Postcard from... Ebro Delta

It sounds like a plot from a spoof horror film, but ever since a South American mollusc, the innocent-sounding apple snail, was accidentally released into the rice fields of the river Ebro Delta in 2009, for local farmers, the nightmare scenario of a plague of voraciously hungry snails has been sadly all too true.

Just three members of the Pomacea canaliculata species can, local media say, eat their way through an entire rice field in one day – and there are an estimated five million of the intruders lurking round the cereal plantations scattered along the lower Ebro's left bank. And they are not easy to wipe out: the apple snail can grow up to 10cm long, has no natural predators and – contrary to what everybody thinks about snails and speed – has an exceptionally fast reproductive cycle. They can even swim upstream.

For three years, farms on the right bank of the estuary of the Ebro, Spain's largest river by discharge volume, have remained thankfully free of the implacable hordes of molluscs with the munchies. Until now, that is, when in the middle of the recent rice harvest, a slew of apple snails – somehow – crossed over. Some 60 nests, each with an average of 300 eggs, have been discovered.

Pomacea canaliculata's sudden, en masse, appearance on the Ebro's right bank is blamed locally on human sabotage – in other words, somebody deliberately releasing them. But whoever was responsible for this ecological and agricultural disaster, despite police searching several cars, they have yet to get on their trail: those of the steadily multiplying apple snails, meanwhile, remain sadly all too evident.