Ants in your Plants: Mutualism benefits both myrmecophyte and insect

What is an “ant plant”? Because we are striving to portray a “real” tropical rainforest, we have several and plants at the Cockrell Butterfly Center, but what makes them so special? Technically called myrmecophytes (from the Greek myrmeco – “ant”, and phyte – “plant”), these plants have a very special relationship with ants, one that is beneficial to both parties. Such mutually beneficial partnerships are known as symbioses or mutualisms, and they are fascinating to evolutionary biologists, ecologists, and lay people alike. So-called “ant plants” typically provide shelter, and sometimes food as well, for ants, and the ants taking advantage of these resources in turn defend the plant against herbivores or other threatening animals, and sometimes even against competing plants. In some cases the ants may provide their host plant with nutrients.

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Ants take shelter inside the passages within the swollen stem of this  Hyndophytum formicarum.

Examples of ant plants abound, particularly in the tropics. They occur in many different families of plants, and involve many different species of ant. Some plant-ant combinations are specific – i.e., the plant has co-evolved with a specific species of ant. In such cases this ant is usually found nowhere else: it has an obligate mutualism with its plant host.  Often the plant has difficulty surviving on its own as well.

More common are facultative mutualisms, where plants provide a resource (usually food) that a variety of ant species may visit. Simply by virtue of their presence on the plant, these ants discourage (or attack) herbivores or other organisms that might harm the plant.

Hundreds of plants bear extrafloral nectaries. These nectar-secreting glands on structures outside of the flower (usually on leaves or the petioles of leaves) are typically most active on new growth, which most needs defense against hungry herbivores. Ants gather at the nectaries to collect the sugary fluid they exude, and kill or chase off insects that try to eat the leaf.

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Ants taking nutrition from extrafloral nectaries on an Inga leaf.

Less common, but still with plenty of examples especially in tropical areas, are plants that provide shelter for ants. These domatia may be hollow stems, swollen petioles, or other hollow spaces on the plant that ants can use as living space.

A few neotropical shrubs in the large genus Piper (black pepper family) have evolved large, envelope-like petioles that house a species of Pheidole ant. Tiny pearl bodies (lipid-containing food bodies) are produced on the inside surface of these domatia – at least when the ants are present. These tiny ants are not aggressive and would not seem to be very effective defenders of their host; however, they have been observed removing insect eggs and small larvae. The real benefit to the host plant appears to be the extra nitrogen that the plants absorb from waste (feces, dead bodies, and other debris) left by the ants inside the petioles.

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Ants taking shelter within the domatia of a Miconia petiole.

Two shrubs in the coffee family, Hydnophytum and Myrmecodia, have very large and complicated domatia. The stems of these epiphytes from the mangrove forests of tropical Southeast Asia and Australia are lumpy and swollen, and almost look diseased. These bizarre-looking structures are riddled on the inside with hollow chambers, much like a Swiss cheese.  If you cut one of the tubers in half, you will find that some of the chambers have smooth surfaces, while others have darker, rough surfaces. The inhabiting ants use the smooth chambers for living space and the rough chambers for dumping their trash. Biologists have observed that the rough-walled chambers are able to absorb nitrogen and other nutrients from the decomposing wastes deposited in them by the ants, while the smooth-walled chambers are not absorbent. Since it is always a challenge for plants without roots in the ground to get enough nutrients high up in the treetops, this is a great adaptation to enhance these epiphytes’ survival. And, not only do the ants provide the plant with extra nutrition – their presence serves to deter things that might eat it.

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Enlarged and swollen stipules on an ant acacia offer shelter for a colony of Pseudomyrmex, an ant species found nowhere else but this plant.

The ant-plant mutualisms that really capture our imagination are those obligate mutualisms where the plant provides both food and shelter for the ants, and the ants are particularly fierce defenders of their plant host.  One of the most famous ant-plants is the bull-thorn acacia from the dry forests of Central America. Acacia is a very large genus of plants in the legume family, especially abundant in dry tropical areas of the world. All Acacia species are characterized by having their paired stipules (little flaps of tissue at the base of each leaf, found throughout the legume family) modified into spines or thorns. In the Acacia species associated with ants, these thorns have become very large and swollen, resembling a pair of bull’s horns. A specific species of stinging ants in the genus Pseudomyrmex has evolved as an obligate mutualist of these acacias.  Worker ants make a small hole at the tip of one of the horns, hollowing out an interior, and the colony lives inside these chambers. These ants are found nowhere else.

