Pitcher Plant Moths and their Pitcher Plant Homes

Discussions about pitcher plants usually revolve around the fact that they trap and eat insects and other animals. However, there are a handful of organisms out there that turn the table on pitcher plants, reminding us that these botanical carnivores can become food themselves. Spend any amount of time surveying pitcher plant populations in southeastern North America and you are likely to encounter at least one such species of pitcher plant eater.

There are three species of pitcher plant moths in the genus Exyra and all of them would not exist if it were not for pitcher plants in the genus Sarracenia. Whereas E. ridingsii and E. semicrocea are largely restricted to southeastern portions of North America, E. fax can be found as far north as Newfoundland. These three species also vary in their dietary specificity. As you can probably ascertain from its distribution, E. fax is a purple pitcher plant (S. purpurea) but will also feed on the southern pitcher plant (S. rosea) in the southern portions of its range. Exyra ridingsii is also a dietary specialist, feeding only on the pitchers of the yellow pitcher plant (Sarracenia flava). Alternatively, E. semicrocea is a generalist and can be found feeding on a variety of Sarracenia species.

An Exyra caterpillar busy feeding on a Sarracenia flava pitcher.

Both caterpillars and adult moths are physically adapted to living within the slippery interior of the pitcher walls. Microscopic analyses of their feet have revealed specialized morphological adaptations that allow them to cling to the waxy walls of the pitcher. The caterpillars may also benefit from their ability to spin silken lines. Interestingly, the moths are only ever found perched upright in the pitchers. Even when they mate (which also occurs within the pitcher), they do so at a 90 degree angle so that neither partner is facing downward. It is thought that they must remain mostly upright in order for their feet to properly cling to the waxy wall.

Regardless of which pitcher plant they are eating, these three moths all behave similarly throughout their lifecycle. The caterpillars are hatched within a pitcher. Immediately they begin feeding on the wall of the pitcher. They will only eat the interior cells of the pitcher wall, leaving a thin layer of tissue on the outside wall. This makes the pitcher look as if it is covered in translucent, brown windows. At some point in their development, the caterpillars will also spin a layer of silk over the mouth of the pitcher. This protects them from predators like lynx spiders and cuts off the pitchers ability to capture prey (more on this in a bit).

As the caterpillars grow, they will occasionally move to new pitchers. At larger sizes, their feeding damage can be quite extensive, damaging the walls of the pitcher to the point that it loses its structural integrity and folds over. This can also serve to protect the caterpillar from predators while similarly reducing the ability of the plant to capture food. After their fifth larval instar, the caterpillars will move to a new, usually undamaged pitcher. In many instances, they will crawl to the bottom and chew a small hole in the side, draining the pitcher of its digestive fluids. They will then pupate just above the drainage hole.

Signs of Exyra feeding damage.

After a period of time that varies between species, adult moths will emerge. The adults are adorable little critters dressed in shades of yellow and black. They are also very secretive and do not leave the pitchers until nightfall. Even then, they only do so to mate and lay eggs in new pitchers. After mating, the female will lay her eggs just below the mouth of a new pitcher and the cycle begins anew. Amazingly, it has been found that the only other stimulus besides the urge to mate that can coax the moths to leave their pitchers is smoke. This is especially true for the southern species as the bogs in which they live are subject to frequent fires. If they were to remain in the pitchers, it is likely that entire populations would be incinerated.

As terrifying as this sounds for the moths, fire is essential to their lifecycle. The pitcher plant bogs of southeastern North America could not persist without fire. When fires are suppressed, these bogs inevitably fill in with more aggressive vegetation such as swamp titi (Cyrilla racemiflora) or any of the myriad invasive species that grow in this region. As bogs become choked with woody shrubs and trees, pitcher plants and other bog species are choked out to the point that they can completely disappear. Fire in these habitats brings more life than it does death.

A population of Sarracenia flava var. rubricorpora showing signs of a thriving Exyra moth population in the form of damaged and bent over pitchers.

Given that the pitchers of pitcher plants function as both photosynthetic organs and a means to obtain nutrients like nitrogen and phosphorus, it stands to reason that damage from pitcher plant moths could harm the plants over the long term. Indeed, high densities of pitcher plant moths can exact quite a toll on pitcher plant individuals. Evidence from multiple sites has shown that heavily damaged pitcher plants can shrink in size over time, indicating loss of energy reserves. In support of this, some have also found that highly damaged pitcher plants go on to produce more pitchers, which indicates that such individuals are prioritizing more nutrient capture. In ecosystems already defined by nutrient scarcity, the effects of herbivory on these carnivorous plants are likely more severe than they are for plants growing in nutrient-rich environments. However, it should be noted that it is a rare case in which pitcher plant moths exact such a toll on plants as to completely kill the pitcher plants they rely on for survival.

That being said, there is plenty of room for concern over the future of both pitcher plants and moths. Only 3% of the bogs that once existed in southeastern North America remain today. Habitat loss means fewer populations of plants and thus less habitat for the moths (and myriad other lifeforms) that rely on them. For these reasons and more, habitat protection and restoration must be made a high priority moving into the future. Please consider supporting a land conservation/restoration organization in your area!

Further Reading: [1] [2] [3] [4] [5] [6] [7] [8]

Encounters With a Rare White-Topped Carnivore

I am not a list maker. Never have been and never will be. That being said, there are always going to be certain plants that I feel I need to see in the wild before I die. The white-topped pitcher plant (Sarracenia leucophylla) was one such plant.

