Panicked callers are ringing a butterfly conservation group because they are running out of food for monarch caterpillars.

Monarch Butterfly New Zealand Trust is predicting a "dire situation" for monarchs this year because of a shortage of mature swan plants and increasing wasp populations.

"People are panicking and calling us because they have run out of monarch caterpillar food," said trust member Jacqui Knight.

Ms Knight said the number of monarch butterflies has been dropping over the last few years mainly because they are being attacked by wasps but could this year also face a shortage of food. The monarch is New Zealand's largest common butterfly and gardeners plant their larval food, such as swan plants, to attract them into gardens.

Advertisement

Ms Knight said there would be 81,000 fewer swan plants this year following the retirement of Mr Warwick Wilson, who is one of the country's largest suppliers of mature swan plants.

Between November 10-30, the trust will be running Big Backyard Butterfly Count, where people are encouraged to count the species they see in the garden.