People growing milkweed or swan plants to sell as food for monarch butterfly caterpillars are being warned not to spray the leaves with pesticides.
The Monarch Butterfly New Zealand Trust says hundreds of caterpillars have died as a result of people buying plants from garden centres to feed them, without knowing the plants had been sprayed.
The trust secretary, Jacqui Knight, says growing the plant under cover will prevent butterflies from laying eggs on them, meaning there is no need to spray.
Ms Knight says growers need to be honest about any pesticide residue on the plants.
She says people also need to plant the seeds earlier so that when the butterflies lay eggs, there is lots of food on the plant for them.