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.