Washington state Tuesday began spraying thousands of acres to try wipe out a virulent strain of gypsy moth that state officials say may be the nation's worst pest threat ever.

The spraying, which parallels efforts in Oregon and British Columbia in Canada, aims to wipe out the caterpillar stage of Asian gypsy moths, believed to have arrived in the United States aboard ships from eastern Russian ports.

The US Forest Service has estimated that the moths, which can defoliate both deciduous and coniferous trees, could cause billions of dollars in losses of commercial timber, recreation and tourism if they were allowed to spread.

Scientists said the Asian strain of the moth is far more dangerous than the European form, which first came to the US in 1869 and is established in 14 Eastern states.

Unlike their European relatives, the female Asian moths are capable of flying up to 20 miles before laying eggs, enabling the mothsto spread more widely and quickly.

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