Among many features of the farm was pasture thick with milkweed. We did not interfere with it. As a kid, it was trivial during the summer to find monarch caterpillars. Normally, we'd bring one or two into the house each year, put them in a jar that we'd refresh daily with milkweed and watch the process of the caterpillar growing, entering a chrysalis and emerging as a monarch butterfly.
Twenty five years later, when my city kids were the right age, I eagerly took them out to the pasture to find monarch caterpillars. The pasture was just as thick with milkweed as I remembered, but where is was once easy to find monarch caterpillars, year after year there were none to be found. Only one year were we able to find any.
I too mourn the decline of the monarch butterfly, but whatever the cause, it can't be a mere 20% decline in milkweed population. My parents' farm still has the capacity to support the hundreds of monarchs it once did, yet as far as I can tell they are gone.