Posted on the Monarch Watch list serve July 19, 2018
I fervently hope that people will not get hung up on studies of ovipositing preference and larval survival on different milkweed species – and not because I don’t believe the studies, but because I think many laypeople tend to draw misguided conclusions from them. These studies advance our knowledge base but do not necessarily need to guide us in making choices.
Our migratory monarchs, east and west, evolved with some 73 species of milkweed in their breeding ranges. Monarchs will lay eggs on most if not all of them, and at least some of their larvae will develop on most if not all of them. I have raised more than 20 species and have yet to find one that doesn’t fit the bill. Some larvae die on every species. Evolution being what it is, more-toxic milkweeds must have their place, and milkweed mortality may simply be the price that monarchs have paid to Charles Darwin for having adopted a toxic genus of plants as their host.
It is good-hearted and well-intentioned, perhaps, to say, “But I don’t want to plant Sullivant’s milkweed because two-thirds of all the caterpillars that hatch will die before they reach adulthood.” But that overlooks the many advantages that this species provides, and it is a purely human emotional reaction to an incredibly complex set of relationships that occur in the wild.
If we were to adopt the “some milkweeds are better than others” approach (which I wholly reject), weighing preference, larval survival and development, and adult fitness, choosing only those species that score “good” or better, we in the East would be planting only common (A. syriaca) and swamp (A. incarnata). We’d have to abandon everyone’s favorite, butterfly weed (A. tuberosa), because it is the least preferred species, even though it has the highest survival (75%, per Pocius). We might leave out poke milkweed (A. exaltata) because it is only moderately preferred, even though it is tops in larval development and fills a niche (shade) that most other milkweeds cannot. We’d ignore whorled milkweed (A. verticillata) because it is so little preferred, even though it provides late-season blooms and tender growth when most other milkweeds have begun to senesce (in the drought of 2012, whorled milkweed was highly preferred by season’s end when common milkweed had turned bone-dry)
As for Sullivant’s, or prairie, milkweed (A. sullivantii), it is highly preferred but scores low on survival (36%, per Pocius). Beyond that, it is incredibly long-lived once well-established in an appropriate habitat; blooms much longer than any other milkweed; continues to produce new, tender growth much longer than most other milkweeds; produces copious nectar that is highly preferred (in my observation) by both monarchs and a host of other pollinator insects; and is remarkably drought-tolerant (again, in 2012, it was still blooming and producing new foliage until season’s end when all other milkweeds except whorled had scenesed. About 90% of all eggs laid in my Waystation that year were on sullivantii, despite there being at least 13 other species to choose from.).
The mathematics of monarchs and milkweeds is an equation of shocking mortality. Tori Pocius’ survival studies were necessarily done on milkweeds growing in the absence of any predators, parasites and pathogens. Put those back into the equation, in the wild, and we find that regardless of milkweed species, 88% to 98% of all monarch larvae will die one way or another before they reach the fourth instar. On average, over time, more than 99% of larvae, pupae and adults will die somehow before they can reproduce, and that would be true even if the population were in equilibrium.
So, if only one in four larvae laid on A. tuberosa will succumb on their diet alone, while two of three laid on A. sullivantii will perish, but monarchs will lay, say, 100 eggs on sullivantii for every one laid on tuberosa, which species will produce more monarchs? Add the fact that sullivantii will spread vegetatively into a large clone of many stems over time, while tuberosa will have to rely on self-sowing alone. Which is better? Well, neither. Both are good, just for different reasons.
Some might still argue that syriaca and incarnata are the better choices because they are preferred, have good survival and good development, hence all the advantages we want over both sullivantii and tuberosa (in this example). And perhaps in the short term, we should use a lot of syriaca and incarnata to achieve the most rapid recovery of the monarch population to sustainable levels. But long-term, and starting now, we need all of our milkweeds, because…
Incarnata cannot fit all the niches of land-use that we need to recover. It is the only species we can use to repopulate very wet places, but it will not persist in any place that is not predominately wet throughout most of the season. Since most of the land-use areas we are talking about restoring are not predominately wet, it cannot be our sole or main reliance.
Syriaca is our least conservative species and is very good food for monarchs. It seems to grow almost everywhere, but in fact it has its own niche, if you will. It is a pioneer species that is highly reliant on disturbance for natural colonization. It is not terribly persistent nor likely to reintroduce itself in habitat that remains undisturbed for long. If our goal is to restore large areas to tall-grass prairie to the benefit of the monarch, common milkweed will only get us started. If you visit any long-established prairie restoration, you will find little or no common milkweed beyond the borders and trail edges where disturbance continues to occur. You will find very little if any in the interior of the mature prairie. And our naturalists will want to do this – restore prairie “the right way,” meaning undisturbed once established, “pure prairie in perpetuity.” Common milkweed can be planted there, or it will arrive on its own, when the restoration is first underway. But it will evaporate over time.
What species will establish and then persist in a mature tall-grass prairie restoration intended to be perpetually maintained only by occasional mowing or fire? Sullivantii and tuberosa.
The fact that common milkweed accounts for 90% of all the milkweed feedstock available to monarchs is a matter entirely of our own doing. Common milkweed cannot once have been so vey common. It is an opportunistic species that would have occupied the margins and boundaries where disturbance occurred (wind-thrown trees, flash-flood scars, heavily browsed thickets, etc.) and would have invaded prairie only with the help of soil exposed by buffalo wallows, badger dens, gopher mounds and the like. For all we know, common milkweed may have evolved with the relatively much greater disturbances caused by the Pleistocene megafauna ripping and tearing at the turf. It may have waxed and waned and waxed again with the introduction of Native American agriculture. But its current super-prevalence is entirely a product of our vast disturbance of the natural environment and destruction of habitat for all other milkweed species. We have forced the monarch to subsist, for all practical purposes, on a single plant. We have made it single-host specific, whereas it is naturally genus-specific to dozens of species. Monoculture is seldom a good thing. Our long-term goal should not be to perpetuate monoculture in different places but to promote diversity in every place.
- All milkweeds are good, but for different reasons. No one milkweed is inherently better than any other except insofar as it is better adapted to a particular place.
- We should value, protect and preserve, and restore all milkweeds for their own sake regardless of our biases toward other valued species, including the monarch. Milkweeds are highly beneficial in many ways and beautiful to behold. They are as much a part of our native legacy and responsibility as de facto stewards of all living things as are any other living things. This is our chance not only to recover the monarch butterfly but also to right the wrong we have done the Asclepias tribe. (In Wisconsin, of our 13 native milkweeds, one (sullivantii) is threatened and four are endangered, one of those (A. meadii) having already been extirpated once and only recently reintroduced on a small scale.)
- All milkweeds are excellent and highly preferred nectar plants serving hosts of beneficial insects, but they have different bloom periods. Some are early and short, some are late and long. Some have flower sizes and forms that serve different insects (whorled milkweed serves many insects that cannot access larger flowers of other milkweeds, or that appear only later when others are done blooming). We shut out many other species if we shut out any one species.
- We should plant as much milkweed as possible of as many varieties as possible where each species will thrive, as it is appropriate to the ecology of the site – not because of our human biases against any milkweed or in favor of any other species, even monarchs. Plant all milkweeds where our knowledge tells us they should grow, maintain those sites using best practices, and let nature re-take its course.