Department of Conservation (DoC) staff are monitoring substantial defoliation of beech trees in Nelson Lakes National Park, believed to be caused by a native moth.

With hundreds of hectares affected, several of the park's valleys had large tracts of bare, dead-looking red and silver beech trees, DoC entomologist Ian Millar said yesterday.

Many trampers had contacted DoC with concerns about the extent of the damage.

An upsurge in numbers of a native moth caterpillar that eats beech leaves was believed to be the cause of the extensive defoliation, Mr Millar said.

Beech forest in the national park recovered after a similar occurrence nine years ago.

Two species of moth - Thiotricha tetrphala and Thiotricha lindsayi, commonly known as the beech case moth - were suspected and specimens of caterpillars found on damaged trees had been sent to experts for confirmation on which was the culprit.

"The moths naturally occur in Nelson Lakes beech forest but for some reason there had been an explosion in their population this year," Mr Millar said. "We are not exactly sure why; it could be due to it being a particularly dry winter or other factors."

Mr Millar said damage to the beech trees would have been caused by millions of caterpillars, which as well as eating the leaves, rolled leaves into a "case" which they carried on their back as camouflage.

In 1997, Thiotricha lindsayi was blamed for causing beech defoliation in the park's Travers Valley, but most bounced back from the damage.

"Large-scale tree damage in forest from increases in populations of certain insects that live in the forest is something that occurs from time to time," Mr Millar said. "The forest usually recovers."

Such occurrences were part of the "natural ecosystem cycle".

DoC wants people to contact it if they notice large areas of dieback on trees outside Nelson Lakes National Park.