Food Loyalty in Bugs

In which I talk about picky eating in bugs, and decide to try an experiment…

Most bugs are picky eaters. Photo © Men’s Health

Another unusual thing about the Hickory Horned Devil (the regal moth caterpillar) is that it eats a whole lot of things besides hickory. According to The Wild Silk Moths of North America (WSMNA), it eats basically any nut tree (Juglandaceae – hickories, walnuts, butternut, pecan), sweet gum, persimmon, all sumacs, sourwood, ashes, sycamore, lilac and cotton. Other sources note that it will also eat oaks and cherries. This is an unusually broad range of food plants for a moth or butterfly caterpillar, and even a broad range for  a giant silk moth caterpillar (a group which often does eat a large number of deciduous tree species), and it notably includes leaves that are pretty different in taxonomy and texture and smell, which means that they are probably chemically different (I’m guessing).

Some of the regal moth's foodplants.

Why are many caterpillars only found on one or two foodplants? Well, science has worked out that it almost certainly has to do with past history. Imagine hundreds of thousands (or perhaps millions) of years ago, a species of moth found that a specific plant or group of plants suited its dietary needs – it had leaves with materials in them that the caterpillars were capable of chewing… could digest… had nutrients that the caterpillar needed to grow, and contained no poisons that could make the caterpillar sick. Perfect! However, if it was too perfect a food, this could be a problem for the plant because the caterpillar populations would thrive, eat a lot of the plants and quickly it would start threatening the plant’s survival. This growing catastrophe would then strongly favor the chance survival of individual varieties of this plant that happened to be a little different genetically so that they contained some different chemicals making those varieties more toxic – in other words, the leaves tasted worse -- to the caterpillars. The worse the plants tasted, the more plants would survive (perhaps this a defense mechanism the broccoli tribe has used against human children). In this way, over a lot generations, the plant would evolve a chemical defense against the caterpillar that was eating it until it wasn’t threatened any more. It’s an evolutionary process called natural selection – perhaps you’ve heard of it.

The back-and-forth “arms race” between plants and caterpillars.

However, natural selection also works for the caterpillar – since the plant was otherwise a good food, lucky caterpillars that happened to have genes making them less sensitive to the plant’s new toxins would tend to survive better, so after a while all the moth’s caterpillars might develop immunity to the new toxin (bad-tasting poison) in the plant. They might even evolve to use the toxin in their own chemistry to help them grow and survive. So then there would be a problem again for the plant, and varieties that had new toxins that the caterpillars didn’t like would survive better… and then the caterpillars… and then the plants… you get the idea. It’s an arms race between plant and caterpillar, and  the two begin evolving together, so the caterpillar’s chemistry is very finely tuned to eat a specific kind of plant that contains a lot of nasty chemicals. It may have even  evolved to use those chemicals to defend itself against caterpillar predators (like birds), because something that is toxic to caterpillars is often also toxic to animals that eat caterpillars. A familiar example is the monarch butterfly, which feeds exclusively on milkweeds, which contain some nasty poisons, which the monarch caterpillar (and butterfly) use to make it poisonous to birds.

 Toxic monarch caterpillar on toxic milkweed plant

Plant-eating insects like moths and butterflies have evolved this way with specific plants for a long time, which makes most of them picky eaters… which is why a non-picky insect like the royal moth is somewhat unusual. How would a moth come to be this way? My best guess (another “hypothesis” of mine) comes from something that I was talking about two posts ago – the fact that regal moths and their relatives seem to be relatively recent “invaders” from the south, taking advantage of ecosystems that are relatively “new,” thanks to past ice ages. In ecology, scientists note that the species that can best take advantage of a new place (where there are not a lot of established, well-balanced relationships between species) are creatures they call “generalists.” Generalists are species that can eat lots of different things,  aren’t fussy about living arrangements, etc. and thus are able to make do in new situations better than other organisms that depend on established relationships with each other. In older ecosystems where all the organisms have long worked out their relationships with each other, these “generalists” may not do as well, but in less-established situations they do better. Curiously, this points to the fact that the regal moth, apparently a relatively recent invader, is somewhat of a “generalist,” despite its unusual appearance (which looks very specialized) and freakishly large size. It’s well-suited to survive in a variety of conditions by eating a variety of foods.

 The thing is, evolution is still going on. WSMNA notes that some studies have found that for regal moth caterpillars “the effectiveness of host plant assimilation varies from population to population.” In other words, regal moths in different areas appear to be evolving preferences for different species of foodplant. The authors go on to say: “A study of localized preferences in this species, with its broad array of natural host plants, would be an interesting topic for further investigation.” Suggestion accepted: I think I will attempt a small (and poorly designed, since I’m winging it) experiment.