The bull-thorn acacia also provides its ant inhabitants with food. Like many legumes, acacias have extrafloral nectaries. In this case, they take the form of little slits along the main rachis of the compound leaf. Ant-acacias don’t just feed their ants sugar, however; they also produce small, protein-rich food bodies for the ants on the tips of the new leaflets. The ants thus have all their needs for shelter and food provided by their host plant. In return, the ants vigorously defend their home against anything that threatens it – leaf-feeding insects or other animals – swarming out of the thorns and stinging the intruder. Because these ants also bite and sting any plant that touches or grows too close to their home, they reduce competition to their host from other plants as well, and because tropical dry forests often burn, the vegetation-free circles around the ant-acacia may be particularly important as a fire brake.  Evidence for how much bull-thorn acacias rely on their fierce little ant defenders can be discerned by tasting the leaves. While most Acacia species are full of bitter compounds, ant-acacias have little need to maintain these chemical defenses, and no longer produce them. Their leaves are not bitter. But if the ant-acacia should lose its ant colony, it is liable to be hit hard by hungry herbivores.

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A species of Azteca ants takes shelter inside the hollow stems of Cecropia plants like this one.

The famous British naturalist Thomas Belt first described this amazing symbiosis, which he observed during his sojourn in Nicaragua in the mid 1800s. Today the food bodies borne at the tips of the new leaflets are called “Beltian bodies” in his honor.

A similar scenario, also from from the Neotropics, is seen in the relationship between several species of Cecropia (a fast-growing, early successional tree) and a species of Azteca ants. Cecropia has hollow stems – with special thin skinned “dimples” in the stem opposite each leaf.  A queen Azteca who finds a young Cecropia not yet colonized bores her way through this thin spot (called a prostoma).  The eggs she lays inside the stem hatch into worker ants, who tunnel through the membranes that divide the stem at each node, and open up more of the prostomata, giving them access to the outside. As the tree grows, the colony moves upwards, eventually making the whole tree a long, hollow shelter for the colony. Cecropia trees also provide food bodies for the ants; these “Mullerian bodies” (named for another 18th century European biologist, Fritz Muller) are produced on special spongy structures at the base of each leaf petiole. The occupying ants also bring small homopterans (aphid relatives) inside the stems of the Cecropia, where they feed on the plant’s sap in a protected environment.  The ants milk these small insects for their sugary exudate (called honeydew) just like farmers milk cows. Azteca ants do not sting, but they can bite, and the workers swarm out in huge numbers to attack any animal that touches their plant.

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Cutting into a hollow stem shows just how extensive the ant colony runs inside this plant.

A relationship very similar to the New World Cecropia-Azteca mutualism is found in the Old World tree Macaranga, also an early successional species, but in the spurge family, a completely different plant family from Cecropia. The ants inhabiting these plants are members of the ant genus Crematogaster (unrelated to Azteca). This is a great example of what is called convergence in evolutionary terms, whereby similar situations in different locations give rise to the same adaptive solutions among unrelated organisms.

Ants are amazing little creatures, and their complicated social behaviors, which often seem to mimic ours, make them particularly good partners for plants in the fight for survival.

Look for the bull-thorn Acacia (sans ants, unfortunately) next time you visit the Cockrell Butterfly Center. We recently received a specimen of Hydnophytum and of Mymecodia from a generous donor. These will eventually be placed in the rainforest. Our Cecropia tree, alas, got too big and died a few years ago. We hope one day to replace it!

 

HMNS greenhouse teaches how to plant a butterfly oasis in your back yard

They float on the wind, decorate your back yard in the spring and summer, and inspire warm emotions with their delicate wings. They seem carefree, at home in any meadow, but butterflies have more specific needs than we might imagine.

