Tigers of Alaska
It’s doesn’t take much to entertain me. Last summer in Alaska, I had a few hours to fill before my Insects of Denali field course with Alaska Geographic would begin. Carrying just my camera, I sauntered off down a trail near the Denali National Park visitor center.
The vibrant yellow flowers of shrubby cinquefoil caught my eye first. Several blossoms had a single small fly basking near their center or walking stilt-like over the bristle of anthers.
Three willow leaves were sewn together in a small chamber. Tiny brown dots of frass (caterpillar scat) spilled out one end and identified it as a feeding hideout rather than a cocoon.
And then I came upon a hillside patch of bluebells. With tall, leafy stems and many clusters of delicate pink and purple bell-shaped flowers, it caught my eye right away. These were tall bluebells, or Mertensia paniculata. While they can be found as far east as Michigan, I think we typically see Virginia bluebells, or M. virginica in the Midwest. An easy way to tell the difference is flower shape: Virginia bluebells have a wide flower that constricts abruptly to a narrow nectar tube. The nectar tubes of tall bluebells are just a little smaller than the rest of the flower.
What also caught my eye about this patch of beauties is that they were being visited by a large tiger swallowtail butterfly. I’ve always loved these lovely yellow butterflies with black stripes, bits of orange and blue, and whimsical tails. Like the bluebells, they are close cousins to my friends back home.
My camera and I had a lovely time observing the butterfly dangle delicately from the flowers, nuzzle its head up into a bell, and then float airily on to the next plant. While this tiger must have been finding nectar to sip, it isn’t useful for transferring pollen. Part of the reason is that only the younger, tightly closed pink flowers contain the pollen, while the older, purple flowers provide nectar. Bees, who have the strength to pry open the pink flowers, are the only known pollinators. They are attracted by the sweet blue flowers, but then visit both shades of flowers on the plant.
Both the tall bush of bluebells with its broad, floppy leaves, and the wide, sunny wings of the butterfly struck me as being out of place in Alaska. When I think of northern plants, my mind jumps to the low, tough, waxy leaves of blueberry, lingonberry, and most other tundra plants. When I think of northern insects, I think of mosquitoes, black flies, and warm-furred bumble bees.
Not surprisingly, Canadian tiger swallowtails have an interesting suite of adaptations to help them survive in their namesake country and in Alaska.
It begins when they are eggs. Female tiger swallowtails lay their eggs on leaves on the south side of trees. This provides more warming sun exposure for the developing larvae. Temperature is important! As the temperature increases from 54 to 86 degrees Fahrenheit (due to normal changes in weather, or just a great location) the larvae can increase their growth rate by up to 500%. There is a catch, though. Larvae on birch trees won’t grow faster, even if it’s warm. Only the more nutritious leaves of aspen trees allow for such rapid growth. The butterflies must choose their host plants carefully, and hope that aspen are available.
Another counter-intuitive adaptation is that if a caterpillar survives a summer cold spell, it will then begin to grow faster. The longer and the colder it was, the faster it will subsequently grow. This is a tricky business. Summer is the most vulnerable time for these insects. When fall comes, the larvae store up cryoprotectants to help them avoid freezing, and then transform into pupae. Once properly hardened off and hidden away, the chrysalis can survive at least seven consecutive days at -2 degrees.
During the summer, though, caterpillars aren’t well-prepared for cold. One risk factor? Larvae with food in their guts freeze at warmer temperatures.
Overall, tiger swallowtails are in a hurry to complete their life cycle in a brief northern summer. The faster that a caterpillar can get to the safety of a cold-hardened chrysalis, the better. Those increased growth rates help, as well as the fact that females lay larger eggs, and the caterpillars pupate at smaller size than their southern cousins. This does mean that the adults are smaller, too, but that’s just the price they pay for making sure their offspring don’t freeze to death.
I was entertained by this pretty yellow butterfly on a warm day in Alaska, and I’m entertained now, too, as its memory helps me escape the gray days of mud season. May its bright wings bring some sunshine your way, too.
Emily’s second book, Natural Connections: Dreaming of an Elfin Skimmer, is now available to purchase at www.cablemuseum.org/books and will soon be available at your local independent bookstore, too.
For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: “Bee Amazed” is now open!