 What I am doing is picking two different foodplants -- pignut hickory and sweet gum -- and testing whether one seems in any way to be a preferred choice for the caterpillars I am raising. Do they accept one readily but not the other? Do a larger percentage survive when fed one than when fed the other? Do caterpillars grow at different rates on the different plants? (etc.) I picked these two plants because both are really common here, and both are on the list of “most common host plants.” They are not closely related plants, so they are likely to have significantly different toxins in their leaves.

Hickory horned devil caterpillars from Charlotte, NC seem to prefer glass to hickory leaves. This being the South, maybe I should have offered them Chick-fil-A,

I will try to gather some data relating to some of the questions above, but don’t expect too much from my informal experimenting here… I may produce some evidence for a local food preference, but it sure won’t be conclusive. Think of this as a test run for a much-better-designed experiment that you might think up and run.

Bibliography:

Tuskes, Paul M., Tuttle, James P. and Collins, Michael M. 1996. The Wild Silk Moths of North America. Cornell University Press, Ithaca, NY.

Wagner, David L. 2005. Caterpillars of Eastern North America. Princeton University Press, Princeton.

Imagining the Moth

The female regal moth I caught on July 26 was dead by last night, having laid 135 eggs (which entomologists call “ova” – scientists like to have their own jargon) . I’ve cut them out of the cardboard container I kept her in and put them in the clean glass container you see above for careful monitoring. The standard egg laying container is a paper grocery bag, but I didn’t have one, so I made due with  a stationary box, which did the job. They are the largest lepidoptera (butterfly/moth) eggs I’ve ever seen and they are a beautiful translucent green, except for a few, which are now partially brown. Those are that way because the eggs are transparent and you can actually see the embryonic caterpillars developing inside – the brown is their heads—I’m guessing some will hatch tomorrow. I have saplings of their foodplants ready in pots – when they are really little I want to grow them inside the house where I can control the environment.

Before you feel too sad for the moth who died for this , I need to say don’t fret – she accomplished her primary mission in life: laying eggs. Giant silk moths only survive a few days as adults in the wild, living entirely off fats that they accumulated as caterpillars. They have no functioning mouthparts and they don’t eat.  They have only two missions – to mate, and to lay as many eggs as possible.

I”ll talk about their mating behavior another time (because it’s an important topic), but their egg laying is also interesting. Regal moths, like many of their close kin, often fly great distances, depositing an egg or two on one tree, then flying a quarter mile to lay another egg. There are a lot of possible explanations for how this behavior evolved.(I like to call these “explanations” “hypotheses” because no one actually knows how the pressures of nature encouraged the behavior’s development, and, frankly, it’s hard to imagine an experiment that would prove the “real” cause. Honestly, what follows is all unproven speculation – intelligent guesses. In science, the next job would be to find some clever way to test these ideas – to design an experiment and then collect solid information – “data..” However, nature – the world outside is a messy, complicated place and coming up with intelligent designs for such experiments is very, very difficult. For now, I’ll stick to the fun, imaginative part – the hypotheses.

 My personal best guess  for why regal moths have evolved to fly long distances and lay single eggs is that, since these eggs grow into very big caterpillars, it would be a bad idea for too many of them to be on one tree because they are likely to attract the attention of predators (such as birds) and once a predator has found one caterpillar, it is going to be primed to see another one nearby. So, this hypothesis says I saved the female moth the effort of flying all over the forest to lay her eggs, so she probably was able to get more laid before her fat-stored energy ran out (fat is really just a chemical battery, if you think about it). She had to lay them all together, which might be disastrous in nature where there are birds, but I’ve got her back – I’ll protect her caterpillars from predators. This makes me the good guy. Really.

However there is another “hypothesis” for why this moth has evolved single egg rather than mass laying behavior and this explanation perhaps means that I’m not being so clever in “farming” these caterpillars. Some of the regal moth’s near relatives, the oakworm moths (species in the genus Anisota), do tend to lay their eggs in big masses, and the caterpillars feed together in big troops, especially when they are young. (Herding together must serve some kind of protective function for these caterpillars, like it does for bison and schooling fish and ducks and starlings and the like, but I have no idea how that works – make your own hypothesis.) Oak moth caterpillars are considerably smaller than regal moth caterpillars at maturity and they grow up faster. Importantly, this means they have less time chance to catch a disease that could spread from caterpillar to caterpillar quickly, wiping out all a moth’s brood in one rapid epidemic.

