Image copyright Richard Duebel

Chestnuts are increasingly being incorporated into vegan and vegetarian diets - as well as being a Christmas favourite. But a small, invasive wasp from China is threatening the chestnut harvest in Spain.

Now government scientists are considering releasing another non-native insect into the environment to keep the wasp population under control.

Under the dense green cover of the Genal Valley in southern Spain, Julio Ruiz, a thirty-something farmer is collecting sweet chestnuts (castañas) with his father, mother and brother.

They are picking up the prickly nuts from the leaf-strewn earth, recognising through experience those that will contain the prized, large-sized chestnut.

But this year there are far fewer chestnuts to gather not just on the Ruiz family's 30-hectare farm, but throughout the 4,000 hectares of the lush Genal Valley famed for its plentiful chestnuts thanks to its micro-climate.

"We've lost at least 30% of our usual production," explains Julio. "It is all the fault of a small wasp. You can't tell there's anything wrong until the damage has been done."

Image copyright Richard Duebel Image caption A micro-climate in the 400-hectare valley is said to contribute to its success at growing chestnuts

At the nearby co-operative collection point and weighing station talk is of little else. The chestnut gall wasp, Dryoscomus kuriphillus katsumatsu (avispilla in local Spanish), is causing huge environmental and economic damage in the Valley where the harvest accounts for 10 million euros' worth of income annually.

"We may be the biggest producers of chestnuts in Andalucia," says Felipe Javier Cabos Aguilar, president of the regional association of chestnut producers shaking his head sadly.

"But that just means we are the most vulnerable."

According to Antonio Pulido, a forest engineer who has been overseeing the fight against the pest, the avispilla is a parasitic insect that comes from China.

Image copyright Richard Duebel Image caption Antonio Pulido and Julio Ruiz inspecs an infected chestnut tree

Image copyright Richard Duebel Image caption A chestnut bud damaged by the Chinese wasp

"It's considered an exotic species that has no natural indigenous predators," he says.

"The main infestation is in the bud where growth is stunted. As a result the flowers and fruit cannot develop and the health and vitality of the tree is compromised. Every variety of chestnut is affected."

Over the last three years since the infestation first arrived in the area, countless trees have died. Over the generations each plot of land, or finca, has been divided between offspring with the chestnut proving the most lucrative of all the crops.

Although cork and other trees grow in abundance, most farmers depend on chestnuts that are exported to France for specialist processing.

In order to limit the damage, some farmers are burning the infected trees to protect those who aren't affected. "This is increasing the risk of forest fires," says Antonio Pulido.

"It is also adding to urban drift as young people in the villages see their future disappearing in front of their eyes."

Image copyright Richard Duebel Image caption Here, sweet chestnuts are prepared at a local restaurant. They are also a major export

Castanea sativa, called the sweet chestnut to distinguish it from the horse chestnut, Aesculus hippocastanus (to which it is only distantly related), offers the region unique potential.

The sizeable vegan and vegetarian markets of northern Europe have already developed an appetite for sweet chestnuts.

Now that potential uplift for a impoverished part of a poor region of Spain is under threat.

In Malaga, at the laboratories of IFAPA-Churriana - an agriculture and fisheries investigation unit of the Andalucian Government (Junta de Andalucia), Juan-Ramon Boyero Gallardo is looking under the microscope at Torimus sinensis, the natural predator of the avispilla.

It also comes from China. He confirms that this parasitoid insect is capable of helping keep the spread of the infestation under control.

The Chinese predator has had a high rate of success in Japan, Italy and North America where the infestation appeared prior to arriving in the Iberian Peninsula.

However, he remains hesitant about releasing the predator into the Genal Valley.

"The problem of introducing a further exotic species such as Torimus sinensis is that it can invade the natural woodland, attack indigenous species, displace others and alter the overall biodiversity," he cautions.

Image copyright Gyorgy Csoka, Hungary Forest Research Institute, B Image caption The oriental chestnut gall wasp has already appeared in other regions of the world, such as North America

Although the unforeseen repercussions might have a negative impact, there has been a limited release of a tiny number of the predator species in the Genal Valley.

Juan-Ramon Boyero Gallardo says he is optimistic that Torimus sinensis and local predators will slow down the spread of the avispilla.

He views the plague (plaga in local Spanish) as just another example of the disadvantages of globalisation.

"We know that the United States have very strict controls over bringing vegetation in to the country, yet they too have been infested" he says.

"With container-loads of produce arriving in the port of Malaga from lots of different countries around the world, there is little we can do to prevent the arrival of exotic species."

He goes on to comment: "If we take a wider view this is another example of the unintentional globalisation of parasites and the problems facing scientists as they search for ways of eradicating, or at least limiting the pest," he says.

However the more pressing need is to prevent the mass destruction of chestnut trees and uphold the economy of the Genal Valley - not to mention the supply of chestnut stuffing and the luxury marrons glaces so beloved at Christmas.