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Speaking of Nature: The life of a tiger swallowtail

  • This is an adult male eastern tiger swallowtail. A female would have a larger expanse of the iridescent blue along the bottom edge of the wings. For the Recorder/BILL DANIELSON



For the Recorder
Monday, June 18, 2018

For me, the month of June is a “quick month” because I am a teacher in a public school. First there are the state exams, then there are the course exams and finally there is the “wrapping up” phase. The anticipation of the coming summer vacation is tremendous, the preparation for it is all consuming and then — poof — it’s over. If I’m lucky, I can relax during the last week of the month.

The problem with this is the fact that June, especially early June, is a fantastic time of year to be outside. Unfortunately, year after year, I seem to miss out on most of early June. Just ask my beautiful wife, Susan, about the grumbling and complaining that she hears as I sit at my desk grading papers on a beautiful weekend instead of being outside. Such occasions are not among my prouder moments, but I am human after all.

This year, the time has zipped by at an astounding speed. The weather has played a part in this, plus the fact that there have been graduations to attend and other miscellaneous social engagements, but I find that as of today, June 18, I am absolutely stunned by how quickly things have transpired around me. What happened to the marsh marigolds? When did I miss the mourning cloak butterflies? Did the peepers sing for more than just a day or two?

So it was with great interest that I noticed an icon of spring cross the road in front of my car on the way home last week. A big, beautiful butterfly fluttered out into traffic and I winced as I saw it take a course that would inevitably result in a collision with the car in front of me, but the butterfly was spared. Had this been 20 years ago, when cars weren’t quite so aerodynamic in shape, the butterfly would have been killed, but this one encountered a pressure wave that carried it safely over the sedan and allowed it to continue across the road unharmed. The butterfly was an eastern tiger swallowtail (Papilio glaucus).

This species is particularly interesting to me because it has two distinct seasons when adults can be seen flying around. The first season begins in mid-May and ends in mid-June. This is the season when pupating caterpillars that spent the winter hibernating have had a chance to hatch into flying adults. They fan out across the landscape, looking for a variety of different host plants that include a large number of familiar trees and shrubs like cherry, willow, ash, cottonwood and lilacs.

Adults will congregate around plants that provide a good source of nectar, and they are particularly fond of plants with red or pink flowers. Males have to do a lot of the work involved in courtship, including tracking down the females, participating in nuptial flights and even enticing them with pheromones. If mating is successful, the females will seek out trees near a good supply of nectar-producing plants and there they will lay their eggs. At just over a millimeter in width, the eggs are so minute that they would go unnoticed by any but the most determined searchers. To my knowledge I have never seen one.

The average time to hatching is about a week and then the tiny caterpillars tuck in. Their goals are twofold: eat and avoid being eaten. As they grow, they must occasionally molt their skins and each period between molts is identified as an “instar.” Among the many books in my personal library I have a copy of “Caterpillars of Eastern North America” (I simply love the fact that such a book exists!) and it features a photo of a swallowtail caterpillar in its fourth and final “instar.”

At just over two inches in length, the caterpillars are narrower at the tail end and have a bulbous swelling just behind the head. This is the only instar that is green in color. In the first three instars, the caterpillars retain their odd shape, but are smaller and brown in color. This combination of color and irregular shape helps them to mimic bird droppings, which affords them with an excellent camouflage. After all, who in their right mind would scrutinize a dollop of bird poop on a leaf? So far I am happy to say, “not me.”

The caterpillars will pupate after they have finished their fourth instar, and there seems to be a general surge of this activity from mid-May to early June. However, different caterpillars may have different luck, so there is always the chance of seeing adults flying at any time during the summer. Then the life cycle repeats itself. You will see the largest concentration of flying adults in the month of July.

This second generation of the year will lay their eggs with just enough time left in the summer to allow their offspring to pass through all four instars and then weave a chrysalis to begin the process of pupating. The arrival of cold weather will stall the process and the caterpillars will go into hibernation instead. The last adults will disappear around mid September and the only surviving individuals to continue the species will be those hibernating pupae.

As the Independence Day fireworks start to burst, so too will the next generation of tiger swallowtails burst out into the world. Two weeks may seem like a long time to wait, but for me it will seem like the blink of an eye. Furthermore, since I have so many of the swallowtail’s favorite host trees in my own yard, I will be sure to blink twice before I dismiss a bird poop on a leaf. It is highly possible that one will turn out to be something quite interesting indeed. It is also highly possible that I will have to ask myself some serious questions about the way I spend my vacation.

Bill Danielson has been a professional writer and nature photographer for 20 years. He has worked for the National Park Service, the U.S. Forest Service and Massachusetts State Parks, and currently teaches high school biology and physics. Visit www.speakingofnature.com for more information, or go to Speaking of Nature on Facebook