       Young oakworm moth (A. osalari) caterpillars. National Park Service photo by Sally King.

This is a familiar problem to anyone who has ever tried to raise a lot of giant silk moth caterpillars  – you have to grow them in crowded conditions (you’re really “farming” them) and various diseases can appear and destroy your project. So perhaps another explanation (hypothesis) for why the regal moth lays single eggs is that this protects long-maturing caterpillars from spreading the diseases a few are likely to catch during a long period of infancy. This could be the explanation, or both explanations could be right or… something else entirely might be the cause. Nature is complicated and it’s hard to imagine all the problems that might come up in the life of a moth.

But it’s still fun to make guesses about the evolution of unusual characteristics in a species. One other feature of regal moth egg laying behavior that I also think is a bit strange is the fact that these moths produce only one generation a year, even here in the south where summers are long. This is actually not typical of most giant silk moths. Lunas, for example, have at least two generations a summer here in North Carolina, and researchers have noticed that they have three in Texas. The first lunas hatch and lay eggs in April here, but the regal moth waits until the end of July or August to hatch, mate and start laying eggs. Why?

We can reject the explanation that the moth has only enough time for one generation a year because it is so big that it takes a long time to grow. It does take a relatively long time to grow, but not really that much longer than a luna, which goes through the whole cycle twice. So let’s add another piece of evidence: its closest relatives (other species in the sub-family Ceratocampinae) also have only one generation (one “brood” as scientists say, or “univoltine” behavior) though many are much smaller moths. The authors of The Wild Silk Moths of North America  have suggested what I think is a convincing hypothesis for this: “In the United States and Canada the greatest number of species occur in the Southeast. Midsummer flight in warm, humid weather maybe an adaptation related to the tropical affinities of the group.” In other words, the authors note that relatives of these insects are a lot more common in the American tropics, so species like the regal moth may represent relatively recent invaders from tropical climates and they may still have hold-over physical adaptations from their not-too distant past, such as favoring the time of the year when North Carolina feels like Mexico or Honduras. In the tropics, the moths lay eggs in the rainy season, since that’s when the food plants are freshest. Perhaps these tropical moths have carried the home country behaviors here.

 This is another perhaps un-provable hypothesis regarding evolution, but I like it because it again makes you expand your imagination to consider yet another dimension that can affect things: history. Consider the fact that only a little over ten thousand years ago (a blink of an eye in biological history, really) most of North America was covered by the expansion of a thick ice sheet. (Ice might have been a mile thick where I live now.) When that happened, where did all trees, bugs and other animals go? They either died out or they moved south to where there was no ice (Florida would have been ice free, but much cooler than now.) Then, when the ice receded back to the north, old species returned and some new species invaded from the south. The regal moth and its relatives may have been just such invaders – they still carry the signs of their history in the way they live.

So, in order to come to grips with how a moth works you need to imagine all kinds of things – what it’s really like out there – and what it must have been like out there a long time ago. Who knows what the most important “causes” were? There were certainly many, all working together and the result of the biological evolution they caused is the moth we have now. To really know we await clever minds, brilliant experiments and … data.

Moths, Memory, and Motivation

It’s this moth’s fault. I’m blogging.  About bugs.

Also about biology.  About science and what it can tell us about our lives and our world.  And, I guess, I’m blogging about me and about why I think this stuff is cool.

To be honest, I have been looking for an excuse to start a blog for quite some time. I write about science for a living. I also work with university scientists and spend a lot of time convincing them  to share what they know with the public – particularly through blogging and other social media. It’s a little bit phony for me to lecture others about this when I don’t do it myself.

The problem I have in doing a blog is that most of the things I know a lot about (and want to write posts on) are subjects that very few other people would want to read about. I set up this blog a couple of years ago to write about epizootic diseases – epidemics that affect wild plants and animals and that are, in some ways as big a threat to the stability of the natural world around us as climate change and human development. It’s an important topic, but the problem is that it’s also astoundingly complicated and something very few people outside of some  landscape ecologists care about. More people should care, but, well, they just don’t. What’s the point in writing something that no one else out there is going to want to read?

In a big way, the problem is public education in science. Science, as most of us know it, seems to be dry, difficult and (I’ll say it) boring. However, it has never seemed boring to me, even though I am not a scientist myself.  I wish I could say this is because I was lucky enough to have had a great science education when I was younger, but, honestly, I didn’t. Though my primary and secondary education wasn’t bad for the time (in the 1960’s and early 70’s), I had virtually no science classes until high school, and science classes there were very weak. I went to an Ivy League school for college, and there science was extremely demanding and technical and… pretty unimaginative. I took the classes I had to and then majored in English. I became a writer. Science, I decided, was disappointing.

