Where are the Eastern Black Swallowtails?

Has there been a drop in the Eastern Black Swallowtail population this year?

I am in upstate New York. The last three years, a friend of mine has been raising Eastern Black Swallowtail butterflies from caterpillars and eggs she finds in her garden. In 2019, she found lots of them in July and August, so much so that she had to buy extra dill at the farmer’s market just to keep them all fed! Last year, she found a few, but nowhere near as many.

This year, she has not found a single black swallowtail egg or caterpillar in her garden, despite providing plenty of dill and other food plants, as well as flowers for nectar.

My observations while out in our local nature preserve seem consistent with this: Last year, for most of the summer I would spot at least one and often several black swallowtail adults on the wing almost every time I went out for a hike. This year, I have seen huge numbers of spicebush swallowtails in flight, but very few black swallowtails. (Since they are both large, dark swallowtails, they can be hard to distinguish, but I have learned to tell the difference as long as I can get a good look at the upperside as they fly past.) (It is possible I’m overestimating the number of black swallowtails I saw last year–I was not as good at distinguishing the two back then.)

We hope our unscientific observations are not an indication of a larger-scale drop in the black swallowtail population, but we’re wondering if there have been any other reports of a lack of black swallowtails this year?

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I am growing several carrot family plants and have had a few black swallowtails in my yard in Michigan, though not as many as last year. I recently went to a community garden where there were plenty of them, but in general I feel like I have seen less butterflies this year. I recently attended a talk on butterflies where the presenter told us many species are in decline due to pesticides, urban expansion, climate change, among other things.

They are all in my front porch! Only two pupae. I’ve noticed a decline in butterfly population overall this year. It’s hard to say why. A few years ago there were tons of Nymphalis l-album in the fall - this year I have hardly seen one, in the spring or in the fall. It’s been very dry here (Winnipeg MB), so I thought that might have something to do with it.

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I have several fennel plants that I planted in my flower beds specifically for Black swallowtails. The numbers are definitely down from the past few years but not to zero by any means. In fact, just last night I noticed a final instar (or penultimate) caterpillar on Daucus carota in a neighbor’s wild field.

In general, most butterfly species are down this year (in my beds) compared to prior years. Monarch numbers are probably less than 50% of last year.

Having lived in upstate New York for the past four years, I would say (anecdotally) that the swallowtail numbers here seem to swing quite a bit from year to year. I think this year has been quite good for spicebush swallowtails and rather poor for black and giant swallowtails. I didn’t have much luck finding black swallowtails this year either. The number of observations on iNaturalist would seem to agree with this drop in numbers – 937 observations in NY from Jan 1 to Sep 15 in 2020 and only 612 observations in 2021 (although this isn’t corrected for user activity on this site or other biases). Similar trends can be observed for giant swallowtails this year too. The numbers in more southern states seem much more stable though. I would not be certain that this is a large scale drop though; could just be a natural fluctuation in the north.