Dogbane Tiger Moth: A Story in Shrewd Survival
Would you rather hear this article? Click on the above audio to take a short “Walk In the Woods” at Harmony Hill!
During so many of our visits, we make a point to talk about the beauty of nature and of just how striking so many species are to the eye. There is so much happiness found in looking out over the rolling hills of the piedmont and seeing the leaves change each fall. There are birds with feathers that shine in the sunlight that make you gasp, like when a Prothonotary Warbler, the jewel of the swamp, perches in front of you. And there are flowers so lovely, like those found on Tulip Poplar, that make you smile involuntarily. Nature is beautiful! And we are her apostles, proclaiming the stunning splendor right in your backyard.
But there is another side to the gorgeous grandeur found around each corner of the woods and the fields. There is a constant struggle of survival going on each and every second of each and every day. A battle of life and death, quite literally, is always happening; even as we sit in the quiet and the elegance of what we view as a peaceful moment in nature. Though we may stand in the dappled sunlight of a relaxing morning, predators seek to capture prey, plants conduct chemical warfare to prevent competition, and other species find ways to outsmart their would-be attackers. Whether by hiding, the use of camouflage, or mimicry, some species have found the best ways to lower the risks of being eaten are by fooling a potential predator. Let’s look at a species you may not be familiar with that uses a couple of methods to outwit hunters, one method that may just surprise you!
Sitting on the ground, in the wide open of a field, a cottony white moth with an ochre wash on its head and leading edge of its wings, is perched on the fallen needles of a Loblolly Pine. Sticking out like a sore thumb, even a seasoned naturalist would wonder if it had a death wish. Sure, the small amount of yellow-brown coloration on its forewing provides some means to blend into the scattered pine straw. But with so much of this moth being white, it can be easily spotted. (As seen in the accompanying photos with this Walk in the Wood.)
The Delicate Cycnia Moth (Cycnia tenera), or Dogbane Tiger Moth, doesn’t particularly worry about blending in or shying away into a hiding spot. Evolution has given this species two adaptations to use in its fight against predation.
Without the effective camouflage we’ve seen exhibited by some of the other species we’ve focused on, it would seem the Dogbane Tiger Moth would be the proverbial “sitting duck” as it loafs out in plain sight. And this moth doesn’t mimic another lepidopteran in order to borrow safety by being a lookalike.
What does it know that we don’t?
What tricks does the Dogbane Tiger Moth have up its sleeve to be able to lower its concern over being eaten?
For the answers to these questions, let’s begin by looking at the diet of the larval form of Cycnia tenera. Now, as naturalists, we all know that caterpillars of butterflies and moths that are capable of ingesting poisonous plants can build levels of the plants toxins in their systems, then use those toxins to become anything from unpalatable to downright deadly if eaten. We can look to the very familiar Monarch Butterfly (Danaus plexippus) and the way it utilizes the toxins found in milkweed plants (Asclepius sp.) to be both distasteful and mildly poisonous to predators. As it turns out, the Dogbane Tiger Moth uses the same approach and, on occasion, the same host plants as Monarch Butterflies.
The primary host plant for Cycnia tenera caterpillars at Harmony Hill is Dogbane (Apocynum cannabinum), a highly poisonous plant with a reputation for being so toxic, the common and scientific names tell of its ability to keep dogs away. The genus name of Apocynum can be roughly translated to “away with dogs”. In fact, civilizations have used the plant to poison animals, including wild dogs, in some way for centuries. And the species name of cannabinum is a reference to the plant being used, much like hemp plants, to make fibers and threads by Native American tribes, like hemp plants. Indeed, one of the common names for Dogbane is Hemp Dogbane; another nod to the way Dogbane can be used like hemp to make fibers.
The toxins in Dogbane effect the heart and are in the family of chemicals known as cardiac glycosides. However, the larvae of Dogbane Tiger Moth are unaffected and have evolved the ability to build up the levels of the glycosides as they eat the plant and grow. Those toxins carry over to the adult form of the moth we see in this Walk in the Wood visit, causing any predator that makes the poor decision to eat it to quickly spit it out or face an unpleasant reaction.
As a side note, it is not out of the question for Dogbane Tiger Moth caterpillars to feed on milkweed plants, like Monarch Butterflies. It was even once believed that Monarch caterpillars fed on Dogbane plants.
Being mostly active at night, the primary predator of Cycnia tenera are various bat species. Bats have learned to not eat Dogbane Tiger Moths through coevolution, with previous generations of bats finding out the hard way to not make a meal of the moth. But that is not where the story ends; nor is it the only way this moth has figured out to outsmart predators.
Bats hunt by echolocation. They use a series of vocalizations to hunt, communicate, and understand their surroundings. There are species of bats with a surprisingly large repertoire of vocalizations for a variety of situations besides just seeking out food on the wing. Well, Dogbane Tiger Moths have evolved to emit ultrasonic clicks that may serve as both a warning to bats to not even think about grabbing them out of the air and to also confuse their hunting vocalizations.
You read that right! This little white and yellow moth uses a type of radar jamming in the battle of survival against the only mammals capable of true flight! While we know the moth uses these clicks to deter bats from capturing them, we don’t exactly know if it is as a warning to not eat the poisonous bat or to disrupt the signal interpretation abilities of bats. It could very well be both! If you will recall, one of the things we have mentioned repeatedly is that things in nature rarely ever have one purpose. Either way, it sends out ultrasonic clicks to effectively remove themselves from the dinner menu of bats. And it does this when it detects bats approaching that have switched to clicks to hone in on a target! That, fellow naturalists, is incredibly fascinating!
A simple observation of a small moth has led us down the path of discovering how some species not only survive, but actively deter predation. A small moth that not only uses poison to prevent itself from being eaten, but also works to beat bats at their own echolocation game! And another example of just how special moments in nature can be without leaving your own yard! Oh, there are some astoundingly beautiful faraway places; but there are mind-blowing stories and amazing discoveries to be found mere steps away from your front doors. Now, get out there and find them!