Is tropical milkweed harmful in Iowa

Asclepias curassavica (tropical milkweed)

From Debbie Jackson, Monarch Watch Conservation Specialist

"So many people are hearing tropical milkweed (Asclepias curassavica) is harmful to monarchs and the migration. It is - in SPECIFIC locations.  It is certainly NOT harmful in Iowa or states that get frost.  Dr. Chip Taylor was asked his opinion on the matter and here is his response."

Dr. Chip Taylor

I’ve been asked by several people to comment on some recent research on Asclepias Curv that has gotten a lot of press.

Here is an example. More bad news for Monarchs - Study shows climate changes devastating effect 

I want to make it clear that I have NO issue with the science, but have distinctly different take on the interpretations and implications of the study.

First of all, A.c., per se, is not bad for monarchs. It’s their ancestral base. It’s the dominant milkweed throughout the tropics and both it and monarchs occur in regions with extremely hot climates. Monarchs use A. c. in central AZ which is about as hot as it gets. If future climates are as hot as AZ in most of the monarch range, there will be few milkweeds of any species and virtually no monarchs. People aren’t putting any of these studies into the larger context - either at a continental scale or in view of future climates.

A.c. gets a bad wrap - partly because it’s not native, partly because it grows all winter along the Gulf coast such that multiple females bearing O.e. spores visit the relatively few plants available thus depositing spores on the leaves in the process of laying eggs. That dynamic has the potential to increase the incidence of O.e., which does depress the population. That said, the easy solution is to cut the A. c. back to the ground twice a year at critical times. If A.c. was well established and abundant along the coast, it would be one thing, but it is almost exclusively found in gardens and quantitatively is a TRIVIAL component of the collective populations of milkweeds.

Looking at this in the context of plant/insect dynamics, the responses of milkweeds to increased temperatures and CO2, as well as stress, which can change the amount of toxins in the plants, the amount of latex or the nutritional quality, are all part of the monarch’s world. Monarchs already deal with a broad spectrum of milkweeds species which vary greatly in their toxicity, responses to stress, latex production, nutritional quality and rates of senescence, etc.

For me, the bigger issue is maintaining the migration itself. With that perspective, and with my eye on future climates, I see all of the issues involving A.c. as distractions from what we should be paying attention to, namely, that we are loosing 1-2 million acres of habitat each year - mostly due to development (at present). Unfortunately, our restoration efforts aren’t keeping pace with those losses. We are losing the battle - this year’s large population notwithstanding.

Here is another point, more obscure, and I won’t explain all the details, but I’m speaking from data, the impact of planting A.c. varies greatly across the latitudes and especially the longitudes from the Rockies to the east coast.

At best, we should be using the concerns about A.c. to leverage the planting of native milkweeds without sending the message, since some people are not very discerning, that all milkweeds are bad - and yes, some people are reacting in that way. Negative messages generally don’t work and haven’t been terribly effective in cutting down the planting of A. c.

We need a more positive approach to how we engage the public about this species.