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.