310318. News. Photo: DOUG FIELD/STUFF Monarch butterflies in the grounds of the Aigantighe Art Gallery

Hundreds and thousands of monarch butterflies flocking to trees at two public Timaru gardens is a sign winter is on its way, Monarch Butterfly New Zealand Trust spokeswoman Jacqui Knight says.

For the past few days the bright orange and black insects have been gathering in their thousands on a Himalayan cedar at the Aigantighe Art Gallery sculpture garden and in their hundreds on cedar and buddleia plants at the Timaru Botanic Gardens, in an annual get-together before the cooler months.

Knight said monarchs gather in large colonies when winter is approaching and settle in one spot.

DOUG FIELD/STUFF Anastasia'Rose Kelly, 4, finds a monarch butterfly in the grounds of the Aigantighe Art Gallery on Saturday.

Cedars are popular as they are a good place for butterflies to leave pheromones behind, encouraging butterflies to return the next year to the same tree, she said.

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The tree is also easy for butterflies to hold on to.

The wintering of the insect follows a busy summer in Timaru where thousands of monarch caterpillars chewed through swan plants, leaving a shortage throughout the region.

The butterflies will stay in the trees and while sexually mature, the butterflies will not act on this until spring.

"They don't start to mate until the season is warm enough again," she said.

This state is called diapause and is known as a period of suspended development in an insect during unfavourable environmental conditions.

Knight said while it is pretty for visitors to see the insects gathering in the trees, it is also sad as it means winter is approaching.

"They are clever."

In North America each autumn more than 250 million monarchs leave the United States and southern Canada and journey south for up to 5000km to their wintering roosts in the mountain fir forests west of Mexico City.

Aigantighe Art Gallery exhibition curator Hamish Pettengell said the annual migration of the butterflies to the gallery's garden is always a popular sight.

"They do it every year," Pettengell said.

"Sometimes it's earlier and sometimes it's later."

He estimated there were at least a thousand or so butterflies in the cedar tree at the garden.

Pettengell encouraged people to visit the sculpture garden and see the butterflies flocked there.

"It's a great opportunity for people to see them," he said.