Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-vasto, āvi, ātum, 1, v. a., to lay waste, devastate: fines, Liv. 6, 4: pervastatis passim agris, id. 8, 19: pervastata Italia, Tac. A. 15, 45: Boii Laevos cum pervastassent, Liv. 33, 37, 6.