I will never forget the first time I laid eyes on one of these plants. It was at a carnivorous plant club meeting in which the theme had been “show and tell.” Local growers were proudly showcasing select plants from their collections and it was a great introduction to many groups which, at the time, I was unfamiliar with. Such was the case for the taller pitcher plants in the genus Sarracenia. Up until that point, I had only ever encountered the squat purple pitcher plant (S. purpurea).

I rounded the corner to a row of display tables and was greeted with a line of stunning botanical pitfall traps. Nestled in among the greens, reds, and yellows was a single pot full of tremendously white, green, and red pitcher plants. I picked my jaw up off the floor and inquired. This was the first time I had seen Sarracenia leucophylla. At that point I knew I had to see such a beauty in the wild.

More like white and red top…

It would be nearly a decade before that dream came true. On my recent trip to the Florida panhandle, I learned that there may be a chance to see this species in situ. Needless to say, this plant nerd was feeling pretty ecstatic. Between survey sites, we pulled down a long road and parked our vehicle. I could tell that there was a large clearing just beyond the ditch, on the other side of the trees.

The clearing turned out to be an old logging site. Apparently the site was not damaged too severely during the process as the plant diversity was pretty impressive. We put on our boots and slogged our way down an old two track nearly knee deep in dark, tanic water. All around us we could see amazing species of Sabatia, Lycopodiella, Drosera, and so much more. We didn’t walk far before something white caught my eye.

There to the left of me was a small patch of S. leucophylla. I had a hard time keeping it together. I wanted to jump up and down, run around, and let off all of the excited energy that had built up that morning. I decided to reign it in, however, as I had to be extra careful not to trample any of the other incredible plants growing near by. It is always sad to see the complete disregard even seasoned botanists have for plants that are unlucky enough to be growing next door to a species deemed “more exciting,” but I digress.

Sarracenia leucophylla flower. Photo by Noah Elhardt licensed by GNU Free Documentation License [SOURCE]

This was truly a moment I needed to savor. I took a few pictures and then put my camera away to simply enjoyed being in the presence of such an evolutionary marvel. If you know how pitcher plants work then you will be familiar with S. leucophylla. Its brightly colored pitchers are pitfall traps. Insects lured in by the bright colors, sweet smell, and tasty extrafloral nectar eventually lose their footing and fall down into the mouth of the pitcher. Once they have passed the rim, escape is unlikely. Downward pointing hairs and slippery walls ensure that few, if any, insects can crawl back out.

What makes this species so precious (other than its amazing appearance) is just how rare it has become. Sarracenia leucophylla is a poster child for the impact humans are having on this entire ecosystem. It can only be found in a few scattered locations along the Gulf Coast of North America. The main threat to this species is, of course, loss of habitat.

A large conservation population growing ex situ at the Atlanta Botanical Garden.

Southeastern North America has seen an explosion in its human population over the last few decades and that has come at great cost to wild spaces. Destruction from human development, agriculture, and timber production have seen much of its wetland habitats disappear. What is left has been severely degraded by a loss of fire. Fires act as a sort of reset button on the vegetation dynamics of fire-prone habitats by clearing the area of vegetation. Without fires, species like S. leucophylla are quickly out-competed by more aggressive plants, especially woody shrubs like titi (Cyrilla racemiflora).

Another major threat to this species is poaching, though the main reasons may surprise you. Though S. leucophylla is a highly sought-after species by carnivorous plant growers, its ease of propagation means seed grown plants are usually readily available. That is not to say poaching for the plant trade doesn’t happen. It does and the locations of wild populations are best kept secret.

Sarracenia leucophylla habitat. Photo by Brad Adler licensed by CC BY-SA 2.5 [SOURCE]

The main issue with poaching involves the cut flower trade. Florists looking to add something exotic to their floral displays have taken to using the brightly colored pitchers of various Sarracenia species. One or two pitchers from a population probably doesn’t hurt the plants very much but reports of entire populations having their pitchers removed are not uncommon to hear about. It is important to realize that not only do the pitchers provide these plants with much-needed nutrients, they are also the main photosynthetic organs. Without them, plants will starve and die.

I think at this point my reasons for excitement are pretty obvious. Wandering around we found a handful more plants and a few even had ripening seed pods. By far the coolest part of the encounter came when I noticed a couple damaged pitchers. I bent down and noticed that they had holes chewed out of the pitcher walls and all were positioned about half way up the pitcher.

I peered down into one of these damaged pitchers and was greeted by two tiny moths. Each moth was yellow with a black head and thick black bands on each wing. A quick internet search revealed that these were very special moths indeed. What we had found was a species of moth called the pitcher plant mining moth (Exyra semicrocea).

An adult pitcher plant mining moth (Exyra semicrocea) sitting within a pitcher!

Amazingly, the lives of these moths are completely tied to the lives of the pitcher plants. Their larvae feed on nothing else. As if seeing this rare plant wasn’t incredible enough, I was witnessing such a wonderfully specific symbiotic relationship right before my very eyes.

Fortunately, the plight of S. leucophylla has not gone unnoticed by conservationists. Lots of attention is being paid to protecting remaining populations, collecting seeds, and reintroducing plants to part of their former range. For instance, it has been estimated that efforts to protect this species by the Atlanta Botanical Garden have safeguarded most of the genetic diversity that remains for S. leucophylla. Outside of direct conservation efforts, many agencies both public and private are bringing fire back into the ecology of these systems. Fires benefit so much more than S. leucophylla. They are restoring the integrity and resiliency of these biodiverse wetland habitats.

LEARN MORE ABOUT WHAT PLACES LIKE THE ATLANTA BOTANICAL GARDEN ARE DOING TO PROTECT IMPORTANT PLANT HABITATS THROUGHOUT THE SOUTHEAST AND MORE.

Further Reading: [1] [2] [3] [4] [5]