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  • Andy Davis

The forgotten study of insect road mortality, from Doug Tallamy's lab

(the image above is from an online article about roadside mortality of monarchs, from Texas A&M -

Hello everyone,

As this blog title indicates, there is a very important study out there that I think has gotten lost in the weeds of the research and conservation world, and so today I'm going to (very briefly) refresh everyone's memory of it. I think it's a study that everyone should know. If you weren't aware of it, I bet you at least know one of the authors - it was conducted in the lab of one of our most important conservationists of our time - Doug Tallamy. Yes, that Doug Tallamy - the scientist who wrote the book "Bringing Nature Home", and who is an avid advocate of all things nature, especially the insects that run this planet. I love Doug's message for how people can conserve insects by making their yards more natural, and I can't say enough good about him.

Anyway, a few years ago, a study was published by Doug and two of his students, William Keilsohn, and Desireé L. Narango, on the topic of road mortality of insects. This is a very dicey subject, and one that I have researched myself, and have covered here in prior blogs. For those readers not up to date on this, there is a growing movement by certain conservation groups to create insect and pollinator-friendly habitats next to roads, including interstates. There is even government legislation being proposed, and bills being considered to this effect. The logic behind this movement is that these roadside strips, collectively, make up a sizeable chunk of real estate (millions of acres), which is not privately owned. Plus, by adding habitat, these areas would no longer need to be mowed, reducing labor costs. And finally, doing this would make roadsides look pretty. All of this would be a boon for insects and pollinators, or so the narrative goes.

This all sounds great in theory, but I, and a number of other scientists have not been on board this train, because there is research showing some very serious negative impacts of roads to insects, that I think is being purposely overlooked. I myself did a study showing that the noise alone from these areas is enough to cause elevated heart rates in monarch caterpillars (see the blog on this). And, I've also done the math on the number of migrating monarchs killed each year on roadsides - it's a staggering number (see that blog too). Then there is the pollution from the car exhaust that the insects breathe in, or eat (the roadside plants take up the chemicals in their tissues). There is even more than this too. Anyway, I think Doug Tallamy is in my camp too, after his group conducted their study.

The paper was published in 2018 in the Journal of Insect Conservation (link here), and was titled, "Roadside habitat impacts insect traffic mortality." I recall reading this paper when it came out and was shocked that it did not cause more of a stir. I could be reading into things too much here, but I wonder if it is because their findings did not support the promotion of insect habitat near roads... hmmm...

Here's a breakdown of the study, and it was really quite straightforward. The researchers wanted to know how different types of roadside habitats affect the number of insects killed on the roads. They evaluated three different roadside habitat types: 1) woodland (i.e. forest next to road), 2) typical grass lawn, and 3) meadows - which is exactly what it sounds like - tall grass, wildflowers, etc, which sounds like ideal insect habitat. Here is a direct quote on this part - "In June of 2015 we selected 30 sites in Delaware, Maryland, and Pennsylvania with a posted speed limit of 70–105 km/h and similar intact habitat on both sides of the road. Sites were separated from each other by at least 400 m and while they varied in annual average daily traffic, all had high traffic volumes." Me again - they also recorded whether or not the roads had a vegetated median of this same habitat. Once the sites were selected, then the researchers conducted walking surveys along the roadsides once a week during the summer, and they picked up every dead insect they found, and brought it back to their lab for identification. Pretty simple, right? The only real difficulty apparently was in trying to identify the remains of all of the mangled bugs.

Here is what they found.

First, a direct quote regarding the insect collections - "We collected 6371 dead insects during this study representing 106 taxa. Scarabaeidae (3007), Bombus (850) and Libellulidae (239) were the most abundant taxa killed by vehicles. Coleoptera (3480), Hymenoptera (1734), and Lepidoptera (500) were the most abundant orders." In other words, the most abundant insects killed on roads were beetles, bees and butterflies.

Now, here is the big result, and if you have access to the paper you can see this for yourself - the habitat type that resulted in the most insects killed was the one that conservation organizations are trying to replicate right now - meadows filled with tall grass and wildflowers. Second on the list was grassy lawns, and the lowest mortality was along wooded roads. Also, in all habitats, whenever there was a vegetated median, the level of mortality doubled! This pattern was the same or nearly so for almost all insect groups.

That's right. Meadows of wildflowers next to roads result in higher insect mortality than there would be if there were just grass next to the road. In other words, turning grass into wildflower meadows along roadways (which is the growing trend) will increase insect mortality, based on these findings. By my interpretation of the graphs in the paper, the level of mortality within meadows is double that of grassy habitat. Or, how about looking at it another way, if you want to reduce insect mortality on roadways by 50%, then remove the insect habitat - mow it down!

Obviously, from reading this blog you can see what my take on this study is, but don't just take my word for it. Here are two more direct quotes from the paper on these results - "In all cases, however, our data suggest that insect mortality from vehicle strikes is unacceptably high and ways to reduce such mortality should be investigated before wide-scale roadside pollinator plantings are encouraged." "...our study suggests an imperative for research on several fronts to ensure that roadside restorations do not function as ecological traps for the very species they are designed to conserve. "

OK, so now you are at least aware of this forgotten study, and I encourage folks to get a copy of it to find out all the details. I guess what you do with this information is up to you.

That's all for now.


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The science of monarch butterflies

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

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