Home-wrecking beetle: who'll pay the piper?

Home-wrecking beetle: Who's pay the piper?

It's a match made in ant-plant heaven: What one partner lacks, the other provides. But when an unwelcome house-guest arrives, the relationship falls apart.

The drama unfolds in Costa Rican tropical forests, where Pheidole bicornis, a species of brown ant, makes its home inside young piper trees, also known as pipe ant-plants. Entering through the petioles -- hollow chambers connecting leaves to stems -- the ants bore tunnels through the tree without harming it. At mealtime they feast on white, swollen "food bodies" -- nutrient-packed cells specially produced by the petioles while the ants are in residence. In turn, the ants protect the piper tree from predators, casting off or devouring the eggs and larvae of opportunistic insects that attach to the leaves or stems.

But sometimes a group of beetle larvae horns in on the ants' turf, infiltrating the piper and rapidly destroying the symbiosis between ant and plant. When Phyllobaenus larvae worm their way into the petioles, they wedge their soft bodies in protective crevices, snacking on ant larvae and using their tough, jaw-like mandibles to crack the heads of the adult ants. Though the intruders damage leaves and offer no protection against other predators, they nevertheless manage to keep the piper producing the same swollen food bodies long after the ant colony's demise.

Parasites that upset or destroy a symbiotic relationship are not uncommon among plants and insects. Consider, for instance, the ability of nectarless flowers to lure bees away from other plants by mimicking the coloration of nectar-producing blossoms. Similarly, the viceroy butterfly mimics the colors of its inedible relative, the monarch butterfly, to fool predators.

But the beetle differs from these and other symbiosis-meddlers in that it manages to freeload without imitating the color or shape of its rival, asserts ecologist Deborah K. Letourneau of the University of California, Santa Cruz. "No one knows how the ant or the beetle triggers [food] production," she says. But Letourneau suspects the beetle larvae use a chemical trigger, possibly similar to one the ants may use, to trick the piper into manufacturing more food.

Letourneau found that even ant-free piper trees produce food bodies when the beetles invade -- another indication the beetles have cracked the ants' food code and usurped it for their own purposes, she reports in the April 13 SCIENCE. In addition, Letourneau told SCIENCE NEWS she has recently identified a related species of Phyllobaenus that infiltrates Costa Rican piper trees with similar success.

Understanding the mechanism behind the beetle's behavior might alter the way scientists view other parasites that sever symbiotic relationships, she suggests. "Every time we think we have understood a system, nature suggests we must have overlooked something. The interactions [in other systems] may be more complex than we think," she says. But to this conjecture she adds: "I can't tell for sure. I've been too engrossed in this project for the past 10 years."