Monarch butterfly lovers should prepare themselves for some very hungry caterpillars.

There are expected to be 81,000 fewer swan plants in full maturity for monarch caterpillars to feast on.

A prominent grower of mature swan plants has retired, leaving a gap in supply.

Monarch Butterfly Trust secretary Jacqui Knight said gardeners need to be aware that small seedlings common in garden centres often can't provide enough food for monarchs.

"People who buy those young swan plants really need to keep them for a year before they can sustain a monarch caterpillar, so we're going to be losing a lot of our monarchs because there aren't the plants around."

She said they're advising people to buy extra plants and let them grow, before putting caterpillars on them.

"Make sure that some are covered up to get established for next year. If you have swan plants that go to seed and you let the seed blow around your garden you're going to have swan plants everywhere."