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  • Andy Davis

The scariest paper about monarchs that I've ever read (and most people haven't)

Hi everyone,

Did that title get your attention? No? Does the deaths of millions of monarchs EACH YEAR get your attention? Ok now that I have it, let me tell you about a paper that scares the hell out of me...

There is a little-known paper in the realm of monarch research that I never thought got as much attention as it deserves. It was published back in 2001 in the Journal of the Lepidopterists' Society, and it was by McKenna, McKenna, Malcolm and Berembaum. The title of the paper did not mention our beloved monarch, which is why it may have gotten overlooked.

This paper was about road mortality of butterflies - in other words, butterflies killed by cars. This is a huge issue in North America, and given the recent efforts to enhance roadside habitats, and 'butterfly corridors' etc., plus having more and more cars on the roads, this is something that is only going to increase. Before this paper came out, there was virtually nothing in the way of scientific research on this subject, and the main goal of the study was to fill in this big gap. For this project the researchers attempted to quantify how big this problem really is (or was back in 2000). Essentially they walked along 13 specific roads in the State of Illinois on a weekly basis throughout one whole summer and fall, and they picked up all the dead butterflies they could find. The roads ranged from "country roads" to major highways, with speed limits between 35-65mph.

Each week they sorted the dead butterflies by species, or at least identified what group of butterflies they belonged to. The family Nymphalidae (which monarchs are a member) was one of the more abundant ones. If you're wondering, the most abundant group of butterflies (by far) were those in the family Pieridae, which are sulphurs, yellows and whites.

Here's the first scary bit from the paper: in the summer, the weekly mortality rate of all butterflies on the busiest roads was about 25 dead per 100 meters of road. The number was smaller on less-busy roads. That doesn't seem like much, but the researchers then extrapolated these rates to the entire state of Illinois,which has "2050 miles of interstate, 276 miles of toll road, 14,892 miles of highway and 120782 miles of other roads" according to their research. They estimated that the total number of butterflies killed on these roads in one week in the summer was 20 million. That's 20 million. In one state in the U.S. In one week.

Now for the monarchs. They found more dead monarchs during the fall, which they attributed to the monarch migration passing through the region then. In terms of numbers, they estimated that about 500,000 monarchs were killed during one week in the fall in the state of Illinois. That's insane.

So that was just for one state in the monarch flyway, and for one week. But what about for the rest of the migration? A while back I did some back-of-the-envelope math to try to answer that question and to get a handle on the bigger picture, that is, how any monarchs die on roadways throughout the flyway and for the entire migration season. So to do this I took the estimate from Illinois and extrapolated it to the rest of the states that fall within the flyway. Essentially what I did was to take the total land area of Illinois (this can be looked up easily), plus the mortality estimate of 500,000, then I just scaled this up the the total land mass of all states in the central flyway. This assumes each state has the same amount of roadways (which they don't), but I was only trying to get a ballpark figure here. Also, since that estimate was only for one week, I then assumed the migration takes about 4 weeks in each state to pass completely through, and I multiplied the whole thing by 4. I won't bore you with the numbers from all of this, but in the end I came up with a staggering number of 25 million. That's how many monarchs I estimated get killed each year on roadways during the entire fall migration. To put this into perspective, in the past year or two the number of monarchs at the overwintering sites in Mexico was estimated to be around 50 million.

I'd like to end on a high note here but I can't. This is VERY. SCARY. STUFF.


The science of monarch butterflies

A blog about monarchs, written by a monarch scientist, for people who love monarchs

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