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Monarch butterflies don’t live just anywhere; they need habitat, too!

As urban sprawl continues to grow, reducing green space and native plant growth, natural butterfly habitats are shrinking. Butterflies require specific plants on which to feed and lay eggs. Caterpillars are finicky eaters.

Soni Holladay, Houston Museum of Natural Science Horticulturist and Greenhouse Manager, will lead a class Saturday, April 18, beginning at 9 a.m., to share information with the public about how best to plant a garden that will attract native butterfly species, creating a backyard butterfly nursery.

Holladay’s main concern is planting tropical milkweed to attract the famous migratory monarch butterfly. Though tropical milkweed is easier to grow, scientists have discovered it may play a part in declining monarch populations.

A parasitic species of protozoan called Ophryocystis elektroscirrha, or simply Oe, grows on the body of their monarch hosts. When infected monarchs land on milkweed to lay eggs, Oe spores slough off and are left behind. Caterpillars, which eat the milkweed, ingest the spores and become infected.

When the protozoans become too numerous, they can overwhelm and weaken individual butterflies, causing them to suffer. Several heavily infected monarch can take a toll on the local population. Oe can kill the insects in the larval or pupal stage, as well, before they can reach full adulthood.

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Tropical milkweed survives the Houston winters, making them a perennial plant and a possible danger to monarch butterfly populations.

Native milkweeds die off every year and grow back in the spring Oe-free as part of their cycle, but the evergreen tropical milkweed remains standing year-round, providing a vector for the protozoan to spread.

“We’re advising everyone who plants tropical milkweed to cut it back once a year or more,” Holladay said. Much like their native cousins, the tropical variety will return later, a healthy habitat for butterflies.

Holladay’s class will offer more details about this and other butterfly-raising issues. After the class, guests will tour HMNS greenhouses and our on-site butterfly-rearing operation. Tickets $23, all ages. Native milkweed plants and other seeds will be available to get you started.

STEMS & GEMS: HMNS’ Erin Mills gives us the buzz on bugs

Editor’s Note: As part of our annual GEMS (Girls Exploring Math and Science) program happening February 21 2015, we conduct interviews with women who have pursued careers in science, technology, engineering, or math. This week, we’re featuring Erin Mills, Entomologist at the Cockrell Butterfly Center.

Erin Mills Cockrell Butterfly Center

HMNS: How old were you when you first become interested in science/technology/engineering and/or math?
Mills: I’ve been interested in Science as long as I can remember, particularly Life Science. I was always fascinated with creatures big and small.

HMNS: Was there a specific person or event that inspired you when you were younger?
Mills: 
My 7th grade Science teacher Mrs. Pierce-Mcbroom was an awesome science teacher! Picture a female Bill Nye! She had lots of exotic pets that she would bring into the classroom and she was very fun! My mother was also a huge inspiration. Whereas most moms squeal and squirm at the sight or even thought of creepy crawly creatures, they didn’t bother my mom one bit! She was fearless and encouraged me to explore the natural world and all of its residents. She was always very proud of me!

HMNS: What was your favorite science project when you were in school?
Mills: 
My mom helped me to make a mini tropical habitat. She helped me pick out plants that live in the tropics and in the end it looked beautiful! It was my first Science fair project and I got an A!

HMNS: What is your current job? How does this relate to science/technology/engineering/math?
Mills: 
As an Entomologist at the Cockrell Butterfly Center I get to work with some of the most amazing bugs from all around the world. Bugs are some of the most important organisms on the planet and my job involves a lot of educating people, young and old, about them and their importance to the ecosystem.

HMNS: What’s the best part of your job?
Mills: Getting to work with the live insects and other arthropods

HMNS: What do you like to do in your spare time?
Mills: 
Anything outdoors: hiking, walking, running, and playing with my little boy and showing him the natural world.

HMNS: What advice would you give to girls interested in pursuing a STEM career?
Mills: 
Keep your mind open to the thousands of possibilities there are with one of these careers. Ask lots of questions and learn as much as you can, and don’t be afraid or too shy to do what it takes to pursue whatever it is that interests you!

