Monarch butterflies are facing a famine in Auckland this summer, according to the trust that works to protect the colourful insects.
The commercial nursery that usually supplies swan plants that monarch caterpillars feed on has stopped the trade.
In the 10 years since Jacqui Knight started the Monarch Butterfly Trust, garden centres have been selling large, well-grown swan plants but this year, she said, the only plants around were small ones.
"For Aucklanders, the difficulty is that there are no well-grown plants, and the ones you can buy are too small to sustain a monarch caterpillar."
Hungry caterpillars need to munch their way through a startling amount of swan plant before they became big enough to form a chrysalis,
The butterflies can produce six to seven generations in one summer, given good conditions, but Ms Knight said with the shortage of food this spring the population would be starting from a lower base.
Breeding can start as early as August. By mid-summer, the monarch caterpillars are under siege by wasps.
"Usually, the wasps attack the monarch caterpillars when they're looking for protein to feed their juveniles.
"Then, towards the end of the season, all the wasps - including the juveniles - are looking for nectar, so they leave the monarchs alone to breed that last generation of butterflies before the winter sets in. "
Christchurch-based entomologist Brian Patrick said monarch butterflies had relied on a hand-up from humans ever since they arrived in New Zealand 140 years ago.
None of the tropical plants they fed on in the Pacific Islands were present in this country, and could not survive the cooler climate, he said.
"Humans cottoned on that there was no plant for the monarch here, so by the 1870s the swan plant was brought in, and planted widely, and now of course we've got resident monarchs living in New Zealand."
Monarch caterpillars are abundant in Dr Patrick's Christchurch garden this season, and he is hoping another commercial grower will fill the gap in the Auckland market.
He is appealing for people to take part in the Big Backyard Butterfly Count underway this month, a national snapshot of butterfly numbers. New Zealand had 62 species of butterflies, and 48 were found nowhere else in the world, but the populations appear to be in decline.
"People can do as many counts as they like from a particular place. Anywhere from their big backyard, to a local school, to parks and gardens nearby; right through to mountain tops - wherever they happen to be."
Dr Patrick has been doing spot surveys of butterflies since he was a 10-year-old, observing the diversity of insect life in the back garden of his childhood home in Invercargill.
This week, for the first time, the results of those 46 years of careful data-collecting will be presented by a Masters student from Lincoln University to the New Zealand Ecological Society conference in Wellington.
Details of how to take part in the survey can be found on the Monarch Butterfly Trust website.
Ms Knight's advice for people wanting to help the monarch butterflies in Auckland this summer is that they should resist pulling out old plants because they're untidy or in the wrong place.
People could also plant a packet of seeds, and protect some plants from the butterflies until later in the season, when they are big enough to sustain a caterpillar and the wasps have lost interest.