As it happens, I just got back from seeing the Monarch grove in Mexico. It's an amazing sight. When the sun comes out from behind the clouds and they all warm up and take off within a couple minutes, the air is so full of them that they darken the sky. And the mountain (in Michoacan province, about 2 hours by car from Mexico City) is beautiful and the people are friendly. If you've seen the Monarch groves on the California coast, Mexico has 2 orders of magnitude more. Well worth the trip.
Our guides, showing WWF slides, told a more optimistic story about numbers. They had declined to very low levels around 2011, but have bounced back substantially. They said the small decline in the last 2 years might be normal variation due to weather. The graphs showed random variation in population by a factor of 2 from year-to-year going back to the 1970s, so it's probably hard to conclude anything from a single year's count.
Still, if you live along the migration corridor, please plant milkweed and don't use glyphosate weed killer.
Note that there are multiple species of milkweed. If you find milkweed unappealing, look into other species. In the late 1980s/early 1990s, I found a couple of monarch pupae on some plants in the wetlands in my back yard. The plant stems were more stiff and the leaves less velvety than the milkweed I was familiar with in the local park.
I asked our local nature center if Monarchs only eat milkweed, why they would pupate on this plant. They identified the plant as https://en.wikipedia.org/wiki/Asclepias_incarnata , a variety of milkweed.
A retaining wall in my backyard has a growth of Cynanchum laeve (honeyvine milkweed) and English ivy - it looks rather nice, and every year I find a handful of monarch caterpillars happily munching away at the leaves. While the vines can spread and be very invasive if not controlled, they co-mingle with ivy very well. I greatly appreciate them being part of my garden.
I grew up next to meadows that were rarely if ever cut, in the 1980's (Northeastern US). As a young child I'd often find Monarch caterpillars, and was sometimes allowed to keep them in a small, strange box molded from glass and known as a terrarium. I'd feed them milkweed until they reached the pupal stage, then watch with wonder as the chrysalis changed from a jade-like, gold-rimmed ornament into a transparent capsule of folded orange-black wings. Finally, when the new insect emerged, we watched and waited (with still more patience) for the wings to dry, leading up to release day, when the butterfly was suddenly gone in a dash toward the sky.
Guided by my grandmother, an amateur naturalist, these were my first experiences leading to a knowledge of change, lifecycle, and above all the beauty to be found in the natural world. This simple gestation process of a curious looking caterpillar is impressed on me like no other early childhood memory. A process taking weeks, it was my first consciousness of time.
Today, those meadows are acres and acres of trimmed lawn. The smooth aesthetic of green grass pocked with dandelion has won over the wildness of a milkweed dotted field. But me, I've forsworn all lawn mowing, and should I ever own a meadow, will let it go to seed.
In fact, herbicides are crucial in the fight against one of the major threats facing monarchs.
The invasive Dog Strangling Vine is related to milkweed and monarchs will mistakenly lay their eggs on it, where they fail to mature. It is a very prolific invasive species, and is significantly outcompeting milkweed in many areas.
There are many different species of milkweed. Please, only plant the species that are native to where you live. If you are unsure what species are native to your area, contact a local plant society, for example the California Native Plant Society, the Oregon Native Plant Society, etc., and ask them. They would be more than happy to help and can tell you where to buy seeds.
edit: here is what I would consider a better guide from the Xerces Society, with links to regional seed suppliers.
It's very disturbing. I used to drive across the country on a yearly basis (i80) and would have to clean my windshield of bugs at many of the gas stops, by the time I arrived the nose of the car would completely covered. Last year I made the same trip, similar time of year as previous trips, and didn't clean the windshield once.
Things are getting really out of wack, and I'm not looking forward to being around for what might become the collapse of our ecosystem.
If it helps, milkweed is also known as "swan plant" due to the shape and colour of the flowers. Can help convince some gardeners to invest, if the word "milkweed" puts them off as it sounds unpleasant.
Almost sounds touchy-feeling, until you get to this part (of the link above)...
"... The Endangered Species Act has tremendous impact on private land ownership. And so we don't want to get at a point as a partner with DNR with our other partners out there, we don't want to get to the point where there is no choice but to do this then it becomes mandated under the endangered species act. We'd rather have voluntary participation."
tl;dr; If the DNR shows up, we'll finally have to do something about our water quality.
My mom used to raise Monarch caterpillars for her elementary school class. I remember that it was always difficult to find milkweed even back then. We had to walk a certain hiking trail to find some and then search the plants for a few caterpillars. I don't ever remember seeing large patches of milkweed, only a few scattered plants with perhaps a dozen or so plants over a roughly one mile hike.
I was in Monterrey and San Luis Obispo last December and ran into the Monarchs on their way migrating down south. It was quite a sight catching them randomly on forest hikes: https://i.imgur.com/315pC3y.jpg
Has anyone ever bothered to ask themselves that the ridiculous propagation of the idiotic ritual killing of every terrestrial arthropod by suburbanites throughout every municipality east of the Mississippi might actually be a major contribution to this outcome?
California has less bugs... In the midwest in most cities/suburbs you have to maintain your lawn for the purpose of keeping the bugs and noxious weeds down. If you don't, it's a code violation. I think the real reason is that mosquitos like to hide in them... Find a way to keep the mosquitos at bay and maybe we can have more natural lawns.
Anecdata. I live in the same place I grew up. As a kid, the monarchs passing through was a fun part of the season. Thousands and thousands of butterflies over a few weeks or so. Now, we are lucky to spot a single monarch.
Some milkweed came up next to my garage a few years ago. I let it go and it spread. This last summer I saw Monarchs around it a couple times. Once I thought I saw one lay an egg and later when I checked the plant, I saw what looked like an egg in that spot. I never saw any caterpillars. There are other insects that also like milkweed and I saw plenty of those.
Our guides, showing WWF slides, told a more optimistic story about numbers. They had declined to very low levels around 2011, but have bounced back substantially. They said the small decline in the last 2 years might be normal variation due to weather. The graphs showed random variation in population by a factor of 2 from year-to-year going back to the 1970s, so it's probably hard to conclude anything from a single year's count.
Still, if you live along the migration corridor, please plant milkweed and don't use glyphosate weed killer.