The affected area is about thousand square kilometres in size.
Scientists at the Kilpisjärvi Biological Research Centre say birch trees could die over a wide area as result of lavae infestation for two consecutive years.
Birch trees recover from the damage very slowly. The destruction seen in the past two years has not been witnessed for half a century.
Smaller outbreaks of the moth occur every ten years. Researchers say tough, cold winters keep the pest at bay. Climate change bringing warmer weather can excerbate the problem of autumnal moth lavae.
Autumnal moth lavae have also been reported near the Tenojoki river in the north of Lapland.
YLE24