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  • Andy Davis

Roadside mortality of butterflies - take two

Although I've talked about this issue in a recent post, I'm going to revisit it here again because a new scientific study just came out last week on this subject, and I'm pretty sure it's something that monarch folks should know about.

It was in the Journal of Insect Conservation, and it was done by a group of Canadian researchers (my kind of folks) - led by James H. Baxter-Gilbert, from the Department of Biology, Laurentian University (but now in Macquarie University in Australia). The paper has the ominous title of "Road mortality potentially responsible for billions of pollinating insect deaths annually". So it was not a paper on monarchs, it actually focused on all insects (even flies!). The authors of this study basically attempted to quantify how many bugs get killed by cars on roads, which is a lot like the study by McKenna et al in 2001 in the Journal of the Lepidopterists Society, that I discussed in the previous post - although for some strange reason, the authors of this new paper did not cite the McKenna study - clearly an oversight.

This new study took place in a fairly remote section of Canada, in central Ontario, along a 2km stretch of the Trans-Canada highway. For the folks who aren't familiar with this part of the world, I can paint you a picture of it - I've travelled this road myself in years past. This part of Canada is mostly flat, and is mostly wooded, with scattered openings of rocky barrens or bogs. There are few towns or other signs of people - in fact, this study took place on a section of highway that passes through a First Nations Reservation, which just means the area was basically untouched wilderness. I might add there are no old fields full of milkweed, or even any farmland at all. It's just rocky wilderness. But there is a steady stream of car traffic through this road, since it is a major thoroughfare for people travelling east or west through Canada. The authors cite information from the Canada government that says about 9700 cars travel this road each day. so it's definitely a well-travelled highway, despite the remoteness.

Here are a couple of pictures of the side of the road from this section of highway, which I downloaded from google maps (just do a google search for Magnetawan First Nation and look at the satelite view or to get photos). You can see that there is not much there. I'll explain later why I'm going to so much trouble to describe this location. Apparently these pictures were taken in the summer too.

So the methods of this study were much like the 2001 McKenna study; the researchers walked this 2km stretch of road throughout the summer months (over two years - 2012 and 2013) on a regular basis, picking up dead bugs on the side of the road. These guys were very thorough too - they collected everything they found, even the wasps and flies. At the end of the censuses they sorted out the specimens according to the type of bug it was, but they didn't record which species they were. So at the end of each day they would have numbers like 47 flies, 56 Lepidoptera..., and so on. At the end of the study they basically just added all the numbers up to answer the original question (i.e. how many bugs get killed...).

The results for the number of bugs that were killed by cars on this 2km stretch of road were staggering. In one year there were over 4000 individual bees and wasps, over 8000 the next year. For Lepidoptera, there were about 2000 the first year and 2700 the second. The total number for all bugs in both years was 117,000 - remember this is just for one tiny stretch of road. Then, the authors extrpolated these numbers in a couple of ways. They came up with an average rate of mortality, per km of road, per day. For Lepidoptera, the rate was about 10 individuals/km/day. Then, they estimated how many bugs die on this one section of highway in Ontario, Canada, plus how many die on all roads in North America, which has 7,600,000 km of road, according to their information.

So,here's the big numbers - based on their exprapolations they figured that about 500,000 Lepidoptera die on this one highway in Canada each summer, and about 9.3 BILLION Lepidoptera die on roads throughout North America each summer. Once again, this is a crazy number, and very scary. Incidentally, the authors did point out that this number is not very precise, given they were extrapolating a small sample to a much larger landscape, but I think we can all agree, the final number is likely in the billions - with a capital B. Maybe the most scary thing about this is that there's not much we can do about this problem.

But speaking of this, here's why I spent so much time describing this location, in the remote Canadian wilderness, and the answer has to do with our current conservation efforts for monarchs. As most folks know, there has been a big push to enhance roadside habitats for monarchs and other pollinators. But I wonder if this is such a good idea, now that these results are in (also the results from the 2001 study). Keep in mind that this Canadian study found tremendous mortality in an area where there was no habitat enhancement at all. It was just wilderness with a road cut through it. So what would happen if we made those areas even more appealing to butterflies? Given these new data, I would think the last thing we should be doing is encouraging more butterflies to take up residence near our roads.

As with the prior post on this issue, I can't really end on a high note here. Maybe researchers should just stop studying this issue so that we aren't made aware of this problem - that might fix things.


The science of monarch butterflies

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

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