One day, as Wooly Bear was eating its lunch of poison hemlock, a monstrous thing happened. A huge, hideous maggot squirmed out of Wooly Bear's body, and then turned into a fly. While this may sound like a scene from Alien 8, it's actually science, not science fiction. And the story gets even better.
The wooly bear is a caterpillar that becomes a tiger moth, and is the subject of study by Professor Rick Karban and his colleagues in the Department of Entomology at UC Davis. The researchers are interested in how this insect copes with a particularly nasty parasite. During the spring, wooly bears munch vegetation voraciously, growing rapidly to become plump and succulent by early summer. This is when parasitic flies inject their eggs into unlucky caterpillars. The fly larva develops inside the wooly bear's abdomen, living off its fat and eventually taking up the whole abdominal space. When the maggot has finished growing, it makes a hole in its host and emerges, Alien-like, from the caterpillar's body, after which it spends some quiet time as a pupa before finally unfolding as an adult fly. The trauma is so deadly to the caterpillars that the flies are known as parasitoids, meaning lethal parasites. In fact, it was several years into their study before the entomologists realized that wooly bears frequently survive the ravages of the fly, and go on to become perfectly healthy tiger moths, mating, and producing offspring. How do they withstand an attack by parasitoids? Their survival seems to be related to what they eat.
In their field site at Bodega Marine Reserve, Rick Karban and his team collected young caterpillars from the two most abundant food plants, lupine and poison hemlock, the stuff that Socrates used to kill himself. They brought the wooly bears back the lab, where they reared them on one or other of the two plants, and followed their fates. Using an ultra-sound monitor, the scientists could determine which caterpillars were infected with fly larvae. They discovered that when the wooly bears were free of parasites, those reared on lupine survived better than those reared on hemlock, whereas the reverse was true for parasitized caterpillars. The ones reared on hemlock were more likely to survive than the ones eating lupine.
What would a wooly bear prefer to eat if it had the choice? The entomologists asked the caterpillars just this question. At Bodega Bay, they placed young caterpillars on sites in the shrubbery where branches of lupine and hemlock intertwined, and then observed where the wooly bears settled to feed. After this choice test, the scientists carried the caterpillars back to the lab where they dissected them in search of fly larvae. The caterpillars that had chosen hemlock were parasitized, whereas those that had preferred lupine were free of infection.
The UCD team concluded that infection with the parasitic fly somehow induces the caterpillar to switch from its preferred diet of lupine, to hemlock which somehow protects it against the effects of the parasitoid. This looks intriguingly like self-medication, although the "medicine" does not get rid of the fly. A hemlock diet might even help the flies grow slightly bigger inside their hosts.
This study is the first case where an insect has been shown to switch food preferences in response to a parasite attack. Biologists working on other species, however, have reported apparent cases of "medicinal" feeding. Chimpanzees, for example, swallow certain types of leaves when they are suffering health problems caused by intestinal worms. The leaves seem to ease the symptoms and might help flush out the parasites.
Ecologists have long been fascinated by the question of why different
animals eat what they do. It is commonly assumed that seeking
nutrients and avoiding toxins are the two major rules that guide
diet choice. It is becoming evident, however, that feeding for
medicinal reasons plays a larger role than was previously considered.
In the bigger picture, studies of many different species over
the last twenty years have demonstrated the importance of parasites
and diseases in shaping animals' behavior and even their social
systems.
Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu