“Huh, that’s weird”, I muttered as I trudged through the mud in the rainforest. Even though the sun was setting, it was still hot and steamy, and sweat was dripping into my eyes as I stared at a tree with bizarre yellow outgrowths...
Watch the video 'Mystery of the Yellow Bulbs: Caterpillars, Ants and Parasitic Plants.'
Some sort of fungus? That was my first thought. After all, I've seen tons of strange looking fungi in the Amazon by now. But something about these yellow bulby-looking things piqued my curiosity, so I walked up to take a closer look.
A tree covered with strange yellow bulbs in the jungle
Inspecting them closer only made things more confusing. They didn't really look like fungi, at least not like any I had ever seen before. Was it a fruit produced by the tree? Plant lenticels? I started to take some pictures.
A closer look at the mysterious yellow "bulbs"
That's when I noticed something really interesting. As I scanned the tree's alien protuberances, my eyes locked on to something I wasn't quite anticipating: caterpillars! I had definitely never seen, or heard of, anything like this before. They appeared to be munching away on the yellow bulbs.
An unknown caterpillar eating one of the yellow bulbs
I quickly noticed there were also ants surrounding these caterpillars. The ants were not attacking the delicate butterfly larvae, so this had to be some sort of symbiotic relationship between the caterpillars and ants. While I had little idea at the time what I was looking at, my background in entomology was telling me one thing: this was something cool.
By this point I had returned to the tree with my colleague, wildlife photographer Jeff Cremer. We took several shots of the caterpillars, ants, and yellow bulbs. This kind of mutualistic caterpillar-ant relationship is known as myrmecophily, and has interested scientists for a long time. Caterpillars belonging to the family lycaenidae have a special structure known as the dorsal nectary organ, which secretes sugars and amino acids. This sweet, nutritious reward is what keeps the ants around and in return, the ants protect the caterpillars by driving off hungry predators and parasitoids.
An attendant ant tapping the caterpillar and receiving a nectar droplet. Filmed thanks to the help of Chris Johns
As I was inspecting this bizarre caterpillar-ant interaction, I noticed something flutter just above my head. A butterfly! Not just any butterfly, I could immediately identify it as a lycaenid and it had a distinct yellow spot on its hind wing that looked remarkably like the yellow bulbs. Was this the adult of the caterpillars!?
A butterfly with a yellow wing spot lands on the tree
Ok, Homework Time
I wrapped up my field work and headed back to the states, but I was dying to figure out what was going on here. To recap, we observed:
- Mysterious yellow bulbs growing on a tree
- Caterpillars eating the yellow bulbs
- Ants taking care of the caterpillars
- Butterfly with yellow wing spot lands on tree with yellow bulbs
I assembled the photos from the trip and starting emailing the top butterfly experts as well as botanists. The responses I received were mostly along the lines of "I've never seen anything like this before" and "there's nothing published on the life history of the butterfly". Ok, so it seemed like we were on to something new here.
An ant protecting two young caterpillars
With the help of some experts, we were able to identify the butterfly as Terenthina terentia, which belongs to family Lycaenidae. While this family contains around 6,000 species, the Neotropical lycaenids are still only partially known and poorly studied (Pierce 2002). Many species of lycaenids are known to engage in relationships with ants (aka myrmecophily) so our caterpillars definitely fit this criteria.
Filming this strange caterpillar-ant-parasitic plant relationship
Several botanists emailed my pictures around to their colleagues until we were finally able to ID the yellow bulbs as a "very unusual and rarely seen" parasitic plant belonging to the family Apodanthaceae. Ever heard of that? Yeah, me either.
Apodanthaceae is a small family of parasitic plants that live inside other plants and they only become visible once the flowers burst through the bark (Bellot 2014). The species we found is possibly Apodanthes caseariae and there is little known about their ecology, what pollinates them, or how they infect their host plants. Our observations appear to be the first record of an insect utilizing Apodanthes as a host plant. The strange yellow bulbs of this plant appear to emerge once a year around October through January and then fall off.
The yellow bulbs later identified as a rare flowering parasitic plant
In January, my colleague Phil Torres visited this site and checked out the caterpillars. You can see the yellow bulb flowers are more developed at this point.
However, our work is far from over and many questions still remain. Is this the butterfly's only host plant? How does it know when the parasitic plant is emerging and how does it find the yellow bulbs? What purpose does the butterfly's yellow wing spot serve?
Perhaps the yellow wing marking helps the butterfly blend in with the yellow parasitic plant and reduces predation by birds. While this hypothesis needs further attention, the similar wing pattern and observed host plant could lend support to the idea of a long-term co-evolutionary relationship between the flowering endoparasitic herb and the Terenthina terentia butterfly. In any case, we'll attempt to continue to pick apart this fascinating Amazon mystery which will undoubtedly result in even more questions!
-Aaron Pomerantz, Entomologist
You can follow Aaron for updates & get in touch on Twitter @AaronPomerantz
Special thanks to colleagues in and out of the field who assisted with this project, especially Jeff Cremer, Frank Pichardo, Chris A. Johns, Phil Torres, Christie Wilcox, Jason Goldman, Trevor Caskey, and Alex Gardels. Thanks to Andrew Warren, Alex Wild, Naomi Pierce, Adrian Hoskins, Sidonie Bellot and Robert Robbins for help with insect/plant identifications and expert input.
Bellot S, Renner SS (2014) The systematics of the worldwide endoparasite family Apodanthaceae (Cucerbitales), with a key, a map, and color photos of most species. PhytoKeys 36: 41-57
Pierce NE, Braby MF, Heath A, Lohman DJ, Methew J, Rand DB, Travassos MA (2002) The ecology and evolution of ant association in the Lycaenidae (Lepidoptera). Annual Review of Entomology 47: 733-771