Yet I kept a wistful eye on science, waiting to see something that would change my mind.  (Eventually it did, but more on that some other time. )

My science education – a personal history

Despite my education, I have been naturally drawn to science.  I grew up with the practice of scientific thinking, though I certainly didn’t know it at the time.  I lived in a beautiful place during my childhood – Ithaca, NY – in a landscape full of well-kept woods and beautiful streams, with impressive waterfalls, gorges and a big lake, and my friends and I spent most of our childhood rambling through it all,  hiking, swimming and fishing… and marveling at the oddities and complexities of the nature around us. We saw things when we played: crayfish, waterbugs, salamanders, wasps, bees, spiders, frogs, toads, fossils, trees, wildflowers, wild berries, hawks, song birds,  snakes, lizards, mushrooms, caterpillars, butterflies… and we were curious about them all. We collected anything that crawled, flew and swam (looking back on it, our mothers were saints) and wanted to know more about them, to understand them because they were alive and strange. We asked our parents for information.They. knew a bit, but not much. We read books, which told us more – the Golden Nature Guides, the Peterson Field Guides – but still not enough. In the end, we taught ourselves.

Like all scientists, we began to specialize. Because our parents had bought us butterfly nets (catching butterflies was an acceptable childhood “hobby” in the 1960’s, like stamp collecting), we started making our own butterfly collections.  We loved this hobby because it was easy and fun to go on “missions” to collecting spots, and because it was both physically and technically challenging (the really cool butterflies were both hard to find and hard to catch – you had to know where to look and you had to be fast).  There was biology to learn – butterflies were tied to their foodplants: black swallowtails hung out near queen anne’s lace, monarchs near milkweed; these flowers grew in different places. There were technical skills to learn – how to catch butterflies without damaging them, how to kill them quickly and effectively, how to mount them and dry them (we found biology supply catalogues, we bought spreading boards and insect pins),  how to preserve the important data concerning where and when we caught them.  It was complicated and seemed grown-up. We became “experts” in something, which was cool. Our parents were (I guess) amused and…  a little impressed.

We quickly found out that something that seemed simple – catching a bunch of pretty colored insects and putting them in boxes – was actually demanding and nearly endlessly complex and mysterious.  A lot of the butterflies that were the coolest, the rarest, the most beautiful, lived in strange places – treetops, the edges of swamps and streams, sunlit clearings in deep woods – and only flew in certain seasons and specific times of the day – early spring, late afternoon.  We learned why – mating rituals, foodplant availability, lifecycle requirements. We didn’t just read, we observed. We learned that the books were not always right – insects are really variable and behave differently in different locales.  We developed hypotheses, collected information that supported or contradicted them. We learned, at least concerning a couple dozen species of butterflies in the part of upstate New York where we lived, how nature worked. Nature taught us the science we needed to use, and science taught us what there was to know. (Not that we knew enough to call it “science, “ of course.) It was like the world had opened up.

And the world kept opening wider: butterflies are a fairly limited group of insects, but their cousins, the moths, are much, much more numerous and diverse.  Some of them – the giant silk moths, the sphinx moths – are even bigger and more spectacular than the biggest butterflies,  yet few people notice them. Moths fly at night and hide during the day, so they live in an unseen world. One of my friends took a vacation at a cabin at one of the local state parks and came back with giant purple-eyed polyphemus moths and ethereal green luna moths that he had caught at the cabin light… we had read about them in books, but they seemed mythological and perhaps part of a forgotten past. Who knew that they had been around us all along?  There was a whole new world out there that we knew nothing about…

And so, as we approached adolescence, we began taking journeys into the countryside where lights were scarce (and woods and fields were broad) and staying out very late on summer nights, having found our own lost world.  There were hundreds and hundreds of species, some drab, some strange and exotic, almost all of them totally new to us, though we thought we knew nature so well. We learned light collecting techniques, baiting techniques to attract different species. We learned a lot more about caterpillars and their biology than we ever had before.  The caterpillars had always been there too, all around us in the trees and plants and bushes, but ingeniously hidden.   We learned how to find them, and marveled at their weird shapes and colors. We learned how to get moths to lay eggs, and how to raise the young. We learned about the predators and parasites that keep the moth populations from exploding (and that really upset a boy trying to raise a cool caterpillar). We began to learn ecology. Again, there was layer after layer of mystery in the world, hidden, but waiting to be explored.

