I have real concerns for the monarch butterfly in NZ (and of course we already know it’s in real trouble in North America).
There are pockets around our country where people see plenty of eggs being laid early in the season, and those people of course say, “there are lots here in my garden” (or similar). But most monarch-lovers are concerned: their swan plants are devoid of eggs.
Firstly, some facts:
• Monarchs do not mate until they are three to eight days old. When they mate, they remain together from one afternoon until early the next morning – often up to 16 hours.
• Male monarchs tend to stake out the milkweed patch (swan plants), waiting for females to come by, whereupon mating will take place.
• Females begin laying eggs immediately after their first mating. Both sexes usually mate several times during their lifetime.
• Female monarchs can smell a milkweed from 2 km away. After laying eggs at one spot, she will move on to find more places to lay eggs.
• A female monarch typically lays 300-400 eggs. In a lab study, one monarch laid 1,179.
• It takes about 28 days for a monarch to go through its metamorphosis and become an adult. Typically, eggs hatch after about five days. The caterpillars go through five instars and form a chrysalis (pupate) after about two weeks. The butterfly ecloses (or emerges, but never "hatches") about ten days later.
Maths and Monarchs
Depending on your region, overwintering monarchs return to your garden in the latter half of the year for the summer (Month 1).
In Month 1, let’s say three female monarchs return from overwintering and lay eggs. Each lays 300 eggs (300 x 3 = 900), but let’s imagine that 90% are lost to predators/parasites/disease. We would now have 90 butterflies, half of which might be female and are laying eggs.
Month 2, 300 eggs from 45 butterflies, 10% survive would give us 1350 adults, 675 females.
Month 3, following this exponential increase might mean we have over 10,000 of each sex.
Month 4, we would have almost 152,000 females, 152,000 males.
My figures are conservative. You can see that the number of monarchs increase exponentially as the season progresses… or rather should increase. But over the past 10-20 years, the social wasps and other factors may have changed the habitat so much that Month 1, for me, is now the equivalent of February. There is hardly time for the butterflies to build up numbers before winter.
Here in Blockhouse Bay, ten years ago, “Month 1” would have been August or September. However, it’s been getting later and later, and now “Month 1” is February. By the time winter comes, instead of there being tens of thousands of monarchs flying around Auckland, it could be closer to a thousand. No wonder I’m not seeing them in historical overwintering sites!
Not all of the butterflies survive the winter. There are hardly enough to sustain the population, and not enough to grow the population! And we hear of overwintering colonies that are being devoured by rats – and with the vagaries in our weather these days...
Where is “Month 1” in your corner of NZ, your street... your community? You might have a thriving population of monarchs in your garden… but for the population to be stable, there needs to be more safe butterfly habitat around/less social wasps. When someone says to me “there are lots here (monarchs) in my garden”, I know what they mean, but I have a nationwide focus:
My experience:
1. I have been driving a car for 60 years. I have memories of driving from A to B and seeing monarchs flying across a street in front of me. These are individual monarchs, and I later learned from Professor Myron Zalucki (at a MBNZT conference) that these were more than likely females, flying off to find the next milkweed to lay more eggs. I hardly ever see monarchs flying from house to house now.
2. In the 20th Century everyone who went to school in NZ learned about monarchs, as it was a simple and captivating way to teach about metamorphosis. Today, a high number of children and adults have never experienced the monarch at school – some of these people have migrated from densely populated urban areas where there is no "wilderness", and English is their second language. There are other people are only interested in indigenous species. Some people spend more time on digital devices rather than experiencing the outdoors.
3. Do you remember when it was the norm to run out of swan plant – we would be desperate to find more swan plants to feed the hungry caterpillars? Today, because the earlier generations of monarchs become food (protein) for juvenile wasps, the swan plant is becoming unmanageable in some areas. “It’s a noxious weed,” some people are saying. Could it become listed as a pest plant?
4. I have had WWOOFers staying with me for 30-something years. Ten years ago, when they looked for monarch eggs it was not surprising to find leaves with three eggs on. The WWOOFer would get so excited! Now, we are lucky if we find any eggs at all.
Rowena Algar-Magee in Australia found one leaf this summer with SEVEN eggs on it!
5. People from close to overwintering sites typically say there are less monarchs each year – but since a report and map published in 2004 of Christchurch overwintering sites there are no official records – this is all anecdotal evidence.
How can you help:
1. Tag monarch butterflies. If you can do so, please join our tagging project. More information here.
2. SHARE! If there are teachers in your immediate network, please share this blog post with them. If you’re on social media, could you post something on your Facebook page? Do you post on Neighbourly? There is a suggested message* below – feel free to copy any images of tagged butterflies from our site.
