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Butterfly lovers are being urged to be careful when buying swan plants (milkweed) from garden centres.
Plants affected with pesticide residue will kill the caterpillars that eventually turn into monarch butterflies.
"Every year it happens," said Jacqui Knight, secretary of the Monarch Butterfly New Zealand Trust.
"People are desperately looking for food for their monarch caterpillars, and end up buying plants affected with pesticide residue. We hear from adults and children distraught to see their caterpillars dying."
