Milbert's Tortoiseshell, Aglais milberti , "In Decline? Why?"

What’s going on with the Milbert’s Tortoiseshells? When I was a child, 40 some years ago, they were one of the most common butterflies, especially noticeable in the early autumn when hundreds would get into open barns and garages and flutter at the windows. Now? I hardly see a one. The caterpillars eat nettles. There are still plenty of nettles. A quick Google search says their look-alike species, the Small Tortoiseshell, Angais urticae- also a nettle eater-are declining in Europe as well. I find this disturbing. Anyone else feeling bothered? Any thoughts?

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the insect apocalypse?

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It’s not a stretch at all to say insects are having tough times right now, but I really do wonder why this particular one is struggling so.

I’ve never seen that many of them, but they are a fairly common butterfly in northern Pennsylvania. I saw several last year and some the year before, but despite looking, I don’t think I saw any this year. There was a lot of flooding late last summer, and I wondered if that had reduced the population this year, but I was surprised to not see one. I can’t speak to a larger decline though. Frankly it wasn’t a very good year for butterflies overall back home–numbers seemed down.

I wonder if they’re another casualty of agricultural insecticide.

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Aren’t red admirals nettle-eaters, too? They, by contrast, don’t seem to be struggling.

Milbert’s Tortoiseshell numbers were also low in Northwestern Ontario this year - I’ve only had two sightings. My best year by far was 2011 (21 sightings). In our case, the low numbers may be a result of a string of really bad winters. The overwintering generation may be struggling to endure all those -40C temps. That said, 2019 was a very good year for some species here: I saw record numbers of both Little Wood-Satyr and Tawny Skipper. Monarchs rebounded spectacularly and Painted Ladies are still on the wing. So, the signals are very mixed at the centre of the continent.


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