HMNS: Why do you think it’s important for girls to have access to an event like GEMS?
Mills: 
I think mentors play a huge part in helping people be successful. This event can help connect girls with mentors and that can have a tremendously positive impact on their futures!

 

Know a girl who loves science? Click here to learn more about how you can get involved with GEMS!

Horticulturalist Zac Stayton bids a fond farewell to HMNS

Editor’s Note: After four and a half years, Zac Stayton, Horticulturist for the Cockrell Butterfly Center, is leaving HMNS for a new job as a Project Manager for the grower Color Spot. I sat down with him this week to discuss his time at HMNS, his favorite projects and what he’ll be up to next.

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Vincent Covatto: So Zac, we’re very excited for you about your new prospects, but sad that you’ll be leaving. Could you tell us a little about what your job has entailed here at HMNS?

Zac Stayton: Basically, the beautification and upkeep of the Butterfly Center itself, from making sure all the plants are blooming, which ends up feeding the butterflies, to maintaining the tropical fruits, pollinating the chocolate trees and the coffee. Pretty much everything that goes into the conservatory itself, from irrigation and upkeep of the waterfall to electrical and everything in between.

VC: So we’ve essentially been running you ragged for the last five years.

ZS: [laughing] You could almost make it a job for a team of four.

VC: Or one Zac.

ZS: [laughing] Yeah, yeah.

VC: What are some of your favorite things that you’ve worked on in your time here?

ZS: Of course Lois and some of the tropical flowers that you wouldn’t normally get to encounter here in the Houston climate. There are some crazy orchids that we’ve got in there. We’ve got one in particular that’s endangered, and almost went extinct when the Japanese invaded a small island off the coast of Taiwan, so it’s very, very rare. Just getting to see some those plants bloom, I mean I’m one of the few people that ever gets to grow these plants, is really a great experience.

Also being able to grow coffee and chocolate, pineapples and vanilla — which you wouldn’t get to do outside of the glassed-in enclosure there in the conservatory.

VC: I remember this past winter we had had a coffee tasting in the conservatory. Was that sort of your own pet project?

ZS: Yeah, that coffee tree was actually kind of a fluke, we call it a volunteer. We took an old [tree] out when I first started and it dropped one single seed and it ended up growing into this tree that we didn’t even mean to plant there. And so, I guess it was maybe last September, October, I looked at it and it was just covered in berries, and it was like, well we’d better do something with all this coffee, rather than just letting it all go to waste.

So I went through and kind of studied how to do it and there’s not a lot of — like a lot of things in the Butterfly Center… you can’t just google these plants and see what to do. So I had to do a lot of calling around.

Here in Houston there’s plenty of coffee shops that will roast the beans for you, but we had undried beans. And it was like, ok how do we get these to the point where we can roast them. So we really had to break it down step by step and do some trial and error, with the whole process — from cracking the beans and drying them and roasting them and then finally grinding them and finally drinking them. It was pretty eye opening to see what actually goes into your daily cup of coffee.

VC: [laughing] Or four…

ZS: [laughing] Yeah depending on the day. Two for me at least. Minimum.

VC: Well I bet that was your favorite cup — the best coffee you’d ever had.

ZS: Definitely. Definitely, yeah, I mean I think everybody in the Butterfly Center got blisters on their fingers just having to — there’s what’s called parchment coffee and you basically have to take this thin layer of parchment off of each of the several thousand beans that we had. And they have machines for that for that kind of stuff in the tropics, but I couldn’t find one, at least in the area. I think the closest one was in Hawaii or Coast Rica, so we had to do it all by hand. And so you definitely get a new appreciation for coffee.

VC: Do you have a favorite plant, either inside the conservatory itself or something you’re really interested in?

ZS: I have a favorite plant family. I like bromeliads. So everything related to — the closest relative would be pineapples, but they also include Spanish moss [so there’s a big range]. I actually grew these in Hawaii before I came here so that’s kind of been my expertise, if you will, I’ve channeled myself into those a little bit more than some of the other plant species.

VC: So what is your new job?

