Apply extract from a dead ant on live pupae (white), and worker ants carry the pupae to the refuse pile.

How does a worker ant recognize a dead ant? The answer has puzzled scientists for years, but new work by Dong-Hwan Choe and his colleagues may have provided an answer.

Choe was born and raised in Seoul, South Korea. After receiving his undergraduate degree there he came to the University of California in Riverside to study entomology: insects, specifically Argentine ants.

“There is better opportunity in the U.S. for studying entomology,” Choe told me. In South Korea, as in much of the world, universities don’t have entomology departments. Instead entomologists are scattered throughout biology and biomedical research departments.

The United States is an exception, perhaps due to its large agricultural industry, for whom pest removal is a lucrative problem worthy of research funding. In urban entomology, the study of insects associated with people and cities, big money comes from pest control research. The United States, for example, contains many termite-susceptible buildings, so research aimed at improving our understanding of termite biology and behavior is relatively well funded here.

It’s ironic that entomologists enjoy studying insects, but funding for research comes from companies that want to kill insects. Choe was funded by a scholarship from the Western Exterminator Company.

His sponsors can be proud, as Choe, working in the laboratory of Michael Rust, found that live ants secrete two chemicals, dolichodial and iridomyrmecin, which rapidly degrade upon an ant’s death. The absence of these chemicals appears to unmask a signal that marks a dead ant, which, once identified, is removed from the colony. (This process is known as necrophoresis.)

When Choe treated baby ants, called pupae, with extracts from dead ants, worker ants carried pupae to the refuse pile. (See photo.) Pupae were then treated with dolichodial and iridomyrmecin and placed in a foraging area near the nest. Workers initially ignored the treated pupae and took longer to bring them back to the nest, suggesting that dolichodial and iridomyrmecin are chemicals that adults use to signify life.

Due to technical limitations, Choe did not put dolichodial and iridomyrmecin on a dead adult worker and seeing if this delays its removal to the refuse pile. (Adult ants are soft and easily damaged, unlike the more solidly constructed pupae. The experimental manipulations damage the adults and cause the release of bodily secretions that could confound the results.)

Choe hopes to remain in the United States for a postdoctoral research fellowship, and then become faculty at a U.S. university. Otherwise, he’ll return to South Korea.

His family in South Korea understands that professional opportunities are good in the United States, although they want him back. With direct flights it’s only a 12-hour trip between Los Angeles and Seoul. Choe and his family have visited one another one or two times a year.

For Choe, the distance is “not a huge deal.” Proving how ants recognize their dead – that’s a different story.

Source: “Chemical signals associated with life inhibit necrophoresis in Argentine ants” by Dong-Hwan Choe, Jocelyn G. Millar and Michael K. Rust, published online in PNAS on May 4, 2009 (doi: 10.1073/pnas.0901270106).