Since 2009, a member of the Moths and Butterflies of New Zealand Trust has walked the same route weekly from October to March, recording butterfly numbers.
That data was used to produce a report, which found common and paper wasps were to blame.
"The most likely drivers are environmental pressures, particularly introduced pest wasps (Vespula and Polistes spp.)." founding trustee, Jacqui Knight said.
She said a wasp survey done in 2025 showed the pests were killing native insects like wētā and young birds, while also "limiting people's ability to work and enjoy the outdoors".
"We are concerned that other species are in decline as well, but the monarchs are much larger and used to grace our gardens every summer - so their absence is much more noticeable."
Knight is urging the government to take the impact of those more seriously and is calling on volunteers to take part in butterfly transect monitoring.
"There are other factors affecting our native fauna as well that need to be addressed. Our work is just beginning."