ZS: [I’ll be working for] Color Spot the largest grower in the U.S. right now, and they have several locations — I thinking six or seven in Texas — but the branch that I’ll be at is in Huntsville. And I’ll be the production manager, making sure the timing goes through. Most the plants there are bedding plants, they’re seasonal.

For example, when Christmas time rolls around it’s going to be my job to make sure that all the poinsettias and mums and things like that are all ready, that they’re nice and red and at a specific time are on the shelves ready for people to buy.

VC: So you’ll be making sure that everything is timed right.

ZS: Exactly.

VC: So you determine when things are planted, when they’re harvested?

ZS: Exactly. There are a couple teams of growers, so I’ll be the Production Manager, from the time the plant gets put in the soil, until the time it gets on a truck headed to the main customers — I make sure it’s all on schedule.

If you’ve ever bought a plant from Walmart or Home Depot, or Lowes or Kroger, any of those big box stores, especially here in Texas, it’s come from Color Spot. Also smaller nurseries around town will buy their plants.

VC: So would you say this job is a continuation of the sort of work you’ve done for the museum or is it more of a fresh start?

ZS: It’s kind of a fresh start. It’s changing gears from tropicals to basically what grows well around here. A big part of it though will be — working here I learned a lot of the butterfly attractants for our area, although I work with tropicals on a daily basis there are a lot of calls asking “well what can I plant in my backyard?” So learned a lot about our native butterflies here and what plants attract them, and what plants you can put in your garden. And that’s a big part of what I’m going to be growing at Color Spot — butterfly attractants, nectar plants — so there is an overlap there.

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VC:  And you have the different pollinators to take into account with that as well?

ZS: Yes, exactly. Everything about pollination and pollinators I learned here firsthand. When I first started I didn’t know much about butterflies — I knew the monarch, the one that everybody knows, and other than that I didn’t know that there were 15 other species of butterflies that are native here, to Houston. That is something I’ve definitely gained from my time here.

VC: Is there anything that you’re really excited about growing or working on at Color Spot?

ZS: There are a lot of new bulbs, tulips and things like that, that we get to do trials with. Before they even hit the market we get to take some of these bulbs that these growers bring from the Netherlands and get to try them out, test them out, in our greenhouses. We get to be on the front line before anyone else [in the area] knows about them — that’s something I’m really excited about.

VC: I’ve been reading through some archival press from when Lois was blooming, and I saw a couple of places where you’d been quoted. I think that’s how some of our Houston audience got to know you — through that experience. Can we count on you to come back down the next time she’s in bloom?

ZS: Oh yeah definitely. I’ll be here of course, I wouldn’t miss it.

VC: Do you think you’ll become pen pals with her?

ZS: [laughing] Oh no, I don’t think so. No more anthropomorphizing Lois, although that was funny and clever when that happened.

The people though, that’s what made Lois what she was. Lois was just a flower, and people would probably gasp hearing me say that, but it was the whole community rallying around Lois that was the coolest part of the whole thing.

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80,000 plus people coming out, plus the people —you know, we had people in Australia saying “get out of the way! We can’t see Lois on the webcam!” So just the fact that it was everybody at the same time, seeing the same thing and it just blew up on social media. And that’s what I though was so cool, everybody so in synch, waiting, just to see what would happen with Lois.

As a horticulturist it was really cool to see just everyone getting around it and thinking it was as cool as I did. Normally, you know I’ll be like “ooo look at this cool plant!” and people couldn’t care less, but to see everyone else sharing that passion — that was the best part about it.

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VC: Is there anything else you’d like to say or share with our readers on the blog?

ZS: It’s been a fantastic four and a half years here, and honestly all of the events that we did, it wouldn’t have been possible without people getting as geeky about plants as me.

From the miracle fruit tasting, and the chocolate and the coffee — to see everybody getting around it, that was the best.

Lois, kind of spurred everybody on to find this new kind of passion in horticulture. There were a lot of parents that came up to me and said “I asked my kid what they want to do and now they all want to be a horticulturist.” And that’s the best thing that could come from it, I think, is a whole new generation of people that find plants as interesting as I do.