Caterpillar Appetite Is Clue to Cure

In an unusual evolutionary adaptation, caterpillars infected by parasites developed an enhanced appetite for chemicals that killed the invaders, researchers reported in the journal Nature this week.

Taste bud-like cells of the parasitized caterpillars grew more sensitive to the chemicals, researchers said, explaining previous observations that the caterpillars changed their eating habits when they got sick, actively seeking out plants to treat their infestation. The parasites, which look like spiny house flies roughly a quarter-inch long, feed on the caterpillars’ internal organs.

“There are no other known examples of an enemy like a parasite being able to change a sense cell,” said the study’s lead author, entomologist Elizabeth A. Bernays of the University of Arizona. “In this case, the change is beneficial to the host and not the enemy.”

The caterpillars, Estigmene acrea and Grammia geneura, are found in the mesquite grasslands of southern Arizona. They grow to a few inches in length and are often called “wooly bears” because of their half-inch-long brown, black or white hair.

Bernays and her colleagues used microelectrodes to record electrical impulses from cells on appendages around the caterpillars’ mouths.

The caterpillars, which transform into brightly colored tiger moths, normally feed on plants that contain pyrrolizidine alkaloids and iridoid glycosides, which are toxic to the parasites. But a bitter taste in chemicals the plants contain sometimes deter the caterpillars from eating them.

“The plants get more ‘tasty’ because of increased response to the important chemicals as well as decreased response to the deterrent ones,” Bernays said.