They counted 12,260 monarchs on the California coast this winter, the third lowest tally in almost 30 years, and the native milkweed that every caterpillar needs vanished from Midwest farm fields long before that

Every Thanksgiving, volunteers walk the same groves along the California coast and count monarch butterflies by eye.

This past winter they covered 249 sites and came back with 12,260 butterflies.

That is up from the winter before it, and it is still the third lowest tally since the counts began.

In the 1980s the same coastline held millions of them.

And most of what went wrong happened 1,500 miles inland, on farmland, to a plant nobody was trying to protect.

There are two monarch populations

Because almost every headline about this animal blurs a distinction that the biologists never blur.

Monarchs west of the Rockies winter in groves along the California coast, and that is the population the Thanksgiving count measures.

Monarchs east of the Rockies fly to oyamel fir forests in the mountains of central Mexico, as far as 3,000 miles from where they started.

The two groups are even counted in different units. One is tallied as individual butterflies, the other as acres of occupied trees.

They also move independently from year to year, so a crash on the California coast says nothing certain about Mexico.

Mixing the two is the most common error in coverage of this species.

What the two counts say

And read separately, they tell a story with more texture than a single collapse.

The western count logged 233,394 butterflies one winter, then 9,119 the next, a fall of roughly 96 percent in twelve months.

The season after that it came back to 12,260, which is a rise on paper and still near the floor.

The eastern population is measured instead by the forest area the clusters cover when they arrive in Mexico.

That figure rose to about 7 acres this past season from roughly 4 the year before, an increase of 64 percent.

Thirty years ago the same clusters covered close to 45 acres, which is the number worth holding on to.

The plant every caterpillar has to have

Which brings this to a weed that most people pull out of the border without looking at it twice.

A monarch caterpillar eats milkweed and effectively nothing else, because the plant carries bitter cardenolide toxins that the caterpillar stores in its own body.

Those toxins are what make the adult butterfly unpalatable to a bird, and the orange warning color advertises them.

So a female lays only on that family of plants, and a field without it produces no monarchs at all.

For most of the last century the Midwest had milkweed standing everywhere, including in the rows between corn and soybeans.

That farmland is where the bulk of the eastern population was actually born.

The decade the rows went clean

Then the chemistry of a cornfield changed, and the date on that is worth getting right.

Glyphosate had been sold since the 1970s, but it could not be sprayed over a growing crop without killing the crop along with the weeds.

Herbicide tolerant soybeans arrived in 1996 and corn followed in 1998, and by 2011 they covered 94 percent of the soybean acreage.

From that point a grower could spray a whole field and leave nothing but the crop standing in it.

Milkweed inside agricultural fields fell by about 81 percent between 1999 and 2010, and estimated monarch production across the Midwest fell by the same share, as the study set out.

Counting every habitat together rather than fields alone, milkweed was down 58 percent across the region.

None of that shows up in a butterfly tally until years afterward, which is why the coastal groves are still walked every November, as the count reports.

What to plant and what to pull

Which leaves the part an individual can actually act on, with one caveat that usually gets left out.

Planting native milkweed in a yard does help, and seed for regional species is sold almost everywhere.

Tropical milkweed is a different matter. In warm states it never dies back, so it lets a protozoan parasite build up on the leaves.

Researchers have tied those evergreen stands to higher infection rates and to butterflies that break off the migration and stay to breed.

So the rule is regional species, and cut back anything still green in midwinter.

Federal protection remains unsettled. A proposal to list the monarch as threatened has sat without a final decision for more than a year.

The same gap between an obvious fix and a funded one turns up around an offshore project or a ground mounted solar site.

A garden will not replace a hundred million acres of farmland, and nobody serious argues that it will.

What a garden does is keep the plant itself in the landscape while the larger argument grinds on.