Monarch numbers tend to see-saw up and down, as do populations of any species in a biological community or ecosystem. Scientists often refer to the 'boom and bust' of a population.
The boom is when the population grows exponentially rapidly, but is then followed by a bust (when the population falls back to a minimal level). It's rather like a roller coaster. Everything depends on the natural predators and parasites in that particular ecosystem, plus the weather, landscape and natural events.
Did you ever consider that a monarch caterpillar is a predator? They are herbivores... and can rapidly destroy milkweed.
Everything is connected
Where there is plenty of milkweed (e.g. swan plant) the monarch will thrive - but so will the aphids. Other monarch predators and parasites will also thrive so this depletes the monarch population. Take a look at this chart and you can see how everything is connected.
When the MBNZT has done research in the past, because we have heard that there is a dearth of monarchs, we also get many people saying 'but I've raised hundreds' etc. And... it is the people who believe they are seeing LESS monarchs that get in touch as they are concerned. The people who feel they are seeing more monarchs do not, of course, raise an alarm. So it is very hard to judge.
Are they REALLY helping?
The sad thing is that a lot of people are trying to 'help' the monarch population by raising hundreds, even saving diseased monarchs, which of course does not help the monarch at all... it just helps the disease thrive, and weakens the monarch population.
This is why the MBNZT is keen to have more of our members (and non-members) participate in MLMP (Monarch Larva Monitoring Project), a citizen science project involving monarch-lovers in NZ. In previous years this project has only been open to volunteers from North America but now New Zealanders can participate and provide useful data for monarch research.
The programme was developed in 1997 by researchers at the University of Minnesota to collect long-term data on larval monarch populations and milkweed habitat. Find out more under the RESEARCH tab on this site.
However, I personally believe that there are fewer butterflies around now than there were 30 or 40 years ago. I am not talking about in my garden but generally, around NZ. Monarchs depend on humans planting and growing milkweed - and humans are concentrated in urban areas. When I was young (I'm now in my 70's) I would see more butterflies in urban areas than I do today.
Male monarchs tend to hang around the milkweed patch (swan plant) as it is where they will find females... laying eggs. Female monarchs can smell milkweed from up to 2km away so when they have laid eggs on one swan plant (and possibly mate with a male or males) they will move on to milkweed nearby and so on, moving around and about the community.
Ten or 20 years ago I would see more on a drive from one side of Auckland to another... or passing through urban areas as I travelled around the country, than I do today.
Another thought is the number of people who comment after I've given a talk to a gardening club or service organisation, that they remember the number of different butterflies (and other insects such as dragonflies) when they were growing up that they don't see today.
We need to do more research! Or rather, we need more citizen scientists! Will you help us?