I think the memory of my natural “awakening,” and the wonder of it all has been a driving force in my life. I began to understand that the world around me, despite technology, our comfortable homes, our cars and planes, is still deeply mysterious, and even the smallest things around us hold great mysteries that are still waiting to be explored, to be experienced and grappled with, to be finally (or at least somewhat) understood.  I guess my adventures with nature made me feel  curious and that curiosity has driven me ever since, whether I was collecting butterflies, or reading great works of literature, or studying the intricacies of human behavior.  As a science writer, it gives me an almost immediate sense of fascination concerning every subject I write about, whether it’s quantum physics, biochemistry, genomics, plate tectonics or patch dynamics, bioinformatics or demographics. There are things there that are unknown but that can be discovered and known. These things are amazing, if you give them your mind.

This is how I am and how most of the people I work with are, but how to get others to share this experience – to understand that science is really the great adventure of our time… well, that’s the problem.

Hence, this blog…

And so,  I looked out the window the other day and saw this moth … and had an idea.

It was not just any moth – it was an extravagant, large, orange and yellow moth – Citheronia regalis, aka the regal moth (for a better photo on the web, see: http://entnemdept.ufl.edu/creatures/bfly/regal_moth01.htm ) and, despite my aforementioned experience in collecting, it was the first time I had ever seen one alive in the wild.  In the world of American moths, it’s a famous moth. I make no exaggeration when I say that as a child I probably would have been willing give you one of my finger in order to get one. It’s not terribly uncommon here in the south (it's not common either – no giant silk moths are), but where I grew up was on the edge of its range, and the only examples I knew of from my town were in a university collection. 

The moth is one of the most beautiful insects in North America, but what the insect is really famous for is its huge, bizarre caterpillar. The caterpillar has its own name – it is popularly known as the “Hickory Horned Devil” -- and I think I can safely claim that it is one of the most unusual-looking insects (beautiful or terrifying or disgusting, depending on your perspective -- see this web video: http://www.youtube.com/watch?v=61thTKLcYhE) anywhere. From my own, admittedly child-like, perspective, it’s just plain cool.

Though I was looking out a window, about a dozen feet away,  I could tell immediately that it was a female (some training never leaves you). I also immediately knew that she had to have in her at least 100 fertilized eggs. I have learned that a newly hatched female giant silk moth will not fly anywhere until she has been successfully fertilized by a male (more about this in a later post). I was not tempted to collect her --her wingtips were damaged (this probably happened when she emerged from her pupa), but her color looked very fresh otherwise, which meant that she still had most of her eggs… I’ve never raised one of these particular bugs, but I know pretty exactly how to do it from rearing other giant silk moths, and I have some good source guides to help me (see brief bibliography at the end of this post).

 I have the technology… So I entertained the idea that I might try rearing the eggs (she’s now laid most of them) from egg to pupa, blogging as I go. Then, yesterday I found another female giant silk moth (a female luna moth, pictured above) while out walking my dog. The universe was clearly telling me to do this project.

Raising large caterpillars is an “interesting” experience where there are regular visuals to share (caterpillars change a lot as they grow), useful instructions to be passed along (for anyone who is interested in doing this too – believe it or not, you can actually sell live pupae to collectors on the internet) and a fair amount of science can come into play. I’m even going to try a simple experiment: as  I mentioned before, while C. regalis has been well-studied, there are still issues involving the biology of local vs. species-wide biology that are poorly understood.

For me, this is the point – I want to try doing and writing about “citizen” or “backyard” science that is accessible to broad audiences (I’m as interested in talking to younger people as I am to adults), on a topic that might-perhaps-maybe be appealing to the kid in all of us. (Or, if you hate gross bugs, you might read for the horror factor.)

And, yes, it’s a project, kids, that you can try doing at home. If I am successful at raising a number of regal moths/lunas to pupa/cocoon, I’d be willing to share them with some of you (for free), so you can try breeding and raising yourself. My only caveat to that offer is that I won’t send these outside the continental United States (where, at least theoretically, these moths can naturally range and not be invasive species). I will send out  at least two of one species per request, first-come-first served. If you’re an adult making the request, that’s fine, but I’d prefer that these be used for some educational purpose, rather than hatching them for perfect specimens for a collection.  Anyway, I’ll post more about that later when I’m closer to having some successfully reared insects.

My regal moth eggs were first laid July 27, 2012 and the luna laid her first egg last night (7/29) and they  are expected to hatch 6-10 days from laying. My next post will be when they do.

Bibliography:

For an excellent book about giant silk moths and their biology, see:
Tuskes, Paul M., Tuttle, James P. and Collins, Michael M. 1996. The Wild Silk Moths of North America. Cornell University Press, Ithaca,NY.