People have told me that butterfly numbers in Vermont seem to be lagging this year. All except for one group that is - the Tiger Swallowtails. These yellow and black butterflies continue to be out en masse but with three similar looking species, they can be difficult to ID.
Which of these butterflies is the less common, Eastern Tiger Swallowtail?
Read on to find out the answer to this weeks’ quiz!
The butterfly on the right is the Eastern Tiger Swallowtail. The butterfly on the left is the more commonly found Canadian Tiger Swallowtail.
Tiger Swallowtails can be tricky to ID due to variable field marks and a new third species, the Midsummer Tiger Swallowtail, that neatly fits into the gap between the slightly smaller Canadian Tiger Swallowtail and the slightly larger Eastern Tiger Swallowtail. However, there are a few reliable traits to pay attention to for ID on this underside view.
First, at the tip of the fore wing on both species is a band of yellow spots. In the Canadian Swallowtail on the left, this band is continuous with each segment appearing blocky with straight lines. The band on the Eastern Swallowtail has more prominent black lines between the segments and each segment is more rounded in appearance.
Second, just above the body on the hind wing, each species has a black V pointing towards the tail end of the butterfly. The bottom line of this V is more prominent in the Canadian Swallowtail taking up over 50% of the wing cell where it occurs. In the Eastern Swallowtail, the bottom line of this V is less prominent, taking up only 10 – 40% of that cell.
You might have some luck with supplementing your ID if you can follow a female to her egg laying site. The Eastern Tiger Swallowtail is generalist using various plants in the Magnolia, Rose, Birch and Willow families, although they prefer Black Cherry and the more southern, Tuliptree. The Canadian Tiger Swallowtail prefers members of the Birch family such as birches and alders, although they will use Apples and Black Cherry as well.
You can read more about Tiger Swallowtails of Vermont from Bryan Pfeiffer, although this resource has not been updated yet to include the newly described Midsummer Swallowtail.
Photo credits:
Canadian Tiger Swallowtail by Kent McFarland (iNaturalist)
Eastern Tiger Swallowtail by Stephen Lewis (iNaturalist)
Ready for next weeks’ quiz? Click here!
