New Zealand butterfly experts say a “concerningly low” number of monarchs are being spotted this winter, with the public being urged to lend a hand.

To survive a drop in temperatures over winter, monarch butterflies go into diapause – often called overwintering or hibernating – whereby groups cluster together in tall trees.

Swarms of butterflies can hang in these large groups from trees for up to six or seven months each year.

Moths and Butterflies of New Zealand Trust (MBNZT) founding trustee Jacqui Knight said over recent years, the overwintering groups observed had been fewer and the sites harder to find.

“It’s very concerning,” Knight said. “The monarch butterfly is not a native but most New Zealanders know it and love it ... if the monarch numbers drop sufficiently, it’s also affecting our native butterflies, moths and invertebrates.”

Introduced species of wasps were contributing to the low levels of monarch butterflies as juvenile wasps were fed caterpillars by their parents, she said.

Members of the public are being asked to renew efforts to track monarch numbers by recording sightings of overwintering sites on a special page set up on iNaturalist – a nature tracking app.

Dr David James, an associate professor of entomology at Washington State University, said “reliable, long-term data on monarch populations was needed”.

The MBNZT is also encouraging people to take part in “tagging”, which involves placing a small, lightweight sticker with a unique code on the hindwing of a newly emerged butterfly.

In the 2024/25 season, more than 4000 monarch butterflies were tagged through the programme.

The tags can be ordered through the MBNZT website and the data help scientists understand the migration paths and flight distances of monarch butterflies.