As the name implies, the frisky hackberry emperor butterfly is a North American native whose larvae feed on wonderful native hardwood trees called hackberries. On a recent foray to visit invasive Tree of Heaven and dastardly spotted lanternflies, I was entertained by lovely hackberry butterflies who shared their secrets for collecting vital nutrients from two unlikely sources. These two nutrients are sugar and salt.
Hackberry leaves are the source of food for larvae of the hackberry butterfly. Tree lore has it that hackberry is a corruption of hagberry, a name given by European colonists to the American Celtis tree due to the resemblance of the hackberry fruit to the fruit of the European cherry tree, Prunus padus.
Let’s think about sugar for a moment. Seems we just can’t live without it. Here’s why. The raw material for cellular respiration and the production of energy for both animals and plants is the sugar molecule, glucose. In addition to sugar, another element vital to life in both butterflies and humans is sodium. This key element is needed for physiological processes ranging from the contraction of muscles and firing of nerves to maintaining the fluid balance of body tissues. Our friends, the butterflies, obtain sugar from the nectar of plants they pollinate. While carbohydrates abound in nectar, it turns out that nectar is notoriously low in other essential nutrients, one of which is sodium. One consequence of consuming a diet rich in nectar is the need to find supplemental sources of sodium to make up for the relative lack of sodium in the nectar of plants. To solve this sodium shortage, butterflies have evolved clever strategies to acquire salt. One of these behaviors is known as puddling. Puddling butterflies place their proboscis on a substrate like wet soil or mud and imbibe sodium rich fluid found on substrate’s moist surface. We met several species of butterflies puddling in a previous episode.
As they feed on plant sap, spotted lanternflies excrete copious amounts of a sugary waste product called honeydew. In their quest for sugar, butterflies like this pretty hackberry emperor take advantage of the sweet bounty provided by the lanternfly. But butterflies cannot live by sugar alone. Sodium is a key element needed by animals for a variety of life processes. This weather-worn emperor tasted my salt-laden hat before it moved to my shirt. Watch as its proboscis moves across my sweat-soaked shirt to lap up sodium and other vital nutrients. Pretty cool on a hot day that an invasive species and sweaty human can team up to help feed a native butterfly.
When I arrived at the Tree of Heaven, I learned that floral nectar was not the only source of sugar used by the busy emperor butterfly. Festooning the bark of the Tree of Heaven were scores of diabolical spotted lanternflies. The bark of the trees was slippery wet with vast amounts of sugary waste product called honeydew, excreted by the lanternflies as they fed on plant sap. In addition to attracting vast numbers of stinging insects including wasps, hornets, and honey bees, emperor butterflies find lanternfly honeydew to be an excellent source of sugar. To my surprise I saw several emperors lapping up the sweet honeydew, taking full advantage of the sugary bounty.
An emperor with a wing likely tattered by a bird strike enjoys a salty treat from my sweat-soaked hat. Image credit: Paula Shrewsbury, UMD
As I quit the tropical heat and humidity of the forest and emerged into blazing sunshine, my colleague noticed one tattered emperor feeding on my sweat-drenched hat. Later, it imbibed salty liquid from my soggy shirt. During this encounter, the salt-seeking emperor displayed a complete lack of apprehension while I shoveled soil and picked rocks from the garden. The drive for salt appears to be strong and capable of overcoming the fear of proximity to a large, potentially dangerous animal. Butterfly experts have discovered that the quest for salt is sometimes stronger in male butterflies rather than females. It turns out that during copulation, in addition to transferring sperm, a male butterfly may donate a large dose of sodium to his mate, obviating the need for her to seek additional sources of sodium. What a guy! And on a humid, hot, late summer day in the DMV, having sweet and salty treats sounds like a pretty good idea.
Acknowledgements
Bug of the Week thanks Dr. Shrewsbury for photographing the emperor butterfly on my head. The fact-filled article “Nutrient acquisition across a dietary shift: fruit feeding butterflies crave amino acids, nectivores seek salt” by Alison Ravenscraft and Carol L. Boggs was consulted in preparation for this story.