Why ARE they so many Eastern Tiger Swallowtails this Year?

This year there are more Eastern Tiger Swallowtails in my garden than I have ever seen before. In years past I’ve seen one, maybe two, Tiger Swallowtails per day in my garden. This year I’m seeing 10, 15, and even 20 at a time in my wildlife garden.

And in every wildlife garden I visit it’s the same there, too. People up and down the East Coast are remarking on this phenomenon, too.

So, I decided to find out what was going on. When I have any questions about butterflies I go to my friend and mentor Pat Sutton who is a walking butterfly encyclopedia, and her passion for creating butterfly gardens is infectious.

Pat in turn posed the question to her friend and mentor, David Wright. His response is fascinating:

Same observations have been reported through out Pennsylvania. I have no explanation. It’s one of the still unsolved mysteries of invertebrate biology. Population fluctuations are natural; they could be the result of internal genetics (as in small mammals like voles) or the could be the result of a bad year for their parasitoids/viruses (more likely).

A simple average temperature increase doesn’t seem to do it. I did do a literature search on the melanic enzyme pathway in dark morph females. It seems that heat cannot create more black females. But the heat can make the black females a more “dusty” intermediate phenotype. Scattered black scales are converted to yellow.

This search was in response to comments on PA-LepOdes that there were more black females being seen this summer, without a substantial increase in Pipevine Swallowtails. After a week these comments ceased. I think we were observing an emergence of a fresh brood that was atttracted to flower-filled in peak bloom. It’s quite impressive to walk through a large meadow in bloom and see 200 swallowtails.

Tiger Swallowtails peak in April/May and again in July/August, although they can be seen anytime from March to October. So it is possible that the large numbers of these butterflies that everyone is reporting is just the result of this normal peak in their population.

But is it possible that the unnaturally cold (and unnatural amounts of snowfall) affected the parasitoids that normally put a damper on the numbers of adult Tiger Swallowtails?

The thing is, there are many questions about nature that scientists don’t yet have all the answers to. But Mother Nature knows what she’s doing, I’m sure. For now, I will just glory in the sight of these beautiful visitors to my wildlife garden.

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