Woolly Bear Caterpillars Self-Medicate -- A Bug First

Ker Than
for National Geographic News
March 13, 2009
Some caterpillars munch on drug-laced leaves to rid themselves of crippling parasites, a new study finds.

The research, which involved furry moth larvae called woolly bears, is the first clear demonstration of self-medication among insects, said study team member Elizabeth Bernays of the University of Arizona.

In the spring, parasitic flies lay their eggs inside woolly bears. When the fly larvae hatch, they feed on the innards of their hosts before exploding out of their abdomens.

But when infected caterpillars eat leaves from senecio and other plants, the animals get bellies full of drugs called alkaloids. Alkaloids familiar to humans include caffeine, morphine, and cocaine.

The scientists are not sure whether the alkaloids attack the parasites directly or if the drug somehow gives the woolly bears' immune systems a boost.

But the alkaloid-laden leaves do cure the insects.

Bernays and her colleagues showed that infected woolly bears eat more toxic alkaloids than their non-infected peers. Healthy woolly bears also ingest alkaloids, but only in small amounts, apparently to make themselves unsavory to predators.

In addition, the team showed that parasite-free woolly bears that binge on alkaloids are more likely to die compared with woolly bears that take the drug in moderation.

"It's a balancing act," Bernays said.

The new finding challenges the idea that self-medication is restricted to relatively intelligent creatures that are capable of learning, such as primates.

For example, chimps can learn which drugs to take to cure their ills and can pass on that knowledge to others.

Something much simpler is probably happening in insects.

When a woolly bear is infected by parasites, its immune system may react by altering taste receptors so that the animals crave more alkaloids, Bernays said.

Insects "have a system that's based on changes to their taste system, rather than the cognitive ability of their brains," she added.

The research is detailed in the journal PLoS One.

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