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How a Tiny Wasp With Invisible Eggs Is Wreaking Havoc on Spain's Chestnut Harvest

Women gather chestnuts in Spain's Genal Valley. Gary John Norman / Photodisc / Getty Images

The Genal Valley in Southern Spain is famous for its sweet chestnuts, and that could give the economically struggling region a huge boost as vegans and vegetarians in Northern Europe develop a taste for Castanea sativa. But a new threat is putting the valley's reputation, and future, at risk, BBC News reported Tuesday.

"We've lost at least 30% of our usual production," farmer Julio Ruiz told BBC News. "It is all the fault of a small wasp."


The wasp in question is the chestnut gall wasp (Dryoscomus kuriphillus katsumatsu), an import from China that first started causing havoc in the region three years ago. The pest has no natural predators in the area, forest engineer Antonio Pulido told BBC News, and has led to the deaths of numerous trees.

"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," Pulido explained.

While other crops grow in the valley, the chestnut is the most lucrative, bringing in 10 million euros a year. The wasps have further consequences for the ecology and culture of the area. For one thing, Pulido said, the burning of infected trees to stop the spread is upping the risk of wildfires.

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

The chestnuts of the Genal Valley aren't the only ones that have been imperiled by the wasp. The insect has caused damage throughout Europe.

"It is very likely that the chestnut gall wasp population originates from very few females which were accidentally introduced into Italy via infected plant material brought from China in 2006," University of Extremadura researcher Raúl Bonal explained in a press release translated into English and published by ScienceDaily. In a study published in May, Bonal and his fellow scientists found that the wasps present in Europe had all come from a few females reproducing asexually. This allowed the species to spread quickly throughout the continent.

The spread was also aided by the fact that the wasps are only the size of a grain of rice, and their eggs cannot be seen by the naked eye. The eggs are laid in the buds of the trees and begin to develop when the buds open the next spring. As the larvae develop, they cause galls to form on the leaves and shoots of the chestnut trees.

"Since the eggs cannot be seen, people take the infection with them in the seedling and we are taking the enemy home with us. As a result, before selling the plant the nursery should keep it 'in quarantine' for at least one year to ensure that the plant has no galls. In this way the plant sprouts in controlled conditions," Bonal recommended.

In the Genal Valley, government researchers are also considering another solution: introducing the wasp's natural predator, Torimus sinensis. This approach has succeeded in curbing the spread of the wasp in Italy, North America and Japan, and a limited number of them have been released in the Genal Valley already, BBC News reported.

But government researcher Juan-Ramon Boyero Gallardo is also nervous about the repercussions of a wide release.

"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 told BBC News.

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