A late burst in the Monarch butterfly breeding season has seen numbers finally lift, but experts say they're still worried about the implications of the approximate two-month quiet period.

But it's what's happening in the United States after reports the West Coast Monarch population has plunged a whopping 86 per cent since last year, that has New Zealand butterfly experts worried.

National Geographic reported the Monarch population had plunged more than 80 per cent over the past 20 years in America.

The magazine put the cause down to humans; climate change and the growing of milkweed plants - relatives of New Zealand's swan plants - higher than normal toxicity of cardenolides, a poison absorbed then used as a deterrent for predators by the butterfly.

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Lepidopterist Brian Patrick says there are now fundamental concerns with the Monarch's unusual behaviour in both New Zealand and the US, that could possibly be showing global implications.

What those implications remain unknown and Patrick, together with Jacqui Knight, of the Moths and Butterflies of NZ Trust, are still appealing for public feedback on any changes in the Monarchs behaviour.

In New Zealand, Monarchs normally frolic in the backyards of urban Kiwi properties between September and March.

However, in a phenomenon they'd never experienced before, the number of butterflies seen around the country was minimal at best leading up to Christmas.

Knight told the Herald last month they'd seen fewer, if any, monarch butterflies and those they were seeing were not laying any eggs.

After appealing for information at the time about sightings, or lack of, she has been buoyed by the response.

The feedback showed there had been few sightings until the past week when there had been a sudden surge.

Monarch butterfly numbers are not only plunging in New Zealand but also in the United States. Photo / File
Monarch butterfly numbers are not only plunging in New Zealand but also in the United States. Photo / File

"But there was this big patch, about eight or nine weeks, where there were no eggs being laid."

She didn't want people who were suddenly experiencing an over-run of eggs or caterpillars on their plants to kill them, but to pass them on to others in their community or family to raise them, to continue to boost the population.

The trust had since set up a Facebook page, Monarch and Milkweed Matchmaking New Zealand, where people can write what they have an excess of, or needed, and do an exchange.

While she was pleased to hear they were successfully breeding and now flying high in the skies, she said they were still concerned about the implications of the eight-week flat period and how it will affect the next season.

She agreed with climate change concerns in North America, and said she couldn't help but wonder whether the turn to become eco-friendly the past few years could be too little too late for the iconic insects.

"I have been aware of concerns for nature or the environment for 40 or 50 years and people are just beginning to change their attitudes now and I think really, I hope it's not too late ... we have to look after our planet better."

Patrick said there were still unusually low numbers of Monarchs around.

"There's a few Monarchs around but they still don't seem to be laying eggs and doing their normal behaviour. We really need the public's observations because it seems like it's a global thing."

While the Monarchs are an assisted native, the swan plant is not and was introduced specifically for the butterfly to survive.

"We're in a unique situation here swan plants are weeds in North America whereas in New Zealand they've been planted particularly for the Monarch."

There so far hadn't been any studies carried out on either the Monarch or the swan plant to be able to monitor its behaviour.

"We just don't know what's happening here. There could be something global in the environment that Monarchs are extra sensitive too, we just don't know.

"What they're doing could be telling us something fundamentally that's happening to our environment and we just don't know, are there too many toxins out there? But it is ironic that a butterfly that is toxic itself could be extra sensitive itself to other toxins.

"Something similar is happening here and around the rest of the world, that sounds like it's something more fundamental, more global, and therefore more of a worry."

He said it was a perfect opportunity for a budding university student to take up the happenings as part of a research project over the next 12 months, as there was currently no funding available for anyone to delve deep into the issue.

"Some aspiring university student, it's their holidays at the moment. They might just pick it up and continue with it for the rest of the year."

Knight said pesticides were also an issue as they were often used by nurseries to help grow baby swan plants to sell to the masses.

"What does concern me is that to raise hundreds and hundreds of swan plants you've got to spray them with pesticides because otherwise you're going to have aphids thinking it's paradise."

On occasion some plants were sold with pesticide still on them, causing a devastating affect on the Monarchs who go onto lay their eggs.

The best way was for members of the public to just grow a few swan plants themselves, that way pesticides wouldn't be needed.

However, there was an unlikely ally . A species of wasp, the aphidius colemanii - which grew to the size of a flying ant, which she used to help kill aphids on her plants.

"There's thousands of wasps but they're not all bad. It's an insect that's been brought into the country deliberately because the only thing it does in the world is it lays its eggs inside aphids and kills them."

People can report sightings, or lack of, on their website, mb.org.nz.