanish farms. “We’ve lost at least 30% of our usual production,” Julio Ruiz tells the BBC from the family’s farm in the Genal Valley. “It is all the fault of a small wasp. You can’t tell there’s anything wrong until the damage has been done.”
The oriental chestnut gall wasp, to give it its full common title, are small insects with black bodies and orangey-brown legs. They come from Asia, but they arrived in Europe in roughly 2002. The exact method by which they first reached European shores isn’t clear, but infested wood is one suspect. Some researchers believe that infected plant material brought to Italy in 2006 may also be to blame for the greater surge in infestation in the European mainland.
The wasp poses no threat to humans or to animals, but it can be a big problem for chestnut trees. The female chestnut gall wasp will lay her eggs in the buds of chestnuts. When the larvae then hatch and later begin to grow (not coincidentally when the host tree’s buds begin to develop) the larvae make the tree develop so-called “galls”: green or pinkish bulges that form a protective casing for the larvae. The larvae then develop and later emerge as adults.
Because the galls disrupt the fruiting process, they can reduce the tree’s chestnut yield significantly. In some cases where trees have been heavily infested, it can induce early leaf drop and even kill trees.
Chestnut cultivators in Europe currently face two major problems. The first is that, until the galls begin to develop on the trees—by which time it is too late to save the fruit—there is no sign of gall wasp infestation. This means that farmers don’t know from one year to the next whether, like this year, it will be a bad yield for them. For those who rely on chestnut farming as a major source of income, that’s a terrible burden.
Secondly, because the gall wasp is an invasive species it has no native predators. That means that this problem isn’t going to balance out until the wasps reach a saturation point, which could well be when most chestnuts are infected, or unless a native predator animal steps in—which is not an impossible scenario, but one we cannot easily control.
This is perhaps doubly injurious, because right now farmers should be raking in a lot of money thanks to the boom in vegan and vegetarian diets and the growing desire for sweet chestnuts.
SO, HOW DO WE FIGHT THE CHESTNUT GALL WASP?
Currently, culling infected trees and those in the immediate surrounds has been one way farmers are trying to stop the pest’s spread. However, that is not proactive and means that farmers are still losing significant numbers of their trees. There is also a secondary hazard to this: farmers have taken to burning infected trees, which is increasing the risk of forest fires.
Researchers in Spain are turning to the natural predators of the chestnut gall wasp for a solution. Torymus sinensis is a parasitic insect also native to China and one that preys on the chestnut gall wasp. It was successfully introduced in Japan and in the USA to combat the gall wasp, and European trials are currently looking at whether introducing it as a more wide-scale prevention measure is viable.
It has already been released in Italy where data gathering is underway, but obviously this comes with risks. For one thing, we are introducing yet another insect species with no natural predators, so we don’t know exactly what it might do in our European habitats and whether it might displace other similar but native species.
And yet, at the moment, this appears to be the leading solution. With farmers under significant pressure from a changing climate and from other pests and blights, it may well be that this is, for now the best we can do.
The chestnut gall wasp illustrates the need for more wide-ranging measures to combat invasive species spreading because, unless we can get ahead of this, it will keep impacting farming communities and our environment.