A Southern Strategy

The votes are in…

… and surprise, surprise, the southerners have simple tastes.

As I noted a couple of posts ago, I decided to try a simple experiment with the hickory horned devils (aka regal moth caterpillars) and how they react to different foodplants. I did this because they are known to eat a wide variety of trees, but some of my source books  say (this is what scientists call “reviewing the literature”) that ” local populations may have different preferences.” I decided to try my caterpillars on sweet gum and hickory, both of which are described in the books as “primary” choices for the moth.  I assumed that they would be willing to eat either, but I was going to test which produced bigger, healthier caterpillars.

Well, as I said, surprise, surprise – the experiment was over almost before it began. When I gave some of the caterpillars pignut hickory (the most common hickory in our area), they clearly refused to eat it. Newly hatched caterpillars sat together on the glass walls of their hatching cage, like kids huddling in a crowd outside a classroom they don’t want to enter, and wouldn’t go near the leaves, though they were fresh and clean. After a day and a half, I began to worry that they were going to die if they didn’t eat, so I also put in some sweet gum leaves, which their earlier hatchlings had already accepted readily. Within an hour, they were on those and eating – you can see this in the picture below.

 1st instar caterpillars on sweet gum leaves. Untouched hickory leaves are in upper left corner.

So, these Charlotte-area hickory horned devils  have a definite preference for sweet gum – I would actually call it a requirement, since I’m pretty sure they would have died before eating the other plant. If I had more caterpillars to and if I was willing to let them die, I would have tried a lot of other plants and I would have pushed the experiment to the bitter end. This might have given me what scientists call “solid experimental confirmation” of the conclusion that my caterpillars exclusively feed on one tree. Actually, a fuller experiment would have been to run the experiment with caterpillars from multiple local moths (to make sure this moth’s caterpillars were not just freakishly picky) and at the same time also running the experiment with caterpillars from a moths caught in other places, say somewhere in the deep south and somewhere in the upper midwest. That would take some work, so forget it for now.

Nonetheless, I have a solid “hunch” that this piece of information is “meaningful.” Here’s why: the food preferences of my caterpillars is clear, it agrees with other “field sightings” I’ve had (the only fully grown hickory horned devil I’ve ever seen here was feeding on a sweet gum tree).  I know from long experience that clear insect behaviors are rarely “anomalies” (something unique).  Insect populations are large in any given area, population interbreeding happens every year (many times a year in some species) and natural selection weeds out “bad” behaviors with a fine-toothed comb. In other words, in the normal word of insect evolution, the major details regarding bugs in a local environment tend to average out to what works best. If my bugs like sweet gum over all other foods, chance are the whole extended family (the whole Charlotte-area hhdevil tribe) feels exactly the same way, just like I can bet you most of my red-state neighbors won’t be voting for any atheist-socialist-gun-control candidates this year (I could be wrong). The regional cuisine of Citheronia regalis  appears to be limited to sweet gum (ideally, deep-fried).

Though the best source books I have don’t say anything about this, a quick web-search of some insect/nature sites, does have some people commenting that southern populations of the moth prefer sweet  gum.  Most of these reference are vague/uncertain, clearly because no one has really studied the matter. The truth is, research in insects in general (except crop pests), not to mention research in something specific like the larval behavior of a single moth,  is not done much or well-funded, so no one really knows what’s going on here. In this age where we know a lot about a lot of biological fine points like individual genes and proteins, it sounds strange to say, but common features of nature around us have not been well-studied – by professionals. So the science of this really depends a lot on the “field reports” of amateurs like me. Think of it as mobilizing a lot of unpaid interns to do data collection.

So, I say, let’s not let our lack of scientific validity stop us from going forward with speculation here:  back to the “tentative data” I have regarding the food choices of a mid-southern population of the regal moth…

Consider the history…

 If southern regal moth populations are much more specifically focused on eating sweet gum, how does this make sense, when other populations (northern populations) are known to eat a lot of other things – and these other foods are all around in the south?