3. Plant! Plant swan plant seed, or if you see plants at the garden centre, take home three and plant them around your house. Plant nectar flowers in spots around the garden too.
4. If you are raising monarchs, or covering swan plants, be careful about HYGIENE. Don't raise monarch diseases - keep everything sanitised and clean.
5. The majority of our work is funded by our financial memberships or donations. If you are not a financial member, we would welcome you – more information here. If you would like to make a donation, there is a green button at the top of this page.
6. Even if you're not interested in monarchs, remember that the monarch is an indicator species: with its large wings and bright colours it is much easier to see than other invertebrates. Are our native invertebrates likewise under threat? Are you not seeing them because you don't know about them, because you don't look for them - or because they've disappeared?
7. If you would like to do more, or have a suggestion, please add your comment below, or feel free to email me, .
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* Read this to understand why the monarch butterfly is under threat in NZ : https://www.nzbutterflies.org.nz/monarchs-and-maths-jacqui-knight/ .
Those are some sobering statistics for our Monarch butterflies. I have noticed quite a fluctuation on where Month 1 begins and if/when it ends. Sometimes I have had monarchs go right through winter.
This year particularly I have noticed a lot of people asking where all the Monarch butterflies are.
We live on the North Shore in Auckland. Last year we had to buy new swan plants as we had so many caterpillars. We put newly hatched caterpillars into a netting cage to protect them from wasps. We released many butterflies to the delight of my granddaughter.
This year 2025, it is Feb. So far not 1 egg on our large aphid free swanplant. In fact I have only seen 1 monarch butterfly all Summer
That's a good way to protect the caterpillars from social wasps. I was wondering what we could do to mitigate the damages from wasps.
Most people already use caterpillar castles (https://www.nzbutterflies.org.nz/product/caterpillar-castles/), mosquito nets over their plants, or have butterfly houses.
Hey Gaylene - that's exactly what I'm talking about. We encourage people to use our caterpillar castles, and of course there are many other ways you can raise monarchs safely. I myself have a butterfly house, but even a mosquito net thrown over a swan plant helps! I have known monarchs to lay eggs on the outside of the netting, where it touches the swan plant, and the caterpillars when they hatch, crawl through.
Great article. Very informative. I have had swan plants growing every year. I had 12 caterpillars this year. This was probably due to having only 3 plants. (I’ve bought more).
Not sure if you have read the book Flight Behaviour by Barbara Kingsolver. It is a novel, but does talk about monarch butterflies arriving in the Appalachian mountains in the USA, usually they winter over in Mexico, but their habitat had been destroyed.
Yes, I love this book! 🙂
I’ve been raising Monarchs in my castle all summer. I’m in Northland. Eggs are few and far between and I make a daily search of my many plants. I’ve probably raised about 50 monarchs and there are several flying round my garden most days, but the egg situation is dire. It’s not so much the wasps this year, as I have dealt to dozens of nests which they conveniently start on a long wooden fence., but it’s the eggs! Lack of that is. I know the ants eat them and there are plenty of ants about. Nobody has needed plant from me this year in my community. Most unusual.
We need more people like you, Lesley!
I haven’t seen a single free flying monarch this year. Early on I found three eggs which I brought inside and successfully hatched two butterflies. That’s it. There have been no other eggs. This is a massive decline. I see wasps scanning our well established swan plants every day. I’m guessing they either eat the eggs themselves or the caterpillars as soon as they hatch.
I think you're right, Tina! They take the eggs and caterpillars back to the nest to feed the juvenile wasps. The wasps are just getting stronger!
Greetings from Taupo.
Each year I grow about 50 plants in my glasshouse and have potted host plants that I put outside to gather eggs. I also do a caterpillar rescue service for people who are running out of food by posting in local community pages.
This year I have only had one response to multiple ads placed and am only picking up a few eggs on my host plants.
Very unusual.
Last year I raised 240 this year to date I have had 11 hatch and about 10 to hatch soon.
Disappointing to see such a marked decline.
It's a sad state of affairs, Clint. 🙁
It's common for there to be three or four adults (mostly females I think) drinking nectar from the zinnia flowers in my (predominantly vegetable) garden in mid-town Whakatāne. I see them over-wintering in a tree on the nearby riverbank. I don't grow any swan plants but there must be lots around Whakatāne because the number of butterflies doesn't seem to have changed in the last four years. At least not here, not yet.
Yes there are/were monarchs overwintering in trees near the campground. It would be excellent if someone could monitor these, noting dates and sizes of the cluster. This information could also be posted to inaturalist. Please contact if you can help.