Apple Snail Plague Resists All Countermeasures in Ebro Delta

Apple Snail
A 2cm apple snail in an Ebro Delta drainpipe. Photo by Josep Lluís Sellart.

Apple Snail Plague Resists All Countermeasures in Ebro Delta
Hundreds of thousands survive all attempts to eradicate the species. The desiccation of 9500 hectares over five months proves useless.
El País: La plaga del caracol manzana lo resiste todo en el delta del Ebro
M. Pérez and F. Balsells reporting from Tarragona and Barcelona, respectively, July 19, 2011

Not even desiccating half the Ebro Delta (Tarragona) or launching quicklime into irrigation channels could remove their shells from rice paddies. The apple slime is giving farmers headaches for a third year since arriving in northern Spain, and it is threatening to extend to the south. It is a very resistant species which reproduces very quickly, and it has slowly but surely expanded its reach since 2009, now affecting 9500 hectares. This despite the efforts of the Valencian legislature, which has spent 3.5 million euros since last year trying to eradicate the species through unprecedented measures: it left the most affected 9500 hectares on the left side of the delta without water for five months and filled drainpipes with lime.

“No country has successfully eradicated this plague,” says Ravindra Joshi, an Indian biologist who has studied the apple snail and its effects on crops since 1980. “Some four years pass between detection of the species and the beginning of crop damage,” he said, anticipating that next year the delta will begin to feel the effects of the animal. “From then it may well be a catastrophe.”

“We don’t know what to do with this bug,” confessed Hernán Subirats, the engineer the legislature entrusted with the plan to eradicate the species. “At least it seems like there aren’t any more than there were last year. If we hadn’t done anything, we wouldn’t have a single rice plant left.” Hundreds of millions survived all the measures, nevertheless. This season, experts have applied a chemical with saponin, a toxic product with effects similar to soap which helicopters and tractors dump into the environment. The treatment, which costs some 160 euros per hectare, has been about 60% effective.

Nor have they affected the millions of lilac-colored eggs which inundate the canals and side of the delta. “An apple snail can lay between 400 and 500 eggs every 10 days. At eight months, those are fertile. After you do the figures, you just put your head in your hands,” Subirats said. There isn’t an industrial method to eradicating them; you can only dive in the water and destroy them by hand. “We’ve tried an oil which cuts respiration. But they release a mucus that pierces the oil and allows them to breathe,” Subirats lamented.

In December 2010, six million apple snails were counted in the Ebro Delta, according to the legislature. Some 450 rice fields were affected by the plague, 140 very gravely, that is to say there were more than 50 snails per square meter. A snail can eat 400 rice roots a night, the equivalent of one plot of land. But this invasive mollusk can advance and even climb above water. Subirats said this is the worst invasion ever in an area which has been threatened by other species before. He is still confident he will find a solution and will cut down the population by the end of the year. “If it did so, Spain would be the first country in the world capable of eradicating this plague,” said Joshi.

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