by Joseph DeSisto
Meet the ornate tiger moth (Grammia ornata):
Tiger moths are some of the most beautiful in North America, with flashy warning colors to startle predators. But the real action, and the story we’re going to talk about today, takes place before the moth ever hatches from its coccoon. Here is a tiger moth’s caterpillar (different species, this time Grammia doris):
Tiger moths all belong to the subfamily Arctiinae, and their caterpillars are sometimes called woolly bears, a reference to their often dense coating of hair. Don’t be fooled by their fuzzy, warm appearance — those hairs are barbed and meant to protect the caterpillar against predators. Birds and other attackers find it hard to get a hold on such a hairy insect. Even if they do, hairs easily detach and get stuck in a predator’s skin, where they can be extremely irritating.
Yet irritating hairs are no defense against predators that eat from the inside out. Tachinid flies, for example, lay their eggs on the caterpillar while it is still alive. The fly maggots, upon hatching, begin to eat away at the insides of the caterpillar, growing at their host’s expense. Finally, when the maggots are ready to emerge as adult flies, they burst out of the caterpillar’s body, killing it in the process.
Once the fly’s eggs are attached, a caterpillar doesn’t have many options — unless it’s a woolly bear. At least one of these caterpillars, Grammia incorrupta, has an unusual little trick. Many caterpillars are specialists, eating only one or a few types of plant — monarchs, for example, eat only milkweed. Woolly bears are just the opposite, and will eat almost anything leafy and green, from dandelions to clover to lettuce.
Despite their catholic appetites, woolly bears understandable avoid poisonous food. When a caterpillar ingests toxins, its body has to break them down and minimize any damage. That can take a lot of energy, energy better invested in growing as quickly as possible.
All of that changes when a tachinid maggot arrives. If given the opportunity, a maggot-laden caterpillar will actually seek out toxic plants, especially those that are full of pyrrolidine alkaloids. Pyrroladine alkaloids are chemicals that many plants make — tobbacco and carrot leaves have them, for example. Like many plant-manufactured alkaloids, they evolved for the exact purpose of killing or deterring insects, but the woolly bear can use this to its advantage.
By stuffing itself with toxins, a woolly bear can kill the maggot inside its body. Experiments have shown that even though caterpillars without maggots can become sick if they eat too much toxic food, caterpillars with maggots have a much higher survival rate if they are allowed to eat food with pyrroladine alkaloids (Singer et al. 2009). As with any medicine, there is a fine balance — overdose can be fatal. But the risk is worth the reward, since otherwise, the odds of survival for a maggot-carrying caterpillar are essentially zero.
The tale of the woolly bear is one of the best-known examples of self-medication by an invertebrate. We now know, however, that many other insects seek out chemicals to help rid themselves of parasites.
Wood ants are known to gather bits of pine resin, hardened sap, to embed within the walls of their nests. The resin is toxic to many of the ants’ diseases, including harmful bacteria and fungi (Chapuisat et al. 2007). Unlike pyrrolidines, resin isn’t an insecticide, so the ants can use it as a preventative medicine, hoarding it in their nests even when diseases aren’t around (Castella et al. 2008).
One of my favorite insect medicine stories comes from the humble fruit fly. Alcohols are usually quite toxic to animals, even humans. Fruit flies, though, are the lead-bellies of the animal kingdom — they need to have a high tolerance for alcohol, since they eat the yeast that grows on fermenting fruit. So when they find themselves full of parasites, they drink a little extra (Milan et al. 2012).
No symbolism there. None at all.
Castella G., M. Chapuisat, and P. Christe. 2008. Prophylaxis with resin in wood ants. Animal Behavior 75(4): 1591-1596.
Chapuisat M., A. Oppliger, P. Magliano, and P. Christe. 2007. Wood ants use resin to protect themselves against pathogens. Proceedings of the Royal Society of London B 274: 2013-2017.
Milan N.F., B.Z. Kacsoh, and T.A. Schlenke. 2012. Alcohol consumption as self-medication against blood-borne parasites in the fruit fly. Current Biology 22(6): 488-493.
Singer M.S., K.C. Mace, and E.A. Bernays. Self-medications as adaptive plasticity: Increased ingestion of plant toxins by parasitized caterpillars. PLoS One 4(3): e4796. doi: 10.1371/journal.pone.0004796