Let’s look at it the other way – do northern populations eat other things because of what is available in the north? Let’s look at range maps for both the regal moth and for the sweet gum tree:

 Though the ranges look pretty much alike (sweet gums can’t take the long, cold winter of the north and regal moths, which pupate in the ground, without insulating cocoons, probably also have an issue with cold), I notice that the range of the regal moth is somewhat larger, going a little further north than the sweet gum tree. So … northern populations of regal moth in, say, New York or Massachusetts,  must eat something besides sweet gum. This may be why the books describe the other plants as known food for the moth. If the moth’s range started in the south, eating sweet gum there, and expanded north, then it had to evolve to eat other foods in order to move further north…

We actually know that the moth started in the south and expanded northwards (as I explained in an earlier post) because we have a geological record of relatively recent (only 13,000 years ago) ice ages when glaciers covered North America about as far south as I live now. When the ice receded, plants and animals (including the regal moth and the sweet gum) moved back in to recolonize the land, scrubbed clean by ice. apparently, the sweet gum has not been as rapid in its northern expansion as the regal moth. Trees can’t fly, so perhaps the range difference is simply the result of difference of the two species spreading at different speeds across the landscape, but the maps suggest different ideas. Notice that the sweet gum isn’t present in higher elevations in the Appalachian Mountains  but the Regal Moth is – this tells us that the sweet gum is probably more sensitive to cold than the moth.

So what happened when the ice receded? It didn’t all happen at once, but when it did, large areas of land were first colonized by more cold tolerant species like hickories and walnuts, sycamores and ashes --  the sweet gums could only arrive when the south became pretty warm. The regal moth, if it had an ancestor living in the deep south (or if it was an invader from Central America, as I speculated before), had to be/become a “generalist” with very flexible food habits if it wanted to take advantage of the new landscape, so it learned to tolerate a wide variety of foods…

But the moth wasn’t always a generalist. Back before the ice, when there was no new, unsettled landscape suddenly open, the moth’s ancestors surely had done what most other moths and butterflies do – evolved a special interdependence with a specific plant or family of plants (see earlier post for explanation of how this works), because that strategy seems to work well. What was the foodplant that the ancestor ate? My “hunch” (based on extremely sketchy evidence) is that it was the sweet gum or its ancestor. One thing we tend to forget about evolution is that, though new gene combinations result in new features, the genes that made the organism what it was before are generally not lost, at least not for a while.  this means that most of  the genes that helped the regal moth ancestor live successfully with a specific host plant are still back there in the genome, and, once the conditions  return that made them useful  in the past, natural selection will encourage those genes and the traits they cause to return. The sweet gum is back… and the regal moth is reverting to its ancient ways, its ancient strategies of survival, its ancient tastes.  I certainly can’t prove it, but that’s my best guess.  Again, it’s fun to start with a little hard information and… speculate.

“Oh, I wish I was in the land of cotton, old times there are not forgotten…”  Now there’s an idea the bugs share.

Bibliography:

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

Petrides, George A., 1988. A Field Guide to Eastern Trees.Houghton Mifflin Company. Boston.

Hatching Magic

Adult male luna moth from "The Butterflies and Moths of North America.". It may be hard to believe, but these moths are much more beautiful in life than in photos, though this one is good.

Well, the luna moth eggs finally hatched today – I was beginning to wonder if the eggs were fertile. Like the regal moth caterpillars, they began hatching in a large first wave, about 30 or so hatching in the first hour. Since the eggs were not layed so rapidly, it leads me to wonder if they emit a pheromone or something that coordinates hatching.

Hatched luna caterpillars -- click to enlarge.

The caterpillars are light green and tiny – they are about one half the size of the regal moth hatchlings (as were the eggs). I have raised lunas before, so I know what they are going to grow up to look like – beautiful fat, green corrugated caterpillars. I'm feeding them sweet gum, like the hickory horned devis, and they appear to like it just fine.

If you have never seen a live luna moth, I have to say that I think it is one of the most beautiful creatures in the insect world. There’s a reason why the Lunesta sleeping pill people use the moth (or a bad cartoon of it) to symbolize their product. I have read numerous stories about people finding one by their front porch light and calling the newspaper or the local natural history museum to report it, assuming that since it is so spectacular looking, it must be spectacularly rare. They’re not rare, in fact, they’re just hard to see.

As I said, I’ve raised this moth before – in fact it’s the last giant silk moth I raised before the current batch. When my children were small (about 20 years ago), I brought them to my mother’s house in upstate New York. It was summer and my oldest daughter was about 6. One night, on a whim, I took her on a drive to visit the places I used to go to collect moths when I was a teenager, and we caught a luna – it was high on a wall, underneath a light and its extravagant, long tails hung down like the wings of a magical fairy or an exotic bird. It was a female too, and I got it to lay eggs and brought them back with us to Arizona, where I raised the caterpillars. The whole thing made  such an impression on my daughter that years later she got a tattoo of a luna on her back.

I can’t say that I was pleased when I first saw the tattoo (no parent that I know of is pleased by their children’s tattoos, though science writer Carl Zimmer may try tell you otherwise) but it did soften the blow a bit when she told me that she got it in memory of that special evening with me, so long ago. I have to admit that the tattoo artist really did a good job, accurately capturing the luna, and the moth is printed on me too, in my memory.

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.