ANN ARBOR—Azteca ants are voracious predators that live on coffee plants and aggressively defend their territories. That’s generally good for the coffee plants, which are protected in the process against all sorts of insect pests.

But the whole system goes awry when parasitic flies called phorids enter the picture. When they get the chance, the flies lay eggs in Azteca ants’ heads, but they also influence the ants’ behavior, with far-reaching results, a University of Michigan graduate student has discovered.

“When phorid flies appear near an ant nest, the ants all run back inside the nest, severely limiting their ability to search for or attack their prey,” said Stacy Philpott, who will present her findings Aug. 5 at the annual meeting of the Ecological Society of America. “The result in itself is surprising since most people think of a parasite as something that kills its host, not as something that has an effect by changing the behavior of its host.”

In experiments conducted on coffee plantations in Mexico, Philpott found that Azteca ants are more efficient predators than other ants, finding and eliminating caterpillars and other potential pests from coffee plants faster than other common ants in the same farms. But when she compared ant attacks on caterpillars on coffee plants where phorids were found with those on phorid-free plants, she found that the numbers of ants patrolling the plants were cut in half when phorids were present. In addition, ants took more than twice as long to carry away caterpillars on plants with phorids, and some caterpillars on those plants escaped ant attacks altogether.

The system illustrates what ecologists call a trophic cascade, in which changes higher up in the food chain cascade or trickle down to affect organisms at lower levels.

“Having phorids in the system cascades down to affect Azteca prey via their interactions with Azteca ants,” Philpott said. “The phorids, however, may also affect how Azteca interact with other species of ants or with other predators such as spiders, with potentially more widespread effects in coffee agroecosystems.”

It’s also possible that birds and other predators on insect pests compensate for the Azteca ants’ reduced activity, said Philpott, who plans to investigate the system further. “The overall impacts on coffee production will depend on the interactions among all these groups of predators.”