Butterfly experts call for help with drop in monarch butterfly sightings, strange winter trend

Monarch butterflies hang in large clumps from trees during the winter – a phenomenon known as overwintering. Photo / Kathy Reid

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.

Naturalist and bug enthusiast Ruud Kleinpaste said he had not seen any hard evidence to suggest monarch butterfly levels were under threat in New Zealand but tagging monarchs offered a way to track the movement of groups.

“In the United States and in Canada, they fly all the way to Mexico to do overwintering in the tall trees there, and then they fly all the way back in springtime – it’s thousands of kilometres,” Kleinpaste said.

“But here in New Zealand, we’ve known for a long time they don’t make those huge journeys, they make short journeys to their nearby places where they hibernate.”

Knight said reducing the use of pesticides in the garden, planting swan plants in springtime and removing wasp nests are ways to encourage monarch butterflies to settle around your home.

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