Shhh! Is that scrape a caterpillar scrap?

A series of staged insect spats reveals the first known acoustic duels of caterpillars.

Larvae of the hook-tip moth, a common resident of birch and alder trees in the northeastern United States, spin silk stitches to create folded-leaf retreats, explains Jane Yack of Cornell University. Should another caterpillar have the impertinence to wriggle too near the masterpiece, an exchange of leaf-scraping and drumming breaks out. The insects use the drumming--so loud a person can detect it several meters away--to compete for territory without violence, Yack and her colleagues report in the Sept. 25 PROCEEDINGS OF NATIONAL ACADEMY OF SCIENCES.

Scientists too often dismiss caterpillars as "eating machines," Yack grumbles. Although naturalists have published the occasional report of noise from various species of caterpillar, previous studies focused on sounds from species that lure ants to serve as bodyguards.

The work with hook-tip-moth larvae represents the first analysis of caterpillar-to-caterpillar conversation, she says, and it gets pretty sophisticated for simple leaf crunchers. "This is exciting because it's opening the door to what caterpillars are saying," she says.

Yack was rearing caterpillars to expand her earlier studies of insect hearing (SN: 1/22/00, p. 54) when she heard ticking sounds. "I thought it was the refrigerator at first," she says. However, recordings of airborne and leaf vibrations confirmed that the scraping and taps came from solitary larvae of the hook-tip moth, Drepana arcuata.

When they hatch, the 2-millimeter-long caterpillars share a communal shelter. As an older, centimeter-long larva, each seeks a leaf of its own. Doubling over part of the leaf and fastening it with silk takes a caterpillar several hours.

Yack and her colleagues staged 53 invasions of occupied leaves. As the intruder approached, the resident tapped and scratched with its mouth parts and with a pair of oarlike projections on its rear.

Intruders scraped and drummed, too, but they usually retreated within minutes, the researchers report. When Yack replaced a resident with a squatter, it typically drummed up a storm and retained possession when the original home builder tried to return.

Caterpillar anatomy suggests that the larvae respond to vibrations of a plant rather than airborne sounds, Yack notes.

Another student of insect vibrations (SN: 3/21/01, p. 190), Rex Cocroft of the University of Missouri in Columbia, calls the work a "lovely study." Because researchers can hear the dramatic choruses of crickets, katydids, and other users of the airway, that's what they first investigated. "The proportion of the insect world using airborne sounds is just the tip of the iceberg," he says.