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.
Sign up to The Daily H, a free newsletter curated by our editors and delivered straight to your inbox every weekday.
